Mechanical Behavior and Energy Dissipation of Sandstone-Shotcrete under Monotonic and Cyclic Triaxial Loading

The mechanical properties of tunnel support structures have been widely studied by researchers, but there are insufficient studies concerning energy mechanism and mechanical properties in the failure process of concrete and rock as binary composites. Based on this, this paper first analyzed the characteristics of stress-strain curves of sandstone ordinary concrete (SNC) and sandstone fiber-reinforced concrete (SFC) in disparate confining stresses. Next, the volume strain, dynamic elastic modulus, elastic-plastic strain, brittleness index, damage constitutive model, inward friction angle, and cohesion of specimens were explored under monotonic and cyclic loads. Finally, the failure mode and triaxial loading energy mechanism were studied based on computerized tomography (CT) detection. The consequences expounded that triaxial compression failure of sandstone concrete could be divided into four stages, and the compression of the specimen changed to expansion during loading. The elastic modulus of specimens increased when the confining pressure increased and decreased with the increase of fiber. As the confining pressure increased, the plastic strain of sandstone concrete at the final failure increased. The fiber was harmful to the growth of the elastic concrete modulus and the elastic modulus first increased and then decreased. With the confining pressure increasing, the brittleness index of concrete was smaller, and the fiber could efficiently prevent the concrete from cracking. As the strain increased, the rule of change with the increase of axial strain was "S". As the confining pressure decreased, the growth rate of the damage constitutive model rose variably. The cohesion of SFC was greater than SNC based on the analysis of the evolution of basic parameters. The pore and crack area after SNC compression failure was larger and deeper than that of SFC after compression failure. When the peak stress of specimens was maximal, the density of the elastic energy reached the maximum. The density of dissipated energy increased sharply after the peak stress.