Technical Questions and Answers Archive

Shotcrete is just a placement method for concrete. Shotcrete placement with proper materials and application techniques should provide monolithic concrete with a 28-day compressive strength of at least 4000 lb/in2 (28 MPa). Thus, any systems that work in concrete should be fine. Either mechanically-fixed or epoxy-set anchors are commonly used in concrete. You should consult with the anchor suppliers for the size and type of anchor appropriate for your specific application.

In normal weather conditions test panels should be undisturbed for at least 24 hours. In cold weather you must protect the panels from freezing and preferably keep the concrete surface temperature above 50 to 55 °F. That allowsthe young concrete to gain enough strength to tolerate movement. Also, your test panels are very small in comparison to current ACI 506.2 Specification for Shotcrete requirements that have a minimum of 16 by 16 by 51⁄2 in. (400 by 400 by 140 mm) dimensions. When coring your smaller panels you should be sure to have the nearest edge of the core 3 in. (75 mm) from the sides to preclude the effect that rebound collection in the corners may cause.

Shotcrete placement conveys concrete materials through a delivery line that ranges from 11⁄2 in. (38 mm) to 21⁄2 in. (64 mm) in diameter. A 2 in. (50 mm) diameter at the nozzle is most common. Wet-mix shotcrete pumps wet concrete through the line. Dry-mix conveys dry concrete materials through the line and adds water at the nozzle to make concrete. Using proper concrete materials, equipment and placing techniques both wet-mix and dry-mix should provide high strength, durable concrete for your pool. Wet-mix is more difficult to pump as it has more internal friction pushing the low-slump concrete through the line. Shotcrete contractors will try to minimize the delivery line length in wet-mix to make pumping easier and reduce the chance for plugs in the line. With proper planning, concrete mixture selection, and delivery line choices, wet-mix shotcrete can be pumped 500 ft (150 m) or more. The shotcrete contractor can increase the pumping distance by using steel pipe for most of the delivery line with rubber hose the last 50 to 75 ft (15 to 23 m), or using larger diameter hose or pipe and then reducing to the final size closer to the nozzle. If the shotcrete contractor is using dry-mix (often called Gunite) they should easily be able to reach over 500 ft as the delivery hose is mostly full of air conveying the dry concrete materials. This means you don’t have nearly the internal friction that would tend to cause plugging of the line, and thus allows for longer delivery lines.
Thus, if using wet-mix, check with the shotcrete contractor and see if they can make adjustments to their delivery lines to allow a greater pumping distance to accommodate your lot. If using dry-mix, they should have no problems running the hose to your back yard assuming they have enough hose.

Shotcrete is a great solution to your wall casting issue. In all shotcrete repair to get the best bond you need to:
1. Chip back to sound substrate – all the rock pockets and voids should be chipped out (or you can use hydrodemolition) to sound concrete.
2. If the chipped-out area is deep into the wall, make sure to have the opening at about a 45° angle from the back of the chipped out area to the surface so that the air flow providing shotcrete’s high velocity can escape and not be trapped.
3. Do not feather edge the perimeter of the repaired opening. Provide a ¾ to 1 in. (19 to 25 mm) roughly square shoulder at the perimeter edge. If this is sawcut make sure the sawed surface is roughened before shotcreting.
4. Thoroughly clean the chipped-out area to remove all dust.
5. Bring the entire chipped out area to a saturated surface dry condition.
6. Do NOT use a bonding agent. It will detract from the inherent excellent bond of shotcrete.
7. Use an experienced shotcrete nozzleman (ACI-certified in the vertical orientation for the process being used) with a quality concrete mixture, and proper shotcrete equipment.
8. Make sure the shotcrete finishers are experienced and do not tear or delaminate the shot sections.
9. Protect the freshly shot and finished sections from freezing or extremely hot weather.
10. Cure the shot sections for a minimum of 7 days. A water cure is preferred to a curing membrane. Either wet-mix or dry-mix would be suitable for your project.

The shotcrete contractor you select for the project should recommend the process they are best suited for based on their crew experience and equipment. Appropriate testing for this type of repair may include compression testing of the materials from shotcreted panels (ASTM C1140 Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels, ASTM C1604 Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete, and ACI 506.2 Specification for Shotcrete), and bond pull-off tests to verify the bond of the shotcreted material to the original substrate. For more guidance on shotcrete and its use in concrete repairs, you may want to review ACI 506R-16 Guide to Shotcrete, as it can give you more detailed information about shotcrete materials, surface preparation, shotcrete crews and placement, testing, protection and curing.

Shotcrete is a placement method for concrete so most admixture or supplemental cementitious materials that can be used in cast concrete will work with shotcrete placement. In fact, shotcrete contractors have been some of the most innovative adopters of new concrete technologies. Silica fume (microsilica) is a ultrafine particle 100 times smaller than cement that enjoys early and wide use in shotcrete due to its ability to make concrete stickier and more cohesive. This facilitates overhead placements and can provide thicker layers. Shotcrete has also used other ultrafine and nanoparticles, like colloidal silica, clay-based particles and the carbon nanotubes you mentioned.

The ultrafine and nanoparticles can provide many benefits to fresh concrete, as well as hardened properties. This may include:
• Improve the pumpability of wet-mix concrete;
• Reduce rebound and dust due to increased “stickiness”;
• Ease the finishing process;
• Reduced permeability by filling pores between cement in the paste;
• Enhanced corrosion resistance for embedded reinforcement;
• Enhanced resistance to chemical attack.

Yes, shotcrete is a placement method for concrete, so any coating appropriate for concrete would be applicable specify new concrete should be a certain age before applying their coating. Generally, the concrete surface should be clean and dry before coating. The surface texture provided on the shotcrete can affect the coating application. A hard, smooth steel trowel finish will tend to be quite slick, and the coating may not bond as well as a floated or sponge finish. A light abrasive blast may be considered to roughen the surface and give more bond. If using a gun or rodded finish for the shotcrete, the coating will generally require quite a bit more material to be able to fill the depressions in the surface.

Both wet-mix and dry-mix shotcrete when using proper materials, equipment and placement techniques will produce high strength, low permeability concrete in-place. Dry-mix shotcrete will tend to have a lower w/cm since water is added to the dry concrete materials at the nozzle. Wet-mix needs a higher w/cm and a fairly high cement paste content to facilitate pumpability. This can make wet-mix more susceptible to plastic or drying shrinkage cracking than dry-mix. However, cracking in either dry-mix or wet-mix shotcrete can be controlled by using fogging of freshly finished surfaces and then early curing of exposed shotcrete surfaces.

Wet-mix still has very low w/cm (0.40 to 0.45) to allow vertical and overhead placement without sloughing or falling out and is lower than much of the form-and-poured concrete. The low w/cm and high velocity impact produces excellent compressive strength and low permeability. Properly placed dry-mix will have similar compressive strength and permeability as wet-mix. strength gain.

ASA is the international trade association dedicated to advancement of shotcrete placement. The American Concrete Institute (ACI) has a long history of certification of individual craftsmen in concrete construction. Nearly 20 years ago ASA worked closely with ACI to establish the ACI Shotcrete Nozzleman certification program to evaluate a nozzleman’s ability to place quality shotcrete. ASA is the primary group that conducts the ACI Nozzleman Certification sessions around the world. We provide a full day of education before the formal ACI certification and have a team of examiners with proven shotcrete expertise to rigorously conduct the sessions to ACI’s high standards. Many engineers specify that all the nozzlemen on their projects must have ACI Nozzleman Certification. Many concrete-related codes, specifications and standards require ACI-certified shotcrete nozzleman for shotcrete placement of structural concrete.

Thus, ASA doesn’t offer an expert opinion on individuals as an association but provide the ASA education and ACI certification upon request. You will find full details of the session services we can provide on our website at shotcrete.org/ education then clicking on the “Shotcrete Nozzleman Certification Program” link. An article that covers in greater detail the overall shotcrete nozzleman education and certification process ACI Nozzleman Certification—Why, Who, When, and How can be found on our website: shotcrete.org/ wp-content/uploads/2020/05/2018Win_Hanskat.pdf

Also, many of our corporate members offer consulting services. You can find them in our free, online Buyer’s Guide at Shotcrete.org/BuyersGuide. On the web page you can select the category, subcategory and geographic location to narrow your search.

ACI Committee 506 is the technical committee responsible for creating and maintaining the ACI documents related to shotcrete. ACI 506R-16 Guide to Shotcrete is an excellent resource for shotcrete information including materials, equipment, crew composition and placing techniques. It is a non-mandatory document that is very descriptive and readable. ACI 506.2-13 (18) Specification for Shotcrete is another excellent resource that has mandatory requirements for the contractor’s shotcrete placements. Additionally, the ACI 318-19 Building Code Requirements for Structural Concrete directly addresses shotcrete for use in buildings.

A past article in Shotcrete magazine describes the additions to cover shotcrete in ACI 319-19: shotcrete.org/wp-content/ uploads/2020/06/2019Fal_HanskatHollandSuprenant.pdf

Another past article dealing with Shotcrete Testing—Who, Why, When, and How can be found on our website here: shotcrete.org/wp-content/uploads/2020/01/2011Sum_Hanskat.pdf

Shotcrete is a placement method for concrete. Concrete mixtures that have been shotcreted have used all different supplemental cementitious materials (SCM) including slag. Silica fume, and fly ash are also widely used. Generally geographic availability and cost of the slag are a deciding factor on if slag is included in a concrete mixture design. Concrete using alternative binders or cements have been used for specialized applications like refractory, or extremely high early strength gain. Here are links to past articles from our Shotcrete magazine that have considered slag in shotcreted concrete mixtures:

The Use of Slag Cement, Alkali-Free Accelerator, and Macro-Synthetic Fibers – https://shotcrete.org/wp-content/ uploads/2020/05/2016Spr_Yurdakul-etal.pdf

Sustainable Shotcrete Using Blast-Furnace Slag – https://shotcrete.org/wp content/uploads/2020/05/2013Fal_Sustainability.pdf

Wet curing of newly placed concrete is certainly important. Your 10-day cure sounds great. Properly placed shotcrete against a properly prepared construction joint will provide concrete that acts monolithically and will not be a “cold joint” as is common in cast concrete. There are three key factors for joint preparation:

1. The joint must be roughened. This should be done when the crew finished for the day using a stiff broom or raking with a trowel.
2. On the subsequent shoot the surface should be cleaned (usually using a high pressure, pressure washer).
3. Before shooting bring the surface to a saturated surface dry (SSD) condition. SSD means the pores of the concrete have been filled with water but there is no running water on the surface.

Following these 3 steps are essential for creating a monolithic, watertight joint. Here’s a link to an article on why shotcrete doesn’t have cold joints: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf

Quality shotcrete placement requires a well-designed concrete mixture, proper equipment, attention to the surface prep, proper placement techniques, and curing. One aspect that can help you ascertain the nozzleman’s expertise is to require they have a current American Concrete Institute Shotcrete Nozzleman Certification. You can verify an individual’s certification status at: https://www.concrete.org/certification/verifyacertification.aspx?d=Ask

WWF should only be used when the area of reinforcing provides the required reinforcement area. It may serve as supplemental reinforcement to bars, or entirely replace reinforcing bars as long as the required area of reinforcing steel is provided in the concrete section. Where multiple sheets (more than 2 at say a corner) overlap you may need to cutout one or more of the layers to allow good encasement. Don’t use steel fibers with WWF as they tend to bunch up where the wires cross.

WWF can be used to provide reinforcing in double curved sections where it would be hard to bend and place reinforcing bars. Also, it may be used where you have a thick concrete cover or unreinforced thickness of a repair area that you wanted to get some reinforcing in place to control depth of any surface or shrinkage cracking.

If the mesh is just being provided to help support shotcrete being shot overhead you may not worry about the area of steel, just adding it in addition to the original design reinforcing. Then a lighter mesh might be used since you aren’t really worried about its ability to be reinforcing.

Casting the floor first is recommended. Note that rebound and trimmings from vertical wall placement are not fully consolidated concrete and must not be used as part of a structural concrete section. Thus, casting first allows:

• No collection of rebound and trimmings from wall construction to get trapped below floor reinforcing steel.
• Provides a stable surface to collect and remove rebound and trimmings.
• Is generally faster placement and allows full consolidation of concrete throughout the floor thickness by immersion vibrators.
• If using wet-mix shotcrete the shotcrete pump can serve as a line pump for pumping concrete into the floor thus requiring no extra equipment.
• Allows proper joint preparation at the floor/wall joint to provide a watertight joint.

You say near freezing, so we assume it did not reach freezing. Though the newly place concrete will not gain strength quickly it should not be damaged. As temperatures rise during the day the warmer temperatures should allow more strength gain. Concrete has a chemical reaction to build strength, that generates internal heat. If freezing weather was expected, we would recommend use of a protective blanket on the concrete surface to help hold the heat in and promote quicker strength gain. Generally, in concrete we like to see the concrete surface temperature at 50 °F (10 °C) or higher in cold weather to encourage strength gain.

Yes, this is definitely a good application for shotcrete. To restore the cover:

• Chip or hydrodemo back to sound concrete. If a reinforcing bar is more than half exposed, chip at least 1 in. (25 mm) back behind the bar.
• Make sure the surface is roughened and clean.
• Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp but water is not picked up on a hand.
• Make sure the shotcrete placement is properly executed. Use of an ACI-certified shotcrete nozzleman is recommended.
• No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
• If chipping out a section do not feather edge. Provide a ¾ to 1 in. (19 to 25 mm)) depth of cut at the edge to provide adequate thickness for the integrity of the shotcreted material at the edge.

This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/ uploads/2020/05/2014Spr_TechnicalTip.pdf

Shotcrete is a placement method for concrete. As the lazy river is a long concrete trough that is intended to be functionally watertight, the best guidance is ACI, 350-06 Code Requirements for Environmental Engineering Concrete Structures. As the predominate stresses in the horizontal direction result from shrinkage and temperature, Section 7.12.2.1 provides guidance on joint spacing based on the provided reinforcement ratio in the concrete section. The closer the movement joints, the less reinforcement required. Chapter 5 of ACI 350.4R-04, Design Considerations for Environmental Engineering Concrete Structures, has guidance on design of joints for water containing structures to help maintain water tightness.

We see both Pre-Bagged and Pre-Packaged terminology used. ASTM C1480 / C1480M – 07(2012) uses the rather unwieldy “Standard Specification for Packaged, Pre-Blended, Dry, Combined Materials for Use in Wet or Dry Shotcrete Application.”  Most suppliers of packaged dry concrete materials for shotcrete have formulations designed for wet-mix applications. You can find our corporate members who supply pack-aged materials on our website in the Buyers Guide (shotcrete.org/BuyersGuide), select the Category, “Shotcrete Materials-Mixture Sales” and the Subcategory, “Wet Mix.” 

Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies. 

Shotcrete is a placement method for concrete. Thus, generating the maturity curves would be based on the concrete mixtures. There are several online resources about the maturity method. One that discusses production of the curves is from the Minnesota DOT and available in PDF format at  www.dot.state.mn.us/materials/concretedocs/MaturityMethodProcedure.pdf. 

Shotcrete is a placement method for concrete. Thus, all non-destructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact Echo and Impulse Response are two one-sided techniques that can provide good results for a portion of the 24 in thickness though would likely not be able to scan the entire depth. Ultrasonic Pulse Velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth you may be able to use a MIRA system. It is sophisticated tomographic system that says it can test from 50mm to 800mm (32 in) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator so be sure to verify the firm can document successful experience with the method.

Shotcrete is a placement method for concrete. It is routinely used for a wide variety of structural applications. It has been used for both initial and final linings in tunnels where it is commonly reinforced with wire mesh, fibers, or reinforcing steel. You may want to review our past Shotcrete magazine articles on tunnel shotcrete at https://shotcrete.org/archive-search/
using keywords such as “tunnel,” “underground,” and “linings.” Also, we have two position papers from our underground committee: “Spraying Shotcrete Overhead in Underground Applications,” and “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes,” that you may find informative. Also, ACI 506.5R-09, “Guide for Specifying Underground Shotcrete,” can provide insight into topics important for using and specifying underground shotcrete.

Unfortunately, I’m not aware of any specific references to guide you in stamping or carving of fresh shotcrete for vertical surfaces. In many ways carving and stamping is a technique that requires more of an artistic flare that a finisher develops by hands-on experience. One key aspect is to minimize the working of the surface as much as possible to prevent tearing or delaminating the shotcrete surface layers. Carving or stamping when the concrete has reached an appropriate level of set (not too hard or soft) must also be judged by the experienced shotcrete finisher.

Shotcrete is a placement method for concrete. Dry-mix shotcrete (the old tradename is gunite) using proper materials, equipment, and application techniques should have easily been able to reach a strength of 4000 psi (28 MPa) in the first month. After 10 years in-place the concrete should be even stronger. The level of concrete strength developed by quality shotcrete should easily accommodate drilling in anchors or concrete screws. In my experience it would have been highly unusual to build an entire 10-story building with shotcrete even in the 1960s. I’d suggest based on the extremely weak material properties and the wall turning to dust what you think is shotcrete may be sprayed plaster or stucco. Those materials don’t have near the same strength as shotcrete and would exhibit many of the problems you have mentioned.

Spray-crete is NOT shotcrete. Shotcrete is high-velocity placement of concrete in thicker structural sections. It appears Spray-crete is a low-velocity sprayed mortar product generally applied in a very thin layer to provide texture to an existing concrete substrate. Since you mentioned the underlaying concrete pad was cast and then the Spray-crete added the cracking could well be originating in the underlaying concrete. Concrete cracks for a variety of reasons, such as drying shrinkage, thermal volume change (summer/winter cycles), inadequate curing, insufficient reinforcing steel, or settlement of the subgrade. You should locate a local professional engineer experienced in concrete slab evaluation who can evaluate your site, materials used, application techniques, and potential causes of the cracking. You may find the Florida Engineering Society and ACEC-FL has a list of firms who offer evaluation services.

Gunite is the original tradename for what we now call dry-mix shotcrete. Shotcrete is a placement method for concrete, so recommendations for curing and protection follow general ACI guidelines for exposed concrete. ASA recommends a minimum of 7 days of continuous (not just a few times a day) wet curing to help control shrinkage issues, increase strength, and reduce permeability in young concrete sections. Lack of curing and exposure to windy, hot, or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength.

Shotcrete placement with quality materials and proper application techniques generally exceeds the minimum 4000 psi 28-day compressive strength ASA recommends. The statement that 3000 psi is the “industry standard” is not true, as the ACI 350 Code for concrete liquid-containing requires a minimum 28-day compressive strength of 4000 psi for concrete intended to have low permeability when exposed to water. The required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604/C1604M provides guidance on taking cores from existing structures. A minimum 3 in. (76 mm) diameter core is recommended. Before coring, it is recommended to use ground-penetrating radar (GPR) or similar equipment to identify the location of reinforcement in the pool section, and then take cores to avoid cutting through the reinforcement wherever possible. The core holes would then need to be filled with a high-strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions.

Regarding cracking, the lack of curing will increase the concrete’s shrinkage and correspondingly the potential for cracking. You should verify that there are no significant cracks in the pool shell before the plaster or other interior coating is applied. If there are cracks, the pool contractor should repair those before proceeding with the plaster or coating. Although proper curing would certainly decrease the concrete’s permeability, generally good-quality shotcrete with proper placement and a strength of 4000 psi will be functionally watertight and not allow any significant amount of water to flow through the uncracked concrete thickness. You will find more detailed information on pool compressive strengths and watertightness of pool shells in our ASA Position Statements.

Shotcrete is simply a placement method for concrete. Most wet-mix shotcrete contractors use a 2 in. (50 mm) diameter delivery hose, so maximum coarse aggregate size should be limited to 3/8 in. (10 mm) nominal. Pumpability usually requires a good paste content. Here’s a link to an article, “Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and Placement,” that should answer many of your questions. It is rare to have wet-mix concrete mixed on site from bulk aggregate and cementitious materials. Most site-batched wet-mix uses dry prepackaged materials that have metered water addition to provide a specific water-cementitious materials ratio (w/cm) for the provided bagged mixture. We also see volumetric mixers used on site that can precisely meter the dry concrete materials and water often with needed water-reducing air-entraining admixtures.

Shotcrete is a placement method for concrete. Thus, you should place movement joints (contraction, expansion, isolation) as would be required by your local design codes for concrete. Joint spacing will depend on the amount of reinforcement used in the section to resist temperature and shrinkage volume changes. Construction joint spacing can be determined by the contractor depending on their production rates. Properly prepared construction joints will act as monolithic concrete as long as the joints are properly prepared and proper concrete materials, equipment, and placement techniques are used by the shotcrete contractor. Construction joints should be roughened, cleaned, and then dampened to saturated surface-dry conditions before subsequent shotcrete placement.

The U.S. Bureau of Reclamation has a nice summary document that you may find helpful in your design.

Shotcrete is a placement method for concrete. Both wet-mix and dry-mix produce quality in-place concrete when using quality materials and proper equipment and placement techniques. There are no compatibility problems with shooting wet-mix over dry-mix. Basically, it is just shooting shotcrete on top of already placed concrete. For proper bond, the surface of the existing dry-mix must be roughened, cleaned, and brought to a saturated surface-dry moisture condition before shooting the wet-mix lining.

Shotcrete is just a placement method for concrete. Thus, the in-place material will have the same characteristics as concrete. You should consult the coating supplier to establish how long they want the concrete surface cured. They may also specify a certain limit for surface moisture conditions. There are several methods for evaluating the moisture content of the in-place concrete. Here are the ASTM standards that deal with surface moisture:

• ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring; Section 5.2, pH Testing;
• ASTM F1869, Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride;
• ASTM F2170, Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes;
• ASTM F2659, Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter; and
• ASTM F3191, Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring.

In addition, The International Concrete Repair Institute (ICRI) has a certification program, “ICRI Concrete Slab Moisture Testing Technician—Grade 1.”

There are many variables that would need to be considered to answer your question. Water flow, depth of the application, and overall geometry can influence the required thickness. This is a question that should be addressed by an experienced engineer who can ascertain the required structural properties to resist the hydraulic loads for the depth and geometry of your application. You may find consultants who are ASA Corporate Members in our Buyers Guide.

Shotcrete is a placement method for concrete. We have shot a wide variety of concrete mixtures from lightweight to heavyweight to the use of alternative cements. Reviewing ACI 304.3R-96, “Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing,” the key to producing heavyweight concrete is the aggregates used. The document also mentions that more cement paste is needed in heavyweight concrete and that helps to increase pumpability, which is important to use with shotcrete placement. Shotcrete will allow you to minimize or eliminate formwork, so it may have benefits of reducing cost and time for construction. Wet-mix shotcrete generally has a low water-cementitious materials ratio (w/cm) of 0.38 to 0.45. Dry-mix shotcrete tends to be even lower with a 0.35 to 0.40 w/cm. Shotcrete materials achieve this using high-range water-reducing admixtures. As concrete hydrates and gains strength, the available free water in the concrete is consumed, and so may be less of a problem for shielding. Reviewing the 304.3R document, it appears reaching the 3.2 g/cc density is fairly easy, and even higher densities may be achievable. You may want to review the ACI 304.3R document, as it provides a good background of heavyweight concrete mixture design and use.

Shotcrete is a placement method for concrete. Thus, evaluating your pool shell life expectancy will be the same as any concrete structure exposed to water. You may find the technical document ACI 201.1R-08, “Guide for Conducting a Visual Inspection of Concrete in Service,” helpful in evaluating your pool shell. Generally, shotcreted concrete that uses quality materials, proper equipment, and placement techniques will serve for at least 50 to 60 years. ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures,” states: “When all relevant loading conditions are considered, the design should provide adequate safety and serviceability, with a life expectancy of 50 to 60 years for the structural concrete.” This ACI 350 Code is for liquid-containing concrete structures such as tanks for water and wastewater treatment but the original pool design may or may not meet the Code requirements.

Shotcrete is a placement method for concrete. Thus, generating the maturity curves are based on the concrete mixtures. There are several online resources about the maturity method. The Minnesota DOT provides a PDF resource that discusses production of the curves.

Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19), “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

Shotcrete is a placement method for concrete. Thus, all nondestructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact echo and impulse response are two one-sided techniques that can provide good results for a portion of the 24 in. thickness, although they would likely not be able to scan the entire depth. Ultrasonic pulse velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth, you may be able to use a MIRA system. It is a sophisticated tomographic system that says it can test from 50 to 800 mm (32 in.) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator—so be sure to verify the firm can document successful experience using the method.

Shotcrete has been used in many dam repairs, including large-diameter draft tubes. Shotcrete is high-velocity (60 to 80 mph [100 to 130 km/h]) placement of concrete. When shotcreting with proper concrete materials, equipment, placement, and curing techniques, along with complete surface preparation, you can expect a tensile bond strength of at least 150 psi (1 MPa) between the existing concrete and the newly shotcreted material. Original Portland Cement Association research by Felt from 1956 showed that 200 psi (1.4 MPa) bond shear strength is required for bonded overlays to act monolithically in flexure. Research by Silfwerbrand in 2003 showed that the ratio of bond shear strength to direct tensile bond strength ranges from 1.9 to 3.1. Thus, using the low value of the range with a 150 psi tensile bond strength yields a shear strength of at least 285 psi (2 MPa), well above the 200 psi needed. You may find more information on the bond between concrete and shotcrete layers in the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” that can be found in our article archive.

Regarding the abrasion, shotcrete displays good toughness in a wide variety of demanding applications. Quality shotcrete should have at least a 4000 psi 28-day compressive strength and, with attention to mixture design using silica fume and a low water-cementitious materials ratio (w/cm), can comfortably reach 6000 to 8000 psi (40 to 55 MPa) or more. Shotcrete also can easily use steel or synthetic fiber to significantly increase the toughness of the in-place concrete.

Finally, because shotcrete requires no formwork or bonding agent for a high-quality repair, you will find the shotcrete process provides an economical solution.

An article about a draft tube modification project can be found in our article archive.

If looking for a low-permeability concrete mixture for shotcrete placement, you should consider using supplementary cementitious materials (SCMs) such as silica fume, fly ash, or slag to reduce the permeability. You should also be sure to require a minimum 4000 psi 28-day compressive strength to assure good paste content and the ability to fully encase your reinforcement. The addition of microfibers and early wetting of the finished shotcrete surface will help to reduce the potential for early-age plastic shrinkage cracking. Proper curing for at least 7 days is also important to help increase strength gain and reduce the potential for drying shrinkage cracking. You can consult our Buyer’s Guide to locate our corporate members, who may consult with you on the mixture design. However, please be aware that many of the panel systems with an EPS core don’t use high-velocity (60 to 80 mph) shotcrete for consolidation and compaction of the sprayed concrete, but use a low-velocity sprayed mortar (LVSM). Because LVSM doesn’t have the compaction of shotcrete impact, it depends on a more sophisticated and expensive cementitious mixture, often with a latex or other admixture to improve adhesion and reduce permeability. You’ll need to consult with the material supplier of the LVSM product to verify the permeability of their in-place product.

Shooting shotcrete against a one-sided form (what you called a temporary wall) is a common way to build a shotcrete wall. Once the shotcrete sets and builds strength, the plywood form can easily be stripped off the back of the wall. Form release agents (not oil) can be applied to the plywood to make the stripping easier. Once the forms are removed and the concrete has gained adequate strength, the walls can be backfilled with compacted soil or gravel, depending on the drainage needs.

We recommend 7 days of curing. Continuous water curing is best, but if impractical, applying a curing compound on the exposed surface at twice the manufacturer’s recommended rate for a good seal is acceptable. If they remove the forms before 7 days, they should also water cure or apply curing compound to that newly exposed surface. The shotcrete needs to build up enough strength to resist the external force of the backfill, so check with the pool designer to see what they need for the required strength of the concrete before backfilling. With most good-quality shotcrete materials and placement techniques, you can expect about 4000 psi (28 MPa) compressive strength in 7 days.

This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete, so standard reinforced concrete design is appropriate for shotcreted sections.

Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.

By “gunite batch truck” we assume you are referring to a volumetric mixer supplying material for a dry-mix shotcrete operation. Gunite is the original tradename for what we now call dry-mix shotcrete. Here’s the description from an ACI Materials Journal (January-February 1991 issue) article about the calibration of volumetric mixers:

“To insure production of quality concrete, each volumetric-measuring unit must be calibrated for each respective concrete ingredient, following the manufacturer’s recommendations and ASTM C 685. These ingredients must be the same as those to be used in actual concrete production. The measuring devices for aggregates, cement, and dry admixtures are calibrated by weighing the discharged ingredient. Devices for water, latex modifier (if required), and liquid admixtures such as air-entraining and water-reducing admixtures generally are calibrated by weighing or measuring the volume of the discharged ingredient. The objective of calibration is to coordinate the discharge of all concrete ingredients to produce the proper mixture.”

ASTM C685/C685M states, “The proportioning and indicating devices shall be individually checked by following the equipment manufacturer’s recommendations as related to each individual concrete batching and mixing unit. Adequate standard volume measures, scales, and weights shall be made available for the checking accuracy of the proportioning mechanism.” Thus, you need to check with your equipment supplier for their recommended procedures to verify batching. Because concrete mixtures always are based on weight of ingredients there you will need to weigh a given volume to confirm the batching is accurate.

The air content test is a measure of total air so includes both entrapped and entrained air. Ten percent is definitely high. The 10% air is likely not representative of the in-place shotcrete. It may have been an issue with the lab mixer introducing more entrapped air for some reason. Estimating the air content from the unit weight test requires a good value for the theoretical unit weight. I’m not sure if you have that with the volumetric batching. You should run the air meter test (ASTM C231/C231M) to measure the air content to get a more accurate assessment.

Regarding the verification of mixture proportions, this is from ASTM C685/C685M:

“7.5 Proportioning Check—Whenever the sources or characteristics of the ingredients are changed, or the characteristics of the mixture are noted to have changed, the purchaser is permitted to require a check of the fine aggregate content and the coarse aggregate content by use of the washout test. Essentially, in the washout test, 1 ft³ [0.03 m³] of concrete is washed through a No. 4 [4.75-mm] sieve and through a No. 100 [150-µm] sieve; that retained on the No. 4 sieve is normally considered coarse aggregate whereas that passing the No. 4 and retained on the No. 100 sieve is considered fine aggregate. Corrections to the quantity of aggregates (per cubic foot or cubic meter of concrete) shall be made if the original sieve analysis of each aggregate is available.”

Because you are only interested in the sand and cement, you can simply weigh the sample of concrete, then wash out all the cement, and then weigh the remaining sand. You would need to bring the sand to roughly the same moisture content as the sand in the truck, so you aren’t including in the weight of excess water in the sand. You should note that with shotcrete impact during placement we will generally lose 50% of the air content, so your final in-place air should be around 5%. That is a reasonable value for good freezing-and-thawing durability.

Dry-mix shotcrete adds mixing water to the dry concrete materials as the concrete materials flow through and out the nozzle. Gunite is the original tradename for dry-mix shotcrete. Though you may not find design information using the old gunite name, you will find numerous current design references to dry-mix shotcrete. This includes ACI 506R-16, “Guide to Shotcrete”; ACI 506.2-13, “Specification for Shotcrete”; ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote”; ACI 372, “Design and Construction of Circular Wrapped Prestressed Concrete Tanks”; ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures”; ACI 350.5, “Specifications for Environmental Concrete Structures”; and seven ASTMs that directly cover shotcrete. ACI 318-19, “Building Code Requirements for Structural Concrete,” has also added specific shotcrete provisions. Dry-mix shotcrete has been used for decades to build structural concrete walls over 50 ft high in circular prestressed concrete tanks that withstand a full head of water pressure. This is a substantially greater water pressure than your 5 ft vault wall would experience. There are no limitations in the dry-mix placement process that would preclude use in high walls. Both dry-mix and wet-mix shotcrete using quality materials, proper equipment, and experienced placement crews will produce in-place concrete of equal strength, durability, and low permeability. However, generally wet-mix shotcrete can offer placement rates up to four times higher than dry-mix. Thus, in thicker, longer walls, wet-mix shotcrete may be more cost-effective because it can be placed faster.

The current version of the ACI “Specification for Shotcrete” is ACI 506.2-13 and purposely eliminated the core grading due to many problems that arose in the field in trying to equitably apply the procedure. ACI 506 has a new document ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote,” that is intended to assist specifiers with evaluating shotcrete core quality for an intended use. So yes, the 506.2-95 version of the specification and the grading system it included is no longer supported as an industry standard.

Regarding grading, a shotcrete score of 5 under the deprecated grading system just because the core broke during coring is unreasonable. The coring operation exerts a massive force on the core, as it basically rips it out of the concrete. We have seen many instances where the core snapped where a large reinforcing bar horizontally crossed the core and effectively created a slip plane that reduced the shear resistance of the core section to the torque created by the coring. In those cases, examining the core hole for any evidence of shotcrete problems would be appropriate and then discounting the core itself.

In summary, my position and the ACI 506 Committee’s position is that core grading should not be used for evaluating shotcrete quality. The ACI 506.6T-17 Technote document should be the current reference for making a reasonable evaluation of shotcrete quality for the intended use.

The engineer designs the wall based on the loads anticipated on it throughout its service life. The minimum spacing for noncontact lap splices as indicated in ACI 506.2, “Specification for Shotcrete,” is:

“Clearance of at least three times the diameter of the largest reinforcing bar; three times the maximum size aggregate; or 2 in., whichever is least”

In your case with No. 5 bars, the 2 in. (50 mm) minimum probably controls and would then provide a 2.6 in. (64 mm) center-to-center spacing. No. 5 bars at 4 in. spacing can be shot properly with proper materials and technique. An ACI-certified shotcrete nozzleman will have had education that explains how this configuration or even closer spacing can be shot properly. Experienced shotcrete contractors doing structural concrete walls do this type of work routinely with excellent results. If you are concerned about encasement of the reinforcing steel in the back curtain of steel you may consider erecting only the back curtain of steel, shooting the wall out to the location of the outer curtain of reinforcement, erecting the outer curtain, and then shooting out to the final surface. Experienced shotcrete contractors have shot in thick sections (36 in. [900 mm] thick or more) with heavy reinforcement (No. 11 at 4 in.) using this technique.

Shotcrete is a placement method for concrete. Thus, cure time is the same as what the coating manufacturer recommends for new concrete surfaces. Because moisture in the concrete surface is part of the concern of bond from manufacturers, a water-based latex may be preferable to oil-based, but you should discuss the appropriate time and materials with your coating supplier.

Shotcrete is a placement method for concrete. Thus, structural properties are equivalent to those of concrete with specified strength. Shotcrete compressive strengths will normally range from 4000 to 7000 psi at 28 days, so your 4800 to 5000 psi (33 to 35 MPa) is very normal. Density of shotcrete placed concrete is the same as cast concrete ranging from 145 to 150 lb/ft³. Walls can generally be built at any required thickness because we will create the wall by bench shooting building the wall from the bottom to the top in vertical lifts.

If you only need color in the exposed shotcrete surface, just putting color in the flash coat should be adequate as long as the flash coat is continuous across the entire area. You may want to specify the final “flash” layer to be thicker, perhaps 1/2 in. (12 mm) to 1 in. (25 mm) thick. This would help assure consistent color and still allow the finishers to produce a consistent final texture. As long as the concrete materials are the same in the production, you wouldn’t need to shoot the final layer all in one day. Also, be sure curing methods are the same for the entire area as varying moisture can sometimes affect the early appearance of the coloring, though it usually balances out over time.

The thickness of the shotcrete placement, site logistics, and the availability of ready-mixed concrete, as well as expertise of the shotcrete contractor would likely determine the most cost-effective method for shotcrete placement. Both dry-mix and wet-mix will produce structural concrete in place with similar physical properties and durability. Wet-mix can generally produce more volume of material placed per hour than dry-mix. You had several basic questions about shotcrete that can be answered by visiting sections of Shotcrete.org. On our website, you will find the informational pages www.shotcrete.org/Resources and www.shotcrete.org/ArchiveSearch beneficial. The Resources page lists many shotcrete-specific articles and web resources. The Archive Search allows you to search through our Shotcrete magazine archive for past articles using keywords. You may also find ACI 506R-16, “Guide to Shotcrete,” an informative primer on all aspects of shotcrete. You can purchase a PDF copy of the Guide from ACI at www.concrete.org/store/productdetail.aspx?ItemID=50616. ACI Committee 506, Shotcreting, also has several other technical documents available for specification for shotcrete, evaluation of shotcrete cores, specifying underground shotcrete, and fiber-reinforced shotcrete you can find on the ACI website.

All surfaces receiving shotcrete should be above freezing. The mandatory requirements of ACI 506.2-13, “Specification for Shotcrete,” specifies:

“3.4.5 Cold weather shotcreting—Unless otherwise specified, shooting may proceed when ambient temperature is 40°F and rising. Stop shooting when ambient temper­ature is 40°F and falling, unless measures are taken to protect the shotcrete. Shotcrete material temperature, when shot, shall not be less than 50ºF. Do not place shotcrete against frozen surfaces.”

Your question then asks about substrate temperatures for sections not requiring bond. The concern of frozen concrete is not only bond, but an issue with freezing of some thickness of the concrete that would prevent strength gain. For requirements on this, ACI 301-16, “Specifications for Structural Concrete,” would likely apply and 5.3.2.1(b) requires:

“5.3.2.1(b) Cold weather—Concrete temperatures at delivery shall meet the requirements of 4.2.2.5. Do not place concrete in contact with surfaces less than 35°F. Unless otherwise specified, this requirement shall not apply to reinforcing steel.”

There is a discrepancy between ACI 301 and ACI 506.2. The ACI 301 value (35°F) is somewhat more conservative, though ACI 506.2 provisions (32°F) have proven to produce quality shotcrete. You may consider asking the Engineer of Record for your project what minimum substrate temperature is acceptable on your specific job.

This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements, you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete so standard reinforced concrete design is appropriate for shotcreted sections.

Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.

Dry-mix shotcrete adds mixing water to the dry concrete materials as the concrete materials flow through and out the nozzle. Gunite is the original tradename for dry-mix shotcrete. Though you may not find design information using the old gunite name, you will find numerous current design references to dry-mix shotcrete. This includes ACI 506R-16, “Guide to Shotcrete”; ACI 506.2, “Specification for Shotcrete”; ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote”; ACI 372, “Guide to Design and Construction of Circular Wire-and-Strand-Wrapped Prestressed Concrete Structures”; ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures”; ACI 350.5, “Specifications for Environmental Concrete Structures”; as well as seven ASTM standards that directly cover shotcrete. ACI 318-19, “Building Code Requirements for Structural Concrete,” has also added specific shotcrete provisions. Dry-mix shotcrete has been used for decades to build structural concrete walls over 50 ft (15 m) high in circular prestressed concrete tanks that withstand a full head of water pressure. This is substantially greater water pressure than your 5 ft vault wall would experience. There are no limitations in the dry-mix placement process that would preclude use in high walls. Both dry-mix and wet-mix shotcrete using quality materials, proper equipment, and experienced placement crews will produce in-place concrete of equal strength, durability, and low permeability. However, generally wet-mix shotcrete can offer placement rates up to four times higher than dry-mix. Thus, in thicker, longer walls, wet-mix shotcrete may be more cost effective because it can be placed faster.

ASA’s “Safety Guidelines for Shotcrete” specifically addresses hose blockages. Use of compressed air to clear blockages or for cleaning the lines is not recommended. The Guidelines state:

“With the variety of shotcrete material delivery systems available, and their placement on individual job sites, the Contractor should establish site-specific safety procedures applicable to the specific delivery systems and site conditions for blockage removal. Any field procedures for clearing blockages should not use compressed air as means to remove or dislodge blockages.”

Using water to clear blockages or the delivery lines when finishing shooting is the recommended procedure. However, if compressed air is used, the hose end must be securely fastened with a substantial fastening system that can routinely and safely handle the forces created if the concrete is discharged explosively. Shotcrete contractors have developed cleanout bins that clamp the hose end into a heavy steel tank and collect the waste concrete from the line for disposal. Others have created clamps that firmly hold the end of the hose to a loader bucket or other heavy piece of equipment, thus depending on the weight of the equipment to hold the hose end. Simply having two people sitting on the hose end is not safe and can result in injury to crew members. Even when holding the hose end with a clamping system on heavy equipment, clearing the line can cause an explosive discharge of concrete with material flying in a wide path from the hose. Unless planned for and contained, the material stream can hit adjacent workers, facilities, equipment, and vehicles.

ASA has closely followed the development of the new OSHA rules for a couple of years before they were put into effect. You can find several articles in Shotcrete magazine that specifically address the rules in consideration of shotcrete application. In the Summer 2016 issue, an article, “OSHA’s New Crystalline Silica Rule–Potential Impact on Shotcrete Operations,” addressed many of the concerns. Unfortunately, with the wide variety of shooting conditions, there are no generic values. The reason for this is that the levels can vary significantly due to a variety of factors, including:

1. The materials used—this includes comparing wet-mix to dry-mix and the variations in variability of concrete mixture design ingredients (for example, silica fume, fly ash, and accelerator).
2. Equipment
   1. Dry-mix gun type (rotary or chamber), using a predampener or not (type of wet-mix pump likely doesn’t make much difference)
   2. Size of air compressor (more air might result in more dust)
   3. Delivery line and hose (1.5 in. [40 mm] hose versus 2 in. [50 mm]) can change volume of flow, and then level of acceleration and nozzle stream dispersion as a function of air volume)
   4. Nozzle type can significantly affect the material stream
3. Shooting location (inside or enclosed, or open air)

With so many variables it is difficult, if not impossible, to get any reliable “generic” number for shotcrete as a whole. Many of our shotcrete contractors are using air quality consulting firms or testing labs who have the monitoring equipment. You may want to note that silica fume is amorphous silica, not crystalline, so it is not hazardous. Most exposure to crystalline silica is through sawing, cutting, or grinding of hardened concrete. We expect that most shotcrete contractors will need to establish a reliable, accurate level by on-site testing because shotcrete is not directly covered in Table 1 of the OSHA rule.

The ACI 350 Code doesn’t really cover repair in detail because it is more about new environmental structures. Shotcrete is concrete; it provides excellent bond to properly prepared concrete substrates and it inherently creates a composite section with the existing concrete that acts monolithically. The supplemental shotcrete could be considered an additional layer of concrete that acts monolithically and thus provides the needed concrete cover. Reference-wise, you could refer to the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” in the Shotcrete magazine archive, as it discusses the bond issue between layers. There are research papers that discuss a 200 psi (1.4 MPa) bond shear stress is needed for a bonded concrete overlay to act monolithically, and that bond pulloff (tensile) test results should be multiplied by 2 or 3 to represent the bond shear capacity. Because we typically well exceed a 145 psi (1.0 MPa) tensile bond pulloff strength, properly applied shotcrete will easily exceed the 200 psi requirement for the bond to act monolithically.

An additional factor in providing additional cover with shotcrete is that shotcrete has very cement-rich paste, along with low permeability, and thus provides a better alkaline environment to combat corrosion of embedded reinforcement, so is effectively giving even better cover than normal form-and-pour concrete.

Shotcrete is a placement method for concrete. If the sand can be used for concrete, it should be acceptable for shotcrete placement. To some extent, the type of shotcrete equipment may make a difference. If shooting wet-mix shotcrete, the concrete mixture must be able to be pumped. For pumpability, we do recommend a smooth gradation of the fine aggregate. If shooting dry-mix shotcrete, the sand gradation has much less effect, and you should be able to shoot most any sand in your concrete mixture.

“Coquina shell shotcrete” is not a well-defined concrete material. Though it may have been promoted to you as shotcrete, it may well have been a stucco-like application accomplished with low-velocity plastering equipment, and thus not high-velocity shotcrete placement. Without physical characteristics of the in-place material (strength, density, type, and amount of reinforcement), it is hard to delineate the structural enhancement the coating may provide. You could have an evaluation by an engineer or testing lab to ascertain the characteristics of the in-place material. Then refer to ACI 216.1-14, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies,” to see whether your composite system has enhanced fire resistance as compared to brick alone. Regarding wind resistance, you would need to have a structural engineer evaluate the composite system for any potential increase in strength against wind loadings.

The design of any concrete structure must consider loadings and environmental conditions. This includes drying shrinkage of the concrete, and daily and seasonal thermal changes that introduce stresses within the concrete sections. Domes are thin-shell concrete structures and stress concentrations are expected around any openings through the shell. Typically, the design engineer will provide additional reinforcing bars around openings and especially at corners of openings to accommodate the buildup of stresses in these locations. Also, chain-link fence is not considered as acceptable concrete reinforcement, as it cannot accept tension in a straight orientation within the concrete. Proper concrete reinforcement is either deformed reinforcing bars or steel mesh with smooth wires laid out in an orthogonal pattern.

You should have a professional engineer with experience in concrete shell design and construction evaluate your dome home for structural integrity. If the cracking is determined to not affect the structural integrity, epoxy or polyurethane grout injection is routinely used to seal cracks in concrete.

There is not a fixed value that one can use for maximum lift height. The maximum lift height when bench shooting is controlled by the concrete mixture (admixtures, aggregate, slump), concrete temperature, size and layout of reinforcement, substrate being shot against, and ambient temperatures. These factors must be evaluated by the nozzleman during the placement. The maximum height is constrained by the ability of the fresh concrete in the lower portion of the lift to carry the weight of concrete in higher portions without creating sagging or sloughing. Hot weather conditions will allow higher lift heights than cold weather in non-accelerated concrete mixtures.

Shotcrete placement requires high velocity for full consolidation and compaction of the concrete. Standard shotcrete equipment with a properly sized air compressor will produce a velocity of 60 to 80 mph (95 to 130 km/h). The 400 psi you stated is not a velocity, but a pressure. Sixty psi is a very low air pressure and may not create the velocity needed for proper shotcrete placement. Shotcrete generally uses compressors that create 100 to 125 psi of air pressure. Also, depending on the shotcrete process used (wet-mix or dry-mix), ACI 506R-16, “Guide to Shotcrete,” states you need an air compressor that can produce a flow rate of at least 200 ft³/min (5.7 m³/min) for wet-mix to 600 ft³/min (17 m³/min) for dry-mix for your 1-1/2 in. diameter hose.

Shotcrete is a placement method for concrete. It is routinely used to strengthen existing masonry or concrete walls. The 5 in. thickness can be easily achieved with either dry-mix or wet-mix shotcrete. If using dry-mix, you may want to review available prepackaged bagged dry concrete materials. A comprehensive listing of our ASA member companies supplying bagged dry concrete materials can be found in our online Buyers Guide. If using wet-mix, the local concrete supplier should be able to provide concrete mixture designs that will achieve the 4000 psi compressive strength requirement. Dry-mix and wet-mix processes use different equipment and skills of nozzlemen are somewhat different. We encourage requiring use of an ACI-certified shotcrete nozzlemen in the process to be used on the project. If looking to select a qualified shotcrete contractor, you may want to review our ASA position paper on Shotcrete Contractor and Crew Qualifications.

We could not identify any specific cases of shotcreting onto metal decking for sound dampening. However, shotcrete is used in a wide variety of overhead applications, and as long as we can get good bond to the underside of the metal decking, should perform well. Depending on the surface profile of the metal decking, you may want to consider attaching studs to the decking to enhance the overall bond of the concrete. You may also want to consider using lightweight shotcrete since the concrete sounds like it is more for acoustic purposes, than for structural.

Proper shotcrete placement of concrete depends on high-velocity impact of the concrete materials for full consolidation. Though not clear from your inquiry, it sounds like the additional concrete was just dumped in place without high-velocity impact. Thus, you simply have normal cast concrete that would require some type of external vibration to densify and properly consolidate the concrete. The bond of cast concrete would be inferior to the bond from shotcrete since you do not have the high-velocity impact driving the cement paste into the previously shot material. Also, adding 20 gal. of water at the site would substantially weaken the concrete from the original design strength of the concrete mixture. Further, feathering edging in a joint is not recommended since you end up with a very thin overlay at some point that may tend to spall or delaminate much more easily at the thinner section. A better approach when running short of concrete is to stop and prepare a joint for later shotcreting. For best bond, joints should be cut at a 45-degree angle, roughened, and then cleaned and wetted immediately before shooting.

Thus, answering your final question, in summary the bond will be reduced, and with the higher water content, the concrete will be weaker than properly shotcreted concrete.

Shotcrete is concrete. PCA’s Design and Control of Concrete Mixtures, 16th Edition states: “In general, some reduction in strength at early ages (one to three days) accompanies the use of retarders. However, increased long-term strength may result from retarding the initial rate of hydration. Excessive addition rates of a retarding admixture may permanently inhibit the hydration of cement.” Thus, if you closely follow the admixture manufacturer’s recommendations for dosage, you may get higher long-term strength than non-retarded mixtures. You may also consider use of the newer hydration control admixtures that essentially stop hydration until activated, and can theoretically put the concrete “to sleep” for up to 3 days.

Shotcrete is just a placement method for concrete. So shotcreting will provide the fire resistance of concrete. For securing the concrete, you can place anchors through the foam into the original basement wall. You can consult with an engineer experienced with shotcrete on anchor size and spacing required for supporting the shotcrete layer.

Shotcrete has been used for over 100 years for slope stabilization on natural soils. Your self-consolidating concrete (SCC) sounds like it could be considered a high-grade soil, so stabilization and protection of the SCC with shotcrete is certainly a good application for shotcrete. The fresh surface of the shotcrete can receive a wide variety of finishes, from an as-shot finish (rough) to a floated or even stamped or carved look. You should consider appropriate amounts of reinforcement (reinforcing bars and/or fibers) to control potential shrinkage cracking, as appropriate for your exposure and serviceability requirements.

Shotcrete placement produces very localized pressure on the one-sided forms we typically use. The 90 lb (40 kg) is a reasonable figure when shooting directly against the form. If shooting thick walls, we typically use a benching approach for placement that puts most of the force directly on the supporting floor (or earth) and putting little force on the form. Thus, the form is more of a way to define the back surface. You will often see shotcrete forms in lower-height applications using thin forming materials such as Masonite, pegboard, or even a stay-form for the formed surface. Often, the larger load controlling the design on a form may be the wind loads expected during the construction period.

Shotcrete is a high-velocity placement of concrete. In most thick walls, as I imagine your foundation walls are, the shotcrete contractor will be bench shooting the walls. This means they will be shooting the full wall thickness in 3 to 4 ft (0.9 to 1.2 m) high lifts where most of the impact forces and weight of the shotcrete is carried by the previously shot material. This results in very low impact forces on the back side of the section. When creating a section with a one-sided form, shotcrete contractors have used thin material, like Masonite, pegboard, or even an expanded mesh material, as we just need to have a surface to define the back of the section.

Shotcrete is a placement method for concrete construction. Shotcrete-placed concrete should be properly cured to provide desired strength and reduce potential shrinkage. ACI 308.1-11, “Specification for Curing Concrete,” and ACI 308R-16, “Guide to External Curing of Concrete” are excellent reference documents. ASA recommends curing a minimum of 7 days, and prefers curing with water, maintaining a continuously wet surface condition for the 7-day period. If using a curing membrane instead of water curing, ASA recommends applying the curing membrane at twice the curing membrane manufacturer’s recommended application rate, and applying in two layers with the second perpendicular to the first. If applying a coating over the final concrete surface, you should check with the coating supplier to verify the duration and properties required before application of the coating.

Shotcrete is a placement method for high-quality concrete. Here’s a link to an article of a rehabilitation of a concrete-supported lighthouse in the Saint Lawrence Seaway (Pointe de la Prairie Lighthouse) that provides a lot of detail on an installation similar to yours, including salt-water exposure in a tidal zone: (www.shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf). Plus, this project also has extreme freezing-and-thawing exposure.

You are correct in suspecting quality issues with your pool. These are the specific issues that lead to poor quality, that can affect the serviceability and durability of your pool.

1. Shotcrete placement requires high velocity and impact for compaction of the concrete. Hand-applying “sloughed-off” concrete would not provide proper compaction needed for producing monolithic concrete sections. The resultant fissures and voids in your pool reflect the lack of proper velocity and compaction.
2. Proper preparation of the substrate is essential for good bond and creating a concrete section that acts monolithically. The surface needs to have any materials that would interfere with the bond removed, be roughened, cleaned, and brought to a saturated surface-dry condition before shotcrete placement. This article from Shotcrete magazine gives more details on how and why surface preparation is important (www.shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf).
3. Concrete cover over the reinforcing bar is critical for maintaining corrosion protection of the embedded steel, and thus providing long-term durability. Low cover will often result in premature corrosion and subsequent spalling of the concrete cover, reducing the serviceability and life of the pool concrete.
4. Curing is important for all concrete, and especially for the relatively cement-rich concrete we use for wet-mix shotcrete. Curing essentially provides additional water to hydrate the cement in the concrete, and produces stronger, less permeable concrete. Not curing concrete yields concrete that is weaker, more permeable, and ultimately less suitable for creating a watertight pool shell.
5. You haven’t indicated the actual time concrete sat for “a while.” Industry standards are that concrete should be placed within 90 minutes of the introduction of water to the mixture unless special precautions are taken. Water is usually added at the ready mix plant. If concrete sits too long it can start to lose workability. At the point of losing workability, some contractors will add additional water on site over and above the concrete mixture design requirements, but this “retempering” produces concrete that is weaker and more permeable than the original design mixture.

Based on your input, you have many good reasons to ask the contractor to provide full remediation of these quality issues.

Your proposed project is a great application for shotcrete. We’ve seen shotcrete used for enhancing structural integrity of historic masonry structures across the country. Often shotcrete is used on the back side of the wall to add structural strength while preserving the exterior appearance. In effect, we build a structural concrete wall in-place behind the old wall. Shotcrete has the natural advantage of not requiring any formwork, and can create a good bond to the existing wall, letting the structure elements work together. Here’s a link to a past article from Shotcrete magazine documenting the restoration of a historic brick building (www.shotcrete.org/wp-content/uploads/2020/01/2009Win_SCM01pg08-12.pdf). If you don’t need to preserve the exterior appearance, you can certainly shotcrete the exterior of the wall using the same approach. You can finish the interior (or exterior) surfaces in a variety of ways to provide the architectural appearance you desire. Shotcrete is a placement method for concrete, so using shotcrete will provide a final structure with the strength and durability of cast concrete. By the way, the term “gunite” is the old tradename for what we currently call “dry-mix shotcrete.” Let us know if you have any further questions.

As you discovered, experience of the shotcrete contractor is key to a successful project. It takes an experienced and knowledgeable shotcrete team (the project manager, supervisor, pump or gun operator, nozzlemen, and finishers) to get a quality job. Answering your specific questions:

1. Can this be done with either a dry or wet mix? Yes, either wet or dry mix will produce good results. You must be sure to properly prepare the substrate including chipping/bush hammering back to sound concrete, fully cleaning the surface and then bringing the surface to a saturated surface-dry condition (SSD).
2. Do you need to bush a recessed area to accept a minimum amount of new material in lieu of a feather edge? Feather edging will create a very thin layer that would have more potential to spall when exposed to shrinkage or seasonal thermal movements. We recommend creating a square shoulder at least 0.75 in. (19.0 mm) deep to create an acceptable thickness of the patching layer.
3. Is a wet mix acceptable to fill these areas given that the aggregate in it is generally up to 0.375 in. or so? With the dry-mix being primarily concrete sand and cement it would seem more practical. Wet-mix with a coarse aggregate can be shot in thin layers, but with a 0.375 in. coarse aggregate may require more finishing due to impact depressions of the aggregate in the shot surface. A dry-mix material without coarse aggregate may be easier to fill in the thin layers. Dry mix is typically less productive in volume placed per hour than wet mix, but in this relatively small-volume repair application, either process should be fine.

ACI 117 provides an excellent guide for tolerances for concrete structures. Although shotcrete is concrete, ACI 117 specifically excludes shotcrete because shotcrete’s unique method of placement permits a wider variety of applications and uses than that of form-and-pour concrete. Shotcrete can generally be finished to the tolerances required for the application. For example, lining a channel might not need close tolerance control, while an Olympic luge/sledding track or skateboard park may require very exacting tolerances. ACI 506.2-16, “Specification for Shotcrete,” in the Tolerances section (and the Mandatory checklist item) requires the specifier producing the contract documents to provide the tolerances required for the project. ACI 506R-16, “Guide to Shotcrete,” Section 3.8, Tolerances, provides a more descriptive commentary. Pertinent portions of that section include:

• Tolerances provide an indication of the finished product expected by the owner, but meeting tolerances may require additional effort and cost. Tolerances given by ACI 117.1R, for placement of reinforcing steel, cover over reinforcing steel, and overall alignment of cast-in-place structural members should be generally the same for shotcrete. Tolerances that require distinct values for shotcrete construction are cross-sectional dimensions, cover, and surface finish (or flatness). Therefore, specifying tolerances that can be consistently achieved are needed so that project expectations can be met at a reasonable cost.
• Specified tolerances should be based on use and function and can be the same as concrete, but are typically broader. Some finished surface tolerances may be waived to achieve proper coverage over existing reinforcement.

Although some shotcrete structures have been allowed greater tolerances than allowed for concrete, shotcrete structures can be built to the same degree of accuracy and tolerance as cast-in-place concrete.

You should review the entirety of Section 3.8, Tolerances, in ACI 506R-16 to get a complete description of tolerances for shotcrete placements.

The minimum strength for safety must be established by the designer based on the specific structural and geotechnical aspects of the project. The minimum strength value may also be influenced on whether using fiber-reinforced shotcrete or plain shotcrete. The early-age strength was tested with a Meyco Needle Penetrometer after creating a time (early) strength curve with minimum tests at 10 minutes and 30 minutes with Needle Penetrometer; then at 3 hours and 6 hours with Hilti Studs; then at 1, 7, and 28 days with cores. Thus, the curve was calibrated for the specific mixture and environmental conditions. It was also useful to identify when early strength (and potentially long-term strength) was lacking.

ACI 506.2-95, “Specification for Shotcrete,” is the deprecated version, and is no longer published (it isn’t readily available on the ACI website). Thus, the Tech Note along with the current ACI 506.2-13, “Specification for Shotcrete,” is the current industry standard for evaluating cores. An engineer may specify use of the outdated ACI 506.2-95; however, they are opening up their exposure because it isn’t the current standard. This is similar to the exposure if an engineer uses a much older version of ACI 318, and not the current one.

Generally, sand/cement in standard concrete mixtures starts breaking down around 400 to 500°F (200 to 260°C) because the portland cement starts to dehydrate. It loses strength with every phase and will have no strength after 1000°F (540°C). Putting it on brick depends on the shape of the brick surface. Uneven surfaces with holes will hold much better than flat brick. If the brick is flat, placing more than a couple of inches (±50 mm) may delaminate and fall off the wall without anchors of some sort. You could probably use a 2 x 2 in. (50 x 50 mm) or 3 x 3 in. (75 x 75 mm) mesh. In refractory, we consider low temperatures at 1700 to 1800°F (930 to 980°C). In the higher temperature refractory, we use calcium aluminate cement and “traprock,” which is a fine crushed limestone aggregate. The mixture ratios would be the same as a typical sand and cement gunning mixture. This is what traditionally has been for coal bunkers and coal dryers or any other sections in the 1700°F (930°C)-plus temperature range.

Shotcrete placement for a concrete wall gives the owner the advantages of concrete durability and the appearance of rock. Shotcrete has been used extensively for cre­­ating false rock faces for zoos, water parks, highways, and retaining walls. However, because shotcrete placement and carving to look like natural rock requires quite a bit of labor, the costs to do so may be more extensive than simply placing boulders.

Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. The specific section on drying shrinkage tests says:

“Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.”

Shotcrete is a placement method for high-quality concrete. Here’s a link to an article of a rehabilitation of a concrete-supported lighthouse in the St. Lawrence Seaway (Pointe de la Prairie Lighthouse) that provides a lot of detail on an installation like yours, including saltwater exposure in a tidal zone. Additionally, this project also has regular freezing-and-thawing exposure (www.shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf).

Another project with tidal zone exposure involved repair of bridge pier pile caps on the East Coast of Florida. The article details the project parameters and testing conducted to verify the quality of the shotcrete placement (www.shotcrete.org/wp-content/uploads/2020/01/2012Win_White.pdf).

Shotcrete compressive strength should be tested using cores from test panels. Shotcrete cannot be shot into closed cylinder forms and be representative of the in-place shotcrete. ASTM C1140-11 and C1604-05(2012) provides the panel configuration and compression testing requirements. However, if using the wet-mix process for shotcreting and one wants to verify the compressive strength, air content, or temperature of ready mixed concrete materials as delivered, rather than as shotcreted in place, samples can be taken from the truck before pumping. These cylinders would follow ASTM C31-15 for making and curing concrete test specimens.

Shotcrete is a placement method for concrete. If the berm is composed of granular materials, one wouldn’t expect the shotcrete to actually bond to the soil. Rather, placing a thin shotcrete layer would create a uniform, relatively impermeable layer of concrete to prevent water from penetrating through and washing out the soil underneath. The emulsion may help to stabilize the soil to help withstand the pressure of shotcreting directly against the berm, but many similar soil stabilization projects will shotcrete directly onto the natural soils. Also, when considering using the emulsion, be aware that some oils may contain ingredients (like sulfur) that can attack the concrete over time. You should consult with an engineer or concrete materials specialist to ascertain whether the specific oil you want to use will have a long-term effect on the shotcreted layer.

Some mines have used chain link mesh in shotcrete in severely deforming ground and claim that it is better in holding the ground than mesh after large deformations, in which the shotcrete sustains major cracking with deformations. Other than for such unusual applications, we do not recommend the use of chain-link reinforcement in shotcrete. It cannot be fixed “tight” and as such is susceptible to vibration and movement during shooting, resulting in shotcrete sloughing and formation of voids in the shotcrete. Also, the mesh interconnections are conducive to the formation of voids during shooting. Additionally, there don’t appear to be any consistent material standards on the strength, flexibility, or brittleness of the steel (or other materials) used in the fencing material, so a designer has no way to establish the tensile or flexural strength of the concrete sections. In brief, don’t use chain-link mesh if you want to produce quality, durable shotcrete.

Shotcrete is a placement method for concrete. When the pool shotcrete contractor uses quality materials, properly sized and maintained equipment, and experienced crew members, the shotcreted pool shell should be watertight. Proper shotcrete application would also not exhibit voids, honeycomb, or major seepage through the pool shell. Before replastering, the pool shell should be evaluated and all defects (voids, cracks, porous sections, deep holes, and so on) repaired using industry-standard methods and materials. You may consider retaining a professional engineer or experienced pool consultant to evaluate the pool shell and make specific recommendations on the appropriate repair for your specific issues. You can use our Buyers Guide (www.shotcrete.org/products-services-information/buyers-guide/) to locate consultants in your area. You also may want to review our pool-specific Position Statements on our website at www.shotcrete.org/products-services-information/resources/. The “Watertight Shotcrete for Swimming Pools” would be particularly informative for you to assist in discussions with your pool contractor.

Because shotcrete is a placement method for concrete, standard concrete repair techniques are applicable. If the repair is being done by shotcreting new concrete on the existing concrete, the repair should include these steps:

1. Chip the poor or weak concrete back to sound concrete.
2. If reinforcing bars are exposed, they should be cleaned of any rust.
3. If more than half of the perimeter of a reinforcing bar is exposed, the concrete should be chipped back to provide at least a 0.75 in. (19 mm) space behind the bar to allow shotcrete to flow around the back of the bar.
4. Before shooting any additional shotcrete, the surface should be cleaned and then wetted to a saturated surface-dry condition (SSD).
5. SSD means the substrate is damp, but there is no flowing water. It should feel damp to the touch, but no water is picked up on your hand when placing it against the surface.
6. Do not use bonding agents, as they can interfere and reduce the bond of the new shotcrete to the old shotcrete surface.
7. When the new shotcrete is in place, the contractor should initiate water curing as soon as possible. In warm, dry, or windy weather, fogging of the fresh surface immediately after finishing would be helpful.
8. Water curing (where the concrete is kept continuously moist) should be supplied for at least 7 days.

Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. Here’s a link to the archived PDF of the article (www.shotcrete.org/wp-content/uploads/2020/01/2011Sum_Hanskat.pdf). The specific section on drying shrinkage tests says:

“Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.”

Shotcrete is a placement method for concrete. Lightweight concrete usually ranges from 90 to 115 lb/ft3 (1400 to 1800 kg/m3). In wet-mix shotcreting, lightweight concrete should use presoaked aggregate to make the mixture pumpable. When it’s pumpable, as with conventional concrete mixtures, it is then accelerated to a high velocity by air at the nozzle and projected onto the surface. Lightweight aggregates can also be used directly in dry-mix, and there you don’t need to worry about pumpability because the dry materials are conveyed through the delivery hose. Water is added at the nozzle.

Here’s the specific reference on lightweight from ACI 506R-16, “Guide to Shotcrete”:
2.1.3.2 Lightweight aggregates—Lightweight aggregates should conform to ASTM C330/C330M if used in shotcrete. The aggregate should meet one of the gradations shown in Table 1.1.1. Wet-mix shotcrete with lightweight aggregate is seldom used and is difficult to pump because the aggregate absorbs water, which reduces the consistency of the mixture. Presaturating the lightweight aggregate before batching improves pumpability. Lightweight aggregate mixtures have been shot for wall and floor construction. Shotcrete is frequently employed for fireproofing structural steel members using lightweight aggregates in the mixture.

We suspect that cellular concrete cannot be shot because it uses injection of a pre-formed foam into a cement slurry and is highly fluid. That would preclude any stacking of material to make a vertical surface, and would instead just be pumped in place like a high slump concrete.

Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Pools will also experience volume change in the concrete due to thermal changes, especially summer to winter seasonal swings. Contraction and expansion joints are common in all kinds of concrete liquid-containing structures especially with walls of this length. Though we can’t provide a firm design recommendation you should consider these factors:

• What are the weather conditions when the pool is anticipated to be built? If during hot summer months, could there be enough seasonal temperature swing to require expansion joints?
• If expansion joints wouldn’t be needed would contraction joints be needed to handle anticipated temperature swings, and drying shrinkage?
• Will the pool be empty for extended times (this could lead to more shrinkage or direct exposure to solar gain or cold conditions?
• Is the pool to be kept full or empty during the winter months? (if the pool is in a geographic region where extending freezing conditions are prevalent)

Overall, the design for a shotcrete pool should be the same as one for a cast concrete pool.

ASA recommends a minimum of 7 days curing to help control shrinkage issues in young concrete sections. Lack of curing, and exposure to windy, hot or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength. However, shotcrete generally exceeds the minimum 4000 psi (28 MPa) 28-day compressive strength ASA recommends, and required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604 Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete gives guidance on taking cores from existing structures. A minimum 3” diameter core is recommended. Before coring it is recommended to use ground penetrating radar (GPR) or similar equipment to identify the location of reinforcing in the pool section, and then take cores to avoid cutting through the reinforcing wherever possible. The core holes would then need to be filled with a high strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions.

There are many variables that can cause cracking. Vertical cracking is often the result of drying shrinkage of the concrete. You said you cured (watered) the wall properly, but don’t give any specifics. ASA recommends a minimum of 7 days of curing, with a wet cure preferred over a spray-applied membrane. You should have a licensed engineer evaluate the structural sections, and determine if there were any problems with the amount or placement of reinforcing in your wall section.

Since shotcrete is simply a placement method for concrete, the R-value is the same as cast concrete. ACI 122R-14, “Guide to Thermal Properties of Concrete and Masonry Systems,” would be a good reference.

By control joints, we assume you mean contraction joints. Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Though shotcrete has a lower w/cm than most form and pour concrete, it will still undergo shrinkage. In being shot on an existing concrete wall the shotcrete liner will be restrained by the bond to the substrate and the restraint of the horizontal volume change from shrinkage can create internal tensile stresses in the concrete. This is likely the reason the designer has specified contraction joints in the section. 30 ft (9 m) spacing between joints is common in new construction of concrete tanks. The question becomes whether the bond of the shotcrete to the existing substrate is high enough to restrain the volume change and prevent cracking along the hundreds of lineal feet of wall you will be lining. The thickness of the lining, the type and duration of curing, the concrete mix design, the strength of the concrete, the strength of the substrate, the quality of shotcrete application, proper surface preparation and exposure to seasonal temperature changes will impact the effect of the volume change of the lining. With the many variables we’ve pointed out you can see there isn’t a clear answer that covers all situations. We recommend you discuss your opinion with the designer or consult with a professional engineer experienced in shotcrete repairs to fully evaluate the specific structural sections you’re shotcreting.

Shotcrete is a placement method for concrete. Shotcrete has been successfully used for over 70 years in thousands of industrial wastewater treatment/storage tanks, as well as replacement lining of sewers and manholes. Thus, exposure of the shotcreted pond to wastewater should be as good or likely even better than the original cast concrete liner. Long term durability of the shotcreted section will be dependent on the concrete mixture design. Many contractors use supplemental cementitious materials (SCMs) like silica fume or fly ash to improve the pumping or shooting characteristics of the mix. These SCMs also help to reduce permeability, increase strength, and thus make the concrete more durable. Fly ash also has the benefit of adding some sulfate resistance that would be beneficial in wastewater exposure conditions. Shotcrete is often shot on geomembranes or directly on the subgrade soils if they are stable enough to hold the impact, and weight of the shotcrete.

Air entrainment will generally slightly reduce the compressive strength of concrete, but significantly increase the resistance to freezing-and-thawing exposure. Dry-mix shotcrete (gunite) is generally a very paste-rich mixture. With modern cements, the normal 28-day compressive strengths easily exceed commonly specified compressive strengths. ASA recommends a minimum of 4000 psi (28 MPa) for shotcrete, and 4000 psi to 5000 psi (28 to 34 MPa) strengths are routinely specified.

1. Air entrainment increases the workability. The small air bubbles act as a form of lubricant to ease internal friction between the concrete mixture components.
2. You should contact one of our material supplier members to see what they offer. You can use our Buyers Guide at www.shotcrete.org/products-services-information/buyers-guide/, and limit your search to “Admixture Sales” with the “Air-Entraining” subcategory.
3. Quality shotcrete shot against a properly prepared concrete substrate should produce a watertight interface. The hydrophilic waterstop at the joint could be considered a secondary method of making the joint watertight. Though not necessary, it is kind of a “belt and suspenders” approach with a relatively low cost to place.
4. In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Sections 3.4.2.1 and 3.4.2.2 that state:

“3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.
“3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.”

For more details on bond between shotcrete layers, you may want to refer to an article in the Spring 2014 issue of Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.” A PDF of the article can be found at www.shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf.

Properly produced concrete material shotcreted in place should gain strength over time, not lose strength. ASA recommends that concrete placed by the shotcrete method have a minimum compressive strength at 28 days of 4000 psi (28 MPa). Coring does damage the sample somewhat, so it is common to require core strength to meet 85% of the specified compressive strength. Cores should be no less than a nominal 3 in. (76 mm) in diameter for representative results because smaller cores (less than 3 in. [76 mm] diameter) are more subject to damage from the core extraction, affecting the reported strength. Thus, at 85% of 4000 psi (28 MPa) the minimum should be 3400 psi (23 MPa). Based on the reported values, and assuming a 3 in. (76 mm) diameter or greater core, the concrete strength is well below ASA’s recommended strength, and the strength you originally specified in 2013.

Shotcrete is a placement method for concrete. Thus, seismic design for concrete is applicable to shotcrete placement. Here’s a link to an article in the Winter 2009 issue of Shotcrete magazine, titled “Seismic Retrofit of Historic Wing Sang Building,” that details the seismic retrofit of a brick building in Vancouver, BC, Canada: https://www.shotcrete.org/wp-content/uploads/2020/01/2009Win_SCM01pg08-12.pdf.

A second article from 1999, “Seismic Reinforcing of Masonry Walls with Shotcrete,” also gives some input on the design: https://www.shotcrete.org/wp-content/uploads/2020/01/1999Fal_Snow.pdf. In general, the structural engineer must evaluate the condition of the existing masonry structure and determine whether the added shotcrete sections will be supplementing the existing capacity or providing the full resistance to seismic loads.

Each manufacturer of concrete constituents needs to have their products tested by NSF if they want NSF 61 certification. Whether the fibers need NSF 61 certification is an issue with the local authority having jurisdiction for exposure of components to potable water supply systems in your state. Generally, this is the state EPA-type agency, but maybe a federal agency if on a federal project.

We don’t maintain a database of manufacturer products that meet NSF 61. However, you can readily identify contacts for the fiber manufacturers who are ASA corporate members with our Buyers Guide at https://www.shotcrete.org/products-services-information/buyers-guide/. When entering the Buyers Guide, you can select “Fiber + Reinforcement Sales” and the fiber type subcategory to get a list of our member fiber suppliers.

ASA recommends a minimum of 7 days curing for all exposed shotcrete surfaces. Wet curing is preferred to supply additional water to the concrete surface. If a spray-on curing membrane is used instead of water curing, the material should be applied at twice the manufacturer’s recommended rate for formed surfaces. Curing is important to allow the concrete to develop as much strength as possible and to help resist cracking from internal shrinkage of the concrete. Low humidity, wind, and exposure to sun will increase the need for proper curing. If the site is dewatered, the groundwater is below the concrete work, and not effective in curing the exposed shotcrete surface. The American Concrete Institute (ACI) has an excellent reference, ACI 308R-01, “Guide to Curing Concrete.” It appears your contractor is not following the industry standards as documented by ACI.

Shotcrete has been used for residential concrete construction, often for domed or other curved shapes. It has also been used to provide a structural skin over internal insulation panels in more rectangular layouts. Since shotcrete is concrete projected at high speed (between 60 and 80 mph [97 to 129 km/h]) many insulation products do not withstand the impact and abrasion associated with shotcrete impact. In many cases when shooting over an insulating foam panel that cannot withstand the high velocity impact, shotcrete isn’t used, but a low-velocity plaster/grout mix is spray applied over the insulation. Here’s a link to the PCA website with a page on residential housing using a foam inner panel: https://www.cement.org/think-harder-concrete-/homes/building-systems/shotcrete. You can also find similar system information doing a web search for “shotcrete foam panel.”

Yes, either dry-mix or wet-mix process can and have been used at this height. Because either process can be used in these conditions, you should use an experienced shotcrete contractor who will pick the best method based on their firm’s shotcrete capabilities. Factors such as the shotcrete contractor’s specific shotcrete equipment, material availability, site constraints, remoteness of the location, and crew experience will influence their choice.

ACI 506.2-13, “Specification for Shotcrete,” is an excellent resource for answering your questions. ACI 506.2 addresses both preconstruction panels and test panels used for material quality control during construction. ACI 506.2, Section 1.5.1.4, requires that when preconstruction panels are required, the Contractor shall “Construct test panels for each proposed shotcrete mixture, each anticipated orientation, and each proposed nozzleman.” Preconstruction test panels vary in size to adequately represent the embedded reinforcement and section thicknesses in the work to be done.

Test panels shot during construction for evaluation of material properties are covered in ACI 506.2, Section 1.6.3.1, which states: “Construct a test panel for each mixture, each nozzleman, and each work day or for every 50 yd3 (38 m3) placed—whichever results in the most panels. The face dimensions of a test panel shall be a minimum of 16 x 16 in. (406 x 406 mm) with a minimum depth of 5 in. (127 mm). For toughness testing in accordance with ASTM C1550, the face dimension shall be 30.5 in. (775 mm) in diameter and 3 in. (76 mm) thick. Shoot test panels in a vertical orientation only unless otherwise specified.”

ACI 506.2-13 has extensive provisions for the submittals, testing, materials, and execution of shotcrete work by a shotcrete contractor. It cites many of the ASTM testing standards appropriate for shotcrete construction. We recommend you review the document in its entirety to become familiar with current industry standards.

Shotcrete is routinely used in creating retaining walls or soil-nailed walls in this fashion. Designers and inspectors often confuse placement of multiple layers of shotcrete in building out a section with cold joints experienced in cast-in-place concrete construction. Unlike cast-in-place concrete, shotcrete provides thorough consolidation and densification by high-velocity impact of fresh concrete material on the receiving surface. The high-velocity impact of shotcrete on a hardened, previous shot layer (or existing concrete surface) provides a strong abrasive blast to open up the surface, and then provides an immediate exposure of that hardened surface to fresh cement paste. As a result, shotcrete exhibits excellent bond to concrete and previously shot surfaces. Thus, the structural action between the sections acts as a monolithic section without any weakened planes.

In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Section 3.4.2.1 and 3.4.2.2 that require:

3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.

3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.

An experienced shotcrete contractor should routinely provide proper surface preparation between shotcreted sections, and use skilled crews with ACI certified nozzlemen to place and cure the shotcrete placements.

Yes, this is definitely a great application for shotcrete. Either wet- or dry-mix shotcrete can easily be used in tunnels with hose lengths of 200 ft (61 m). An experienced shotcrete contractor will be able to select the appropriate process based on the site, availability of material, and their particular equipment and trained shotcrete crews.

ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete,” is a good reference for general use of fibers in shotcrete. If looking at underground applications for shotcrete, ACI 506.5R-09, “Guide to Specifying Underground Shotcrete,” will also offer guidance. The ACI 506 documents are available in hard copy format on our bookstore website with ASA member discounts (www.shotcrete.org).
Because shotcrete is a method for placing concrete, many, if not most, of the properties of fiber-reinforced concrete, as found in ACI 544 committee documents, are applicable to shotcrete.

Shotcrete is concrete placed at high velocity to achieve compaction. It is dependent on the projection of material with air velocity of 60 to 80 mph (97 to 129 km/h) from the nozzle to consolidate the concrete material in place. It cannot be hand-applied. You may consider hand-applied pre-packaged mortar mixtures to achieve your results, although strength and durability may be less than a similar section with shotcrete because the hand-applied material is not fully compacted. Another alternative is to create an inner cylinder and cast concrete in the space between the box and the form. The concrete could then be vibrated for consolidation.

Shotcrete is a functionally impermeable material when properly designed and placed. Thus, if water accumulates between the rock substrate and shotcrete, depending on temperatures and thickness of the shotcrete, it may freeze. As with all concrete, shotcrete material can be air-entrained to enhance resistance to freezing-and-thawing exposure. You should consult with an experienced engineer to determine whether the type of rock, geometry of the sections, and anchoring of the shotcrete (such as with soil nails or rock bolts), along with the bond of shotcrete to the rock, will provide the desired performance.

Recent research into velocity of the material stream shot from the nozzle is approximately 60 to 80 mph (97 to 129 km/h) in the middle of the stream. Outer portions of the stream are slowed and show speeds of about 45 mph (72 km/h). Here’s a link to the Technical Tip published in the Fall 2013 Shotcrete magazine that provides more complete documentation of the research: www.shotcrete.org/wp-content/uploads/2020/01/2013Fal_TechnicalTip.pd.

ASTM C1140/C1140M-11, “Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels,” is the appropriate ASTM standard for producing and coring test panels. ASTM C1140 specifies panel size as a minimum of 24 x 24 in. (610 x 610 mm) with a minimum 3.5 in. (89 mm) depth. Without more information on the materials used in the shotcrete and the type of shotcrete, it is impossible to identify what may be causing the lower compressive strength tests.
The compressive strength should increase between 7 and 28 days on a curve equivalent to cast concrete. Strength degradation between 7 and 28 days may be a result of poor shotcrete application or problems with coring or curing of the samples.

In construction of shotcrete dome roofs with inflatable forms, the structural thickness of the dome is built out in layers to prevent overloading the support offered by the inflatable form and foam. Thus, your nozzleman qualification panels should be representative of the dome construction methods. This would include shooting orientation (vertical and varying slopes), shooting procedures (layers), and with the most congested reinforcing. When shotcrete is applied in layers, all you need to do is wait for the first layer to stiffen sufficiently (usually called initial set), before applying the next layer. It is not necessary to wait for days before applying the next lift.

Experienced shotcrete contractors with qualified crews have often shot structural concrete sections 24 to 30 in. (610 to 762 mm) thick with reinforcing bars up to No. 11 in size. There is no real limit to how thick you can build a shotcrete “wall.” Though earlier versions of ACI 506R, “Guide to Shotcrete,” did recommend limiting reinforcing bars sizes, modern equipment, concrete mixtures, and shooting techniques have proven that large-diameter steel reinforcing bars can be properly encased on a routine basis by experienced shotcrete crews. Thus, ACI 506R was updated to reflect the successful industry practice.

The existing shotcrete can be removed by many methods, such as using a Hoe-Ram. The existing soil nails could be removed as suggested or left in place and trimmed back to some distance behind the new plane. We would assume that this work would be done from the top down installing new soil nails and shotcrete facing in lifts of approximately 4 to 5 ft (1.2 to 1.5 m).

We are not aware of any specific applications of shotcrete to glass. The glass would have to be tough or strong enough to withstand the sprayed application. We have seen shotcrete stick to smooth surfaces like glass and glossy paint, but have not seen any data on the bond. In general terms, better bond is achieved with roughened surfaces so sandblasting the glass would likely improve the bond.

FThe shoring design should be done by a competent licensed professional engineer who specializes in earth retention systems. This is not a question that can properly be answered by ASA. You can search for such a professional in our Buyers Guide section of the ASA website www.shotcrete.org. Another resource is the FHWA Manual for Design and Construction of Soil Nail Walls.

It is difficult to make an assessment of a situation like this with a few photos and the description given. Based on your statement that the contractor has an excellent 30-year reputation, we’d suggest you discuss these concerns with the contractor. If his response does not give you a full explanation, we suggest you engage an independent professional familiar with shotcrete installations and swimming pools to give you an opinion. You can use ASA’s Buyers Guide at www.shotcrete.org to find a consultant.
There certainly is reason to question the quality of the installation based on the description. However, it is not unusual for the pool shell contractor to leave a temporary opening in the shell to relive potential groundwater pressure and prevent floating of the empty pool shell. These holes are, or should be done, in a professional manner to allow complete watertight sealing when filled. In some cases, the openings may include a pressure relief valve.

From what you have described, the original work may have been done by the shotcrete process or hand-applied. Typically in the shotcrete process, over a large area at the thickness of 5 in. (127 mm), you would expect to see reinforcing mesh or reinforcing bars.
The material can be patched. If the areas are small, it may be prudent to hand patch. Shotcrete is most effective in larger applications. It is important to select a good repair mortar or mixture design.
ASA does not have a recommended finish. Generally in this type of application, the goal would be to match the existing surfaces as close as reasonably possible.
Many patches are done on bridges and other structures without regard to water penetration.
We do not have certified contractors; however, many of our corporate contractor members have ACI Certified Nozzlemen on their teams and the Buyers Guide on the ASA website can be searched by location and specialty. Also on our website, we have a position paper on “Shotcrete Contractor and Crew Qualifications” with a handy checklist summary that would be useful in qualifying potential contractors for your project.

Polystyrene is available in various densities and the denser the product, the less likely that the shotcrete process will cause any damage to it. The question of the structural value of the polystyrene needs to be addressed by a competent structural engineer. It is not something that we, as a shotcrete association, feel qualified to comment on.

Pozzutec 20+ is intended for use in ready mixed concrete. Although it may work with dry-mix shotcrete, there are other products that are designed for use in the wet and dry shotcrete processes.

There have been many prior projects in the city of Los Angeles where columns and pilasters are incorporated into perimeter basement walls. This would likely require a preconstruction mockup panel and might require a waiver from the City Engineering Department. It is extremely important to ensure that the work is done by a qualified and experienced shotcrete specialty contractor who has documented experience doing similar work.

Shotcrete, both wet- and dry-mix, have been used to repair and reline sewer structures for many decades and has held up well as a lining or repair material. It is important to have the work done by a qualified shotcrete specialty contractor and to specify a durable concrete mixture design. The following are links to articles which may also be of interest to you:

• “Rehabilitation of Sanitary and Storm Sewers Using Shotcrete”
• “Shotcrete, Airplanes, and Automobiles”
• “Advances in Shotcrete Technology for Infrastructure Rehabilitation”
• “Ability to Access Restricted Space and Difficult-to-Reach Areas, Including Overhead and Underground”

Shotcrete has been placed against fluid-applied waterproofing and other smooth surfaces, which would likely be similar to the surface you are concerned with. An experienced shotcrete contractor should be able to install this shear wall, taking care to apply the shotcrete in a sequence which inhibits the material from sagging or sloughing. The key is hiring a good, well-experienced shotcrete contractor.
The reinforcing bar size and spacing can help provide support for building up the shotcrete. The spacing should be no greater than 12 in. (305 mm) on center each way and must be rigid.

Shotcrete has been used in similar structural upgrades for decades quite successfully. There are many ways to ensure that the moisture from the fresh shotcrete would not impact the existing partitions. Methods for protection include use of green board (as used in a shower or bath), plywood, painted coatings, moisture-resistant sheeting, and so on.
Shotcrete is placed with a low water-cement ratio (w/c) and the water is needed by the shotcrete to hydrate the cement and harden. As long as the existing surface or treated surface does not actively absorb the moisture, the shotcrete will use the available internal moisture to hydrate the cementitious materials.

This seems to be two questions:
1. The repairs should be accomplished as recommended in ICRI Technical Guidelines: the area to be repaired should be chipped out to sound concrete, sand­blasted or waterblasted to remove any bruised material, dampened to a saturated surface-dry condition, and patched with a reputable and known repair mortar.
2. ACI 506 defines “shadow” as any porous areas behind an obstacle, such as reinforcement. Proper shotcrete application by an experienced nozzleman with adequate equipment and crew will create shotcrete sections that have minimal shadowing and voids. However, sometimes upon stripping of the backside forms there is a noticeably darker coloration of the concrete surface immediately behind reinforcing bars. This darker coloration often does not have porous areas, but is an indication that the reinforcing bar is close to the form and, although good, dense material has been placed, it has a slightly higher cement paste content and thus appears darker on the surface. Sounding the area of discoloration should identify any porous areas that should be repaired, as mentioned previously.
It should be noted that this is not unique to shotcrete. Cast concrete that is not properly placed and fully consolidated often exhibits voids and rock pockets that need to be repaired upon stripping of the forms.

Shotcrete is a method of placing concrete and the properties of properly placed shotcrete are equal to those of cast concrete. We have seldom seen the use of epoxy-coated reinforcing bar in retaining walls with or without waterproofing. (The California Department of Transportation builds many retaining walls with cast concrete and with shotcrete and seldom uses epoxy-coated reinforcing bar or waterproofing.)
It is important that you use a contractor qualified and experienced in this type of work and who uses ACI Certified Nozzlemen and a qualified crew.

This is a classic performance-versus-prescription situation. The owner or customer should specify the desired end product and the contractor should choose the means and methods. Although they may both provide a great product, what works for one contractor or what he/she is comfortable doing may be completely different than another contractor.

Shotcrete would work well either for the stack exterior or as an interior lining. The 2 in. (51 mm) thickness could be easily applied on either the outside or inside surfaces.
For the 9 ft (3 m) diameter stack, it would be easier and more cost-effective to scaffold and gun the inside of the stack. The work could be done with less difficulty, from an interior hanging platform suspended with sky climbers. There would be less cost involved with rigging than there would be with scaf­­folding or rigging around the outside circumference of the stack.
For the 4.92 ft (1.5 m) diameter stack, there is much less room to work inside, so unfortunately it may need to be gunned on the outside.

This is a very challenging potential installation and there could be several potential approaches. For the shotcrete to bond to the existing concrete, the existing surface should be properly prepared, removing any unsound concrete, then roughened and cleaned to allow for a good bonding interface. You mentioned using rock anchors or bolts. These should be installed before any shotcrete.
For each 6 in. (152 mm) layer, a layer of welded wire reinforcement or structural fibers should be used. These, in conjunction with the rock bolts, should ensure the stability of each layer of the shotcrete from falling.
The surface reinforcement should not be installed before most of the area is within 6 to 8 in. (152 to 203 mm) of the final surface.
There is currently similar thicknesses being placed on the East Side Access in New York City to build back the overbreak for the initial tunneling to the “A-line” or profile that was intended for the mining.

Shotcrete is a process of placing concrete. The need to use galvanized or any other type of reinforcing is, or should be, based on the needs of the project and the preference of the structural design engineer. For liquid-containing concrete structures, the designer may review ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures,” which has specific concrete material, cover, and reinforcing requirements for concrete exposed to a variety of chemical and saltwater exposures. (The aforementioned ACI Committee 350 publication may be purchased from the American Concrete Institute at www.concrete.org”.)

Shotcrete is a means of placing concrete and the same precautions should be taken. Once the shotcrete had taken final set, exposing it to water and submerging it in water should enhance its curing. If it is a pool or other deep structure, you should ensure that the shotcrete or concrete is strong enough to withstand any hydrostatic loading due to the filling of the structure.

Shotcrete is a method of placing concrete. Thus, testing for selenium appropriate for concrete is suitable for shotcrete. The Portland Cement Association (PCA) has published a paper on testing of cement for various constituent components, including selenium, to meet NSF 61 requirements. Their testing showed “Values for arsenic, cadmium, selenium, and silver were all below detection limits.” The paper can be found at PCA’s website.

The ICC (International Code Council) Building Code requires a 7-day wet cure, which is good practice for concrete or shotcrete. The Aquron Technical Data Sheet mentions prewetting the surface prior to application, but we do not see the direction on the timing. You may want to question the product representative regarding how to accomplish the ICC curing requirements and also get the best results from their product.

Shooting around embeds can be very challenging. The most important factor is to have the work done by a shotcrete subcontractor who has done this successfully in the past and has ACI Certified Nozzlemen who are also experienced in this type of work. It has been done successfully on many projects in the past, but we do not have a published procedure to do this work.

Shotcrete can be placed as thin as 1 to 2 in. (25 to 51 mm), but will do little more than to provide a new surface to apply the plaster. To do a proper job you need to remove all loose and deteriorated existing shotcrete and should likely add in a layer of reinforcement or use structural fibers (either steel or synthetic) in the shotcrete mixture. The surface preparation should be done to the standards outlined by the International Concrete Repair Institute (ICRI).

A properly designed shotcrete composite wall system would certainly create a quiet atmosphere in the interior of the house. There are 3-D shotcrete wall systems on the market which use shotcrete on both the interior and exterior surfaces with a foam material in the center for insulation and vapor barrier. Attempting to do something like this to an existing structure would require a lot of analysis. Shotcrete is a method of placing concrete and adding shotcrete to an existing wall would significantly increase the weight of the wall and could overload the footings or impact other parts of the system. A structural engineer well-versed in residential construction should be consulted before attempting such a modification.

We would suggest that you refer to an ACI document, ACI 506.1R, “Guide to Fiber-Reinforced Shotcrete.” Carbon fibers are suitable for use in shotcrete in suitably designed mixtures.

Soil and rock stabilization is an excellent application for shotcrete. However, ASA as an association does not provide engineering design. We recommend consulting with a geotechnical engineer familiar with the local geology and soil conditions to evaluate potential lateral earth forces from the claystone. Once potential loads are established, a consulting engineer experienced with shotcrete in soil nailing applications will be able to design the soil nail facing. You can check our online Buyers Guide to find a consulting engineer experienced with shotcrete.

Shotcrete is a method of placing concrete and the properties of shotcrete are equivalent to those of cast concrete. The type of work you are describing is done in many cases with the shotcrete process and is commonly called “ratproofing.” As you have engaged an engineer, we would suggest you follow his advice and he should determine the thickness required. Providing a row of dowels around the stem wall to tie the shotcrete to the wall should eliminate any significant separation between the shotcrete and the stem wall. Shrinkage of concrete between the walls may cause some minor hairline cracking, but nothing to allow ingress of rodents or insects.

Shotcrete is a method of placing concrete. The surface texture of roughness varies considerably depending on the application and the abilities of the installer. The surface can vary from a rough nozzle finish to a smooth trowel finish and many variations between these two extremes. In buildings, the typical finishes are wood float, rubber float, or trowel finish. The owner and the architect determine what finish will be required and generally specify the finish in the construction documents. The documents which might be of help to you are ACI 506R, “Guide to Shotcrete,” ACI 301, “Specifications for Structural Concrete,” and ACI 117, “Specification for Tolerances for Concrete Construction and Materials.”

Multi-layer buildout of shotcrete sections is very common and has decades of successful performance in existing structures. Shotcrete applied to a properly prepared, existing hardened concrete substrate (such as a previously shot shotcrete layer) develops an excellent bond. The high-velocity impact of shotcrete on the surface is in effect like sandblasting, and opens up the receiving surface immediately before exposing it to the fresh cementitious paste. Cores taken through multiple layered shotcrete sections exhibit no signs of reduced bond. Often it is nearly impossible to identify where one layer stops and the next starts.
Incremental placement of reinforcing bars in layered application is also common. Proper shotcrete consistency, nozzleman technique, and air velocity will force fresh cement paste around the back of the bar and fully encase the reinforcing bar, even when in contact with the previous hardened concrete surface.
Shooting a 24 in. (610 mm) thickness at one time with two layers of reinforcement in the mostly overhead orientation of a dome would require use of special concrete mixture designs with chemical accelerators, and would be very difficult to execute with consistent quality. Also, depending on the formwork design, unbalanced loading on the dome by shooting very thick sections adjacent to sections not yet shot would be a potential concern.
For more information on the performance of shotcrete in layers, you can review this article from Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.”

Shotcrete properly applied to a well-prepared existing concrete surface will create an excellent bond and structurally act as a monolithic system without joints or layers. Pulloff testing of shotcrete applied to concrete will often fail in the underlying concrete substrate, and not at the bond interface or within the shotcrete section.
Shotcrete is a method of placing concrete and should have similar watertightness characteristics. These characteristics can be enhanced with admixtures and supplementary cementitious materials such as silica fume. Please refer to the images of cores from shotcrete applied to existing concrete.

The answer is no; shotcrete will bond well to sandblasted reinforcing bar on overhead or vertical applications. Shotcrete, like conventionally placed concrete, can be placed over uncoated black bar or bar that is coated with rust inhibitors. In repair areas where there is heavy scale on the reinforcing bar and spalling of the concrete, the repair can sometimes include some sort of reinforcing bar treatment or inclusion of a rust inhibitor in the shotcrete mixture. It depends on the situation and the assessment of the design engineer as to what is necessary.

There are many ways to approach this situation. It would be difficult, but not impossible, to install all of the work in a top-down sequence and end up with an architecturally uniform surface.
Approach 1: Install a minimal initial layer top-down with either fibers or welded wire reinforcement. Install dowels from the caissons into the structural facing layer. Install the facing from the bottom up with preplaced V-strips to make the pattern. Finish to the outermost face of the detail strips. Alternately tool the details, but likely more like 1 in. (25 mm) instead of 2 in. (51 mm).
Approach 2: Install the structural wall top-down, encapsulating the outer reinforcing steel to a plane at the depth of the detail strips. Prepare the surface by sandblasting or water blasting to create a favorable bonding surface. Install detail strips to the face of the roughened wall. Place and finish the finish layer to the depth of the detail strips.
If the base layer is properly prepared, the bond should be very good and adding dowels would be redundant. There is nothing wrong with redundancy and if so, the minimum layer thickness would be 2 to 3 in. (51 to 76 mm).
The nature of this work will mandate the use of a highly qualified shotcrete subcontractor who has experience in installing similar-quality architecturally significant walls.

Although similar in dimensions, fiberglass reinforcing has distinctly different structural properties when compared to conventional steel reinforcement. This is a question better answered by the fiberglass reinforcing industry or the structural engineering community. As the American Shotcrete Association, we do not get involved in the engineering design of structural sections. However, it should be pointed out that properly designed and applied shotcrete provides a very corrosion-resistant environment around embedded steel reinforcement, providing excellent long-term durability in normal exposure conditions.

As you have noted, shotcrete has been used extensively to reinforce unreinforced or under-reinforced masonry walls and rock walls. It has been used on the Crater Lake Lodge to strengthen and stabilize a rock wall foundation and any number of other projects. In California, shotcrete has been used to strengthen or repair walls since the 1933 Long Beach Earthquake. It was used to strengthen the California State Capitol (3 ft [0.9 m] thick brick walls) in the late 1970s and all of the older unreinforced masonry walls for the San Francisco School District. To the best of our knowledge, there have been no failures of shotcrete strengthening on the West Coast in the past 80 years.

Shotcrete is simply a placement method for concrete, so characteristics of concrete that are resistant to erosion are equally applicable to shotcrete. ACI 210R-93, “Erosion of Concrete in Hydraulic Structures,” has guidance on flow characteristics that lead to erosion of concrete. Also, ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” Sections 4.6.2 and 4.6.3, also provide guidance on concrete mixture characteristics helpful for protecting against cavitation erosion. Properly designed shotcrete mixtures can easily meet the ACI 350 4.6.3 concrete requirements.
In 2000, Rusty Morgan compiled a list of some 37 water supply tunnels that had been lined with shotcrete (a copy of the data sheet can be supplied upon request). Shotcrete was not the final lining in all of these tunnels and not all the inverts were lined with shotcrete. The evaluation does not document the water velocity in these tunnels, but could be ascertained by contacting the project owners.
It should be noted, however, that the 16.4 ft/s (5 m/s) water flow rate is not particularly fast. The water velocity needs to be in excess of 39.3 ft/s (12 m/s) before cavitation erosion can be expected (refer to A. M. Neville, Properties of Concrete) and cavitation would be the most likely cause of erosion of the concrete surface.
Supplemental cementitious materials including microsilica, fly ash, and slag will generally reduce the porosity of the hardened concrete. Microsilica is used in many shotcrete mixtures, as it helps to reduce rebound, as well as gives the fresh concrete better adhesion and cohesion that can allow for thicker or overhead placements.

Properly screeding and finishing overhead shotcrete is very challenging. The contractors who do this type of work properly have very well-trained and skilled tradesmen throughout the crew, including the nozzlemen, rodman, and finishers. Shotcrete that is not screeded and finished properly will likely suffer bonding and other issues.

Efflorescence is common on many exposed concrete and cement mortar applications. Generally it is seen when cracks in concrete or mortar are exposed to water rather than accumulating within the crack. The basic mechanism creating efflorescence is when concrete is exposed to water for a long time; excess free lime (calcium hydroxide) in the cement paste goes into solution with water (leaches). Then when that water eventually leaves the crack and dries on the surface, the white residue of calcium hydroxide creates what is termed “efflorescence.”
It is very common to see efflorescence on brick structures where the mortar joints are exposed to rainwater that leaches out the calcium hydroxide and the resulting white efflorescence is highlighted on the dark-colored face of the brick. In concrete tanks, it is often found in cracks that can accumulate water for a sufficient time to leach the calcium hydroxide. The bottoms of vertical cracks or low areas in horizontally oriented cracks often show the greatest buildup of efflorescence. These can be surface cracks that are exposed to rainwater or through wall cracks that are exposed to water contained within the tank.
Although the tank was cured properly to help deal with long-term drying shrinkage, surface cracking on shotcrete often results from early-age plastic shrinkage cracks. These are shallow cracks that form within hours (or minutes, in extreme conditions) of placement due to rapid evaporation of water from the exposed surface of fresh concrete (common in exposed floor slabs or in your case the fresh shotcrete wall surface).
To answer your question regarding when it will stop, the answer is it won’t unless the cracks are sealed, or water is prevented from getting into the cracks. Cement-rich shotcrete has more than enough free lime to continue the leaching for decades. Although surface-applied coatings may initially span small cracks, as the walls of tanks expand and contract due to filling and emptying, and undergoing daily and seasonal thermal changes, the surface cracks will open and close slightly and eventually mirror through the coating. Coatings designed to tolerate moving cracks would likely be much thicker than the 7 mils used on your project. If the cracks are through-wall cracks that are seeping from the contained water, the crack will need to be sealed, most commonly by injection of polyurethane grout or interior surface coatings.
To answer your question on how to prevent this in the future, early-age plastic shrinkage cracks can be reduced by fogging the fresh shotcrete surface to keep the surface humidity high and reduce evaporation of the water at the surface of the concrete. Also, using fibers in the shotcrete can help reduce plastic shrinkage cracking. In hot or windy climates, placing the final layer of shotcrete during the coolest or calmest time of the day may help, too.
To answer the question if additional paint would seal the cracks, simply coating with an additional 7 mil (0.2 mm) coating would provide a temporary seal, but more than likely the crack will mirror through after some period of exposure. A coating designer would need to evaluate the crack widths and potential movement to design a coating system that would provide a long-term seal.
Finally, the efflorescence caused by exposure to rainwater is generally only a visual defect and doesn’t affect the long-term structural integrity or durability of the tank. Many owners tolerate efflorescence on the tanks and simply clean it off when it becomes objectionable.

Wet-mix shotcrete is a placement method for concrete. Ready mixed concrete used for wet-mix shotcrete needs to meet the requirements of ACI 506.2-13, “Specification for Shotcrete.” ACI 506.2 specifies concrete materials shall meet ASTM C94/C94M, “Standard Specification for Ready-Mixed Concrete.” ACI 506.2 also requires you shall batch, mix, and deliver wet-mixture shotcrete in accordance with Specification C94/C94M, or Specification C1116/C1116M if fiber-reinforced. Further guidance may be found in ACI 506R-05, “Guide to Shotcrete,” and ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete.”

OSHA requires that the personnel in aerial man baskets be tied off with the appropriate harness and lanyard. When you are in a JLG or other type of man lift, the only place to tie off to is to the basket or boom bracket. This question may be better answered by studying current OSHA documents.
We cannot recall any of our members being cited for using an air lance or blow pipe.

At the point at which concrete/shotcrete temperature is starting to rise and the mixture is stiffening up, adding water should not be allowed. Water should only be added when the mixture is stable and only up to the water specified in the approved mixture design. In warm or hot conditions, retarders, set stabilizing admixtures, or ice may be needed to keep the mixture stable for the period of time to transport and pump the load.

Shotcrete is a method of placing concrete and the surface finish can be as smooth as that of cast concrete. Even with a nozzle finish, shotcrete erosion or cavitation should not be an issue at the stated velocities. Examples of smooth shotcrete surfaces can be found in many Shotcrete magazine articles and in particular (“Restoring the Century-Old Wachusett Aqueduct”).

The best information on this subject can be found in ACI 506R-05, “Guide to Shotcrete,” and likely in past articles in Shotcrete magazine. The distance that can be pumped is a function of too many parameters to fit a rule of thumb. The distance that can be pumped is influenced by the equipment being used, the vertical lift, the available compressed air, and other factors. We would suggest that you consult with one of our corporate members (www.shotcrete.org/BuyersGuide) in the area of the project and get their input.

The best guidance on this subject can be found in ACI 506.5, “Guide for Specifying Underground Shotcrete” (available through the ASA Bookstore), and some articles from past issues of Shotcrete magazine might be of interest:

• “Shotcrete Spraying Machines for Immediate Support in Tunnels”
• “Slope Stabilization in an Open Pit Mine”
• “Where Are We Now with Sprayed Concrete Lining in Tunnels?”
• “The Danger of Fallouts in Overhead Shooting”
• “Incline Tunnel—S&S Quarries, Inc.”
• “Reaching 20 MPa (2900 psi) in 2 Hours is Possible”
• “Rapid-Setting Cement in Shotcrete”

With properly qualified nozzlemen, a good shotcrete mixture, and high-quality accelerator added at the nozzle, the re-entry time can be minimal—normally 24 hours.

In the case of installing a lining inside of an existing pipe, there are robotic methods available, such as spin lining, where the cementitious material is cast from a rotating head as the carrier is moved along the pipe. For pipe smaller than 42 in. (1067 mm) diameter, the spin lining is likely the best method.
For pipe larger than 42 in. (1067 mm) and up to 6 or 8 ft (1.8 or 2.4 m) diameter, either hand shotcrete nozzling or spin lining are applicable. For pipes much larger than 6 to 8 ft (1.8 to 2.4 m), hand nozzling is likely the best solution.
In the case of clearance between a wall to be shotcreted and an obstruction, 3 ft (1 m) is a good rule of thumb, but a qualified and experienced shotcrete contractor can use modified equipment to place quality shotcrete in tighter spaces. A recent article on shotcreting in confined spaces can be found here.

You mention that you had originally wallpapered over a cement interior basement wall. It is not clear that the cement interior wall was placed using the shotcrete process. If it was installed with the shotcrete process, then the application was flawed due to improper surface preparation or application. There are many concrete repair products on the market which could be used to repair the surface. Many of these are troweled on by hand or sprayed. Check with a local building supply company or on the Internet.
With respect to the garage floor, we would not recommend the shotcrete process for a thin overlay on a horizontal surface. Again, there are many products on the market that are designed for resurfacing floor slabs. Check with a local building supply company or on the Internet for potential products.

Shotcrete rebound varies for many different reasons, many of which you mention in your question. The water seepage must be controlled or the shotcrete will likely not adhere to the surface and will slough off as the water saturates the fresh shotcrete. Accelerator will help, but it is difficult, if not impossible, to achieve good results against a seeping surface. ACI 506R-05, “Guide to Shotcrete,” estimates approximate range of shotcrete losses from 10 to 30%. Some other factors affecting the percentage of rebound are:

Mixture design

• Shotcrete process (wet- or dry-mix)
• Concrete mixture design and materials (for example, microsilica will tend to create less rebound; more than 30% coarse aggregate can cause more rebound)
• Plastic concrete properties (air content, slump)
• Nozzleman competence
• Vertical placement generally has less rebound than overhead
• Thickness of buildup per layer

Reinforcing grid

• Size and spacing of reinforcing
• Stability of reinforcing grid

The embedment depth of anchors in shotcrete would be the same as it would be for conventionally formed and placed concrete. Shotcrete is essentially a method of placing concrete and the same rules would apply. As with any anchoring system, it is important to make sure that you are anchoring to sound material.

A pH of 5.7 is slightly acidic, while 9.7 is quite alkaline. According to PCA’s “Design and Control of Concrete Mixtures,” most inorganic acids have no adverse effect on concrete. Organic acids (such as tannic acid) can significantly reduce strength when present in higher concentrations. Some alkaline materials, such as sodium hydroxide, in higher concentrations may cause a quick set. However, because this occurs in the rainy season, another factor that may have an impact is an increase in dissolved solids. PCA states that solid contents exceeding 50,000 ppm can increase water demand, accelerate set, lower compressive strength, and increase permeability of the hardened concrete. The appropriate test for acceptable non-potable concrete mixing water is ASTM C1602/C1602M, “Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete.”

All of your points are valid, but the Engineer of Record or the owner makes the final decision on recognizing a composite system or ignoring the value of the initial layer. As your experience shows, shotcrete provides an excellent bond between freshly placed layers and properly prepared concrete or shotcrete substrates. There are many articles available in the Shotcrete magazine archives—found on our website, —that may provide the designer or owner more information to allow them to make their design decision.

This detail is normally only used on very large competition pools on the order of 164 ft (50 m) in length. It takes a lot of skill, technique, and care to properly encapsulate the waterstop and it should only be attempted by a shotcrete contractor with experience in this application. The successful encapsulation of the waterstop is more challenging with the dry-mix process than when using wet-mix shotcrete. The techniques for the proper encapsulation are generally developed by the individual shotcrete contractor and there is no specific “guideline” available for encasing waterstops.

Yes, shotcrete would be an excellent method to fill the voids, open mortar joints, and gun an overlay over the irregular stone foundation. The use of shotcrete would be dependent on the access and ability of the applicator to safely place the shotcrete. A tight or low crawl space would make it difficult. We would suggest cleaning out loose materials with compressed air and water prior to the shotcrete placement. We recommend installing either a 2 x 2 in. (51 x 51 mm) 12-gauge or a 3 x 3 in. (76 x 76 mm) 11-gauge wire mesh over the stone foundation and gunning the shotcrete in place to fill in the mortar joints, creating a shotcrete overlay over the entire stone surface.

If the directive from the state is to cast (or shotcrete) both the slab and the walls monolithically, this would be a difficult task with either shotcrete or cast concrete. If the directive is to cast the floor monolithically, and then the walls monolithically, shotcrete could certainly be used and would be considered a monolithic placement. Once the state’s intent is clarified, this question should be posed to a shotcrete contractor who might be the actual contractor on the project for their input.
As this is a fertilizer plant, there may be additional considerations due to the potentially aggressive nature of the fluids introduced into this pit.

Your question does not indicate the orientation of the application. If the shotcrete is being applied to a sloped surface for a channel or slope the rebound should be incidental. If shooting a vertical wall, the amount of rebound is relative to the skill of the nozzleman, the quality or nature of the mixture, the shotcrete process being used (wet-mix or dry-mix), the stability of the wire mesh, and other parameters. The range could easily vary from 5 to 20% on vertical walls relative to the aforementioned listed parameters.

There are many ways to attach a shotcrete facing to a soil nail shoring system or an MSE wall system. For the attachment to a soil nail wall system, you could review the Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls.” For MSE wall systems, you should consult with the MSE wall system vendors. Shotcrete facing systems are commonly used on both types of walls, but it is beyond the scope of our association to provide further guidance.

The Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls” should address this issue. Many soil nail wall systems incorporate a drainage ditch at the top of the wall that catches the runoff and takes it to the ends of the wall. Your concept of a catch basin and drain between the layers is not something we have seen in the past and we are not qualified to express an opinion on this. We have seen systems with catch basins at the top of the wall and the drains behind the initial layer of shotcrete requiring notching the subgrade. To answer your question, yes, a 6 in. (152 mm) pipe can be fully encased in shotcrete between the layers. Complete encasement of an embedment of this size needs an experienced shotcrete nozzleman with properly sized equipment, appropriate concrete mixture design, and a trained shotcrete crew. The issue of appropriateness of the approach is better answered by a licensed professional engineer familiar with soil nail systems or retaining walls, and shotcrete/concrete.

You need to address this question to a professional engineer who is knowledgeable in the characteristics of shotcrete and concrete. Shotcrete is a method of placing concrete and the same parameters used in concrete design apply to shotcrete placements. Many pools of this size are designed and built without expansion joints, but it is beyond the scope of our association to provide further guidance.

Shotcrete is a method for placing concrete, so an MSDS for concrete or its constituent components would be appropriate. MSDS sheets for cement, aggregates, and concrete are readily available from cement manufacturers, aggregate suppliers, and concrete producers, as evidenced by a simple web search. This is the type of issue that can be discussed and effectively addressed by networking with other contractors and suppliers in the shotcrete industry. This is a primary benefit of actively participating in ASA—you or your organization should join ASA and attend committee meetings. ASA meetings are held three times a year. Check our Calendar for the next available meeting.

Shotcrete is concrete, pneumatically applied. So the same R-value fire ratings for concrete would apply to shotcrete. The standard is Joint ACI – TMS 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

Shotcrete is a method of placing concrete. Fibrous shotcrete will have very similar, if not identical, properties as fibrous cast concrete. Expansion and contraction joints should be similar in shotcrete to those needed in cast concrete. ACI 224.3R-95, “Joints in Concrete Construction,” covers joints in many different applications. The closest relevant document for eliminating joints is ACI 360R-10, “Guide to Design of Slabs-on-Ground,” where, in Section 8.3, it states:
“To eliminate sawcut contraction joints, a continuous amount of reinforcement with a minimum steel ratio of 0.5% (PCA 2001) of the slab cross-sectional area in the direction where the contraction joints are eliminated is recommended.”
This 0.5% reinforcement is consistent with the provisions of ACI 350-06, “Code requirements for Environmental Engineering Concrete Structures and commentary,” for the minimum reinforcement for temperature and shrinkage without contraction joints.
You can refer to ACI 506 series documents regarding shotcrete, and possibly the FHWA SA-96-069R “Manual for Design, Construction, and Monitoring of Soil Nail Walls” for additional guidance. Copies of the ACI 506 series documents are available in the ASA Bookstore.

Installing a shotcrete lining requires a somewhat dry substrate and certainly is not compatible with running water. The water needs to be blocked or diverted.
A means of blocking the inflow is to inject a swellable urethane grout through the openings in the existing pipe. The grout, if done properly, will expand upon contact with water and seal the outside of the pipe. Another means of diverting the water is to install drainage material over the inflowing area to collect the water and remove it from the pipe. The shotcrete can then be applied over the drainage material.

To ensure a good bond between the shotcrete shell and the overlay, the surface should be cleaned and allowed to dry before application of the overlay material.

Boiled absorption and volume of permeable voids testing (ASTM C642) may be required for structures that need enhanced liquid-tightness or resistance to aggressive environmental exposures. The test is sometimes used to provide an overall indication of the quality of the shotcrete mixture, particularly in dry-mix. However, many factors, including admixtures and aggregate, as well as shotcrete placing, can affect the porosity of shotcrete, so it should not be considered an absolute measure of shotcrete quality. When required, the mean average of tests on three specimens from a test panel, or from in-place shotcrete, should be less than or equal to the specified boiled absorption and/or specified volume of permeable voids limits at the specified test age with no single test greater than the specified boiled absorption plus 1%.

The International Building Code (IBC), Section 1913, allows for larger bars as long as it is demonstrated in a Preconstruction Test Panel. However, the Local Building Code likely takes precedence over the IBC. You may want to present IBC Section 1913, which requires anything over a No. 5 (No. 16) bar to be proven in a Preconstruction Test Panel.
There have been many projects shot in Los Angeles County subject to the LADBS with bar sizes larger than No. 8 bars. ASA is not in a position to give you project references, but perhaps our local members can.
Properly encasing No. 10 (No. 32) bars can be challenging, and should only be attempted by qualified contractors using ACI Certified Nozzlemen who have previous successful experience doing this type of work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

The normal repair is to pressure wash with at least 3000 psi (21 MPa) of water pressure to remove any dirt and laitance on the surface of the concrete. Given the rather small size of the hole, it can be hand-patched with a nonshrink hydraulic cement with at least 4000 psi (28 MPa) 28-day compressive strength to plug the hole. After the patch is completed, roughen the surface that will receive the plaster.

Shotcrete is concrete, pneumatically applied. So the same fire ratings for concrete would apply to shotcrete. The standard is ACI 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

Shotcrete is a method of placing concrete. The basic guidelines for placing concrete or shotcrete can be found in documents ACI 305R-10, “Guide to Hot Weather Concreting,” and ACI 306R-10, “Guide to Cold Weather Concreting.” There are some basic “rules of thumb,” such as 40°F (4°C) and rising for starting concrete/shotcrete operations and 40°F (4°C) and falling for stopping concrete/shotcrete operations. With proper planning and procedures, concrete or shotcrete can be placed at below-freezing temperatures and at very hot temperatures, but only with proper planning, procedures, and likely at some cost.

Shotcrete is often used in similar applications. The question of bending is a structural engineering question. Shotcrete is a method of placing concrete and the properties of shotcrete are similar if not the same as cast concrete. To achieve a composite wall, you must ensure that the existing surface is properly prepared to maximize the potential bond between the overlay shotcrete and the existing wall. Shotcrete placed against a properly prepared existing wall should achieve great bonding strength without the use of bonding agents. Drilled and grouted dowels also contribute to the system, working as a composite wall.

We are not aware of a specific standard for pools. However, ACI 350.1-10, “Specification for Tightness Testing of Environmental Engineering Concrete Containment Structures,” specifies a volume loss of 0.05% of volume per day conducted over a 72-hour test period for “hydrostatic tightness testing of open liquid containment structures. Specifics of conducting the test can be found in the ACI 350.1-10 document.
It should be noted that the pool should be filled and allowed to saturate for 3 days before beginning the measurements.

Properly designed shotcrete (both concrete materials and reinforcing are important in the design) placed by an experienced shotcrete contractor can certainly be used to provide a somewhat watertight lining for your pond that will be serviceable, durable, and require little to no maintenance for decades to come. We would suggest you consult with an engineer or shotcrete contractor experienced in this type of shotcrete work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

Design of a concrete mixture to be placed by the wet-mix shotcrete method is essentially the same as normal cast-in-place concrete mix design. The major differences with shotcrete mixtures are:

• The maximum coarse aggregate size is generally limited to about 3/8 in. (9.5 mm);
• They use a fairly low water-cementitious material ratio (w/cm) and slump to allow shooting on vertical surfaces without sloughing;
• The potential to use an accelerator that can be added at the nozzle; and
• The pumpability is an important workability characteristic.

Since you desire a high-early-strength mixture, using fly ash as a supplemental cementitious material (SCM) wouldn’t be recommended because it slows set and strength gain at early ages. Microsilica may be beneficial for early strength gain. Consideration should be given to using accelerator added at the nozzle. There is some guidance on concrete mixture design in ACI 506R-05, “Guide to Shotcrete”; however, because local materials (aggregates, cements, SCMs) can vary significantly, you should consult with an engineer or concrete testing laboratory familiar with shotcrete to produce and test a mixture design to meet your requirements.

Long expanses of concrete canal lining exposed to the sun and weather would experience significant internal tensile drying shrinkage stresses. Regular contraction joints help to relieve the internal tension created by concrete shrinkage. If no contraction joints are provided, shrinkage will still occur and the concrete lining will produce its own contraction joints, better known as “cracks.” Unfortunately, the resulting cracking will be random and can vary significantly in size and length. Thus, contraction joints are a good approach to help induce cracking at regular, controlled locations. If the client doesn’t want contraction joints, they need to understand that cracking will be much more extensive and likely more noticeable.
Theoretically, with the same percentage of embedded reinforcement, cracking between a 2 or 4 in. (51 or 102 mm) should not be substantially different. Of course, the 4 in. (102 mm) thick shotcrete section would require twice the concrete material and twice the embedded reinforcement to maintain the same percentage of reinforcement. A 2 in. (51 mm) thick section could have some difficulty in maintaining adequate cover over embedded reinforcing bars. The designers could also consider using fiber-reinforced shotcrete to help control shrinkage and temperature stresses, although fairly high dosages are needed for effective elimination of reinforcing bars. More guidance on fiber-reinforced shotcrete is available in ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete”. A 2 in. (51 mm) overlay is absolutely the least possible and 3 or 4 in. (76 or 102 mm) is far more normal in practice.
Canals are generally specified to have a natural gun finish, a rough broom finish, or a light broom finish.

Shotcrete is a method for placing concrete. Thus, the concrete placed by the shotcrete method has the same physical properties as cast concrete with the same mixture proportions. Either working stress or ultimate strength methods used for concrete design are applicable. Local building codes may require a particular design approach.

Shotcrete can and has been used to overlay previously installed shotcrete or concrete that has cracked over time. It would be advisable that you engage an engineer knowledgeable in geotechnical engineering and concrete properties to formalize a solution. It is important that the cause of the cracks be determined and adequate reinforcing be designed to ensure that the cracks do not propagate through the overlaid shotcrete.

Shotcrete can be applied with many different textures. The nozzle finish shown is very rough, even for a natural gun finish. Nozzle finishes can be done smoother than this. Another technique would be to use a broom to make it smoother after it is shot. Other finishes include wood float, rubber or sponge float, broom, and smooth trowel finishes. There are many examples of finishes shown in articles in Shotcrete magazine.

Many of our members add them by hand at the mixer and have had good success when using an adequate mix time. Another method is to have the mix blended at a bag mix plant with the fibers.

There are many products in the marketplace for repairing cracks. Because shotcrete is simply a method for placing concrete, any method for concrete crack repair would be applicable. It would be wise to use a product that filled the cracks and is able to tolerate thermal movement in the future (not a brittle product). Many injectable polyurethane grouts can accomplish this. Surface-applied coatings would need an adequate thickness and elasticity to tolerate moving cracks. We suggest that you contact one of our corporate members who is familiar with your area and get their specific advice. Please refer to “ASA’s Buyers Guide”.

Phosphate-bonded refractory materials were routinely shot in cyclone boilers in the 1970s. These phosphate-bonded materials don’t have a cement bond, but achieve a chemical bond when heat is applied. Without knowing the precise formulation of the mixture and grain sizes involved, we cannot tell you definitively that your specific material can be shotcreted. However, there is a long history of successful past experience with phosphate-bonded refractory materials being shot with the dry-mix process. You may want to consider a field trial before construction to verify your specific mixture works with your dry-mix shotcrete equipment.

The proposed overlay will be similar to any repair where shotcrete is placed over existing concrete. Proper surface preparation is essential for allowing good bond. Guidance on surface preparation can be found in ACI 506R-05, “Guide to Shotcrete”. It also appears you are suggesting tapering the thickness from 15 to 0 in. (381 to 0 mm). Feathering thickness down to 0 in. (0 mm) is not encouraged, and a minimum thickness should be established. Because the overlay section will be quite thick and experience differential shrinkage from the previously shot material, the overlay will require additional reinforcement to accommodate temperature and shrinkage stresses. You should consult with an engineer experienced in shotcrete design to establish the proper amount of reinforcement. The required reinforcement and cover over the reinforcement will control your minimum overlay thickness.

Shotcrete can and has been used to increase the size of columns and thicken overhead slabs while providing great-looking linear or curvilinear finishes. Examples of curvilinear finishes can be found in past Shotcrete magazine articles. You can search the Shotcrete magazine archives.

Shotcrete is simply a placing method for concrete. Thus, the in-place material properties are essentially the same as cast concrete. A specific value for the shear is beyond the scope of our Association because many design and material properties can affect the shear capacity. We would suggest you engage a Professional Engineer who specializes in Underground Shotcrete. You should consult our Buyers Guide to find such a consultant. ACI 506R-05, “Guide to Shotcrete”, would be a helpful primer to learn more about shotcrete.

Shotcrete is simply a placing method for concrete. Thus, the mixture design and material properties are the same as concrete. We are not aware of any software specific to shotcrete in any units. ACI 506R-05, “Guide to Shotcrete”, has guidance on desirable mixture characteristics (aggregate grading, supplemental cementitious material [SCM], and so on) that would be helpful in developing a concrete mixture design for shotcrete placement.

Shotcrete is a method of placing concrete and the characteristics of shotcrete are those of cast concrete. Although the enclosed swimming pool will increase the interior humidity, the high humidity should have no detrimental effects on the exposed shotcrete, and may even be beneficial in reducing long-term drying shrinkage of the wall.
Both cast-in-place and shotcreted concrete are commonly used for construction of water tanks with constant exposure to water under significant hydraulic pressure. Using good construction techniques with good-quality concrete to build the tank’s walls produces walls with no moisture evident on the exterior face of the tanks. Simply having a high-humidity atmosphere is a much less severe exposure and should not result in interior air moisture being transmitted into and through the shotcrete wall. Any coatings considered for aesthetics should follow the manufacturer’s recommendations for drying time of concrete before application. If there is a concern about the permeability of the shotcrete wall, a premium shotcrete mixture including silica fume might prevent some issues on this application.

Shotcrete could be used for this application. The thickness of the overlay would be dependent on the material used. A potential concern would be the lines of the existing block showing on the new surface. We would suggest that you search for and review various ASA Shotcrete magazine articles as well as ACI 506R-05, “Guide to Shotcrete”

ACI 506R-05, “Guide to Shotcrete,” Section 5.6, on Alignment Control (refer to ASA Bookstore: https://shotcrete.org/bookstore/ProductDetail.aspx?itemid=506R-05) gives specific guidance on proven methods to establish the line and grade of the surface, as well as proper material thickness and cover. Common methods are use of ground wires, guide strips, depth gauges, and depth probes. Please refer to “ASA’s Buyers Guide” (https://shotcrete.org/products-services-information/buyers-guide/) and contact one of our members who provides supplies to the shotcrete industry

Shotcrete is a method of placing concrete and properly placed shotcrete should have a service life similar to cast concrete. Generally, concrete structures in normal environmental exposures are expected to have service lives from 50 to 100 years. With particular attention to materials and construction methods, some concrete structures, such as the new San Francisco Bay Bridge, have been designed for a service life up to 150 years. The first step in achieving a long-lasting, high-quality installation is to engage a highly qualified and experienced shotcrete contractor. There are many other factors that influence service life, including using the right mixture design for the anticipated exposure conditions.

For concrete piers, the reinforcing steel is generally secured to the piles with reinforcing bar grouted dowels. For steel piles, the reinforcing bar is generally secured with Nelson studs.

Generally, the interface between sequentially placed layers of shotcrete is not considered a cold joint because the shotcrete abrasion, velocity of impact, and high paste content make excellent bonding conditions. Cores taken through layers of shotcrete on shotcrete often show that it is virtually impossible to ascertain one layer of shotcrete from the next. Please refer to ACI 506R, “Guide to Shotcrete,” for information on joints in shotcrete.

Your proposed solution is certainly reasonable and is used regularly. Shotcrete has been used to strengthen both URM and tilt-up panels to accommodate enlarged openings. Shotcrete is a method of placing concrete and the in-place properties would be the same. The applicable code would be the code you would use if you were to strengthen this wall with concrete. Designs using the ACI 318 Building Code and Commentary are fully applicable to shotcrete placement, although compressive tests for acceptance are secured using cores from shotcrete test panels per ASTM C1140/C1140M and C1604/C1604M, rather than cast cylinders.

Shotcrete hoses can be cleaned out using either water or air. In many instances, the site conditions make cleaning with water not feasible. When cleaning with air, the free end or discharge end of the hose should be secured to something to ensure that the hose does not whip as the material and cleaning ball or rag discharges.

Yes, it is possible to shoot 24 in. (600 mm) thick tunnel linings. There are various ways of doing this, depending on the reinforcing steel configuration. One method we have successfully used for shooting tunnel linings this thick with a double mat of reinforcing bar (1 in. [20 mm] diameter bars at 6 in. [150 mm] on center, vertically and horizontally) is to bench gun shoot the walls up to the spring line with a wet-mix silica-fume-modified shotcrete (without accelerator) and then ribbon-shoot (2 ft [0.5 m] wide strips) overhead using the same mixture but with the option of using an alkali-free accelerator added at the nozzle.
If the shotcrete requires a smooth finish (equivalent to a cast-in-place concrete finish), then the initial shotcrete is shot to within about 1 in. (30 mm) of the final shotcrete thickness and allowed to set and harden. Following that, a final non-accelerated finish coat can be applied that can be trimmed to shooting wires with a cutting rod, closed up with a darby, and then trowelled with either a magnesium or steel trowel, depending on the required finished surface texture.
Such work can be done with a remote-control manipulator arm (robot) or, for more precision, with hand nozzling out of a basket on a manlift (provided the tunnel floor is sufficiently smooth for operation of a manlift). The bottom line: hire a contractor who has experience in conducting such work.

This provision has been an issue for shotcrete contractors in the region for many years. ASA and ACI Committee 506, Shotcrete, do not endorse the concept stated in the “Los Angeles Bulletin.” Unfortunately, this provision has shown up in other areas around the country.
A good shotcrete practice is to limit lift height to that which can be placed without sloughing or sagging and to place subsequent lifts at such a time that the previous lift is sufficiently firm to support the subsequent lift. ACI 506R-05, “Guide to Shotcrete,” Section 8.5.8, specifically addresses this point. The following is a link to ASA’s bookstore:  https://shotcrete.org/bookstore

ASA does not have such a document. The concrete cover for embedded reinforcing steel is subject to the local Building Codes and may be increased by the structural plans and specifications produced by an Engineer or Architect for a specific project. ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” covers concrete structures intended for water containment and would be applicable to pools. The following is a link to ACI’s bookstore: www.concrete.org/bookstore/ProductDetail.aspx?itemid=35006.

Compressive strength at 1, 3, and 7 days can be important to all for subsequent operations. In general, 7-day strengths are a good indicator of the ultimate 28-day strength. The need for early strength is an engineering and construction sequence issue, not a normal or typical shotcrete requirement.

There are prepackaged materials commercially available for impact and abrasion resistance. Please contact material suppliers from “ASA’s Buyers Guide” for product information: www.shotcrete.org/Buyers Guide.

UL designs are typically for the hourly fire proofing ratings on structural steel members such as I-beams, wide flange beams, and vessel skirts. The beams and columns are tested for specific fireproofing products, beam sizes, and configurations. The thickness of the steel and other considerations factor in the evaluation; therefore, there is no blanket UL design number that you can use for steel plate. You can get guidance on the cover needed for different fire ratings in ACI 216.1-97/TMS 0216.1-97, “Standard Method for Determining Fire Resistance of Concrete and Masonry, Construction Assemblies.”
Remember that shotcrete is a process for applying concrete. You may also consider looking for a similar concrete UL design and submit it for consideration. Applying the shotcrete at a greater thickness to compensate for any variances should be proposed and presented to engineer or the owner for consideration.

ASA has taken the position that structural shotcrete is shotcrete that meets or exceeds a compressive strength of 4000 psi (28 MPa). Looking at pertinent ACI Codes related to watertight concrete, as we would expect in a pool, we find ACI 318-95, “Building Code Requirements for Structural Concrete,” introduced a provision in 1995 that required: “Concrete intended to have low permeability when exposed to water shall have a Minimum f´c of 4000 psi (28 MPa)”. Similarly, ACI 350-01, “Code Requirements for Environmental Concrete Structures,” first issued in 2001 required: “Concrete intended to have low permeability when exposed to water, wastewater, and corrosive gases shall have a Minimum f´c of 4000 psi (28 MPa)”. Since ACI 350 is more directly applicable to water-containing structures, the 2001 date is probably the most relevant, though ACI 318 introduced the concept in 1991. We do, however, see shotcrete specified at lesser levels for different types of uses.

Stress distribution from external loads through the tank wall with the shotcrete lining will depend on the geometry of the tank and the structural function of sections to be removed. A professional engineer experienced in shotcrete and concrete tank design should be consulted to ascertain the structural capacity of the completed wall. It would certainly be important to create a good bond plane by roughening the surface and removing any loose or fractured materials and using sufficient drilled dowels to make the existing 12 in. (305 mm) wall and new 6 in. (152 mm) overlay work well together. Also, it might help to specify the use of a shrinkage reducing admixture.

Shotcrete has been used in the Northwest to strengthen and overlay existing rockery walls. The need to remove the weathered material is dependent upon the need for the overlay to bond with the existing wall, which is an engineering issue and not a shotcrete issue. Shotcrete can and is shot successfully against soil and other weathered surfaces.

Yes, shotcrete would be suitable for this application. A well-installed shotcrete lining will be durable and protect the steel from impact, abrasion, and from the acid attack that occurs from sulfur in the coal. Shotcrete is used to cover both the steel hopper walls and to encase the steel beams. Calcium aluminate cement is typically recommended for coal bunkers because of the mild acid condition that occurs that can attack the steel. Whether or not the steel beams are galvanized or not is irrelevant because the shotcrete will not bond well enough to any steel surface without welded studs and mesh to hold it in place. The beams will need to have studs welded and mesh installed around the beams for the shotcrete placement. With galvanized steel it is often necessary to grind off a spot of the galvanized coating at the spot of each stud weld location to properly weld the studs.

This type of repair is commonly done using the shotcrete process. The extent of the repair is an engineering issue, not a shotcrete issue—shotcrete can and is used for patches and overlays. The shotcrete will adhere to the properly prepared existing concrete. It is installed such that the weight of the plastic shotcrete does not;exceed the adhesion to the existing surfaces; if additional material is needed, it is added at the initial layer or layer set up. The minimum thickness is related to the material used for the repair and the need to establish cover on the existing or added reinforcing. Some repair mortars can be placed as thin as 1/2 in. (13 mm).
Please find a link to a paper on “Concrete Repair by Shotcrete Application”.
The success of the shotcrete repair will be highly dependent upon using a qualified shotcrete contractor and doing an excellent job of preparing the surfaces. Where the reinforcing is exposed, you should require that it be chipped out the entire perimeter allowing for a space of 3/4 in. (19 mm) behind the reinforcing bar so that the repair material can completely encase the reinforcing.

SFRS is routinely cored from shotcrete test panels or in-place shotcrete linings without difficulty. The shotcrete should, however, have a minimum compressive strength of about 10 MPa (1450 psi) at the time of coring.
Round panel testing of flexural toughness of fiber-reinforced shotcrete to ASTM C1550 is often specified and used for quality control (QC) purposes in tunneling and mining projects in North America and elsewhere (for example, mines in Australia) virtually every day of the year.

The addition of fiber will not increase the bond of shotcrete to the concrete shear wall. A 1.5 in. (38 mm) thick layer of properly designed and applied shotcrete should have adequate bond to a properly prepared concrete substrate without additional mechanical anchors. However, exposure conditions, geometry of the wall, shrinkage potential of the shotcrete mixture, application technique, and curing—as well as the age and quality of the shear wall concrete substrate—may affect the bond. These factors should be considered by an engineer experienced with shotcrete overlays in deciding whether additional anchoring is advisable.

Shotcrete is normally expected to meet or exceed 4000 psi (27.6 MPa). We are not aware of any specific MSHA requirements. We would suggest you consult a tunnel or mining engineer who is well-versed with shotcrete. Cores taken from field-shot test panels are generally used for the evaluation of compressive strength of shotcrete (ASTM C1140/C1140M-11).

The face or surface of shotcrete walls as described are typically established with ground wires or screeds, which assist the person using the shotcrete rod in cutting the wall to the proper plane. The tools typically used to achieve a troweled surface are the shotcrete rod, wood floats, and steel trowels. Typically, shotcrete walls are a minimum of 4000 psi (27.6 MPa) 28-day compressive strength and the aggregate varies from sand only to a blend of sand and 0.375 to 0.5 in. (9.6 to 13 mm) aggregate. Consult our Shotcrete magazine archives for examples at www.shotcreteweb.wpengine.com.

No, there is no reason to assume that the shotcrete would contain asbestos. Shotcrete linings typically contain sand and cement. Asbestos was commonly used for pipe insulation and high-temperature industrial uses and not for shotcrete.

The friction coefficient n of well-finished shotcrete for use in Kutter’s equation (and, more streamlined, Manning’s equation) is generally used as 0.012. Shotcrete is used not only to improve flow characteristics of brick, corrugated metal, or any other pipe construction but can also be conventionally reinforced as a structural liner to eliminate the need for liner plates or other pipe-lining alternatives.

This question should be addressed by a qualified engineer with experience in designing swimming pools and well-versed in shotcrete technology. Shotcrete is a method of placing concrete and the parameters that work for concrete cover work for shotcrete.

Good practices for placing shotcrete or concrete include:

1. Predampening the soil that the concrete/shotcrete is placed against;
2. Ensuring that the reinforcing bar temperature is not too high; and
3. Early curing of the shotcrete surface and maintenance of curing for the specified duration. If no duration is specified, a 7-day wet cure is recommended.

Using fibers in the shotcrete/concrete can also help control surface early-age plastic shrinkage cracking.

There are a wide variety of epoxies and polyurethanes used for crack injection. Smaller crack widths would require a lower-viscosity material to penetrate the crack. You should contact an engineer or injection specialist experienced in shotcrete and cracking issues to evaluate the cracking and make a specific recommendation for repair. Proper concrete mixture design, placement techniques, and early water fogging and curing can help to reduce plastic shrinkage and drying shrinkage cracking in the future.

Shotcrete is a placing method for concrete. Wet-mix shotcrete will be very similar in density to fully consolidated concrete when the concrete mixture designs are similar. Properly mixed and shot, dry-mix shotcrete may have a slightly higher density. Properly designed, placed, and cured, both concrete and shotcrete will give an excellent service life.

Shotcrete would be an excellent process to repair or overlay your wall. You are correct in saying that it could not only repair but also reinforce and enhance the strength of the wall system. It is important to remove all deteriorated brick and sandblast or water-blast the surface if you are looking for a good bond between the shotcrete and the existing brick. Dowels epoxied or grouted into the existing brick are often used to mechanically tie the shotcrete overlay to the brick wall and also stabilize the new reinforcing steel in the shotcrete overlay.

Shotcrete has been used extensively for the purpose of relining existing culverts. You are correct to visualize the existing culvert as a form for building a new structure. Because it is a stay-in-place form, it may actually act as a composite structure. Shotcrete is a method of placing concrete and will have similar, if not better, durability and life span if installed professionally with good mixtures. You can locate numerous past articles on durability of shotcrete that have appeared in Shotcrete magazine in the magazine’s archive on the ASA website, www.shotcrete.org. Similar work has been done in California, Colorado, and other states. This approach is currently being used as permanent tunnel lining in many places, including many of the current New York Transit projects.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

This has been done in the past to improve hydraulic capacity and provide better wear resistance in the invert. Typically, the minimum cover over the corrugation is 2 to 3 in. (51 to 76 mm) with a welded-wire fabric either welded or otherwise attached to the corrugated pipe. The cover could likely be reduced with the use of structural fibers of either steel or synthetic material. Steel fibers and wire mesh should not be used together. Care must be taken to specify the required finish. This application would likely benefit from a smooth trowel or light broom finish. A light broom finish is preferable from a safety standpoint, as a trowel finish creates a very slippery surface both during construction and for the maintenance at a later date. In addition to the hydraulic and wear characteristics, once shotcreted, the entire pipe will become a composite section with improved structural characteristics. It should be noted that the pipe must be large enough for workers to work in safely and productively. This would mean an absolute minimum of 48 in. (1219 mm) and preferably larger.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

It is very common in drill and blast operations to blast shortly after the application of shotcrete. There are certainly risks involved, but a knowledgeable and experienced mining crew working with or for a knowledgeable, experienced contractor would not have any problem with this type of application.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

Assuming this is an oscillator on a robotic arm, it should not be disabled. Good nozzling technique, for either wet or dry, requires the nozzle to be moved in a constant overlapping circular pattern. This allows for better encapsulation of reinforcing bar and produces a more uniform surface; and, particularly for dry process, it is required for final mixing of materials that occurs on the surface.

Without proper nozzle technique, which requires oscillation, you will not get uniform, homogeneous shotcrete.

Shotcrete would be a great substitute for the retaining wall. Design-wise, the shotcrete is equivalent to concrete because shotcrete is really just a way of placing concrete.

The No. 4 (No. 13M) at 18 in. (450 mm) on-center spacing is not a problem. A No. 4 (No. 13M) bar can be easily encased by a qualified, experienced nozzlemen using either wet- or dry-mix shotcrete. In longer walls, or anywhere where reinforcing bars are lap spliced, the lap splice bars should be spaced apart. ACI 506R-05, “Guide to Shotcrete,” Section 5.4, provides some good guidance on optimizing reinforcing bar layouts for shotcreting. On the issue of lap splices, it says: “If the design allows, direct contact of the reinforcing splices should be avoided. Non-contact lapped bars should have a minimum spacing of at least three times the diameter of the largest bar at the splice.”

Thus, with No. 4 (No. 13M) bars in a lap splice, you should have the reinforcing bars spaced 1.5 in. (38 mm) apart at the splice to allow the shotcrete material to flow around the bar during shooting.

Chapter 8 of ACI 506R-05, “Guide to Shotcrete,” also provides a lot of guidance on proper shooting techniques for a variety of applications, including walls.

The article “Shotcrete Testing—Who, Why, When, and How” in ASA’s Summer 2011 issue of Shotcrete magazine should help answer most of your questions on testing of shotcrete. ACI 506R, “Guide to Shotcrete,” and ACI 506.2, “Specification for Shotcrete,” also have helpful information on shotcrete testing. Most competent testing labs should be able to test the compressive strength of cores extracted from shotcrete panels or sections, as they are very similar to concrete cylinder tests. If conducting more advanced testing, you may want to consider selecting a lab experienced with shotcrete.

Shotcrete is concrete applied using the shotcrete process. Therefore, any recycling potential that applies to concrete would apply to shotcrete.

Appropriate thickness of the shotcrete layers is impossible to generalize because it depends on many factors, including:

1. The type of shotcrete (wet- or dry-mix);
2. The texture and stiffness of the receiving surface;
3. The physical properties of the fresh concrete used, including a) w/cm ratio; b) slump; c) use of accelerator; d) type of supplementary cementitious materials used in the mixture (microsilica, fly ash, and slag); e) fibers used in the mixture; and f) mixture temperature;
4. Weather conditions—Is it hot or cold, dry or wet, and/or windy or calm?;
5. The shotcrete equipment used: a) type of nozzle; b) distance from the receiving surface; and c) air pressure and air volume;
6. The orientation of the shotcreting (vertical/sloped/overhead)

Experienced shotcrete contracting firms using ACI Certified Nozzlemen have a wealth of experience in evaluating all these factors to achieve the proper results. You may consider subcontracting the shotcrete work to an ASA member contractor with experience in this type of work. You can submit your project details for bids from our ASA Corporate Members using the Web form at www.shotcrete.org. For further reference, ACI 506R-05, “Guide to Shotcrete,” provides some general discussion of the shooting techniques that may be appropriate. Retaining an engineer or shotcrete consultant experienced in shotcrete application may be of value to assist in evaluating your specific factors and recommend the best solution.

The ability to clean graffiti from the surface will, to some extent, depend on the finish texture. A rough texture will be difficult to coat successfully with a sealer or paint. Commercially available anti-graffiti paints have been used very successfully on shotcrete tanks with relatively smooth float finishes. We do not have any recommendations on the brand of sealer or coating.

We are not aware of specific tolerances for shotcrete in swimming pools. Shotcrete is a method of placing concrete and the cover over reinforcing steel should be the same as that for cast concrete. With respect to the depth of the pool, this would be a building code issue, not a shotcrete issue.

Shotcrete has been used for canal lining throughout the United States. The Bureau of Reclamation published a study on Canal Lining Test Sections constructed in the Bend, OR, area and studies the durability at 5 and 10 years. Shotcrete is a very viable means of placing canal linings. Basically, shotcrete is a method of placing concrete. Care should be taken to ensure that the mixture is designed to withstand the local environmental conditions, such as using air-entraining admixtures to ensure durability due to exposure to freezing and thawing. ACI 506R-05, “Guide to Shotcrete,” contains a lot of useful information in evaluating and using shotcrete in a variety of applications, including canal linings. If liquid-tightness and long-term durability of the canal lining are important, provisions of ACI 350/350R-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” should also be considered in the design of canal lining reinforcement, cover, and joints.

Structurally sound concrete that contains up to 20% fly ash in the total cementitious materials should not present any problems for subsequent bonding of shotcrete. Concrete with fly ash contents up to 30% have been used in recent years without any reported problems with strength and bond. Although concrete mixtures with higher levels of fly ash (up to 55%) have been proposed, we don’t have direct experience with their bonding characteristics. We suspect it would be fine as long as the base concrete develops adequate compressive and tensile strength. This could be confirmed by a simple bond strength test of shotcrete on the concrete substrate in question.

The existing surface needs to be properly prepared, removing all soft or deteriorated material back to sound concrete. For extensive defects in the existing concrete, chipping hammers may be required. For removal of light surface carbonation or laitance, a strong, high-pressure water blast or sand/bead blasting may be adequate. Depending on the thickness of the shotcrete, reinforcing may be required in the overlay. Specific details of the repair are best developed by an engineer experienced in shotcrete repair.

Structurally, using proper quantities of either welded wire or fibers should work well. If fibers are used, they should be specified by an engineer who has the experience to specify the type of fiber and either performance requirements or dosage levels. The advantage of fibers is that they are uniformly distributed through the section, whereas the welded-wire reinforcement can be difficult to maintain in the proper location within the pavement section. The proper thickness should also be determined by a qualified engineer, as soil and groundwater pressures can impact the required thickness.

It is always a good practice to scale off the loose material from the rock face, particularly when dealing with shale or clay stone, as they degrade when exposed to the air. In addition to scaling the rock face, it should be washed down with air and water prior to gunning. As for expansion joints, they are not normally used when gunning over natural rock. The shotcrete is typically gunned continuously across the hillside without any expansion joints, with a natural gun finish following the natural contours of the rock face. With an anchored mesh in place, the use of fibers is not necessary. In many applications, fibers can be used in place of or in addition to mesh.

We are not aware of any recognized standard for abrasion testing or acceptance specifically for this application. ASTM International has several abrasion tests for concrete and mortar that include:

• ASTM C418-05, “Standard Test Method for Abrasion Resistance of Concrete by Sandblasting”;
• ASTM C779/C779M-05(2010), “Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces”;
• ASTM C944/C944M-99(2005)e1, “Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method”; and
• ASTM C1138M-05(2010)e1, “Standard Test Method for Abrasion Resistance of Concrete (Underwater Method).”

Also, ACI 350-06, “Code Requirements for Environmental Concrete Structures,” Section 4.6, has requirements for protection against erosion.

The equivalent dosage of fibers to replace embedded steel reinforcement needs to be evaluated by the design engineer for the specific project or application. Guidance for the designer is available in ACI 506R-05, “Guide to Shotcrete,” and ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete.”

The insulation itself should provide a vapor barrier. Various additives can be used with the shotcrete to improve its permeable properties, such as silica fume or a commercial waterproofing additive. It is also not uncommon to use a plaster coat over the shotcrete to provide improved water resistance and an architectural finish. The density and uniformity of the shotcrete can be influenced by the competency of the shotcrete applicator. It is always advisable to engage a competent and experienced shotcrete contractor to ensure the best possible results. You can search for a contractor with certified shotcrete nozzlemen from our Buyers Guide at www.shotcrete.org/products-services-information/buyers-guide/ or submit a bid request through our Online Bid Submittal Tool at www.shotcreteweb.wpengine.com/pages/secured/ProjectBidRequest.aspx.

Shotcrete has been used in many cases to repair, rehabilitate, and strengthen pipes, culverts, and tunnels. It is not uncommon to use shotcrete to strengthen a culvert under a highway or roadway section. Shotcrete is a method of placing concrete at a high velocity. The shotcrete placed inside the existing pipe can be designed for strengths from 4000 to 10,000 psi (27.5 to 69 MPa), depending on the amount you are willing to spend on the shotcrete products. We cannot speak to the acceptance by the governing body, but it has been done successfully and often in the past. It is vitally important that the shotcrete contractor be competent and experienced in installing the lining. Your specification should require evidence of similar previously completed projects with current references.

There is a difficult balance between wet curing too early or too late. You should not add water too early (before the material sets), as this would increase the water-cement ratio (w/c) of the material on the surface. You also do not want to add water during the finishing process, as this would also work the water into the surface and increase the w/c at the surface. Good practice would be to use an evaporative retardant, which generally also serves as a finishing aid during the finishing process, and then get the wet cure set up as soon as possible.

Shotcrete is a method of placing concrete and therefore the material has the same basic characteristics of concrete. Shotcrete is often used for canal, channel, and ditch lining. It is important with shotcrete (concrete) that the subgrade the material is placed over be compacted and stable. The thickness, strength, and reinforcing needs to be designed and specified by a professional engineer familiar with this type of structure or pavement. For budget numbers, you should contact one of our contractor members, who can be found in the Buyers Guide on the Web site at www.shotcrete.org.

Shotcrete is the same as concrete when evaluated as a material and its exposure to potable water. In the U.S., many admixtures and cements for concrete have been tested and certified to meet the NSF 61 standards for materials exposed to potable water. In my experience, potable water stored in concrete tanks with direct exposure to the concrete (no coatings) has not exhibited any significant rise in alkalinity. Exposure of a tunnel in a groundwater aquifer would seem to have much less contact area per volume of water contained in the aquifer, such that any rise in alkalinity would be miniscule. Because concrete in the U.S. is universally accepted for the storage and transport of potable water, I’d assume that the use of shotcrete in your tunnel would be perfectly acceptable.

It is not uncommon to encapsulate lattice girders or steel sets in fibrous shotcrete. The skill of the nozzleman, the size and density of the reinforcing, and the characteristics of the mixture and the accelerator are the most important factors in achieving good encapsulation of reinforcing bar or these more complicated applications around lattice girders or steel sets. With welded-wire reinforcement, you should have a 4 x 4 in. (100 x 100 mm) or greater spacing. With reinforcing bar, you should use the minimum diameter possible at a minimum spacing of around 6 in. (150 mm). Preconstruction mockups should be considered to prove the competency of the nozzleman and the mixture. Please note that the best nozzleman cannot succeed without a good, workable mixture.

It is possible to use a penetrating sealer on shotcrete in the same manner as cast-in-place concrete. We are not aware of research on the durability of such a sealer and do not know if it would enhance the freezing-and-thawing characteristics. A high-quality shotcrete mixture that is properly placed will exhibit excellent freezing-and-thawing characteristics with or without a sealer.

Shotcrete has been used to successfully rehabilitate sewers for over 50 years.

The thickness to be used is an engineering issue and beyond the scope of our association. We would recommend a 2 in. (50 mm) minimum thickness reinforced with either polypro­pylene fibers or a light-gauge welded wire reinforcement. The surfaces must be cleaned thoroughly to remove grease, oils, and other substances deleterious to good bond. Bonding to brick is not a problem.

Finish is another consideration. The added liner thickness will reduce the size of the culvert. If capacity is not an issue, it is recommended to leave the new shotcrete lining with a nozzle finish. If capacity might be a problem, then a float or trowel-smooth finish may be necessary.

We are not aware of any testing done specifically for the sound absorption performance of shotcrete. Shotcrete is a method of placing concrete and, once hardened, it should have similar parameters as cast concrete. With shotcrete, one has the ability to use many different finishes, which might influence the sound absorption characteristics. A smooth troweled shotcrete wall would be the most similar to a formed cast-in-place wall. On the other end of the spectrum, a nozzle-finished wall would likely absorb far more sound. A recent design of a concert hall at a major university was to be built with oval concrete or shotcrete perimeter walls covered with fabric curtains for sound purposes. The ceiling was to be suspended nozzle-finished shotcrete.

This is a great application for shotcrete. Once drained, the existing surfaces should be cleaned by water blasting or sandblasting to provide a good bonding surface. The shotcrete can be submerged within a few hours or upon reaching the final set (hardened). One factor to be concerned about is the chemical reaction and alkalinity of the area around the shotcrete during the curing period. A good solution would be to submerge the pool for a period of at least a week, drain and waste the water, flush the shotcrete surfaces, refill the pool, and test the pH before reintroducing the fish. This should eliminate the danger of the alkalinity to the fish.

Although some have tried, there are not and should not be specific recommendations for lift height or time between lifts of layers. Shotcrete is a method of placing concrete, and concrete properties vary with many parameters, such as admixtures, ambient temperature, concrete temperature, slump, and age of concrete, to name a few. The lift height is also influenced by the surface on which you are shooting (rough, porous, smooth, dense, and so on); the orientation being applied (vertical, sloped, or overhead); and the size and density of the reinforcing steel, if it is present. Regardless of the period of time between lifts or layers, the receiving surface must be clean and moisture-conditioned to create a good bond between lifts or layers. As you can see, there are too many variables to spell out recommended guidelines or rules of thumb for lift heights or time between lifts or layers. The goal is to place the lifts or layers in heights or thick­nesses that do not slough or sag. The time between lifts or layers is the time required for the initial lift or layer to support the subsequent lift of layer. These decisions must be made on the job on a daily and hourly basis by a properly trained and experienced nozzleman and shotcrete foreman. These decisions may vary during the day to meet the current situation. It is critical that the shotcrete is placed by a shotcrete contractor with trained and experienced crews who is experienced and successful in the type of work being installed.

In structural applications, most of the impact force from nozzling shotcrete is directed toward compacting the shotcrete in place rather than against the formwork. This was the subject of a study conducted by Marc Jolin of Laval University, Quebec City, QC, Canada, and reported in the Fall 2008 issue of Shotcrete magazine. There is virtually no hydrostatic pressure on the forms from the application using the shotcrete process. A copy of this study can be viewed on the ASA WebIt is fine to place shotcrete in two layers on 2 consecutive days, although simply placing two layers on 2 consecutive days won’t prevent long-term drying shrinkage cracking. For the best bond, the surface of the shotcrete on Day 1 should be given a rough broom finish to provide a rough texture for the Day 2 shotcrete to bond to. On Day 2, before shooting, wet the surface of the Day 1 shotcrete to prevent a hot, dry surface from absorbing water from the fresh shotcrete. Please note that the surface needs to be dampened but allowed to dry to an SSD condition. A surface that is too wet can inhibit good bonding. It is essential to moist-cure the shotcrete as soon as it has finally set to help reduce early-age shrinkage cracking. On a hot, windy day, you may need to fog the surface soon after placement with a pressure washer using a fogging nozzle to reduce the rapid evaporation of water from the surface of the shotcrete. Wet curing with a wetted burlap overlay or drip system for at least 3 days (preferably 7 days) is recommended to help reduce the potential for longer-term drying shrinkage cracking. Using macrosynthetic fibers in the shotcrete mixture will also help reduce early-age shrinkage cracking. Because you are in Florida, unless you are shooting in the dead of winter, you may also want to consider using a concrete mixture with up to 20 to 25% fly ash. This will slow down the hydration of the cement and resultant set time to give you some more time to finish the surface and get proper curing started. Fly ash also helps reduce the concrete permeability and increases the long-term strength and is generally less expensive than portland cement. If you use a concrete mixture with silica fume (also called microsilica), it will increase the water demand of the mixture during hydration and has a greater tendency for early-age plastic shrinkage cracks. Thus, if you use silica fume, you will need to pay close attention to keep the surface wet through fogging and then wet curing as soon as it is practical. As previously mentioned, a 2 x 2 or 3 x 3 in. (50 x 50 or 75 x 75 mm) wire mesh would be preferred to reduce congestion of the reinforcement. Stay away from rolled mesh, as it is very difficult (even nearly impossible) to get to lay flat. Sheets of welded wire mesh are recommended. ASA recommends a minimum 28-day compressive strength for shotcrete of 4000 psi (27.6 MPa). A 3000 psi (20.7 MPa) mixture will have a higher water-cement ratio (w/c); therefore, there is more water in the mixture, which will significantly increase the potential for drying shrinkage cracking in the final surface. A 4000 psi (27.6 MPa) mixture is easily achieved with current portland cements and normal supplemental cementitious products such as fly ash. Finally, you mentioned that you will be shooting the surface of a house. You haven’t provided any details about what you are shooting the shotcrete on, but the substrate must be rigid enough to not vibrate when shotcrete hits the surface. If it is not rigid enough, the vibration of adjacent areas of freshly shot plastic shotcrete could cause cracking. This would be more of a problem in the Day 1 coat of shotcrete, but cracks that form in the Day 1 shotcrete would create a weaker section and increase the likelihood of mirrored cracking in the Day 2 layer. Again, please note: While it is appropriate to wet down the Day 1 shotcrete prior to application of the Day 2 shotcrete, it is important to let the wetted Day 1 shotcrete dry back to an SSD condition before application of the Day 2 shotcrete. If the Day 2 shotcrete is applied to a wet substrate (with liquid water on the surface), it will fail to meet the specified 150 psi (1 MPa) bond pulloff strength requirements for the project.

Yes, shotcrete can be applied in a 36 in. (0.9 m) area. Keep in mind, however, that it’s difficult to get as nice a gunning pattern as you would like when you are that close to the receiving surface. When you cannot back off from the wall, there is a tendency for a more irregular gunning surface, which would require more cutting and screeding to get an aesthetically pleasing result.

Shotcrete is routinely used to seal shale after excavations. It is typically done as soon as possible after the excavation because the shale will deteriorate when exposed to the air. When shotcreting, it is considered good practice to wet the receiving surface prior to gunning to create a saturated surface-dry (SSD) condition so the substrate will not draw moisture from the newly placed shotcrete. A good SSD condition is where the surface is wet without any standing water on it. Gunning over wet shale should not be a problem unless the water seeping from the shale is moving. If that is the case, we would recommend installing weep holes with plastic pipe at the locations where the water is seeping from and using an accelerator to flash-set the material immediately around the weep-hole pipe. It is also a good idea to install weep holes at regular intervals along the excavation or exposed hillside. It is important to use a qualified shotcrete subcontractor for this or any high-quality shotcrete installation. A qualified shotcrete contractor will use ACI-certified nozzlemen and should provide you with a résumé of similar, successfully installed projects, along with the up-to-date contact information of representatives from the owners or engineers involved in those projects. The ASA Buyers Guide (www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/) is an excellent source of shotcrete contractors.

In structural applications, most of the impact force from nozzling shotcrete is directed toward compacting the shotcrete in place rather than against the formwork. This was the subject of a study conducted by Marc Jolin of Laval University, Quebec City, QC, Canada, and reported in the Fall 2007 issue of Shotcrete magazine. There is virtually no hydrostatic pressure on the forms from the application using the shotcrete process. A copy of this study can be viewed on the ASA Web site at www.shotcreteweb.wpengine.com/archivesearch/ArchiveSearch.asp.

In short, the shotcrete contractor is responsible for maintaining proper cover over the reinforcing steel. The reinforcing bar installer should set the steel in the proper location for achieving the required cover corresponding to the final desired shape. If the shotcrete contractor finds that he cannot maintain proper cover with the reinforcing as placed, however, he needs to communicate to the designer/owner/general contractor that the reinforcing needs to be fixed before he shoots the section in place. There is no excuse for placing shotcrete with less than the specified cover, as shooting it with reduced cover will obviously create a section that has much less durability than intended by the designer.

Properly added and mixed air-entraining admixture in concrete will actually reduce the permeability of concrete. This is because the small, well-formed air bubbles from air-entraining admixtures are not interconnected as larger, entrapped air bubbles may be in non-air-entrained concrete. Thus, the reported higher permeability of the air-entrained shotcrete is not a material flaw but must be from poor shotcrete application. Air entraining from 4 to 7% air is advantageous for enhanced resistance to the freezing-and-thawing cycles of saturated concrete and should be specified by the designer in areas subject to significant numbers of freezing-and-thawing cycles annually. The reported high permeability and resultant failure to pass a water-tightness test could be investigated by taking cores of the “porous” material and conducting a petrographic analysis of the core. Based on the reported results, I strongly suspect that the in-place shotcrete has major issues with sand pockets, overspray, and rebound.

Refer to ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete,” at www.concrete.org for guidance on fiber types and dosages. Glass fibers are seldom (if ever) used in shotcrete because they tend to break under the high velocity required for shotcrete. Steel or macrosynthetic fibers should be used at about 0.4 to 0.5 volume percent to control hardened shotcrete cracking, 50 to 66 lb/yd3 (30 to 39 kg/m3) for steel (specific gravity (SG) of 7.85), and 6 to 7.5 lb/yd3 (3.6 to 4.5 kg/m3) for macrosynthetic polypropylene (SG of 0.91). Fiber suppliers can provide more technical guidance for their products. You can locate fiber suppliers by visiting the ASA Online Buyers Guide at www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/.

When gunning in tight areas, you have to allow for the bend in the hose and the length of the nozzle, which will require at least 2 to 2.5 ft (0.6 to 0.8 m). An area 3.5 ft (1.1 m) wide is a very tight area to gun in, but it can be done. In areas that restricted, it’s not possible to always maintain a 90-degree shooting angle, so you will get much more rebound from the deflection when shooting at less than desirable angles. Also, with the dry process, you have to reduce the air pressure and volume to keep from blowing the material off the walls. Ideally, you would like the nozzle to be at least 3.5 to 5 ft (1.1 to 1.5 m) from the receiving surface, depending on the nozzle you use. For a tight area like you are proposing, in addition to reducing the air pressure and running it slowly, we would recommend using a double-bubble nozzle, as it has a wider spray pattern, allowing the nozzleman to be closer to the receiving surface and still get an adequate spray pattern. A double-bubble nozzle is also flexible, which will help in extremely tight areas. You can locate organizations that sell a range of nozzles by visiting the ASA Online Buyers Guide at www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/.

Most of the industry does not endorse the use of polymer-modified additive in shotcrete. Please refer to ACI RAP Bulletin 12 and ACI 506R for further information and insight from the American Concrete Institute (ACI) at www.concrete.org. Shotcrete applied by competent contractors to properly prepared surfaces exhibits excellent bond characteristics to the substrate. Additionally, a good shotcrete mixture that is properly applied will yield a durability equal to or superior to cast concrete. There are many examples discussed in various articles of Shotcrete magazine at www.shotcrete.org/archivesearch/ArchiveSearch.asp.

Fibers are typically added to shotcrete linings for canals, channels, and other water structures in lieu of conventional reinforcing, such as welded wire reinforcement (WWR). For your “large amount of concrete via the shotcreting process,” we assume that you are using the wet-mix shotcrete process.

Temperature/shrinkage reinforcement is typically placed in thin sections governed by the structural concrete provisions of ACI 318 at a rate of 0.15 to 0.18%. Please be aware that if the lining is intended to be liquid-tight and has movement joints spaced at greater than 40 ft (12 m) apart, a reinforcement ratio of at least 0.50% is recommended by ACI 350 for concrete liquid-containing structures. Assuming that the section does not need to be liquid-tight and using the ACI 318 requirements, let’s consider the tensile capacity of a conventionally reinforced section and provide an equal or greater tensile capacity with fibers. Assuming a 3 in. (75 mm) thick lining with an assumed 28-day compressive strength of 4000 psi (28 MPa), using a WWR of 6 x 6 x W2.9/W2.9 in this section provides a percentage of steel of 0.161% and a tensile capacity of 3770 lb/ft (5610 kg/m). (Asfy = 0.058 in.2/ft [0.12 mm] x 65,000 psi [448 MPa] = 3770 lb/ft [5610 kg/m].)

Then, we assume that the direct tensile strength is 75% of the flexural strength (modulus of rupture [MOR]). For 3770 lb/ft (5610 kg/m) in a section 3 x 12 in. (75 x 300 mm), we have 3770/(12 × 3) = 105 psi (0.72 MPa). Then, we need an average residual strength (ARS) (ASTM C1399) of 105/0.75 = 139.6 psi (0.963 MPa) = 140 psi (0.965 MPa).

Using a macrosynthetic fiber, one can achieve these results using 4 to 5 lb/yd3 (2.4 to 3.0 kg/m3) in wet-process shotcrete. Fiber manufacturers will provide exact dosages to meet the ARS requirements.

Using steel fibers, approximately 43 lb/yd3 (25.5 kg/m3) will provide an equivalent area of steel to the WWR of 6 x 6 x W2.9/W2.9 in a 3 in. (75 mm) thick concrete section. Using steel fibers, however, may require a flash coat to cover the fibers that will protrude and rust over time. The corrosion of the fibers will only reach a carbonation depth of 0.05 to 0.10 in. (1 to 2 mm) but may result in staining the lining.

These calculations assume a thickness and strength. You must adjust for your conditions.

The project as described sounds very feasible for a structural shotcrete application. As we understand, the concerns are:

1. Detailing construction joints—Please refer to ACI 506R, “Guide to Shotcrete,” Paragraph 5.7, Joints. Typically, shotcrete joints are beveled to increase the surface area of the bonding surface and reduce the likelihood of trapping rebound. Other considerations for construction joints should follow the principles of cast-in-place concrete. Shotcrete is a method of placing concrete.
2. Curing—Shotcrete is concrete consisting of smaller aggregates and generally higher cement content. Good curing practices should be followed as they should be with cast-in-place concrete. Considerations should include the temperature and humidity when evaluating a curing program. High temperatures with low humidity will require significantly more effort than high temperature with high humidity. The key is to ensure that sufficient moisture is available to hydrate the cement during the curing period.
3. Plastic shrinkage gaps or cracking—The shrinkage characteristics of shotcrete are similar to cast-in-place concrete. Shotcrete is composed of smaller particles and higher cement but generally places at a low water-cement ratio (w/c) or less than 0.45. Shotcrete is somewhat more prone to plastic shrinkage cracking due to the surface not being protected by a form in its early stages. If the finished surface is subjected to high ambient temperatures, low humidity, or high winds, it will tend to dry quickly on the surface and exhibit more plastic shrinkage cracking. In these environmental conditions, fogging of the exposed shotcrete surfaces soon after shotcreting may help to reduce or eliminate the plastic shrinkage cracks. Plastic shrinkage cracks are generally superficial in nature and can be repaired if necessary.
4. Slump to be reduced to 2 ± 1 in. (51 ± 25 mm)—This is a good range if measured and treated properly. It is important to ensure that the shotcrete material has enough slump at the nozzle to properly encapsulate the reinforcing steel and is stiff enough to stay in place without sloughing or sagging. The slump at the nozzle is far more relevant than the slump at the pump.

The important factors influenced by slump are maintaining the proper water-cementitious material ratio (w/cm) and consistency at the nozzle to ensure good placement. The most important consideration is to ensure that you have an experienced shotcrete contractor who has a history of success with similar projects with respect to the size and complexity of the installation. You can locate shotcrete contractors on the ASA online Buyers Guide at www.shotcrete.org.

For a potentially suitable wet-mix shotcrete mixture design for marine structure repair, go to the ASA Web site (www.shotcrete.org). Click on Shotcrete magazine and search for “Shotcrete Classics: Deterioration and Rehabilitation of Berth Faces in Tidal Zones at the Port of Saint John.” This mixture design worked well for over 1.2 miles (2 km) of ship berth face repair over a 10-year period. Note: Because of high freezing-and-thawing exposure, the shotcrete was required to be air entrained. While the original mixture design called for 7% air content as shot, it was subsequently modified to require an air content of 7 to 10% as batched (at the point of discharge into the shotcrete pump) and an air content of 5 ± 1.5% as shot (into an air pressure meter base). Such shotcrete has provided good freezing-and-thawing resistance. You should be aware that your local materials (coarse and fine aggregates and cement) may have different properties in the concrete mixture, however, as compared to the mixture discussed in the article. It is recommended that a local engineer, testing laboratory, or concrete supplier be retained to develop a concrete mixture using local materials that meets the performance requirements of the mixture design mentioned in the article. Also, test panels constructed with the mixture, nozzlemen, and equipment to be used in the shotcreting are highly recommended to verify the strength performance of the shotcrete.

Shotcrete will adhere to properly prepared asphalt concrete. Shotcrete bond is generally related to the preparation of the surface that you want to bond with. If the surface is dirty, the shotcrete will not bond very well.

The surface should be cleaned using a high-pressure water blaster or sandblasting to remove any loose material and rust. If the metal bin material is thick enough, you might want to consider welding metal studs or nuts to the bin to secure the reinforcing steel or mesh. The amount and type of reinforcement is beyond the scope of our organization and we suggest getting guidance from a qualified engineer. You may gain some insight from the design of the reinforcing used in the soil nail walls.

The grading of fine aggregates, natural or manufactured, should be in compliance with the combined aggregate gradations in ACI 506R or ASTM C1436. Using crushed washed sand will be more difficult than using natural washed sand due to the more angular particle shapes. Due to the more angular particles, crushed sand will likely require a higher paste content to successfully convey it through the shotcrete hose.

In general, we would recom­mend a minimum of 2 in. (50 mm) of shotcrete. Either fibers or wire mesh or both should be used in this application. Please note that there are different types of synthetic fibers (microsynthetic and macrosynthetic). Refer to ACI 506.1R for information on fiber-reinforced shotcrete.

If the surfaces are steep or vertical, wire mesh should be used, but provisions need to be included to stabilize the wire mesh. This would likely require attachment points through the waterproofing system.

A minimum clearance for the reinforcment off the existing surface should be 0.75 in. (19 mm) or one bar diameter, whichever is greater, to allow a good flow of material around the reinforcing steel.

Yes, this sounds like a good use of the shotcrete process. Your sequence sounds like a good plan. A sample Structural Shotcrete Specification is available from the Shotcrete magazine archive on the ASA Web site (www.shotcreteweb.wpengine.com/).

There are really two questions here: 1) Underground fiber-reinforced versus mesh reinforced; and 2) slope protection fiber reinforced versus mesh reinforced.

1. Underground fiber reinforced versus mesh reinforced: it is not clear what the alternatives are that you are considering, but sprayed concrete has a good, solid track record for ground support. If it is a simple comparison of steel mesh versus steel fiber reinforcement, then the issue is one of a design approach.
   Wire mesh and bolts have a longer history and are simple to design as a rigid structure. To install mesh and bolts, however, requires working under unsupported ground. Mechanized spraying of concrete is done with the operator under supported ground and therefore is intrinsically safer.The design of fiber-reinforced sprayed concrete as ground support is approached differently. The sprayed concrete is allowed to deform to a certain extent before coming to rest with the ground forces finding a new equilibrium. The extent of this deformation depends on the energy absorption of the sprayed concrete structure, which is provided for by the fibers.Steel fiber-reinforced sprayed concrete is by far faster to place and therefore has economic benefits. As the fibers are discontinuous, there is merit in considering this structure less susceptible to corrosion and consequential durability issues. We recommend consulting ACI 506.1R and ACI 506.5R.
2. Slope protection fiber reinforced versus mesh reinforced: for slope protection, both fiber-reinforced and wire-mesh-reinforced shotcrete work well and are durable, reliable, and cost effective if done properly. Care must be taken with wire mesh reinforcing to ensure that it is maintained in the middle of the section and not on the ground where it is not effective. Wire mesh can also be difficult to install on an irregular surface and require more shotcrete material to cover the area and the mesh. The wire mesh can be an asset to the installer in providing a grid to support a scaffold system. In many applications, the choice of wire mesh or fibers should be left to the installer with the engineer specifying the minimum requirement for each.

Concrete, when applied using the shotcrete process, or cast-in-place, needs to cure for 7 days. Water is the best curing method (7 continuous days). Curing compound can be applied, but the membrane film that is formed will have to be removed by sand or water blasting (5000 psi [34.5 MPa] is recommended) before the plaster or tile can be set (it will create a bond breaker if not removed). There are curing compounds with a dissipating resin, which means after about 30 days in the sun, the material will break down. In either case, it is a good practice to pressure wash the concrete surface to remove the grit and dust out of the pours so that the plaster and tile will have a good bond. This is normal, everyday concrete curing practice that helps to prevent shrinkage cracks. The concrete being applied should have a water-cement ratio (w/c) of 0.35 to 0.45. Having the w/c at 0.40 at a 2 to 3 in. (50 to 75 mm) slump will keep the water demand low to help minimize the shrinkage. Wet-fogging freshly placed concrete before the curing process begins will also help prevent shrinkage cracks.

Shotcrete, like concrete, likely will not adhere to surfaces that are painted and dirty from the demolition activities. There should be no issue to the time durability. Shotcrete is pneumatically placed concrete and has great long-term durability characteristics if placed properly.Shotcrete has been installed in many adverse environments at a thickness of 2 in. (50 mm) with fibrous reinforcement and provided a long service life. Many irrigation districts line their canals with shotcrete and it has provided decades of great service in freeze-thaw exposures.

Consult with a local engineering firm that is qualified to develop a coring plan, obtain cores, and perform testing in accordance with ASTM C42/C42M or ASTM C1604/C1604M. Please refer to ASTM C42/C42M for further guidance.

Vertical formwork can generally be removed the day following the shotcrete installation. If the formwork is supporting a load like a soffit form, the form should not be removed until the shotcrete has attained full strength such that it can support the weight of the member.

Your question regarding the installation of the tile should be directed to a professional who installs tile.

Type FA shotcrete uses a fine aggregate meeting the requirements of ASTM C1436 Gradation #1. Type CA shotcrete uses a combined aggregate gradation meeting the requirements of ASTM C1436 Gradation #2. The decision on which type to use depends on the application, shotcrete thickness, specification requirements, and perhaps the shotcrete equipment to be used, that is, wet- or dry-process. For example, one may want to use Type FA if using dry-process equipment and placing thin sections, or when a smooth finished surface texture is required. For thicker sections, Type CA shotcrete may provide the best properties for the application. The choice of which to use depends on the application, equipment, and experience of the contractor.

Shotcrete is well suited to the application you have described. You need to determine the characteristics that you want from the shotcrete (strength, toughness, freeze-thaw durability) and include these in the specification. The 1 in. (25 mm) seems very thin for a long-term installation. Please be aware that the material costs (in most cases) will be a small part of the total cost of the installation. You should also make sure that you have a good specification for surface preparation. If the existing surface is not properly prepared, the added shotcrete will not bond well and the installation will not last very long.

The ASA Online Buyers Guide (www.Shotcrete/BuyersGuide) is an excellent source to locate members within the field of shotcrete whom are listed as shotcrete consultants.

Shotcrete is concrete and the same rules apply with respect to cold weather applications. Cold weather is defined in ACI 306R, “Cold Weather Concreting” as “a period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40°F (4.4°C) and 2) the air temperature is not greater than 50°F (10°C) for more than one-half of any 24-hour period.” ACI 306R is an excellent reference that provides recommendations for cold weather concrete placement and protection. A copy of ACI 306R can be purchased online at ACI’s Web site, www.concrete.org, from the Bookstore and Publications tab. You can also download articles regarding cold weather placement from ASA’s Web site, www.shotcrete.org—click on “Shotcrete magazine,” go to the article search page, and type in “cold weather.”

Shotcrete is concrete, and therefore if designing structures based on the ACI 318 Code, cover for conventional reinforcing steel should be those suggested in ACI 318 for concrete against ground. If the shotcrete is a “temporary” support, with further placement of “final” support, then these requirements do not apply.

It is generally good practice to let the shotcrete cure for the full 28 days before attempting to apply coatings or overlays. We would recommend you get a recommendation on the cure time from the manufacturer of the gauging product before doing the work.

The new shotcrete layer can be added to the existing shotcrete or installed after the existing shotcrete is removed. The decision to remove or not remove the existing shotcrete is beyond the scope of what we can comment on. If the existing shotcrete is left in place and overlayed, it should be thoroughly cleaned and roughened to create a good bonding surface. Because this is in a marine environment and you are considering the use of wire mesh, you need to make the new layer thick enough to have sufficient cover on the reinforcing steel. Alternately you could consider the use of fiber-reinforced shotcrete and silica-fume-enhanced fibrous shotcrete. Please note that there are many types of fibers on the market. We recommend that you review some of the Shotcrete magazine articles on fibrous shotcrete and on shotcrete in a marine environment on the ASA Web site. We suggested two papers for reference. The first is by Gilbride, Bremner, and Morgan on the Port of Saint John, and the other is by Morgan on the use of fibers that cover marine repairs. You mentioned using “L-anchors” at 2 to 3 ft (0.6 to 0.9 m) spacing. The use of grouted anchors with a reasonable embedment is quite common, but the design of such anchors is again beyond the scope of what we can advise.

It is not unusual to have variations in the tone of color for shotcrete or concrete walls that have been stained due to variation of the texture or density of the surface being stained. An acid-based stain typically results in more consistent shading. When anticipating that a wall will be stained, extra care needs to be taken in the curing process. It is generally recommended that walls to be stained should be water-cured to avoid any interaction between a curing compound and the stain material. If a curing compound is used, it must be completely removed prior to applying the stain material. Consult the stain supplier for more information.

Shotcrete is basically concrete that is pneumatically applied. Shotcrete can be used as an overlay for an existing wall to provide structural strengthening and a smooth look. Again, we suggest that you visit ASA’s Web site and search previous Shotcrete magazine articles for finishes. Before the shotcrete is applied, the wall must be stabilized from any anticipated future movement. Relatively thin layers of shotcrete or concrete will not withstand future wall movements without distress and cracking. The recycled content of most shotcrete mixtures is limited to the substitution of fly ash or other pozzolans for a percentage of the cement in the mixture. To properly place shotcrete, this substitution is generally limited to approximately 25% of the cement content.

a) Shotcrete can be finished in a wide variety of ways. It can be left with anywhere from a very rough to a very smooth finish and a huge variety of other finishes. We suggest you visit ASA’s Web site, click on the tab for Shotcrete magazine, and search the previous articles for finishes and swimming pools. You will find a lot of photos of great-looking walls. Not all shotcrete contractors are proficient in providing these attractive finishes. You need to discuss this with your current shotcrete contractor and/or interview other shotcrete contractors to make sure the chosen contractor can provide what you are looking for. We also suggest you look at work these contractors have previously completed. You can also locate contractors online at ASA’s Buyers Guide, www.Shotcrete.org\BuyersGuide.

b) If the pool house foundation is a footing trenched into the ground 4 ft (1.2 m) deep, dry-mix shotcrete would not be a good solution. If the foundation is 4 ft (1.2 m) above grade, then it could be done with shotcrete against a one-sided form. This would be considered structural shotcrete and not all shotcrete contractors are qualified to place shotcrete for structural walls. Again, we suggest you ensure the chosen contractor is qualified to do the work.

Yes, it is feasible and economical to construct structural walls such as a 5 ft (1.5 m) high floodwall. Shotcrete is a method of placing concrete and has similar, if not identical, properties after placement. As you can imagine, shotcrete needs to be shot against something such as a one-sided form, gabion baskets, earth, expanded metal lath, or just about any structurally sound thing you can think of. The economy of the system is dependent upon the site conditions and the ingenuity of the contractor. An example of a similar structure is on the east side of I-880, south of Dixon Landing Road in Milpitas, CA. This project, a flood control channel, involved trapezoidal channel sections, vertical wall sections, and a combination of sloped walls with a vertical extension. If you have further interest, you should contact an organization with experience in this area. An excellent source is the directory of Corporate Members in the ASA Buyers Guide.

This is an engineering question, not an application question. Precast pipe is sized for multiple uses and services. Depending on this service (depth of cover or loads), creative reinforcing bar placement and higher compressive strengths can reduce the wall thickness significantly. For example, success has been realized using elliptical steel to reduce concrete section thickness. Fiber reinforcement is secondary reinforcing and is not a suitable replacement for reinforcing steel. Given the short life required of the culvert, and assuming fairly equal loading on the circumference, a 3 to 4 in. (76 to 101 mm) section with judiciously placed reinforcing bar, and silica fume (8 to 10% of cement for higher strength up to 10,000 psi (69 MPa)]) would be sufficient. The resulting culvert’s life would probably be much longer than 10 years. In the end, an engineering call should be made, but the material will perform.

We are not able to advise you on the structural integrity of a remodel of a spa or any other structure and would suggest you consult with a local engineer who is familiar with pools and spas. Shotcrete is often used to overlay or patch structures and the success of such overlays and patches is highly dependent upon the quality of the surface preparation prior to the application of the shotcrete. With respect to referrals of independent professionals, we would suggest that you use the directory of Corporate Members in the ASA Buyers Guide.

You certainly have adverse conditions to work with! There are products on the market based on calcium aluminate cements that will tolerate the temperatures you mention and are durable. These products can be placed using the shotcrete process. A list of companies who supply this product can be found at www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/.

Shotcrete would work well for the overlay of the walls. In most cases, floors are placed by a conventional cast-in-place method. Either the wet- or dry-mix procedure would work well for the walls. To ensure good bond of the shotcrete to the walls, the walls should be cleaned and prepared to assure that the shotcrete is bonding to sound material rather than contaminates such as dirt or weathered material. You might also want to consider reinforcing the walls, but you should consult with an engineer on how to do this and with what material. If you were to use the wet-mix process, you could use the same equipment to place the floor as you are using for the walls.

Shotcrete can, and often is, finished to provide nice printable wall surfaces. To be the same general texture of the concrete block wall, you should specify a wood or rubber float finish. You can access the ASA Buyers Guide at https://shotcrete.org/products-services-information/buyers-guide/ to locate organizations regarding budget or conceptual pricing.

This is a commonly used technique in the Western U.S. and Canada, and has been used successfully from Stanley Hall at the University of California at Berkeley, Berkeley, CA, to the Baltimore Hilton Convention Center near Camden Yards, Baltimore, MD. There are a number of suppliers of waterproofing materials to choose from for this application. In selecting a supplier, be sure there is field service available to inspect the project before placement of the shotcrete.

There is no stated maximum thickness for shotcrete used in shear walls or any other type of wall. Walls have been successfully placed to a thickness of 36 in. (914 mm) for some time. The two main concerns are the heat of hydration and proper encapsulation of the reinforcing steel. Because shotcrete mixtures typically contain more cement per cubic yard or cubic meter than formed and poured placements, there will be more heat generated by the shotcrete mixture. The ability of the nozzleman to encapsulate the reinforcing will be a function of proper mixture design, proper selection of shotcrete equipment, and the skill level of the nozzleman and the crew.

Yes, if care is taken to prepare the receiving surface properly. The receiving shotcrete edge must be sound (no loose or unconsolidated material), clean (no traces of laitance or gloss), rough, and dampened to a saturated surface-dry condition. If these steps are followed, there should be no concern about the soundness of the joint.

Most shotcrete slopes are placed without moisture barriers and are constructed to ensure that water pressure does not build up behind the slope and create hydrostatic pressure on the backside of the shotcrete. This is generally done with drainage material and weep holes or vents near the base of the shotcrete slope. Please bear in mind that shotcrete slope paving alone is not generally considered as a retaining wall. If shotcrete slope paving is to be used as a retaining structure, it is generally done in conjunction with soil nailing, tie backs, or some type of structural footing. If the shotcrete is intended to be used as a structural wall, a structural engineer must be consulted to be sure all structural issues are addressed.

There are many ways of maintaining the thickness of shotcrete. When placing shotcrete over a rough rock excavation, the thickness will vary with more material filling in the voids than covering the high points. Some methods of checking or maintaining the thickness are as follows: stabbing the plastic shotcrete with a depth gauge; preinstalling pins to the desired thickness; and using groundwires or shooting wires that would create an even plane over the length of the wires.

Successful cold weather placements require more than just modifying a mixture. The mixture temperature, condition of the substrate, and the placing and curing environment are also important considerations. Generally, one is discouraged from trying to apply shotcrete if substrate temperatures are too cold and the ambient temperature is at 40 °F (5 °C) and falling. There are, however, exceptions for extreme situations such as shotcreting in permafrost ground conditions, where it is not possible (or advisable) to heat up the substrate. In such conditions, special accelerated dry-mix shotcretes (in conjunction with the use of heated materials) have been successfully used. This type of work is highly specialized and not recommended for the novice.

Accelerators can be added to shotcrete mixtures to help overcome cold weather conditions. The accelerator can be either a liquid accelerator added with the mixing water at the nozzle or a dry-powdered accelerator in prebagged dry-mix shotcrete. Caution is advised when using accelerators containing calcium chloride, as the use of these materials may accelerate corrosion of reinforcing steel. More information can be found in ACI 306R, “Cold Weather Concreting,” available from the American Concrete Institute, www.concrete.org.

AA typical wire mesh for such applications is 2 x 2 in. (51 x 51 mm) by 12 or 14 gauge; 3 x 3 in. (76 x 76 mm) by 11 gauge; or 4 x 4 in. (102 x 102 mm) by 10 gauge. It is important that the mesh be secured such that it does not move during the shotcrete placement. The mesh will tend to be pushed away from the back surface by the pressure of the shotcrete application.

As a general rule of thumb, brackish or salt water should not be used as shotcrete mixing water. High chloride ion contents can cause rapid setting of the shotcrete (which can make finishing difficult) and longer-term reinforcing steel corrosion-induced cracking, delamination, and spalling. Other components of brackish water can also be damaging to the fresh and hardened shotcrete. For a detailed statement on what constitutes acceptable contents of various dissolved chemicals for concrete/shotcrete mixing water, refer to the Portland Cement Association publication Design and Control of Concrete Mixtures, Chapter 4, “Mixing Water for Concrete.” One could also consult ASTM C1602/C1602M for limits on the composition of nonpotable water for use in the production of shotcrete.

This type of application is very common. Either wet- or dry-process shotcrete can be used successfully. The mixture should contain a minimal amount of coarse aggregate and be rich in cementitious material to minimize rebound. Generally either 2 x 2 in. (50 x 50 mm) 14 gauge or 2 x 4 in. (50 x 100 mm) 12 or 14 gauge welded wire fabric is used. The wire fabric needs to be spaced off the surface of the steel pipe to allow the shotcrete to encase the wire properly. This can be accomplished by welding studs or nuts on the pipe surface and securing the wire to them.

Shotcrete is a method of concrete placement, not a special type of concrete. The fire-rating of a concrete wall constructed by shotcreting or pouring will be the same. The important consideration is the requirements of the Underwriters Laboratory (UL) Fire Resistance Directory. The directory will provide guidance. UL ratings provide the most widely accepted criteria.

Each retaining wall needs to be engineered for the specific job conditions. It is fairly common, however, to see two layers of reinforcing bars in a wall of this thickness. In addition to reinforcing the wall, the steel would help support the shotcrete during placement. If drying shrinkage crack control is an issue, synthetic fibers may be added. Shrinkage in shotcrete mixtures may be higher than placed concrete with a 1 in. (25 mm) maximum-sized coarse aggregate due to smaller coarse aggregate size in shotcrete mixtures, higher fine aggregate content, and higher cement/cementitious material content. This may be partially offset by a slightly lower water-cementitious material ratio in a shotcrete mixture.

Yes, this can and should be done in shotcrete. Shotcrete has been used to successfully line brick sewers for 75 years. Shotcrete has been used to line over $40 million worth of brick sewers in Atlanta alone. Large brick sewers have been lined with shotcrete in most of the major midwestern cities. All of them were designed using the existing sewer as a one-sided form. Properly designed and constructed, shotcrete will provide the owner with a new concrete pipe or permanent tunnel lining and the associated expected longevity.

Each retaining wall needs to be engineered for the specific job conditions. However it is fairly common to see two layers of reinforcing bars in a wall of this thickness. In addition to reinforcing the wall, the steel would help support the shotcrete during placement. If drying shrinkage crack control is an issue, synthetic fibers may be added. Shrinkage in shotcrete mixes may be higher than a poured concrete with a 1″ (25mm) maximum sized coarse aggregate content, and higher cement/cementitious material content. This may be partially offset by a slightly lower water/cementitious material ratio in a shotcrete mixture.

The overlay can be placed successfully with either a dry-mix or wet-mix shotcrete process. The preference of the shotcrete subcontractor is likely related to his/her past experience and what they are best suited doing. The advantages of dry-mix process are beyond the scope of a simple answer. The process is well described in ACI 506R Guide to Shotcrete. The entire 6 inch thickness can be placed in one layer using the bench gunning technique. The number of vertical lifts would depend upon the height of the wall and the nature of the surface that the shotcrete is being placed against.

Shotcrete, like any other concrete mixture, will continue to gain strength as long as there are unhydrated cement particles present along with sufficient temperature and moisture. Strength development will generally be quite slow at the ambient temperature reported. The inclusion of supplementary cementitious materials in this mixture is a benefit in this instance as strength will increase as long as calcium hydroxide is available from the hydration of the cement. The specified strength should eventually be attained as long as the ambient temperature does decrease further and some form of moisture is available to the shotcrete.

A limestone coarse aggregate will generally be harder and more angular than what you are used to seeing in shotcrete mixtures. It really shouldn’t be a problem to use. In dry-process gunning, it is considerably more abrasive so there is more wear and tear on equipment, such as hoses, bowls, and wear plates, but it generally guns fine. In wet-process gunning, a sharper aggregate may not flow as easily through the hoses as smoother sand and pea gravel aggregates would. These are issues that the shotcrete contractor will have to address. They should have no effect on the quality of the in-place shotcrete.

There are several different finishes that are specified for shotcrete. One is a natural gun finish, which is the natural finish as sprayed (often used in slope protection). Another is a cut-down finish, which is cut-to-grade with the edge of a trowel or cutting rod (this finish is often flashed and sealed with a light gun finish to seal and texture the surface). Often in concrete repair, a trowel finish is specified where the shotcrete is cut down with the edge of a trowel or cutting rod to grade after the initial set of the material, and the surface is lightly flashed and toweled. Several different finishes can be achieved with shotcrete, but it should not be pushed or floated with the flat part of the trowel, as is done with poured concrete. It is important to wait for the initial set of the material and to use the edge of the trowel to cut the high points or shave the surface to achieve the grade or effect desired. Several excellent articles describing shotcrete finishes and finishing techniques are available as free downloads from the ASA website: www.shotcrete.org. One article, Technical Tip: Technical Tips on Shotcrete Finishes, written by Denis Beaupre, describes the different finishes that can be applied to shotcrete. Another article of interest would be Finishes for Retaining Walls by Marcus H. von der Hofen. Go the Publications section of the ASA website, click on “Click here to search the archive of Shotcrete Publications” and type “Shotcrete Finish” in the search window.

Shotcrete is used extensively for zooscapes, water parks, museum exhibits, swimming pools, and spas. A shotcrete water feature, although more expensive than PVC liner, would provide a long-term, more aesthetically pleasing alternative to a new PVC pond liner. Shotcrete is very versatile and can be shaped to replicate natural rock ledges or boulders. A properly designed and built water feature would provide a low-maintenance, durable solution.

ASTM C 1604, Standard Test Method for Obtaining and Testing Drilled Cores, covers cores that are obtained for determination of length, compressive strength, or split tensile strength. In addition to discovering the thickness of the applied shotcrete and its strength, a visual assessment can be made to evaluate the shotcrete quality, workmanship, shotcrete-to-substrate bond, and condition of the reinforcement. Shotcrete core strength is affected by core orientation relative to the direction of the shotcrete application. Therefore vertical, sub-horizontal, and overhead application of the same shotcrete may show variability. If obtaining cores for determination of compressive strength, cores containing wire mesh or reinforcing bars may not be used. Also, if a sample has been damaged in the process of removal, it cannot be used for strength determination. Cores must have a diameter of at least 3.0 in. unless otherwise permitted by the specifier. Cores with diameters less than 3.0 may demonstrate somewhat lower strengths and have greater variability. They may also be more sensitive to length-diameter ratio. Cores with length-diameter (L/D) ratios greater than 2.1 must be sawed to produce a capped or ground specimen with a L/D ratio between 1.9 and 2.1. Strength results from cores with L/D ratios less than 1.75 must be corrected as detailed in ASTM C42. A core having a length of less than 95% of its diameter before capping or a length less than its diameter after capping or grinding shall not be tested unless otherwise directed by the specifier. To avoid introducing the effects of moisture gradients of wetting and drying, extracted cores are to be stored in a sealed plastic bag at all times except during end preparation and a maximum of 2 hours prior to capping. Prior to capping, it is a good idea to determine the density of each core. Reported results should include the following: length of the core as drilled reported to the nearest ¼” (5 mm); length of the test specimen before capping or grinding reported to the nearest 0.1 in. (2 mm) and average diameter to the nearest 0.01 in. (0.2 mm); compressive strength reported to the nearest 10 psi (0.1 mpa) if the diameter is reported to the nearest 0.01 in. (0.2 mm) or nearest 50 psi (0.5 mpa) if the diameter is reported to the nearest 0.1 in. (2 mm); direction of the application of the load with respect to the horizontal plane of the shotcrete as placed; moisture conditioning history; date and time of test; nominal maximum size of the shotcrete aggregate; if determined, the estimated density; and any deviation from the stated test method and the reason for the deviation.

According to ACI 506R-05, the core grading method in ACI CP-60(02) is only to be used for nozzleman evaluation. (This is typically done in ACI Shotcrete Nozzleman Certification sessions and/or in preconstruction testing.) The core grading method should not be used to evaluate structures.

Regarding sample size for compressive strength, the core length-to-diameter ratio should be in the range of 1:1 to 2:1, with length-to-diameter core strength correction factors applied as per the requirements in ASTM C 42, Clause 7.9.1. Shotcrete test panels are typically between 3.5 to 5 in. (89 to 127 mm) deep. Thus, either 3 or 4 in. (76 to 102 mm) diameter cores should be drilled for compressive strength testing, depending on test panel thickness. We would also suggest referring to ASTM C 1604/C 1604M for securing and testing cores of shotcrete. This new test method allows smaller core diameters for shotcrete in an effort to provide for increased length-to-diameter ratios. Care should be taken when interpreting the compressive strengths using smaller-diameter cores because of the possible presence of voids, which may result in compressive strengths that are not representative of the actual in-place shotcrete.

Removal is probably not called for in this situation. Spider web cracking usually is an indication of crazing, a form of plastic shrinkage cracking. Crazing generally occurs when the combination of temperature and humidity creates a rate of evaporation at the surface of the concrete that is higher than the rate of bleed water exiting the concrete. Because the surface has very little, if any, tensile strength at this time, crazing cracks start to form. The good news is that crazing is an aesthetic problem. It affects only the very top surface and does not extend deeply into the concrete. Crazing cracks are more apparent when the surface is damp.

To avoid or limit crazing, be conscious of the weather conditions during placement. If there will be high temperature, low humidity, and moderate to high winds, measures such as fogging and/or erection of windbreaks may be required during placement. Synthetic fibers will help inhibit the formation of crazing cracks. Curing must begin as soon as possible, especially in these conditions.

A bonding agent is not required or recommended. A properly prepared substrate in a saturated surface-dry condition (SSD) is the optimum condition for application of shotcrete. Bonding agents may act as a bond breaker in some circumstances.

Yes. Heavily-reinforced shotcrete has been used in California for over 50 years in response to the need to retrofit structures to resist earthquake damage. The shotcrete contractor must demonstrate his ability to shoot test panels with the same reinforcement as designed into the project. By using an experienced and qualified shotcrete contractor, it is possible to achieve cost savings of almost 30% and time savings approaching 50%.

The inclusion of wire mesh must be considered on a case-by-case basis, depending on the thickness and orientation of the shotcrete. Thin sections may well not have any wire mesh. In aggressive environments, at least 2 in. (50 mm) of shotcrete must cover the mesh. The mesh size should be at least 2 x 2 in. (50 x 50 mm) and preferably 4 x 4 in. (100 x 100 mm) to allow for proper encapsulation. Overhead shotcrete usually includes wire mesh for thicknesses greater than 2 in. (50 mm) in case the shotcrete debonds from the substrate. The mesh must be mechanically anchored.

Some designers are eliminating wire mesh and relying on synthetic fiber reinforcement for shrinkage crack control. The use of synthetic fiber eliminates the concern over cover and corrosion in aggressive environments. Specific recommendations on the amount and type of fiber should come from the manufacturer.

The requirements for material temperatures are the same for both wet and dry shotcreting. Refer to Sections 8.7 and 8.8 of ACI 506R-90, “Guide to Shotcrete,” for recommended shotcrete temperatures during placement. Additional information is available in ACI 506.2-95, “Specification for Shotcrete,” in the sections on hot and cold weather shotcreting. Generally, concrete mixtures should be maintained at temperatures above 50 °F (10 °C) and below 100 °F (38 °C). Ambient temperatures should be maintained in a similar range.

Regarding surface temperatures, concrete should never be placed on a frozen substrate. Practical experience in Canadian mines has lead to a suggested minimum temperature of 40 °F (4 °C) for the rock receiving the shotcrete. Without special measures, cold temperatures will cause the shotcrete to set more slowly and result in slower strength development. Remember that in thin sections, the shotcrete will lose its heat more quickly in cold conditions.

ASTM C 94, “Standard Specification for Ready Mixed Concrete” states that concrete must be unloaded within 90 minutes of contact between water, cement, and aggregates, or before the mixer drum has revolved 300 revolutions—whichever comes first. This limit, however, may be waived by the purchaser if the concrete has sufficient workability that it can be placed without the addition of water. In hot weather, the 90-minute limit may be reduced by the purchaser.

The short version of this discussion is that performance specifications provide a list of desired results. The contractor takes this list and selects materials and methods to produce the desired results. The contractor assumes responsibility for results. Prescriptive specifications are very specific as to what materials, proportions, and methods of installation are to be used. The specifier assumes responsibility for the results. The contractor must be able to demonstrate compliance with the specification. Which method is better? The answer to this question is highly dependent on the nature of the project. However, in general, performance specifications produce a higher probability of achieving the desired results as the contractor is better able to use his expertise as it applies to project conditions.

There are four basic reasons to require preconstruction qualification testing:

1. To prove the suitability of the fresh shotcrete mixture design for the intended use;
2. To verify the proposed mixture will produce the required strength and any other specified hardened shotcrete properties;
3. To prove the ability of the nozzleman (and blowpipe oper­ator, if required) to place dense, homogeneous shotcrete completely encasing the reinforcing steel under field conditions; and
4. To prove the desired surface finish can be achieved.

This testing must be discussed in detail with the shotcrete contractor in advance with a clear understanding of the expected outcomes and the process for any required adjustments. Requiring ACI Nozzleman certification is an important requirement in screening for qualified nozzle operators. However, it is not a guarantee that the nozzleman has applied shotcrete under the same conditions to be encountered on your project. Therefore, a preconstruction plan is an important part of critical projects. Other prequalification testing may be necessary depending on the nature of the work.

There are a number of ways to treat these stains. Successful treatment will depend on the specific material in the stain and the depth of the stain. The first step would be to try to draw out the material from the surface by applying a poultice of finely ground kitty litter, cement powder, or talc and allow the surface to dry. Repeat this application if necessary.

Next, try a scrubbing a nominally dry detergent powder into the surface. Allow the powder to dry and rinse off the surface. Follow this treatment with a liquid detergent scrubbed with a bristle brush into the surface. Allow the liquid to remain in the surface for 1 to 2 days, then rinse thoroughly. Should the staining persist, you may want to try a proprietary stain remover specifically intended for use on concrete.

Muratic acid is also an option. However, muratic acid can have deleterious affects on the concrete if not thoroughly removed. Because of its potential to attack concrete aggregates and mortar, along with the hazards inherent with applying and removing acid, muratic acid should only be used with the guidance of an experienced consultant. Following a thorough power washing, the surface should be mechanically roughened to ensure proper bond with the shotcrete.

Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following final set, it should be wet cured for at least 4 days, preferably 7 days if possible. The exposure to rain would prove beneficial as the rain would assure the presence of moisture for continued curing.

ASTM C 33 contains a variety of aggregate gradations. Gradations recommended for shotcrete applications can be found in ASTM C 1436, Standard Specification for Materials for Shotcrete, or ACI 506, Guide to Shotcrete. Note that ACI 506 includes the caveat that “aggregates failing to comply with gradations shown in Table 2.1 may be used if preconstruction testing proves that they give satisfactory results or if acceptable service records are available.”

Shotcrete is a method of concrete placement, not a product. Therefore, concrete placed by the shotcrete method will exhibit the same characteristics as concrete placed by other methods. Mixture designs and proportions for shotcrete are modified for high-velocity placement. The high velocity provides some performance improvements over conventional cast-in-place methods when properly placed.

There are areas that have no problem getting values lower than the most stringent ACI requirement of 0.06% for prestressed concrete with no special adjustments. It would be prudent to test each of the proposed shotcrete constituents to determine their soluble chloride ion content. The most likely suspects are the aggregate and water sources. Typically portland cement and silica fume would contribute little, if any, detectable chloride ions. Assuming this would be a dry-process application, the only admixture other than the silica fume might be an air entraining agent, which would not provide any chloride ions. This leaves only the aggregates and water as the sources. At a minimum, the aggregates and water should be tested by a qualified laboratory for soluble chloride ion content. Alternate sources of aggregates and water may be required based on the laboratory results.

If this statement was true, there would be a lot more shotcrete projects! The truth is that shotcrete is a method of concrete placement, not a special material. The materials, mix designs, and mix proportions may vary between the shotcrete method and the conventional concrete form and pour method, but the thickness and reinforcing of the structure will be very similar.

There is a subtle difference between the two methods that might affect thickness requirements. Shotcrete is generally placed directly onto the undisturbed soil, joining with the soil to provide the shell for the pool. To use the form and pour method, over-excavation would be required to accommodate two-sided forming. The walls would then have to withstand the forces of backfilling. This may result in a thicker wall requirement. The final decision regarding wall thickness, however, should be made by a structural engineer.

Shotcrete is widely used for swimming pool construction. In some areas it is virtually the only method used. Successful shotcrete swimming pool construction is a result of having an appropriate design, selecting a qualified contractor with certified nozzlemen, selecting appropriate materials and shotcrete mixture design, and following industry recommendations for placing, finishing, and curing.

Shotcrete containing silica fume will tend to be more adhesive (sticking to substrate surfaces) and cohesive (adhesion to itself). This will result in quicker build-up (greater thicknesses per pass) and possibly reduced need for acceler­ators. Silica fume additions also result in dramatic reductions in rebound, particularly with the dry-mix process.

Shotcrete made with normalweight aggregates will have a density of approximately 145 lb/ft3 (2323 kg/m3).

No. Shotcrete is a method of placing concrete. Therefore, any applicable certifications would apply to concrete regardless of the method of placement.

ACI 506.2, “Specification for Shotcrete,” recommends that a test panel be produced for every 50 yd3 (38 m3) of shotcrete placed or one per day, whichever is less. A minimum of three cores are to be cut from the test panel for compressive strength testing in accordance with ASTM C 42, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” Testing must be performed in accordance with ASTM C 1140, “Standard Practice for Preparing and Testing Specimens from Shotcrete Panels.” The average of the strength results from the cores must be at least 85% of the specified strength with no individual core less that 75% of the specified strength.

The application of shotcrete can be done successfully with either method. The dry-mix shotcrete process tends to be more favorable for lower volume placements. It is also a more flexible method, allowing for more frequent relocations of equipment. Equipment is more easily cleaned at the end of the placement. The nozzleman must exercise great care in adding the necessary amount of water while shooting.

The wet-mix shotcrete method is more favorable for larger volume placements. Rebound is substantially less than in the dry-mix shotcrete process. The nozzleman does not have to be concerned with controlling the water addition. This method is less efficient when there is a requirement for frequently starting and stopping placements. The wet shotcrete mixture has a limited “pot-life.”

Remember, shotcrete is not a special product. It is a method of placing concrete. All the recommended practices for concrete placed by any other method, such as curing and protection, also apply to shotcrete.

For over 30 years in Canada we have been designing wet mix shotcrete for exterior exposure (rock-slope stabilization, tunnel portals, canals and beams, infrastructure rehabilitation, etc.) to have air content at the point of discharge into the pump to be in the 7 to 10% range. Pumping and the impact on shooting reduces the air content in the in-place shotcrete by about half. i.e. we find the in-place air content in the shotcrete to consistently be in about the 3.5 to 5.0% range. (Only about 1 to 2% air content is lost in pumping; the rest is lost in impacting on the receiving surface).

The air content is measured either by digging out the in-place shotcrete (or dig it out of a shot test panel) and reconsolidating it in the base of the air pressure meter in the ASTM C231 test and conducting the test. Alternatively the shotcrete can be shot directly into the air pressure meter base. It provides virtually the same value as obtained with dug-out shotcrete (as described above), provided the nozzle is held perpendicular to the air pressure meter base, and at the appropriate distance for proper consolidation of the shotcrete.

Testing on numerous projects has demonstrated that shotcrete with 3.5 to 5% in-place air content has a good air voids system ( air content, spacing factor and specific surface), when analyzed in the ASTM C457 test. Such shotcrete has been demonstrated to have good freeze/thaw durability in the ASTM C666 test and deicing salt scaling resistance in the ASTM C672 test. More importantly, feedback from the field demonstrates that such air entrained shotcrete with many thousands of cycles of freezing and thawing in the field over several decades display good durability. There are many research and case-history examples in the published shotcrete literature to support these observations. (See references 1 and 2 below)

With respect to the use of very high air contents at the pump (15-22%), this has been more of a research initiative, used on only a few projects in Quebec, and is not common practice, nor in this writer’s opinion, necessary.

There is another benefit which accrues from the use of air entraining admixtures to get 7-10% air content in the shotcrete discharged at the pump. As any concrete user knows, as the air content increases, the slump goes up. For shotcrete mixes (which have high cementitious contents and low rock contents compared to concretes) this makes the mix easier to pump and shoot. Thus it is common to shoot air entrained wet mix shotcrete at 100 to125mm (4 to 5 inch) slump. On impacting on the receiving surface, as the air content is reduced by about half, the slump of the in-place shotcrete is also instantaneously reduced by about half. (This can be demonstrated by digging the shotcrete out of the in-place material, or a test panel and conducting a slump test on it). We refer to this phenomenon as the “slump killing “process and have used it to advantage on many shotcrete projects. With a good air entrained shotcrete mix design (particularly when silica fume is used) we commonly shoot vertical sections as much as 500mm (20in) thick at 100 to 125mm (4 to 5 inch) slump in a single pass with no problems of sagging or sloughing (fall-out), without having to resort to the use of accelerators.

Finally, there are a few situations where 7 to 10% air content in the shotcrete at discharge into the pump may not work. These are situations where excess air content reduction could occur during shotcrete conveyance, such as dropping shotcrete down a pipe from the surface in an underground mine and catching it in a kettle or remixer unit. In this case, air, if needed, is best added underground in the remixer. Also, pumping shotcrete long distances (particularly pumping shotcrete downhill) may result in excessive loss of air content in the line, which could cause a slump reduction in the line and possible pumping problems. Other than for situations such as these, we always use 7-10% air content in the shotcrete at the point of discharge into the pump (even if it is not needed for frost resistance reasons) because of its enhanced pumping and “slump killer effects”.

Reference 1: Morgan, D.R., “Freeze-Thaw Durability of Shotcrete”, Concrete International, Vol. 11, No. 8, August, 1989, pp 86-93

Reference 2: Morgan, D.R., Kirkness, A.J., McAskill, N. and Duke, N., “Freeze-Thaw Durability of Wet-Mix and Dry-Mix Shotcretes with Silica Fume and Steel Fibers”, ASTM Cement, Concrete Aggregates, Vol. 10, No. 2, Winter 1988, pp 96-102.

See ACI 506R, Sec. 1.7 (ACI document). Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete, mostly due to less and/or smaller coarse aggregate in the shotcrete mix.

The easy answer is that shotcrete material is the same as concrete material and that the same rules or guidelines would apply to shotcrete as to concrete. We usually tell our customers to present this question to the painters. The curing process and chemical reactions are greatest in the first 28 days. Generally a paint or coating is not applied until after the curing of the shotcrete is complete, or mostly so, and the moisture content of the shotcrete is below a point specified by the coating manufacturer.

When trying to find a contractor in your area, please visit the Corporate Member page of this website. When constructing water ponds, the liner is always under the concrete just in case the concrete cracks not on top. Master Builders makes a product called Master Seal 345 which is designed to waterproof the concrete before the shotcrete is placed. Using a macro synthetic fiber for strength, flexural and to control shrinkage cracking will help. It comes down to proper prep work prior to placement and curing of the concrete (7 days of water) to control cracks. Bentonite shotcrete could be a possibility or perhaps plastic shotcrete (cement and bentonite shotcrete).

Typically, the thickness is a minimum of 3 inches and slope lining in the 6 to 8 inch range is often installed. The reinforcing is also variable with the lightest sections with no reinforcing or a low dosage of polyfibers or light welded wire fabric and the heavier sections with rebar. Basically, a lot of different designs can be used. We are not aware of any widely used standards.

The ACI 506R-90 Guide to Shotcrete, Section 5.4.2 is the publication you are looking for. Amongst other things it states: “If the design allows, lapping of the reinforcing splices should be avoided. Lapped bars should be spaced apart at least three times the diameter of the largest bar at the splice”. If laps are not permitted by the design, then it is best to lap the bars one on top of the other (relative to the shooting orientation), rather than side-by-side, to facilitates proper encapsulation with shotcrete.

This inquiry involves a lot more than just mix design. First, additives to the mix by themselves will not keep the shotcrete from cracking. To minimize leakage for the proposed application, he will have to use either a waterproofing membrane on top of the shotcrete, or plaster like would be used on a swimming pool. Putting a membrane behind the shotcrete would only serve to keep ground water from entering the pond through the back side. The other aspect to be addressed is the fact that all concrete shrinks, and that is what causes the cracks. So anything that can be done to minimize shrinkage should help. To name just a few items: avoid shooting on a windy and or low humidity day; use aggregates in the mix that have a good record regarding shrinkage; avoid excessive cement content in the mix; use reinforcing steel (mesh or rebar); synthetic fibers help reduce early plastic shrinkage; proper curing is absolutely essential!

The method of using 6″ diameter by 12″ long wire mesh cylinders has not been used regularly in several years. The most accepted means of taking samples is as specified in ACI 506 documents which generally require a sample panel of approximately 18″X18″ by 4″ thick from which cores are taken. The cores should be taken at a minimum distance from the edge of the thickness of the panel to yield fair test results. ACI 506.4R-94 references under testing of shotcrete, ASTM C 1140-03 (Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels. Also ASTM C42/C 42M-03 (Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. Standard 18″X18″X4″ panels are typically made. ASTM C 1140-03 states a 24″X24″X4″, cores are to be taken 1 core diameter plus one inch from any side of the test panel.

Please refer to the following articles:

Morgan, D.R. “Freeze-Thaw Durability of Shotcrete” Concrete International, Vol. 11, No.8, August 1989, pp 86-93.

Shotcrete magazine Vol. 4, No. 5, Fall 2002, pp. 32-38

Shotcrete magazine Vol. 5, No. 2, Spring 2003, pp. 30-37, “Freeze-Thaw Durability of Shotcrete,”

Gilbride,P., Morgan, D.R. and Bremner,T.W. “Deterioration and Rehabilitation of Berth Faces in Tidal Zones at the Port of Saint John”, ACI, Concrete in Marine Environment, SP-109, 1988, pp.199-227.

Gilbride, P. Morgan, D.R. and Bremner T.W. “Performance of Shotcrete Repairs to the Berth Faces at the Port of Saint John”, Third CANMET/ACI International Symposium on Performance of Concrete in Marine Environment,1996, pp 163-174.

Morgan,D.R., Rich L. and Lobo, A, “About Face-Repair at Port of Montreal”, Concrete International, Vol. 20, No.9, September,1998, pp. 66-73.

The bottom line is that with a properly designed, air-entrained shotcrete, properly applied by qualified nozzlemen, you should be able to get a good quality product, with long-term freeze thaw durability every bit as good as a quality, air-entrained cast-in-place concrete.

Pools are built with both processes. Some find it easier to shoot pools with the wet method. But, when properly done, there should be no difference in performance between wet and dry process shotcrete. Depending on the complexity of the pool, the wet method placement can be faster than the dry method. It comes down to the experience of the contractor and their crews, for a good quality placed pool shell. The nozzleman plays a key role in the placement of well placed shotcrete in both methods. The geographical area may determine the economics of which method is used. Curing of the in-place concrete shell is the same for both processes (water curing for 7 days). Wet concrete has a 90 minute window from the time it is batched at the plant until it placed. Temperature of the material and the air temperature can increase or decrease the set times of the concrete. Typically Ready-Mix companies hold back 10-15 gallons of water in the mix so that the contractor can adjust the slump of the concrete on site. Adding 1 gallon of water over the design mix (amount of gallons of water per yard of concrete) can decrease the strength of the concrete by 200psi.

If you have additional concerns, the following questions should be asked:

• Does the contractor have a good track record of shooting pools with the wet method?
• How many pools have they completed with the wet method?
• Can you provide a list of past completed jobs?
• How do they plan to incorporate the trimmed concrete into the shell? (The rebound and the trimmed concrete play a key role in the final quality of the pool shell.)
• What concrete mix design do they plan to use?

More information is needed before answering this correctly. Shotcrete will stick to strand board, but you need some type of reinforcement (wire mesh) to hold it all together. A good cement plaster mix at a lower velocity would be more economical. Contractors who have shotcreted a house in the past will tell you that it is too time consuming for the money involved.

If the shotcrete is applied correctly, the durability factor is better than cast in place concrete. The 506 and the ASTM documents have references on this subject. There have been papers written on durability and permeability. Countless culverts have been very successfully relined with shotcrete, not only concrete culverts but also brick lined and galvanized metal culverts. If you broaden the definition of culvert to include tunnels you would most likely be identifying where the largest volume of shotcrete is used as a rehabilitation method. To answer questions of durability, shotcrete should be thought of a process or method of placing concrete. Shotcrete in place is concrete. The higher cement content of shotcrete and the impaction of its placement mix design for mix design of other placement methods create a higher strength and more dense, thus less permeable concrete.

From the limited info given, it sounds like an ideal shotcrete application. But, with many caveats, such as: TOTAL deteriorated substrate removal, thorough removal of all aggregate/substrate that may have been fractured during removal of deteriorated concrete (heavy sandblasting and/or high pressure washing), using a replacement concrete mix with similar properties as the original, thorough cleaning or removal and replacement of corroded rebar, etc. It is suggested to discuss this with a shotcreter in the area that has experience with a similar application. It is also recommended to review related ACI and ICRI publications.

There is a paper by Denis Beaupre about this issue in the May 1999 issue of Shotcrete magazine. The simple answer to bonding layers of shotcrete is the same as bonding layers of concrete in typical repair applications. Bonding agents are not recommended. The bond strength between shotcrete layers is generally superior to cast interface because of the impact of velocity and the matrices that form at the bond plane and provide a denser, therefore stronger interface. The key in any bonding situation is primarily dependent on the surface preparation before application of the next layer. The surface must be clean and free of latence and any other unsound materials and should be roughened or textured (gun finish is sufficient) to provide sufficient keying or mechanical locking as required. The surface should be SSD and overspray from progressive application should be controlled. ACI International and the International Concrete Repair Institute can provide direction for surface prep. AASHTO/AGC/FHWA Task Group 37 Report, “Guide for Shotcrete Repair of Bridges and Structures” contains spec and procedure information that should be useful.

All concrete must be cured to ensure full and proper hydration of cementitious components control of shrinkage. Shotcrete is concrete placed pneumatically, therefore must be cured, as all concrete must be. The tunnel environment presents positive and negative conditions. The humidity in an underground space is generally high in humidity and constant in a moderate to cool temperature. Both conducive to slow egress of moisture from the concrete and “natural” curing. The negative in tunnel construction is ventilation air which is generally of high volume and high speed, which tends to dry the surface and “pull” important moisture out of the sprayed concrete. Most tunnels can tolerate extra water in the work space, therefore misting or spraying water onto the concrete surfaces, especially overhead, is the most practical method of curing. Sprayed on liquid membranes are effective as long as their interference with bonding of additional layers of concrete, sprayed or cast, is not an issue. Recommended reading: “Understanding and Controlling Shrinkage and Cracking in Shotcrete” by D.R. Morgan and C.Chan, published in the ASA Shotcrete magazine.

ICS, 3-D panels are structurally reinforced styrofoam panels that, in conjunction with properly applied shotcrete, become a superior building system. This is a proven panel with a global track record and much experience among ASA members. They are located in Brunswick, GA.

We often use shotcrete in lieu of cast in place concrete without using different design factors. Shotcrete is simply a method of placing concrete. Properly designed and constructed, the same reinforcing steel used for cast-in-place concrete or precast concrete should be able to be used with shotcrete constructed retaining walls. The only differences would be in the reinforcing detailing, in that the rebars should be tied in a configuration that makes them suitable for proper encapsulation with shotcrete. Avoid bundled bars or other conditions not conducive to proper shotcrete encapsulation. See “ACI 506R-90 Guide to Shotcrete” for guidance, except that it is possible to use much larger diameter bars than indicated in that document, as has been described in several articles. (See for example the article by James Warner on “Dealing with Reinforcing” in the Winter 2001 of Shotcrete magazine.)

There are many textures that can be applied to the finish surface of the shotcrete. The least expensive is the natural nozzle finish which is rough and tends to absorb light as opposed to reflecting light and standing out. On the other extreme is carved and stained simulated rock as found in zoos and amusement parks. Stamping or rolling also creates a great finish. The broom finish is also very common. Color and textures are options and the owner or designer needs to decide on the value and effect he/she is looking for. Whatever finish, texture, pattern, color, stain, lump, bump, or crease that can be applied to concrete also applies here.

There are two ways to insulate the outside of concrete spas. The first way is to shotcrete the spa and then glue Styrofoam to the outside of the concrete shell or to spray the insulated foam to the outside surface. The second way is to use the ICF (insulated concrete form). You would only have to use one side of this form system. This system would act as the outside form so that the shotcrete could bond to the foam. This type of system has foam insulation thicknesses from 1 to 4 inches thick. Yes, it can be fitted to form circles. Each ICF system is different, so some research would be needed to see which system would work the best. Since most spas are formed up before they are shot, the ICF system would serve two purposes: forming and insulation in one step.

Basically, shotcrete is a method of placing concrete that does not require forms. As a matter of fact, shotcrete requires the concrete mix to be proper every time. With formed concrete walls, the ready mixed concrete going in can be substandard and still appear to be okay. Shotcrete also provides a more dense concrete less susceptible to water penetration. The most glaring difference will be the quality of the materials used. Most poured walls are designed for a compressive strength of 2500 to 3000 psi. Typically they are placed with a water/cementitious material ratio of 0.60 and higher. Curing is almost unknown in the poured wall sector. Protection only occurs in the coldest weather. By the very nature of the process, shotcrete will have a much lower w/cm ratio. This will produce a wall with higher compressive strength and have the attributes of lower w/cm ratio concrete, i.e. reduced permeability, less shrinkage, increased durability. With proper curing and protection, the shotcrete mixture will produce significantly better long-term performance. The shotcrete process should allow for easier addition of insulation to the walls as well. This is especially important if the basement is to be used for more than just storage.

Thicknesses depend on the structure and surface (surface prep is the key to proper bonding of shotcrete) the shotcrete is being applied to. Depending on the application 1/4 flash coat to 1 inch thickness can be the minimum. As far as shotcrete up to the steel windows, you have to consider that cracking may occur off of each corner. This can be minimized by adding additional reinforcement at those locations. It is common to tool in a joint around the windows so that we could apply a caulk later. The caulking will assure a waterproof seal between the window and the concrete during temperature changes that may create some expansion and contraction. You do not have to depend on the trim work to create the weather and water tight seal the architect requires.

There are four common types of sheet pile sealing: 1.) all seams were welded to keep the ground water from seeping in, 2.) the sheet pile surface was sandblasted for bonding, 3.) wire mesh was tack welded to the sheet pile and 4.) rebar was tack welded to the sheet piles. This was done prior to the shotcrete layer. In each case the shotcrete is used as a coating to keep the water from touching the piles and in the third and fourth examples, it is used as a structural coating as well.

Temporary lagging of shotcrete must meet some standard as it is the shoring holding back the earth. If reinforcing is used in the design of the temporary shoring it must be fully encapsulated to provide the design strength of the lagging as specified in the design. A temporary structure may have a low safety factor but the strength of the rebar and shotcrete must meet the design specifications. Many times it is more important to do good shotcrete for the temporary shoring just because it has a lower factor of safety and therefore less allowance for poor construction practices.

The most commonly used estimates for pH of concrete are 13 for plastic (fresh) concrete and about 10 for hardened concrete with a little age to it.