What is Rebound Hammer?

Rebound hammer is an instrument or a device, which is used to assess the relative compressive strength of concrete based on the hardness at or near its exposed surface. Rebound hammer is also known as Schmidt’s Hammer or Swiss Hammer as it is invented by Ernst Schmidt, a Swiss engineer.

Rebound Hammer Test Procedure

In the procedure of rebound hammer test, the plunger of rebound hammer is pressed on the surface of concrete, so that a spring controlled mass with a constant energy is made to hit the concrete surface. At the same time, a spring controlled mass rebounds back. This extent of rebound on a graduated scale is measured for the surface hardness. This measured value is designated as Rebound Number or a Rebound Index.

Selecting a Surface for Testing

1- The surface should be smooth, clean and dry.

2- The loose surface should be rubbed off with a grinding wheel or stone, before testing.

3- The point of impact should be at least 20 mm away from the edge or sharp discontinuity.

4- The test should not be conducted on the rough surface resulting from incomplete compaction of concrete, loss of grout, spoiled or tooled surface.

5- Age of the Specimen should be at lest 14 days old

Surfaces to Avoid

1- Frozen
2- Honeycombed or excessively porous
3- Known to have rebar 2 cm or less below the surface
4-  Carbonated on the surface

Uses Of Rebound Hammer Test

1- It is used to investigate concrete hardness. Hardness measurements provide only the information on the quality of the surface layer of the concrete only.

2- It is low cost and has a simple operating procedure.

3- The rebound hammer can be used for the concrete surface which is at the horizontal, vertical or intermediate angle. Care should be taken that the hammer should be provided perpendicular to the surface under test.

4- It is easy to use because the rebound hammer is light in weight, compact and can be easily carried in hand anywhere.

5- It is used to test in-situ concrete as well as fresh concrete after final set. It is used to assess the in-place uniformity of the concrete.

6- It is used to find out the exact location of poor quality and deteriorated concrete.

7- Rebound hammer test is used to estimate in-place strength if a correlation is developed.

What is Rebound Hammer Test Graph?

Rebound hammer test graph describes the correlation between the strength of concrete and rebound number. Where “a” is the angle of the position of rebound hammer device. Always use rebound hammer perpendicular to the plane of a concrete component or any other materials.

 

It is necessary to calibrate the rebound hammer before carrying out the test on hardened concrete. The suitable methods for checking the calibration of the rebound hammer is to test the concrete cube of size (150 mm * 150 mm * 150 mm) with the help of rebound hammer and then with the compression testing machine (CTM). If the strength results of rebound hammer test exactly match with CTM results, then it is ready to use. But if the major difference found in results, then we need to calibrate the rebound hammer before testing.

How to Check the Quality of Concrete Based on Rebound Number?

The table below shows the quality of concrete based on the average rebound number or rebound index

Average Rebound Number Quality of Concrete
> 40 Very Good
30 -40 Good
20-30 Fair
< 20 Poor and/or Delaminated
0 Very Poor and/or Delaminated

 

For more information watch this video

 

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