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2013FallShotcreteEMag

Shotcrete FAQs (991 mm) start to 15 in. (381 mm) thick at the 54 in. (1372 mm) an adequate thickness and elasticity to tolerate moving depth. What would you recommend for this overlay to bond cracks. We suggest that you contact one of our corporate and not “pop loose” or cause crack transfer to pool plaster? members who is familiar with your area and get their specific advice. Visit www.shotcrete.org/pages/products-services/ Answer: The proposed overlay will be similar to any repair Buyers-Guide/index.asp. where shotcrete is placed over existing concrete. Proper sur- face preparation is essential for allowing good bond. Guidance Question: How might one add fibers to a gunite (dry-mix) on surface preparation can be found in ACI 506R-05, “Guide application? I have heard of some companies adding them by to Shotcrete.” It also appears you are suggesting tapering hand at the base of the auger and others who poured them over the thickness from 15 to 0 in. (381 to 0 mm). Feathering their sand and mixed them in with a loader before loading it thickness down to 0 in. (0 mm) is not encouraged, and a into the truck. Is there a more efficient way to add them to a minimum thickness should be established. Because the dry mixture so that they are distributed evenly throughout? overlay section will be quite thick and experience differential shrinkage from the previously shot material, the overlay will Answer: Many of our members add them by hand at the mixer require additional reinforcement to accommodate tempera- and have had good success when using an adequate mix time. ture and shrinkage stresses. You should consult with an Another method is to have the mix blended at a bag mix plant engineer experienced in shotcrete design to establish the with the fibers. proper amount of reinforcement. The required reinforcement and cover over the reinforcement will control your minimum Question: We have a 17 mile (28 km) long TBM tunnel for overlay thickness. water that will drive our underground powerhouse. Is there a recommended shotcrete surface texture we could use? Our Question: I am interested to know if any shotcrete contractors contractor is using 0.31 in. (8 mm) aggregate, but they are have shot a magnesium phosphate material (dry-process) getting an undulating surface. Can you provide some clarity before and, if so, could you detail the special requirements as to what we should ask our contractor to try and achieve? necessary in placing such a unique product? Answer: Shotcrete can be applied Answer: Phosphate-bonded refractory materials were routinely with many different textures. The shot in cyclone boilers in the 1970s. These phosphate-bonded nozzle finish shown is very rough, materials don’t have a cement bond, but achieve a chemical even for a natural gun finish. Nozzle bond when heat is applied. Without knowing the precise for- finishes can be done smoother than mulation of the mixture and grain sizes involved, we cannot this. Another technique would be to tell you definitively that your specific material can be shot- use a broom to make it smoother creted. However, there is a long history of successful past after it is shot. Other finishes experience with phosphate-bonded refractory materials being include wood float, rubber or shot with the dry-mix process. You may want to consider a field sponge float, broom, and smooth trial before construction to verify your specific mixture works trowel finishes. There are many with your dry-mix shotcrete equipment. examples of finishes shown in articles inShotcrete magazine (www.shotcrete.org/pages/archive-search/Archive-Search. Question: I am an engineer working on a project involving asp?query=finishes&srchtype=ALL). shotcrete and earthwork. The shotcrete that was placed has some expansion cracks, which we expected. I would like to Question: I am an engineering technologist working on a know the best way to repair them. Is there some type of water- landslide project where shotcrete had been applied to stabi- proof coating/grout that can be applied between the cracks? lize the sandstone head scarp at the crest of the slope. The Part of the cracks will be continuously under water. The shot- shotcrete was applied in 1998. After a recent inspection, it crete is the surfacing material for a diversion ditch at a mine, was noted that the surface of the shotcrete had some cracking and we need to recommend some remediation solutions to in some sections. How can this be repaired? Can the cracks our client. simply be filled with a grout/mortar mixture of some sort or do the cracked sections have to be removed entirely and Answer: There are many products in the marketplace for shotcrete be reapplied? repairing cracks. Because shotcrete is simply a method for placing concrete, any method for concrete crack repair would Answer: Shotcrete can and has been used to overlay previously be applicable. It would be wise to use a product that filled the installed shotcrete or concrete that has cracked over time. It cracks and is able to tolerate thermal movement in the future would be advisable that you engage an engineer knowledgeable (not a brittle product). Many injectable polyurethane grouts in geotechnical engineering and concrete properties to for- can accomplish this. Surface-applied coatings would need malize a solution. It is important that the cause of the cracks 76 Shotcrete • Fall 2013


2013FallShotcreteEMag
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