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.
The tests can be performed in horizontal, vertically upward, vertically downward or any intermediate angled positions in relation to the surface .The devices are furnished with correlation curves by the manufacturer.
ASTM C805 now states that these references to the relationship between the rebound number and compressive strength provided by the manufacturer shall be used only to provide indications of relative concrete strength at different locations in a structure. To obtain greater accuracy of test results, it is recommended that the user develop a correlation for the device on each concrete mixture design to be tested and at the intended test angle.
Procedure to extract the appropriate result:
- Prior to the test, the rebound hammer should be tested against the anvil to get reliable results for which the manufacturer of the rebound hammer indicates the readings on the anvil suitable for different types of rebound hammer.
- On applying light pressure on the plunger it will release it from locked position and allow it to extend to the ready position for the test.
- Press the plunger to the surface of the concrete, instrument being perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts.
- Take an average of about 15 readings.