Influence of rock cradle block geometry on rebound hardness

The Schmidt hammer (SH) testing of cylindrical rock cores requires the use of a cradle block (core holder) to hold the cores firmly during the test. For this purpose, the related International Society for Rock Mechanics (ISRM)-suggested method and American Society for Testing and Materials (ASTM) standard suggest the use of an ‘arch-shaped’ or a ‘V-shaped’ cradle block. However, there is little or no detailed quantitative information in the literature regarding the potential influence of cradle block type on the measured rebound values (R values). With this in view, an investigation was carried out regarding the influence of cradle block geometry on R values, by employing the two cradle blocks specified in the preceding Standards. Using an L-type Digi-Schmidt hammer, SH tests were performed on NX-size cylindrical cores of 20 different rock varieties, both in ‘oven-dry’ and ‘wet’ state. Although the observed differences were usually small, results showed that the magnitudes of the mean rebound values determined in the arch-shaped block (R _A) were higher than those determined in the V-shaped block (R _V), except in one case. For the oven-dried samples, lowest and highest differences between the means of R _A and R _V values were 0.37 and 5.92, respectively. In the case of wet samples, the differences ranged from a low of 0.33 to a high of 4.70 in R-units. Statistically significant positive linear correlations were observed between R _A and R _V values, although the degree of correspondence was relatively lower for rocks having uniaxial compressive strength <50 MPa. The results of additional SH tests carried out on two different engineering materials also indicated that higher mean rebound values could be attained in the arch-shaped block. The results of the present study did not confirm the findings of a previous study where it was found that the V-shaped cradle block gave consistently higher mean rebound values than the arch-shaped one. The possible reasons for this discrepancy were discussed.