Hot weather concreting with admixtures

Abstract

The adverse effects of elevated temperatures on the properties of the fresh concrete, namely increased water demand, shorter setting times and increased slump loss, are briefly discussed, and the possible use of chemical admixtures (ASTM C 494) to overcome these adverse effects, is examined in the light of present knowledge. The effect of water reducing and retarding admixtures (type D, ASTM C494) on plastic shrinkage and plastic shrinkage cracking, and the effect of fly ash class F (ASTM C 618) on slump loss, are also discussed. It is concluded that type D admixtures accelerate, rather than slow down the rate of slump loss in concrete subjected to prolonged mixing, but such admixtures are advantageous when used to reduce water demand and to delay setting times. Fly ash was found to reduce significantly the rate and amount of slump loss of concrete with and without admixtures. It is suggested, however, that under hot weather conditions, the combined use of class F fly ash and type D admixtures is to be recommended. Both retarders and fly ash were shown to increase the susceptibility of fresh concrete to plastic shrinkage cracking. Accordingly, when used, extra care should be taken to protect the fresh concrete from drying.