REBOUND HAMMER TEST:

The test is based on the principle that the rebound of an elastic mass depend on the hardness of the surface against which the mass impinges.

• The hammer consist of a plunger connected with a spring driven metal mass. The plunger is held against at 90⁰ to the smooth concrete surface, firmly supported and pressed. This will impart a fixed amount of energy.

• After releasing, the metal mass rebounds, the plunger being still in contact with concrete. The distance covered by the metal mass or amount of setback (rebound) is noted on a scale which gives an indication of the concrete strength. That means larger is the rebound, higher is the strength of concrete.

• The rebound hammer test is sensitive to local variation in the concrete. For instance, the presence of large piece of aggregate immediately underneath the plunger would result in an abnormally high rebound number. For this reason, it is desirable to take 10 to 12 readings spread over the area to be tested, and there average value must be taken.

• The test can be conducted horizontally, vertically upward or downward or at any intermediate angle. At any angle rebound will be different for the same concrete and required separate calibration or correction chart.

The above graph shows that the hammer test is done in any direction and hence different compressive strength can be calculated for different direction. but must preferred direction for rebound hammer test is in horizontal direction.