Q.: Our company builds hundreds of wet-mix shotcrete pools a year in the Southwest, using a pea gravel mix that we buy from either of two ready mixed concrete suppliers. Recently we have had a number of failures in which the shotcrete has crumbled away. This happened when the weather was very hot and I believe the problem is due to high concrete temperatures and maybe to hot cement in the mixes. We have never had this trouble in previous years when we used ice as part of the mixing water. Could hot cement be the problem?
A.: If hot cement has an effect it is a minor one. Even if the cement being used were at a temperature of 185 degrees F it would raise the temperature of the final mix only a few degrees F. (This is because the cement is only a fraction of the mix and because the cement has a much lower specific heat than water.) We suggest you go back to using ice as part of the mixing water.