Rebound Hammer test is a Non-destructive testing method of concrete which provide a convenient and rapid indication of the compressive strength of the concrete. The rebound hammer is also called as Schmidt hammer that consist of a spring controlled mass that slides on a plunger within a tubular housing.

Principle of Rebound Hammer Test

Rebound hammer test method is based on the principle that the rebound of an elastic mass depends on the hardness of the concrete surface against which the mass strikes.

Rebound hammer test is done to find out the compressive strength of concrete by using rebound hammer 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.

Procedure to determine strength of hardened concrete by rebound hammer.

i) 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.

ii) 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.

iii) 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 15 readings.

Objective of Rebound Hammer Test

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

Advantages and Disadvantages of Rebound Hammer Test

The advantages of Rebound hammer tests are:

1) Apparatus is easy to use

2) Determines uniformity properties of the surface

3) The equipment used is inexpensive

4) Used for the rehabilitation of old monuments

The disadvantages of Rebound Hammer Test

1) The results obtained is based on a local point

2) The test results are not directly related to the strength and the deformation property of the surface

3) The probe and spring arrangement will require regular cleaning and maintenance

Flaws cannot be detected with accuracy

Factors Influencing Rebound Hammer Test

Below mentioned are the important factors that influence rebound hammer test:

1) Type of Aggregate

2) Type of Cement

3) Surface and moisture condition of the concrete

4) Curing and Age of concrete

5) Carbonation of concrete surface

Type of Aggregate

The correlation between compressive strength of concrete and the rebound number will vary with the use of different aggregates. Normal correlations in the results are obtained by the use of normal aggregates like gravels and crushed aggregates. The use of lightweight aggregates in concrete will require special calibration to undergo the test.

Type of Cement

The concrete made of high alumina cement ought to have higher compressive strength compared to Ordinary Portland cement. The use of super sulphate cement in concrete decrease the compressive strength by 50% compared to that of OPC.

Type of Surface and Moisture Condition

The rebound hammer test work best for close texture concrete compared with open texture concrete. Concrete with high honeycombs and no-fines concrete is not suitable to be tested by rebound hammer. The strength is overestimated by the test when testing floated or trowel surfaces when compared with moulder surfaces.

Type of curing and age of concrete

As time passes, the relation between the strength and hardness of concrete will change. Curing conditions of concrete and their moisture exposure conditions also affects this relationship. For concrete with an age between 3days to 90 days is exempted from the effect of age. For greater aged concrete special calibrated curves is necessary.

Carbonation on Concrete Surface

A higher strength is estimated by the rebound hammer on a concrete that is subjected to carbonation. It is estimated to be 50% higher. So the test have to be conducted by removing the carbonated layer and testing by rebound hammer over non-carbonated layer of concrete.