Test on hardened concrete – Non-Destructive Test

REBOUND HAMMER

AIM
To assess the likely compresive strength of concrete by using rebound hammer as per IS: 13311 (Part 2) - 1992.

PRINCIPLE
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.

APPARATUS
FIG. 11 : REBOUND HAMMER 

Rebound hammer

PROCEDURE

1. 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.
2. 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.
3. 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.)
4. Take the average of about 15 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.