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2013WinShotcreteEMag

Shotcrete FAQs As a service to our readers, each issue of Shotcrete will include selected questions and provide answers by the American Shotcrete Association (ASA). Questions can be submitted to info@shotcrete.org. Selected FAQs can also be found on the ASA website, www.shotcrete.org/pages/products-services/technical-questions-archive.htm. Question: Which method of placing concrete provides a longer well-versed in shotcrete technology. Shotcrete is a method of service life—traditional cast-in-place concrete with two-sided placing concrete and the parameters that work for concrete forms or shotcrete? cover work for shotcrete. Good practices for placing shotcrete or concrete include: Answer: Shotcrete is a placing method for concrete. Wet-mix 1. Predampening the soil that the concrete/shotcrete is placed shotcrete will be very similar in density to fully consolidated against; concrete when the concrete mixture designs are similar. 2. Ensuring that the reinforcing bar temperature is not too high; Properly mixed and shot, dry-mix shotcrete may have a slightly and higher density. Properly designed, placed, and cured, both 3. Early curing of the shotcrete surface and maintenance of concrete and shotcrete will give an excellent service life. curing for the specified duration. If no duration is specified, a 7-day wet cure is recommended. Question: I have a swimming pool that appears to have Using fibers in the shotcrete/concrete can also help control shrinkage cracks in the floor. I have tried to inject an epoxy, surface early-age plastic shrinkage cracking. but the cracks are too small. Do you have any suggestions? Question: Can shotcrete be effectively used to repair holes in Answer: There are a wide variety of epoxies and polyurethanes an old 8 ft (2.4 m) diameter storm sewer tunnel constructed of used for crack injection. Smaller crack widths would require stone/brick/mortar? One of the holes is completely worn a lower-viscosity material to penetrate the crack. You should through to the earthen backfill material. The other two holes contact an engineer or injection specialist experienced in shot- are missing the innermost layer of stone masonry, but the outer crete and cracking issues to evaluate the cracking and make a layer of masonry is still in place. specific recommendation for repair. Proper concrete mixture design, placement techniques, and early water fogging and Answer: The friction coefficientn of well-finished shotcrete curing can help to reduce plastic shrinkage and drying shrinkage for use in Kutter’s equation (and, more streamlined, Manning’s cracking in the future. equation) is generally used as 0.012. Shotcrete is used not only to improve flow characteristics of brick, corrugated metal, or Question: To keep shrinkage cracking sufficiently tight so as any other pipe construction but can also be conventionally not to cause reflective cracking in the plaster layer, what is the reinforced as a structural liner to eliminate the need for liner proper depth of reinforcing steel from the shotcrete surface in plates or other pipe-lining alternatives. swimming pool applications? Question: A circular concrete tank built in the 1980s is to be Answer: This question should be addressed by a qualified resurfaced due to cracks in the exterior shotcrete lining. It has engineer with experience in designing swimming pools and been proposed that the existing surface will be hydroblasted. Is there a concern that the shotcrete may contain asbestos? Answer: 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 STRUCTURAL and high-temperature industrial uses and not for shotcrete. SHOTCRETE Question: We are a shotcrete contractor in Gold Coast, Aus- SYSTEMS, INC. ferent types of tools for cutting the shotcrete. How do you-tralia. We have noticed that in the United States, you use dif LICENSE #579272 A maintain a plum wall with the shotcrete rods? When a project requires a smooth finish or steel trowel finish, what are the tools JASON E. WEINSTEIN, P.E. and processes that are typically used? Finally, for a structural VICE PRESIDENT wall, what is the typical psi (MPa) and size of aggregate used? Answer: The face or surface of shotcrete walls as described 12645 CLARK STREET (562) 941-9916 are typically established with ground wires or screeds, which SANTA FE SPRINGS, CA 90670 FAX (562) 941-8098 assist the person using the shotcrete rod in cutting the wall to the proper plane. The tools typically used to achieve a troweled 68 Shotcrete • Winter 2013


2013WinShotcreteEMag
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