How to calculate strength of concrete in rebound hammer test

The rebound hammer test is a non-destructive test for the strength of concrete for field testing of concrete.

As per the Indian code IS: 13311(2)-1992, the rebound hammer test has the following objectives:

1. To determine the compressive strength of the concrete by relating the rebound index and the compressive strength
2. To assess the uniformity of the concrete
3. To assess the quality of the concrete based on the standard specifications
4. To relate one concrete element with other in terms of quality

The instrument can be used on both horizontal and vertical surfaces. The main point to be noted is that the graph had to be read appropriately. These days, the hammer is equipped with a digital scale to make life easier.

The IS: 13311(2)-1992 elaborates the procedure in a more detailed manner, making it easier for even beginners to understand the concept.

The point of impact should be at least 20 mm away from any edge or shape discontinuity. For taking a measurement, the rebound hammer should be held at right angles to the surface of the concrete member.

The apparatus is placed on the spot where the test is desired. The hammer is allowed to drop freely. After it strikes the concrete surface it rebounds back. The raise of the rebound is recorded. This test is repeated at different locations and the rebound value is recorded.

The compressive value of concrete is obtained by doing a cube test of the test cube of the batch of concrete on which the rebound test is conducted. The value of rebound and compressive strength of concrete is compared and the value of compression is adjudged from the values of compression test and rebound values.

Around each point of observation, six readings of rebound indices are taken 2nd average of these readings after deleting outliers as per IS 8900: 1978 becomes the rebound index for the point of observation. ( This outlaying is a calculation based method, can be discussed on one’s interest).

Now, we will calculate comp. Strength from rebound number. There are three positions of taking reading

1. Vertical Downward:- Weight of Hammer is increasing stress on spring of hammer
2. Vertical Upward:- Weight of hammer is decreasing the stress on spring of hammer
3. Horizontally :- Weight of hammer have no effect on spring.

This is why, there are three graphs in Rebound Number-Compressive Strength graph of Rebound Hammer.

You will have to calculate the strength from the graph as per position of Rebound Hammer.

For Example,

For Vertical Upward position of Hammer

If 15 Rebound number readings are

48,42,43,43,44,44,45,45,45,45,46,47,48,50

, as per IS:8900, Clause 5.1.1(TESTS FOR TWO OR MORE OUTLIERS , AT LEAST ONE OUTLIER AT EACH END), 42 and 50 are Outlaying values.

Removing those, Average Rebound number is 45.27

From the graph( fro Vertical Upward) , Compressive Strength Will be 40.54 Mpa