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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 the slab with water from a hose or they use a lawn sprinkler, once the concrete is 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.Bill Palmer responds: To start with, a set retarder does nothing to help concrete cure. It simply makes the concrete set more slowly, which would actually make proper curing a more critical concern, since the concrete would have more time to dry out.

For concrete to reach its full strength, it needs water to hydrate the cement. If it dries out, then the resulting concrete is soft, even 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 (a condition called dusting).

There are three important variables in determining how quickly the concrete will dry out: temperature, relative humidity, and wind speed. Thus, 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 to either prevent that water from evaporating or add enough supplemental water to make up for the evaporation. To prevent evaporation, you can use curing blankets, plastic sheeting, or membrane-forming spray-on curing compounds. Curing compounds can be reasonably effective when the evaporation rate is not too high. Using a pigmented compound allows you to see that you have complete coverage; using white pigment in hot weather helps by reflecting the sunlight.

A better way to ensure proper curing, though — as your local finishers have pointed out — is with water. The water can be ponded, sprayed, or misted onto the surface. To keep it wet, many concrete contractors lay down burlap to hold the water on the surface. But if you try this, don't let the burlap dry out or it could have a negative impact in hot weather by holding heat in.

How long to keep the concrete wet depends on the air temperature and the mix: You want it to have reached sufficient strength on the surface before you let up. Typically, about seven 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.

Bill Palmer, P.E., is editor of Concrete Construction magazine.