A low cost, non-destructive test that would provide an immediate indication of the compressive strength of concrete in place was non-existent up until about a decade ago. In answer to this need a Swiss engineer, Dr. Ernst Schmidt, developed a concrete impact test hammer in 1948 and secured worldwide patents. Since it had been established that a relationship exists between the rebound of a steel hammer and the compressive strength of concrete, he developed a spring-loaded hammer to directly relate the magnitude of the is rebound to concrete compressive strength. There are four types of test hammers: the standard test hammer intended for general concrete construction; a smaller version intended for testing small concrete products and impact sensitive areas; a test hammer intended for use on mass concrete and relatively thick sections such as airfield runways; and a pendulum type hammer designed for testing materials of low hardness and strength, including concrete of a cube strength of from 700 to 3,500 psi. New types for use under water and for automatically recording results are being developed. The points chosen for tests must be carefully selected to provide reasonably accurate results. Ideally, the spot to be tested should be a concrete surface that is smooth and uniform, preferably one cast against a form. Areas exhibiting honeycombing, scaling, rough texture or high porosity should be avoided. The impact of the test hammer would be affected by the elastic deformation of concrete sections less than 4 inches thick; therefore, such sections should be backed up with a heavy mass. Small air pockets near the surface being tested result in low rebound values, while tests made over a hard aggregate will result in a high rebound value. As a result of these variables, it is advisable to take about 15 impact reading in any one area to arrive at a reasonably representative rebound value. The compressive strength is then determined by using the best 10 out of the 15 readings.