Q.: A swimming pool contractor who is a customer of ours wants an oversanded mix to use in shotcreting. We have suggested one of our oversanded mixes which contains 730 pounds of cement, 180 pounds of fly ash, 700 pounds of 1/2-inch stone, and 1800 pounds of sand. We often supply this mix at a 6-inch slump to other customers who use it with a small concrete pump. This customer is afraid that his finish coat, which contains marble dust, will not bond properly to a shotcrete containing fly ash. Should we reproportion the mix to eliminate fly ash?

A.: We don't see any problem with your mix. You might caution the contractor to keep the shotcrete moist so that the surface cures well and also to keep the surface clean. He could accomplish both by keeping it covered with polyethylene.