The Effect of High Temperature Treatment on the Mechanical Behavior of Rocklike Samples with Varying Joint Infill Conditions under Different Strain Rates

Abstract

The transition in the mechanical behavior of heat-treated intact rock samples and rock samples containing joints with different types of infill materials was investigated at varying strain rates. Both dynamic compression [using split-Hopkinson pressure bar (SHPB)] and static uniaxial compressive strength (UCS) tests were carried out. A series of 114 SHPB tests were conducted on preheated samples under different temperatures (i.e., 30°C–300°C) for 5 h, under different strain rates (i.e., 53–130 s−1), along with static 57 UCS tests (at 10−4 s−1). Fracture initiation and progression in the samples during loading were continuously observed through high-speed imaging. The evolution of strain was assessed using digital image correlation analysis. Rate dependency existed in the strength behavior of all samples, with a general trend of increasing strength as strain rate increased. However, the rate dependency in the mechanical response of samples significantly depended on treatment temperature. Intact and epoxy-grouted samples showed higher rate dependency than unfilled and cement-grouted samples for temperatures lower than the transition temperature. With increasing temperature, the rate dependency in intact and epoxy-grouted samples reduced significantly and eventually aligned with that of ungrouted and cement-grouted samples. The fracture mechanism transitioned from being tensile-dominated to shear-dominated with increasing temperature in all samples, with crack-initiation and propagation showing significant dependence on the treatment temperature. The damage at the macroscopic level generally increased in all samples with increasing strain rates at all temperatures. More macrolevel cracks were observed in the samples at lower temperatures than those above the transition temperatures. Rocks can be subjected to high-temperature conditions in different structures, such as nuclear waste repositories or during open fires. Further, these rocks could also be subjected to dynamic loading, such as blast loading or rapid transport. The dual effects of temperature and loading rate can change the chemophysical characteristics of rocks, resulting in premature failure of rock-supported structures during service. The present experimental study captures the failure mechanism in rock under high strain rate loading and temperature variation. The obtained data from experiments could be used to analyze the response of rock structures and improve the design of support systems in the field with more accuracy and confidence. Further, the knowledge of the efficiency of grouts in improving the mechanical response of low-quality rocks under high-temperature and high-loading rate conditions is of utmost importance for the practical design of rock structures in the field. This is important because the mechanical behavior of grouts depends on the existing temperature of rocks and the loading rate, as observed in this study.