Abstract
In this paper, the fracture behavior and acoustic emission (AE) characteristics of reinforced concrete under mixed mode I-II load conditions are studied using three-point bending beams with a straight offset notch. Different offset ratios, different crack-depth ratios, and different sizes of reinforced concrete specimens and a set of concrete specimens as a control group were explored. The critical loads, the crack mouth opening displacements, the final failure angles, and the crack trajectory was recorded to analyze the fracture behavior of specimens under different factors. The 3D crack source locations by AE technique was adopted to locate the damage during the fracture process, which was in a good agreement with the crack propagation path obtained through the fracture test. A criterion, based on the simultaneous appearance of the rise of RA (rise time/peak amplitude), the decline of AF (counts/duration time), and Ib-value (ratio of weak events to strong events), was proposed to identify the three critical conditions in the mixed mode I-II fracture process, which include the initial crack at the notch tip, crack at the bottom of mid-span, and the peak load. Then, the fracture process was divided into four stages: initiation of micro-crack, stable crack propagation in a single path, stable crack propagation in double paths, and unstable crack propagation. The AE hits can accurately distinguish the moments of crack at mid-span and peak load. The modified AE parameters analysis method was utilized to classify the cracking mode at different stages of damage and the results showed that the shear cracks followed the tensile cracks, the proportion of AF to RA for common specimens in this study can be set to 80: 1, while that of small size specimen with a span length of 600 mm can be set to 40: 1. The results can provide a theoretical basis for the establishment of remote monitoring and early warning systems for large structures.
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