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.
Q: I am using a superplasticizer in my concrete. I have heard that since this is a water-reducer, the need for curing water is reduced. Is this correct?
A: This is a perfectly logical, but incorrect conclusion. Superplasticizers (aka high-range water reducers), along with their cousins the normal- and midrange water reducers, have the ability to reduce the amount of water needed to achieve the desired level of workability. None of these products reduces the amount of water required to hydrate the cement. In fact, superplasticizers are so powerful that the resulting water content required for workability often is less than the amount of water that would sustain maximum hydration of the cement. This is usually the case with high-strength/high-durability mixtures with low water-to-cementitious materials ratios. Another way to look at it: Mixtures with low initial water contents can tolerate very little loss of water due to evaporation. For these types of mixtures, a thorough wet cure is even more important.
Q: What tests can I use to determine if the curing has been effective?
A: Cylinder or core tests traditionally have been used to indicate curing effectiveness, which is meaningful when compression strength is the most important concrete characteristic. But when the properties of the concrete surface are critical to performance, strength tests by themselves can be misleading. This is because strength tests are influenced primarily by the concrete properties near the midheight of a core or cylinder. Curing has a greater effect on the strength of the concrete at the cured-surface than at the depth, and in many core tests, this top surface is cut off and discarded before testing. (Curing and protection measures that keep the concrete warm are more likely to influence compression strength than measures that keep the concrete surface wet.)
The most direct way to measure curing effectiveness is to evaluate a surface property that is influenced directly by curing, such as abrasion resistance (ASTM C 779), surface hardness (rebound hammer ASTM C 803), rate of absorption (ASTM C 1585), or deicer salt scaling (ASTM C 672). With the exception of surface hardness via rebound hammer (C 803), none of these are easy or inexpensive tests, but all will show significant variation with the type and duration of curing. We may see more use of these tests—or the continued development of improved methods, such as embeddable moisture sensors—as the industry transitions from prescriptive to performance specifications.
— 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.