Sufficient curing time should always be allowed for concrete to develop adequate strength before it is loaded. Curing should be started as soon as it is possible to apply a curing medium without damaging the surface and should continue as long as is face temperatures for normal mild-weather conditions are summarized in Figure 1. however, during hot weather, curing should continue for at least 7 days, regardless of the type of concrete or the temperature. In mild weather, the curing time can sometimes be reduced to 2 days when high-early-strength Portland cement (Type III ) is used. Even this time may have to be increased, through, in very hot weather.
In any case, concrete temperature should not be allowed to fall below 50ºF (10 ºC) at any time during curing . rapid cooling may induce thermal cracking, especially during the first 12 to 24 hours.