Fracture toughness of rock-concrete interfaces and its prediction based on acoustic properties

He relevance of the subject is determined by the need to solve the problem of ensuring the safety and prevent failure of facilities containing an interface between rock and concrete. These include mine shafts, hydroelectric dams in mountainous areas, reinforced concrete tunnel supports and others that are subjected to both static loads from overlying rocks and soils and dynamic loads from explosions and earthquakes. We perfomed laboratory tests according to the International Society for Rock Mechanics (ISRM) methodology on specimens with interfaces between gypsum stone and sand-cement mortar. The fracture toughness coefficient KIC of the interfaces in the specimens was investigated. The cylindrical specimens were 40 mm in diameter and 150 mm long with a V-shaped notch in the middle part. The specimens bending strain measured using a three-point pattern allowed the KIC to be determined based on the maximum force at 5-6 cycles. The average KIC value for interface between rock and concrete proved much lower than that for rock and even for specimens made entirely of concrete. For the specimens without concrete, the average value was 1.327 MPa×√m, and for fully concrete specimens, 0.858 MPa×√m. The average value KIC for the specimens with concrete was 0.323 MPa×√m, which was 4 times lower than that for the specimens without concrete and 2.5 times lower than that for the concrete specimens. The formation of a calibrated fracture during testing results in a relative increase in the internal mechanical loss factor Q-1, determined by the resonance method, by up to 30%. This allows estimating KIC fracture toughness coefficients of rock-concrete interfaces using Q-1. The obtained results can be used in actual practice in the design, operation, and organization of nondestructive testing and monitoring of industrial mining facilities that include these interfaces.