The Schmidt hammer in rock material characterisation: Fid-Bau Portal

The Schmidt hammer in rock material characterisation

Aydin, A. / Basu, A.

The Schmidt hammer provides a quick and inexpensive measure of surface hardness that is widely used for estimating the mechanical properties of rock material. However, a number of issues such as hammer type, normalisation of rebound values, specimen dimensions, surface smoothness, weathering and moisture content, and testing, data reduction and analysis procedures continue to influence the consistency and reliability of the Schmidt hammer test results. The paper presents a critical review of these basic issues and results of tests conducted on granitic rocks of various weathering grades in the light of the conclusions of this review. It was found that a very good correlation exists between L and N hammer rebound values and that both hammers are fairly sensitive to the physical properties, particularly to dry density though less so to effective and total porosities. The N hammer, producing a lesser scatter in the data, proved to be more efficient than the L hammer in predicting uniaxial compressive strength and Young's modulus. The exponential form of the correlation curves was found to reflect microstructural changes during the course of weathering and the differences in the probing scales or mechanisms in the means of measuring these mechanical properties, and could be generalised to other crystalline igneous rocks. The possibility of predicting weathering grades from rebound values was also explored. The changes in the rebound values during multiple impacts at a given point produced a better indication of the weathering grade than a single impact value. It was concluded that increasing the impact energy and plunger tip diameter should significantly reduce the scatter in coarse-grained weathered rocks and hence improve the reliability of the Schmidt hammer as a rock material characterisation tool.