A rebound hammer is an instrument to determine the strength of rock or concrete. Since this is an essential element of structural design and is specified for compliance purposes, it is vital to know the strength of the concrete. This hammer is also known as the Schmidt Hammer in honor of its inventor Ernst O. Schmidt who invented this measuring device in the 1950s.
The rebound hammer test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. When testing the impact bolt of the rebound hammer strikes the surface of the concrete with a defined force. After the impact, a metal body, the so-called hammer rebounds. The rebound distance is measured by a sensor and passed on to the display unit as an electrical signal.
6 Steps to measure the strength of concrete with a rebound hammer:
- Use a grinding stone to smoothen the test surface.
- Perform a few test impacts with the rebound hammer on this smooth, hard surface before taking any measurements which you are going to evaluate. Perform a consistency check on the test anvil.
- Ensure all settings are correctly done.
- Position the hammer perpendicular to and against the test surface. Push the concrete test hammer against the surface at a moderate speed until the impact is triggered. Be aware that the plunger generates a recoil when it deploys. Therefore it is recommended to hold the rebound hammer with both hands, perpendicular to the test surface, before triggering the impact.
- Each test surface should be tested with at least 12 impacts dropping the highest and the lowest and then take the average of the remaining. The individual impact points must be spaced at least 25 mm apart. The British Standard BS 1881: Part 202 advices to test on a grid pattern with a spacing of 20 to 50 mm.
- Interpret the test results.