Concrete contractors who have occasional trouble with the formation of plastic cracks will be interested to know that this difficulty can be virtually eliminated. Plastic cracking is generally caused when a change in weather conditions brings about an increase in the rate at which water is evaporated form the fresh concrete surface. Such cracks usually appear when wind velocity, relative humidity, high air temperatures, or a combination of all three, cause water to evaporate from the concrete surface faster than it is replaced by bleeding. Since the likelihood of plastic cracking is greatest whenever the rate of evaporation exceeds the rate at which water bleeds or rises to the surface of concrete, there are consequently two means of controlling the problem: by reducing the rate of evaporation and /-or increasing the rate of bleeding.