Effect of grain size or anisotropy on the correlation between uniaxial compressive strength and Schmidt hammer test for building stones

To provide guidance for using Schmidt hammer rebound number (N-type hammer: R-N) to predict uniaxial compressive strength (UCS) of heterogeneous building stones, this paper using sandstone (with fine or coarse grain size) and gneiss (with 0 degrees, 45 degrees, 90 degrees inclined anisotropy) respectively explored the effect of grain size or anisotropy on the correlations of UCS-R-N. Based on the regression analysis, no significant formula of UCS-R-N can be developed for samples with vertical anisotropy. The results show that coarse grain size or 45 degrees inclined anisotropy results in increasing discreteness of UCS and R-N data. Then, grain size variation or multidirectional anisotropy deteriorates the statistical performance of equations between UCS and R-N. Finally, the accuracy of estimated UCS through R-N declines significantly due to the varying grain size or anisotropy orientation. Using the empirical equations ignoring grain size or anisotropy can yield unacceptable error for calculating UCS through R-N. The present finding, thus, implies that the Schmidt hammer test should be performed on building stones with single grain size magnitude or unidirectional anisotropy. Also, the Schmidt hammer test parallel to the anisotropy should be used with much care for predicting UCS as no significant correlation. (C) 2021 Elsevier Ltd. All rights reserved.