7.18 : Non-destructive Tests for Concrete Strength

The rebound hammer test, also known as the Schmidt hammer test, is a non-destructive technique for evaluating the hardness of concrete and, indirectly, the strength of concrete. It operates on the principle that the rebound of a spring-driven mass from a concrete surface correlates to the surface's hardness. The device comprises a mass within a tubular housing, a spring mechanism, and a plunger that strikes the concrete. Upon release, the energy imparted to the mass by the spring causes it to rebound, with the travel distance of the mass providing a rebound number. This number, however, is influenced by the presence of aggregates and voids beneath the surface and the angle of the hammer, making multiple readings necessary for accuracy.

On the other hand, the penetration resistance test, or Windsor Probe test, measures concrete strength by measuring the penetration depth of probes shot into the concrete. This test, which slightly damages the surface by creating small holes, is performed using a gunlike device that discharges the probes through three holes in a template affixed to the concrete. The depth of penetration is inversely related to concrete strength, with the average penetration of three probes providing the measure. This method is considered to yield a more accurate estimate of concrete strength than the rebound hammer as it assesses the material below the surface.