ISRM Suggested method for determination of the Schmidt hammer rebound hardness: Revised version

With its portable, simple and affordable attributes, the Schmidt hammer (SH) is an ideal index apparatus, which underlies its increasing popularity and expanding range of applications. The SH rebound hardness value (R) is perhaps the most frequently used index in rock mechanics practice for estimating the uniaxial compressive strength (UCS) and the modulus of elasticity (E) of intact rock both in laboratory conditions and in situ. The SH is also widely used for estimating the UCS of discontinuity walls and assessing the workability, excavatability and boreability of rocks by mechanical means (cutting, polishing, milling, crushing and fragmentation processes in quarrying, drilling and tunneling). In the three decades since the earlier ISRM suggested method for conducting the SH test was published [1], researchers have sought to establish correlations between the SH rebound values (R) and the UCS and E for different rock types. A critical review of the basic issues was recently conducted by Aydin and Basu [2], which considered the influence of hammer type, the direction of hammer impact, specimen requirements, weathering, moisture content and testing, data gathering/reduction and analysis procedures. Understanding the operation of the apparatus and the mechanisms and modes of indentation upon hammer impact are crucial in addressing these issues, determining how the data scatter can be reduced, and settling upon an acceptable or expected degree of scatter.