Development of a mandrel indentation test to evaluate surface hardness of dimensional building stones and comparison to Vickers hardness

The present research introduces a new method namely mandrel indentation test (MIT) to evaluate surface hardness of rocks and building stones and comparing its results with the Vickers hardness test results. For performing the test, first, its apparatus was designed and built from hard and rigid steel in a turning workshop. Then, it was used on 50 common carbonate building stones including 30 limestones and 20 travertines in the laboratory. Also, surface hardness of the stones was evaluated by the Vickers hardness test. The values of mandrel indentation hardness (MIH) for the building stones were obtained between 31.75 and 113.85 kgf/mm2, and the values of Vickers hardness (HV) were obtained between 286.11 and 1361.98 kgf/mm2. The developed correlation analysis results together with the obtained Pearson, Spearman, and Kendall correlation coefficients indicated that the values of the surface hardness resulted from this approach is strongly correlated to the values of the Vickers test by a direct linear relation. The stablished simple regression analysis has confirmed the results of the correlation analysis by a determination coefficient of 0.841. Five well-known statistical indexes were used to assess accuracy performance degree of the developed regression model. As an important finding of the research, the introduced approach has high potential and accuracy for evaluating surface hardness of the studied building stones. Finally, a classification system was proposed to classify different building stones based on their surface hardness. Based on the classification, the studied building stones are classified as very soft to moderately hard with low hardness ratio.