Comparison of the mode I fracture toughness of different cemented paste backfill-related structures: Effects of mixing recipe

The mode I fracture toughness is an important property in evaluating the resistance of the laminar structure to the tensile failure. The various interfaces existing in underground cemented paste backfill (CPB) structures depict the potential failure planes. To better assess the stability of CPB structures, the semi-circular bending (SCB) test is conducted to compare the fracture toughness of the CPB matrix, the short-term CPB/long-term CPB interface, and the CPB/rock interface in this study. In addition, the effects of the mixing recipe of CPB (cement and fiber content, water-to-cement ratio) on the development of fracture toughness are investigated as well. The results obtained from the semi-circular bending test show that the mode I fracture toughness of the CPB body is generally higher than that of the interface structures owing to the denser capillary structure. Besides, the specimens prepared with a lower water-to-cement ratio and cement content experience a higher effective stress and thereby increasing the fracture toughness. Moreover, the addition of fiber increases the fracture toughness of the CPB body (due to the bridging effect) but decreases that of the interface structures by reducing the contacting area between diverse particles. These findings are significant to the stability analysis and the prediction of the stress distribution in CPB structures.