Concrete NDT Test ( Rebound Hammer Test ) This is the most common method adopted for checking strength of concrete . The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. The plunger of hammer is pressed strongly and steadily against the concrete surface at right angles to its surface, until the spring loaded mass is triggered from the locked position. The spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The distance traveled by the mass as a percentage is defined as rebound number.