Rebound hammer test (Schmidt Hammer) is used to provide a convenient and rapid indication of the compressive strength of concrete. It consists of a spring controlled mass that slides on a plunger within a tubular housing.
The operation of rebound hammer is shown in the fig.1. When the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass with a constant energy is made to hit concrete surface to rebound back. The extent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A concrete with low strength and low stiffness will absorb more energy to yield in a lower rebound value.
Fig: Operation of the rebound hammer
The rebound hammer test method is used for the following purposes:
(a) To find out the likely compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.
(b) To assess the uniformity of concrete.
(c) To assess the quality of concrete in relation to standard requirements.
(d) To assess the quality of one element of concrete in relation to another.
Rebound hammer test method can be used to differentiate the acceptable and questionable parts of the structure or to compare two different structure based on strength.
Principle of Rebound Hammer Test:
Rebound hammer test method is based on the principle that the rebound of an elastic mass depends on the hardness of the concrete surface against which the mass strikes. The operation of the rebound hammer is shown in figure above. When the plunger of rebound hammer is pressed against the concrete surface, the spring controlled mass in the hammer rebounds. The amount of rebound of the mass depends on the hardness of concrete surface. Thus, the hardness of concrete and rebound hammer reading can be correlated with compressive strength of concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.