A study of rock response to failure in the context of the bending properties and comparison with uniaxial tensile and compression behaviour

The phenomenon of rock bending occurs during underground exploitation, construction of underground excavations and tunnels, and even rising heading-shafts. It is also common in building engineering, e.g., in the case of floors. Rocks and concretes as granular materials on the aggregate scale are fractured as a result of exceeding shear and tensile strength. In a complex state of stress-bending, crack propagation occurs from tensioned to compressed fibres. Three-point bending tests of medium-grained quasi homogeneous and isotropic sandstone were tested for strength and deformation properties of rocks. The E deformability modules for compressed and tensioned fibres as well as strains at failure were determined. The results of three-point bending were compared with the results of uniaxial compression and direct tension. Clear differences were found in the values of strengths, moduli of deformation and strains at failure. The bending strength B of about 9.5MPa is almost 3 times greater than the direct tensile strength σT of about 3.2MPa and is 1/10 of the ultimate uniaxial compression strength σC . With three-point bending, the values of the moduli E are equal to: for tensioned fibres about 6.7GPa, for compressed fibres 14.6GPa; in uniaxial compression tests about 13.0GPa and in direct tensile tests 4.8GPa. Rock material was also failure at various strains values at the ultimate strength. In the case of three-point bending tests, the strains at failure were equal to: for tensioned fibres about 0.125%, and for compressed fibres 0.065%; in uniaxial compression tests ϵz were equal to about 0.63% and in direct tension tests 0.07%.