Rebound hammer test is conducted to find out the compressive strength of concrete by using rebound hammer. The rebound of an elastic mass depends upon the hardness of the surface when it strikes. When the plunger of rebound hammer is pressed against the surface of the concrete, the pring controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete.
The spring-loaded hammer must travel with a consistent and reproducible velocity. The rebound distance of the steel hammer from the steel plunger is measured on a linear scale attached to the frame of the instrument.
originally found here:-www.sigmatest.org/Rebound-Hammer-Test-Delhi.html