One of the best known effects of high temperatures on fresh concrete is the acceleration of stiffening. The rate of stiffening is higher if the concrete temperature is high or the water-cement ratio is low. Pound for pound, water does more than any other ingredient to raise or lower the mix temperature. Plastic shrinkage cracking is highly troublesome in hot, dry climates. Such cracking should be distinguished form presetting cracks caused by the hardening reactions. This cracking from setting can be avoided by using dry mixes and effective compaction. If cracks do develop they can be healed by vigorously reworking the concrete promptly. Plastic shrinkage cracking, on the other hand, is caused by tensile stresses produced by evaporation of mixing water. It is necessary to prevent evaporation of water until the fresh mix has developed sufficient strength to resist the tensile forces that develop. It is often contended that retarders tend to increase plastic shrinkage cracking and so their use in hot, dry weather may be questionable. In some applications, however, they offer the only practical solution to the problems of handling the concrete. Another approach argues that it is possible to use retarders without promoting plastic shrinkage cracking. The placement of bridge decks in hot weather illustrates the problem and the solution. Bridge decks are particularly difficult. Setting of concrete in bridge decks can be delayed by using a retarder. Decreasing amounts are used as the placement progresses so that all of the concrete will set after the entire deck has been placed. The longer the hardening is delayed, however, the more likely will be the development of plastic shrinkage cracks. Protection against evaporation must be provided by fogging with a fine mist of water.