The Schmidt Rebound Hammer is principally a surface hardness tester with theoretical relationship between the strength of concrete and the rebound number of the hammer. Rebound hammer consists of releasing the plunger from the locked position by pressing it gently against a hard surface. To conduct the test the plunger is pressed strongly against the concrete surface under test. This releases the spring-loaded weight from its locked position, thus causing an impact. While the hammer is still in its testing position the sliding index is read to the nearest whole number. This reading is known as hammer rebound number. Each hammer is to be calibrated for equivalent cube strength verses respective rebound numbers.