If curing is to be fully effective there is more to the problem than adequate moisture and favorable temperature. A great deal of research effort has been expended to establish such important factors as the optimum time to start application of the curing medium, the duration of the time moisture is made available and, where durability is of prime importance, the desirability of a drying period following moist curing. Much of the resulting knowledge is difficult to apply to practical site conditions. The optimum time at which to start moist curing, for example, is not easy to establish. The general rule is that the concrete should have settled in the form and all free moisture should have disappeared from the exposed surface. However, this rule is not applicable to entrained air concrete. The duration of curing is difficult to estimate exactly, since concrete is made with an extremely wide range of properties and placed under widely varying atmospheric conditions. Since the purpose of curing is to enable concrete to reach its design strength in a given time, the period of continuous curing necessary should be specified by the engineer or architect.