Fracture properties of mixed mode I-II cracks in concrete after sustained loading

Abstract

To investigate the fracture properties of mixed mode I-II cracks in concrete after sustained loading, the basic creep tests were conducted on concrete beams with five initial mode mixity ratios under three-point bending (TPB) loading or four-point shearing (FPS) loading, two sustained load levels, 80 % of the initial cracking load and the initial cracking load, for a period of 90 days. After creep tests, the specimens were unloaded and immediately loaded up to failure under the quasi-static TPB/FPS loading. Meanwhile, the entire crack propagation process was monitored using the digital image correlation technique. The numerical simulations based on the Norton-Bailey creep model were conducted to capture the time-dependent mechanical response of concrete under sustained loading. The experimental and numerical results showed that the pre-crack did not initiate in the creep tests unless the applied sustained loading exceeded the initial cracking load observed under quasi-static loading. It can be attributed to the viscoelastic characteristics of the concrete, which caused a decrease in the average stress around the pre-crack tip. A load increment was necessary to compensate for the reduced stress, resulting in an increase in the initial cracking load and the crack resistance of the structures. Moreover, sustained loading showed no significant effect on the peak load, fracture angle, initial cracking angle and crack propagation path. It suggests that sustained loading primarily induced a viscoelastic response of the concrete around the pre-crack tip rather than promoting extensive crack growth. Therefore, it is reasonable to approximate crack propagation paths of the concrete after sustained loading by considering the paths observed under quasi-static loading.