Here are the answers in the same order as your list.
- Shotcrete is just a placement method for concrete, so there is no “off-season.” However, as with cast concrete, cold weather placement needs more planning and material delivery controls. As wet-mix shotcrete material is predominately delivered by ready-mix trucks, the concrete should be delivered at 50°F (10°C) or higher. Then once shot, if temperatures are less than 50°F, the concrete should be protected by insulated blankets or the area enclosed by vented heaters to keep temperatures above 50°F. We also don’t want to shoot onto frozen surfaces.
- We use small line concrete pumps standard in the concrete industry.
- Concrete pumps and air compressors are generally diesel-powered engines. Pump engines range in horsepower from 75 to over 200 hp, depending on the pump size. Your application is of relatively low volume, so one of the smaller pumps should do fine. We also use relatively small air compressors, and many contractors who work in residential or metropolitan areas use equipment equipped with sound reduction. The sound at the point of placement where the air-accelerated material stream exits the nozzle is generally just the sound of the airflow.
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 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.
