Rebound hammer test is done to find out the compressive strength of concrete by using rebound hammer as per IS: 13311 (Part 2) - 1992. The underlying principle of the rebound hammer test is:
The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the Spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.
Importance of Residual Life Assessment
Conducting RLA is important to maintain the efficient operation of a process plant unit and avoid failure of critical equipments, which lead to costly downtime problem. Such equipments include pressure vessels, piping, storage tanks, valves, pumps, compressors, boilers, turbines etc. One common characteristic of all such equipments is their operation under very harsh conditions, like high temperature, pressure etc.
The importance of conducting RLA studies also arises from the regulations that are in place for a number of industrial equipments, like boilers, turbines etc. Also, conducting RLA studies properly can safeguard human lives.
Procedure To Determine Strength Of Hardened Concrete By Rebound Hammer
Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer.
Apply light pressure on the plunger - it will release it from the locked position and allow it to extend to the ready position for the test.
Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound reading in that position.)iv) Take the average of about 5 readings.
Interpretation of Results
The rebound reading on the indicator scale has been calibrated by the manufacturer
of the rebound hammer for horizontal impact, that is, on a vertical surface, to
indicate the compressive strength. When used in any other position, appropriate
correction as given by the manufacturer is to be taken into account.
Table:- Average Rebound number and quality of concrete
Average Rebound Number
Quality of Concrete
Very good hard layer
30 to 40
20 to 30
Rebound Hammer Test Equipment
A simple equipment known as Rebound Hammer or Schmidt Hammer is used for this purpose. Surface hardness measured during the test give an idea about the soundness and quality of cover concrete. Locations having very low rebound numbers indicate weak surface concrete and may be affected by corrosion. The quality of concrete may be interpreted as shown in the above table.