The rebound (Schmidt) hammer is one of the oldest and best known methods of comparing the concrete in various parts of a structure and indirectly assessing concrete strength. It is actually a measure of surface hardness. The rebound hammer should be considered as a means of determining variations of strength within a structure rather than an accurate means of assessing strength. The surface under test should be clean and smooth. Dirt or other loose material on the surface can be removed using a grinding stone prior to the test. Rough surfaces cannot be tested as they do not give reliable results.
If it is to be used as a means of assessing strength the correlation between rebound hammer reading and strength needs to be determined for the actual concrete under investigation. A procedure for doing this is given in BS EN 12504-2. The values obtained by using the hammer may be influenced by the particular operator, so care must be taken when considering results from different operators.