Published on 30-Dec-2021

Rebound Hammer Test: An Overview

Rebound Hammer Test: An Overview

What is Rebound Hammer Test?

Rebound Hammer Test is a non-destructive testing method of concrete that provides a convenient and rapid indication of the compressive strength of the concrete. The rebound hammer is called a Schmidt hammer that consists of a spring controlled mass that slides on a plunger within a tubular housing.

How does the rebound hammer test happen?

The operation of the rebound hammer test happens as follows:
When the plunger of the rebound hammer is pressed against the surface of concrete, a spring controlled mass with constant energy is made to hit the concrete surface to rebound back. The extent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A block of concrete with low strength and low stiffness will absorb more energy to yield a lower rebound value. Refer to the following image for better understanding: 


 Objective of the Rebound Hammer Test:

The rebound hammer test procedure could be used for:

(i) assessing the likely compressive strength of concrete with the help of suitable correlations between rebound index and compressive strength,

(ii) assessing the uniformity of concrete,

(iii) assessing the quality of the concrete in relation to standard requirements, and

(iv) assessing the quality of one element of concrete in relation to another.

Principles of the Rebound Hammer Test:

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 rebound depends upon the surface hardness of concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound is read off along a graduated scale and is designated as the rebound number or rebound index.

Procedure of the Rebound Hammer Test:

The Rebound Hammer Test is conducted in the following steps:

→ Checking of the apparatus: The rebound hammer should be checked against the testing anvil before the commencement of the test to ensure reliable results. The testing anvil should be of steel having Brinell hardness of about 5000 N/mm2. The supplier or the manufacturer of the rebound hammer should indicate the range of the readings on the anvil suitable for different types of rebound hammers.

→ The procedure of obtaining Correlation between Compressive Strength of Concrete and Rebound Number: The most satisfactory way of establishing a correlation between compressive strength of concrete and its rebound number is to measure both the properties simultaneously on concrete cubes. The concrete cube specimens are held in a compression testing machine under a fixed load, measurements of rebound number taken and then the compressive strength determined as per IS: 516- 1959. The fixed load required is of the order of 7 N/mm2 when the impact energy of the hammer is about 2.2 Nm. The load should be increased for calibrating rebound hammers of greater impact energy and decreased for calibrating rebound hammers of lesser impact energy. The test specimens should be as large a mass as possible in order to minimise the size effect on the test result of a full-scale structure. 150 mm cube specimens are preferred for calibrating rebound hammers of lower impact energy (2.2 Nm), whereas for rebound hammers of higher impact energy, for example, 30 Nm, the test cubes should not be smaller than 300 mm.

If the specimens are wet cured, they should be removed from wet storage and kept in the laboratory atmosphere for about 24 hours before testing. To obtain a correlation between rebound numbers and the strength of wet cured and wet tested cubes, it is necessary to establish a correlation between the strength of wet tested cubes and the strength of dry tested cubes on which rebound readings are taken. A direct correlation between rebound numbers on wet cubes and the strength of wet cubes is not recommended. Only the vertical faces of the cube as cast should be tested. At least nine readings should be taken on each of the two vertical faces accessible in the compression testing machine when using the rebound hammers. The points of impact on the specimen must not be nearer an edge than 20 mm and should be not less than 20 mm from each other. The same points must not be impacted more than once.

→ Rebound Hammer Test Procedure: Test Procedure

For testing, a smooth, clean and dry surface is to be selected. If the loosely adhering scale is present, this should be rubbed off with a grinding wheel or stone. Rough surfaces resulting from incomplete compaction, loss of grout, spalled or tooled surfaces do not give reliable results and should be avoided.

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 test can thus be conducted horizontally on vertical surfaces or vertically upwards or downwards on horizontal surfaces. If the situation demands, the rebound hammer can be held at intermediate angles also, but in each case, the rebound number will be different for the same concrete.

A rebound hammer test is conducted around all the points of observation on all accessible faces of the structural element. Concrete surfaces are thoroughly cleaned before taking any measurement. Around each point of observation, six readings of rebound indices are taken and an average of these readings after deleting outliers as per IS:8900-1978 becomes the rebound index for the point of observation.

Points to remember to conduct a perfect rebound hammer test procedure: 

These are the following points to remember to ensure a perfect rebound hammer test procedure:

→ The concrete surface should be smooth, clean and dry

→ The loose particles should be rubbed off from the concrete surface with a grinding wheel or stone, before hammer testing

→ The test should not be conducted on rough surfaces as a result of incomplete compaction, loss of grout, spalled or tooled concrete surface

→ The point of impact of the rebound hammer test procedure should be at least 20mm away from the edge or shape discontinuity

→ Six readings of rebound number are taken at each point of testing and an average value of the readings is taken as rebound index for the corresponding point of observation on the concrete surface.

Advantages & Disadvantages of Rebound Hammer Test:

The advantages of the Rebound Hammer Tests procedure are:

Apparatus is easy to use

Determines uniformity properties of the surface

The equipment used is inexpensive

Used for the rehabilitation of old monuments

The disadvantages of the Rebound Hammer Test procedure are:

The results obtained are based on a local point

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

The probe and spring arrangement will require regular cleaning and maintenance

Flaws cannot be detected with accuracy

Influencing factors for the Rebound Hammer Test procedure:

Here are the following factors for the Rebound Hammer Test procedure:

Type of Aggregate: The correlation between the compressive strength of concrete and the rebound number will vary with the use of different aggregates. 

Type of Cement: The concrete made of high alumina cement ought to have higher compressive strength compared to Ordinary Portland cement.

Surface and moisture condition of the concrete: The rebound hammer test works 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. 

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.

Carbonation of concrete surface: A higher strength is estimated by the rebound hammer on the concrete that is subjected to carbonation. It is actually estimated to be 50% or higher. Hence, the test needs to be conducted by removing the carbonated layer and testing by rebound hammer over the non-carbonated layer of concrete.  

Conclusion:

We hope this blog article has helped you understand the rebound hammer test procedure. To know more about the rebound hammer test, visit here. 

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