The Hammer Rebound experiment measures the surface hardness of concrete with estimated correlations to assess structural compressiveness. While the constraints of the exam may result in great differences in the projected power, the exam has become very famous with ease of conduct and low original price. Rebound Hammer exam is a non-destructive concrete test method that offers a comfortable and quick hint of the concrete’s compressive strength. Also known as the rebound hammer is the Schmidt hammer which is made up of a fountain-checked mass which falls into a tube plunger.

Testing of the rebound hammer according to IS: 13311 Part II norm is performed.

Avantech Engineering Consortium has undertaken health supervision initiatives for power plants, building managers, building authorities, etc. and undertaken the rebounding of the hammer tests as required by the initiative.

The objective of the 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 others in terms of quality

Rebound hammer testing technique may be used for distinguishing between appropriate and questionable structural components or comparing two distinct strength-based constructions.

Principle of Rebound Hammer Test:

The technique of rebound hammers bases on the idea that an elastic mass is rebounded according to the hardness of the concrete ground the mass pushes against. The wind regulated mass in the hammer rebounds when the rebounding hammer plunges against the concrete ground. Depending on the hardness of the concrete surface, the quantity the mass bounces.

A graduated scale reads the recoil value and is indicated as the recoil or rebound index. Therefore, the hardness of the rebound hammer reading and the concrete compression strength can be associated with that of concrete. 

Correlation between compressive strength of concrete and rebound number:

In order to achieve the correlation between concrete compressive strength and rebound amount, the most appropriate technique is to use compression test machines to test the concrete cubes and to concurrently use a rebound gun. The rebound amount of the concrete cube is drawn first and then on the compression measuring device, the compression strength is evaluated. When the effect energy of the hemmer is about 2.2 nm, the necessary set force is in the order of 7 N / mm2.

For the calibration of rebound hammers with higher energy impact and for the calibration of rebound hammering with lower energy impact the load should be increased. In order to minimize the size influence on the test results of a full-scale structure, the test specimens must be as big as possible. For the purpose of calibrating rebound hammers with lower energy impact (2.2Nm), 150 mm cube specimens are preferred, while the test cubes should not be less than 300 mm for the rebound hammer with higher energy impact, for example, 30 nm.

The concrete cube specimens should be kept for about 24 hours before being removed with the rebound hammer from the curing pond at room temperature. To achieve a correlation between rebound numbers and the strength of wet-curated and wet-testing cubes, the strength of wet-testing cubes to which rebound readings are measured must be related.

It is not advisable to directly link rebound numbers on wet cubes with wet cube strength. It is necessary to test only the vertical faces of the cubes as cast. On each of the two vertical faces accessible on the compression test machine, at least nine measuring data should be taken with the rebound hammer. The impact points on the specimen shall only be 20 mm nearer to each other’s edge and not less than 20 mm. There must be no more than once effect on the same points.

Points to Remember in Rebound Hammer Test:

1) Smooth, tidy and moist concrete floor.

2) Before testing, loose objects with a grinding wheel or rock of the concrete layer should be brushed away.

3) The hammer rebound experiment should not be performed on the surfaces of the ground, spalled or tooled concrete panel as a consequence of unfinished compaction.

4) At least 20 mm away from the border or form discontinuity shall be the effect of the rebound wedge on the concrete surface.

(5) At every test point, six rebound number measurements are drawn and a median measurements ‘ value for the respective observer stage on the concrete surface is given as rebound index.

Advantages and Disadvantages of Rebound Hammer Test:

Rebound hammer tests have the advantages of:

1) An easy-to-use appliance 

2) Determines surface uniformity properties 

3) The appliance is cheap 

4) Used for the reconstruction of old monuments

Disadvantages of Rebound Hammer Test are as follows:

1) The obtained results are based on the local point 

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

3) Regular cleaning and maintenance of tests and spring arrangements are required 

4) The errors can not be precisely identified.

Factors Influencing Rebound Hammer Test:

The significant variables that affect the rebound hammer test are stated below:

 1) Type of aggregate 

(2) Type of cement 

(3) Texture and humidity of the wood 

(4) Cure and concrete age 

(5) Concrete ground charcoal.

Source: https://nondestructivetestingindia.com/lab/onsite-rebound-hammer-test/