Rock mechanics engineers design structures built in rock for various purposes, and therefore need to determine the properties and behavior of the rock. The UCS of rocks is one of the important input parameters used in rock engineering projects such as design of underground spaces, rock blasting, drilling, slope stability analysis, excavations and many other civil and mining operations. ISRM (1981) testing of this mechanical property in the laboratory is a simple procedure in theory but in practice, it is among the most expensive and time-consuming tests. This calls for transportation of the rock to the laboratory, sample preparation and testing based on the international standards. In order to carry out these standard tests, special samples, such as cylindrical core or cubical samples, need to be prepared. Preparing core samples is difficult, expensive and time-consuming. Moreover, the preparation of regular-shaped samples from weak or fractured rock masses is also difficult. Under these circumstances, the application of other simple and low-cost methods to carry out the above tasks with acceptable reliability and accuracy will be important. Therefore, indirect tests are often used to estimate the UCS, such as Schmidt hammer, point load index and sound velocity. Indirect tests are simpler, require less preparation and can be adapted more easily to field testing (Feneret al. 2005).
The Schmidt hammer rebound hardness test is a simple and non-destructive test originally developed in 1948 for a quick measurement of USC and later was extended to estimate the hardness and strength of rock. The mechanism of operation is simple: a hammer released by a spring, indirectly impacts against the rock surface through a plunger and the rebound distance of the hammer is then red directly from the numerical scale or electronic display ranging from 10 to 100. In other words, the rebound distance of the hammer mass that strikes the rock through the plunger and under the force of a spring, indicates the rebound hardness. Obviously, the harder the surface, the higher the rebound distances. (Torabiet al. 2010; Schmidt, 1951).