Q: I am a concrete contractor in the Southwest and to beat the heat in the summertime, I cast my slabs early in the morning, starting before dawn. By avoiding the heat of the day, doesn't this reduce the need for curing?
A: Early morning slab pours have a lot of advantages, not the least of which are fewer worries about ready-mix trucks getting caught in heavy traffic. But when it comes to controlling the loss of moisture from the slab surface, there are a couple of factors that can really take you by surprise.
First of all, even though the sun cannot heat the concrete surface in a predawn pour, cement and aggregates can retain heat gained by day so the concrete itself still can be pretty warm. In dry climates the summer air temperature usually drops at night, and as air temperature approaches the concrete temperature, the rate of evaporation actually increases. In Phoenix, for example, the lowest temperature of the day in the summer often is around 5 a.m. or 6 a.m.
The effect of dropping air temperature is combined with the effect of wind speed. In many locations scheduling a night pour can take advantage of calm wind conditions, since lower wind speed usually means lower evaporation. But be careful of relying on this because the wind usually starts to pick up shortly before sunrise.
Cool air before dawn coupled with a fresh breeze means that drying conditions start to get severe around the time the slab is being finished, requiring rapid application of curing actions such as fog-spraying, evaporation reducers, curing compound, and curing covers. The key is to get the slab placed, finished, and protected before the conditions become severe. This can really take an inexperienced crew by surprise when they have been successful in preventing drying and cracking for three or four hours, only to see the cracks start to appear at dawn around the time the weather really feels pretty comfortable, and long before the heat of the day.
— Kenneth C. Hover, Ph.D., P.E., is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N.Y., and a popular speaker at Hanley Wood's World of Concrete.