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with architectural stone face using shotcrete. We have already installed the soil nails and a 4 in. (100 mm) thick shotcrete wall reinforced with welded wire mesh. The shotcrete was continuously wet cured for 7 days and due to cold weather, we are waiting until spring to shotcrete an additional 8 in. (200 mm) thick wall face and 5 in. (125 mm) nominal depth architectural stone facing. The shotcrete is wet-mix process with fibers, silica fume, 0.375 in. (9 mm) aggregates, and fines, and the architectural facing is being sculpted by hand. Since shotcreting the 4 in. (100 mm) wall face 2 months ago, there has been a lot of cracking and efflorescence appearing. To avoid shrinkage cracking when installing the additional 13 in. (330 mm) of shotcrete, we are thinking of adding a shrinkage-reducing admixture (SRA) or even a product that is supposed to eliminate efflorescence. Do you have any recommendations regarding admixtures or any other products that might help reduce shrinkage cracking? Answer: SRAs have been used in shotcrete, and should help reduce long-term shrinkage potential. You may also try reducing the cement content and thus the paste portion of the mixture. Because you’ve added silica fume, you probably are getting good strengths, and may find a lower cement content doesn’t reduce the concrete strength too much. Also, reducing a very high cement content may reduce the potential for autogenous shrinkage of the paste. You mentioned 7 days of wet curing; that is good. Is it possible the cracks were initiated at an early age as a result of plastic shrinkage cracking? We know silica fume is a very “thirsty” supplemental cementitious material, so combined with evaporation at the surface, there may have been a potential to initiate some very thin cracks at an early age. The cracks may then be exasperated by drying shrinkage. To help reduce early-age plastic shrinkage cracking, the contractor may find fogging the surface immediately after finishing helpful. Question: Can we find an appropriate and easy way to evaluate the shrinkage performance of shotcrete? Answer: 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 (www.shotcrete. org/media/Archive/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.” Question: I am an architect and we have a client that is planning to do some major landscaping to his yard. He would like to consider shotcrete to create stone-type walls and outcroppings. Can you advise us on this? Would this be an appropriate application? Should he also just consider having a landscape company install boulders in lieu of a shotcrete-type landscape? Answer: 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. STRUCTURAL SHOTCRETE SYSTEMS, INC. LICENSE #579272 A www.structuralshotcrete.com JASON E. WEINSTEIN, P.E. VICE PRESIDENT 12645 CLARK STREET (562) 941-9916 SANTA FE SPRINGS, CA 90670 FAX (562) 941-8098 www.shotcrete.org Summer 2017 | Shotcrete 69


2017SummerShotcreteEMag
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