Q.: Because shotcrete usually has a higher cement content than normal concrete, do I have to protect shotcrete from freezing as long as for normal concrete?
A.: Although shotcrete typically has a higher heat of hydration because of its higher cement content, the fact that it is placed in thin layers usually offsets the heat of hydration benefits in cold weather. Therefore, protection requirements for shotcrete in cold weather are generally the same as those for normal concrete.
According to ACI 506, "Guide to Shotcrete," a minimum strength of 500 psi is required to protect shotcrete from freezing the same as that required for normal concrete. Shotcrete placed by either the dry or wet process must be protected from freezing before the shotcrete reaches this minimum strength. Temperature during curing should be above 40 F, and ACI does not recommend using water curing in a freezing environment. Remember, also, that once protection is removed, low temperatures will prevent the shotcrete from getting stronger.