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A rebound hammer is an instrument used to measure the strength, resistance, or elastic properties of concrete.  The rebound hammer is a non-destructive testing instrument, where the rebound of the spring-driven stack is measured with the concrete surface after its impact on it. The result of the rebound hammer is mentioned as a rebound number and also correlates with the hardness of the surface of the concrete.

A rebound hammer is a concrete test hammer also known as Schmidt Hammers. Rebound hammers are flexible tools for evaluating the quality and strength of hardened concrete. It is majorly used to estimate compressive strength; rebound hammers can provide considerably more value. The complete assessment of concrete structure is somewhere more than just strength. The rebound hammers are easy to use and portable.

The Rebound hammer is a non-destructive testing mechanism where the rebound of spring-driven mass is calculated with the impact on the concrete surface. The rebound hammer test can be performed in many arrangements; downwards, upwards, vertical, horizontal, and at any transitional angle. Rebound hammer testing provides a cost-effective solution and practical solution for evaluating concrete strength. Rebound hammer helps structural engineers and professionals in classifying concrete strength accurately and make decisions then accordingly.

A Rebound hammer comprises a spring-loaded hammer of steel which is when released hits the steel plunger which is in touch which is in contact with the surface of the concrete. The hammer loaded with spring travels with a reproductive and consistent velocity. The rebound distance is then measured on a linear scale attached to the mountings of the instrument.

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It is often important to test concrete and its structures after the concrete has hardened in order to determine that the concrete structure is suitable and appropriate for its required purpose. Concrete testing using a rebound hammer is done as per the guidelines of Indian Standards IS-13311.

Advantages of rebound hammer test are-

• Rebound hammer is simple and easy to work.
• Inexpensive and cost-effective method to perform.
• It is a fast and portable procedure.
• 100% non-destructive method.

A rebound hammer helps in determining the uniformity of concrete, verifies the quality of concrete depending upon standard specifications, and relates concrete elements with others. The hammer test can also be performed to differentiate and compare questionable and acceptable parts of the concrete.

Factors affecting the performance of the Rebound hammer test are-

• The smoothness of the concrete surface is to be tested.
• The concrete surface should be clean and dry.
• Internal moisture of concrete.
• Loose particles should be removed from the concrete using a grinding stone or wheel before starting the test.
• Type of cement, mold, and other materials used to make concrete.
• Type of coarse aggregate.
• Age of Test specimens.
• Carbonation of the surface of the concrete.
• Age of the concrete (concrete should be 14 to 56 days old).

The rebound hammer test is constructed on the principle that rebound of mass depends on the strength and hardness of the concrete surface on which mass of hammer strikes. When the plunger of the hammer is pressed against the surface of concrete, the mass of spring controlled in the hammer rebounds. The quantity of rebound of mass depends on the hardness and strength of the surface of the concrete. Thus, the hardness and strength of rebound hammer and concrete results can be correlated with the compressive strength of concrete.

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