As the cement in concrete hydrates (chemically reacts with the mixing water), heat is generated that in turn increases the temperature in the concrete. This heat generation and subsequent temperature rise can be both a benefit and a disadvantage. The two most important factors influencing temperature rise in concrete elements are the amount of cement in the concrete mix and the size of the element. The larger the element, and the more cement in the mixture, the more heat will be generated. Exposing concrete to elevated temperature, either from internally generated heat or intentional external heat often used in precasting operations, is not harmul in general. What is problematic is developing excessive thermal gradients in concrete. This causes differential expansion, which can lead to cracking. Preventing cracking is a complex matter. It involves establishing a maximum allowable thermal gradient--the difference between the hottest and coolest portions of the concrete. A conservative guideline for this would be 35 degrees. This can be problematic for high strength mixtures that use a lot of cement. One way to lower the temperature is to remove some of the cement and substitute a pozzolan, such as fly ash, in its place. Pozzolans only generate about half the heat of cement. The use of Type II or IV cement can also lower heat. Another method is to substitute ice for some of the mix water, or keeping aggregate stockpiles wet.