Rebound hammer test is one of the most popular and simple non-destructive concrete tests. The Non-destructive test is the process of detection of defects in concrete without its destruction. The determination of concrete quality is necessary for both fresh concrete and hardened concrete. The rebound hammer test is the common test for determining quality for existing structures.
The rebound hammer is also known as the Schmidt hammer. It determines the hardness of the concrete, helps in repairing and quality checks of RCC structures. Moreover, It provides a simple and quick inspection of concrete structures.
Also Read : Non destructive tests on hardened concrete
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Relevant Codes – Rebound hammer test
- IS 13311 (Part 2): 1992
- BS 6089-81
- BS 1881-202
Objectives of Rebound hammer test
The main objectives of the Rebound hammer test include
- This determines the compressive strength of the concrete in terms of the rebound index.
- Also finds the uniformity of concrete.
- evaluates the concrete quality with standard requirements.
Test Procedure
- The concrete surface should be smooth and dry.
- The plunger is pushed against the concrete surface.
- The spring inside the device rebounds. It depends on the hardness of the concrete surface.
- The graduated scale shows the reading of extending.
- This is the rebound number or rebound index.
- The lower rebound value means low compressive strength. ie, they are weak structures.
- An average of six reading should be taken.
The correlation between the concrete strength and the rebound hammer shown in a graph is called the rebound hammer graph.
Correlation between compressive strength of concrete and rebound index.
It is essential to calibrate the rebound hammer before testing. For this, we test a specimen of 150mm * 150mm * 150mm size using a compression testing machine (CTM). Fix the load rate at 7N/mm2, when the hammer result is 2.2 Nm. Test the vertical surface of the specimen using a rebound hammer. The results from the CMT and the rebound hammer should not have a major difference.
Result Calculation of Rebound Hammer tests
The rebound number assess the strength of the structure. As the rebound number increases, the strength also increases. This depends on the
- Cement type
- Aggregate type
- Surface condition
- Age of concrete
- Moisture content, etc.
The quality of concrete concerning the rebound concrete is as follows.
| Average Rebound Number | Quality of concrete |
| >40 | Very Good |
| 30-40 | Good |
| 20-30 | Fair |
| <20 | Poor / Delaminated |
| 0 | Very Poor / Delaminated |
Thus the rebound number predicts the quality of concrete. The rebound hammer apparatus is very easy to use. Also, it determines the uniformity of the concrete surface. Thus it can be used for the rehabilitation of old monuments. It produces a convenient indication of compressive strength