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.