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Modified Direct Shear Test for Determining Shear Strength of Rock and Concrete

This article proposes a modification in the direct shear test (DST) to improve the boundary conditions of the test to avoid the undesirable tensile fractures and to promote a single horizontal shear fracture. The modified direct shear test (MDST) basically consists of adding two inclined notches in the test specimen of the DST. An ideal geometry was proposed based on numerical analysis and experimental tests on concrete. Experimental tests on homogeneous rocks, for low normal stresses, were performed with the ideal geometry, and it was obtained for most of the cases studied: (i) a single horizontal shear fracture (without tensile fractures); (ii) striated surface appearance in the most sheared area, and (iii) force versus displacement curve approximately linear with abrupt rupture. A summary and guidance for performing and verifying the MDST were provided. Through the proposed modification, a better shear strength test method for rock and concrete was achieved.