We have very small shrinkage cracks in some shotcreted pools. No water is lost through these cracks but they cause cracking in the plaster applied over the shotcrete. Some pools have three or four cracks running the length and width of the pool but others have none at all. Water curing has not seemed to help. We have had no cracks in some pools made in the winter using calcium chloride in the mix. Normally we use an 8bag-grout mix with 100 pounds of fly ash per yard at about 3or 4-inch slump. We also use 3/8-inch rebars, 1 foot on centers, in our pools. What do you suggest to minimize these shrinkage cracks?
Calcium chloride does not diminish shrinkage cracking but may tend to increase it slightly. If the pools you built with calcium chloride cracked less than those without it the chances are that it was for some other reason. For example, perhaps the pools with calcium chloride were constructed in cool weather when the concrete was not so likely to lose moisture during the curing period. Possibly the chief cause of cracking of shotcrete is inadequate curing. Since shotcrete is relatively thin it dries rapidly. The American Concrete Institute "Recommended Practice for Shotcreting" (ACI 506-66, reaffirmed 1972) says: "Good curing is particularly important for the very thin sections, rough surfaces, and low-slump mixes associated with shotcrete. It is generally recommended that surfaces be kept continuously wet for at least 7 days." An ACI standard specification on shotcreting currently in the process of being adopted, "Specification for Materials, Proportioning and Application of Shotcrete" (ACI 506), discusses curing. We believe that the wetter you can keep the shotcrete the better.