Q.: I’ve recently moved my contracting business to the South. Concrete finishers here tell me that a slab poured outside on a hot day won’t cure properly unless it’s kept wet. They do this by having someone periodically spray it with water from a hose or by setting up a lawn sprinkler, once the concrete has set up enough that the top layer won’t wash away. Is this really necessary? Would adding a set retarder to the mix accomplish the same thing?
A.: To start with, a set retarder does nothing to help concrete cure. It simply makes the concrete set more slowly, which would actually make curing more critical. For concrete to reach its full strength, it needs water to hydrate the cement. If it dries out, then the resulting concrete is soft, almost chalky in an extreme case. This is most common on the surface of a slab. If it dries out even momentarily, it will be weakened.
There are three important variables in determining how quickly the concrete will dry out: temperature, relative humidity, and wind speed. Therefore on a hot, dry, windy day, the concrete will dry quickly, and that’s when curing is most important.
Most concrete has plenty of water when it is placed, so the key is either to prevent the water from evaporating or add enough water to make up for the evaporation. The ways to prevent evaporation are with curing blankets, plastic sheeting, or membrane-forming sprayed-on curing compounds. Curing compounds can be reasonably effective when evaporation is not too high.
The better way to cure concrete, though, is with water curing. Water can be ponded, sprayed, or misted onto the surface. To keep it wet, many concrete contractors use burlap to soak up the water. How long to keep it wet depends on temperature and the mix—you want it to have reached sufficient strength on the surface. Typically, about 7 days is sufficient with Type I cement—less in warm weather.
I always tell people that concrete is sort of like a baby: when it is very young, if you keep it warm and wet (rather than dry) it will grow up to be a strong and responsible adult. Neglect it, and you’ll have to live with a problem child for many years.