Experimental study on acoustic emission and surface morphology characteristics of concrete under different fracture modes

In this work, experiments were employed to study on fracture behaviors of concrete under different fracture modes using cracked straight-through Brazilian disc (CSTBD) specimens, focusing on revealing the characteristics of acoustic emission (AE) and surface morphology. The results show the failure modes of CSTBD specimens under different fracture modes contain tensile coalescence, mixed coalescence, as well as shear coalescence. Based on the AE count, peak frequency and stress, the loading process can be dived into four stages (i.e., microcrack closure, linear elastic deformation, plastic deformation, post-peak failure). The b value can be considered as a precursor index of concrete structure instability. A novel method is proposed to distinguish the diving line of RA-AF distribution based on the splitting tensile test and direct shear test, and the evolution trends of the microcrack propagation modes with different curing regimes are consistent. Meanwhile, as the fracture mode moves from pure mode I towards mixed-mode I-II and pure mode II, the fracture surfaces become uneven and rough, the fractal dimension presents an increasing tendency, the standard deviation of asperity height and slope angle exhibits an increasing tendency, and the distribution of the asperity direction frequency becomes disorganized and fluctuates widely. In addition, according to the proposed 2D micro-element model, tangential deformation plays a crucial role in the fracture process, and a larger tangential deformation leads to a rougher fracture surface.