The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
User agreement: The answers provided to submitted questions are intended for guidance in planning and executing shotcrete applications. This information is intended only for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations, and who will accept responsibility for the application of the material it contains. The American Shotcrete Association provides this information based on the best knowledge available to them and disclaims any and all responsibility for the information provided. The American Shotcrete Association will not be liable for any loss or damage arising therefrom.
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StructuralQ We are looking for the application of shotcrete on tidal waters. We are located on Lower Puget sound in Washington state and need examples where this has been used and is holding up under the moving tides. The examples don’t have to be applied to our area; the same conditions may apply to other saltwater areas.
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: (shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf). Plus, this project also has extreme freezing-and-thawing exposure.
StructuralQ We are planning on placing shotcrete over rock face and are concerned about water seeping out of the rock and forming ice behind the shotcrete. Could you please offer some insight?
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.
StructuralQ We are proposing a vertical support of excavation structure using reinforced shotcrete to retain a 10 ft (3 m) high sandy soil. What are the design criteria to choose the reinforcement and the thickness of the shotcrete? Also, what is the minimum reinforcement and shotcrete thickness you would recommend?
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 shotcrete.org. Another resource is the FHWA Manual for Design and Construction of Soil Nail Walls.
StructuralGeneralQ We are working on a design-build project located in southern California of which the perimeter walls of the underground parking structure are planned to be shotcrete. Do you recommend any particular ACI publication that would be applicable to the shotcrete operation or does ASA have their own publications, similar to ACI, that are more appropriate to shotcreting?
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
StructuralGeneralQ We are working on a large infrastructure project in New York state. We have access and approval issues for a new concrete mixture design. Our understanding is we can specify a Pre-Bagged or Pre-Packaged Shotcrete mix for a wet-mix application. Is this the correct terminology? Are there companies that produce this type of product?
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.”
StructuralQ We have 18 ft high, 12 in. thick (5.5 m high, 205 mm thick) walls to shotcrete and need horizontal cold joints to place the shotcrete in three pours (three height sections). How do we create the joint?
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.
StructuralQ We have a 6 in. (152 mm) thick tilt-up concrete wall that needs to be upgraded to achieve a 4-hour fire rating. We would like to add shotcrete to achieve that rating. What is the hourly rating per inch of shotcrete? We were hoping that 2 in. (51 mm) of shotcrete would provide the desired rating.
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.
StructuralQ We have a backwash tank on a waste water treatment plant which is made by a secant wall. The lower area for this structure will receive a shotcrete liner approximately 12 ft (3.7 m) tall on average, the interior perimeter of the structure includes 104 lineal ft (32 m) of unreinforced and 440 ft (134 m) reinforced sections which are a 12 in (300 mm) minimum thickness. The drawings call for vertical control joints with waterstop approximately every 30 ft (9 m). We don’t believe the control joints are necessary and could achieve the same desired performance with one monolithic installation of the shotcrete. Are the control joints really necessary when you are installing the shotcrete against a solid secant wall which does not contain any control joints?
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.
RepairStructuralQ We have a cast-in-place wall with extensive rock pockets and voids from inadequate vibration during casting. One option is to tear down the wall and replace, however we are wondering if shotcrete can be used to repair the deficiencies. If so, what are the recommended procedures to prepare and shotcrete the repairs?
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.
StructuralQ We have a client who wants to reline the surface of a brick drying room made from bricks and used to dry bricks. The temperature doesn’t exceed 400°F (200°C). Apart from the usual shotcrete best practice, do you recommend the addition of any additives to make the shotcrete/reinforcement more suitable for the heating and cooling cycles?
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.