Q. We're on a job that requires a fast forming cycle. If we calibrate our rebound hammer for the concrete being used on the job, are the hammer test results accurate enough to determine when forms can be safely stripped?

A. The one study we found says no. In the study, researchers computed within-test variability of the rebound hammer test at concrete test ages of one to three days. They also studied the ability of the test to determine early-age strength development of concrete for form removal purposes. The rebound tests were performed on plain concrete slabs. Companion cylinders and cores taken from the slabs were tested in compression. After analyzing the test data and finding a large within-test variation, the researchers concluded that the rebound hammer wasn't a satisfactory method for reliably estimating early-age concrete strength development. Reference G.G. Carette and V. M. Malhotra, "In-Situ Tests: Variability and Strength Prediction of Concrete at Early Ages," In-Situ/Nondestructive Testing of Concrete, ACI SP 82, 1984, p. 111.