Fibers effects on the concrete damage, crack tortuosity andcrack recovery

Through the disc splitting testing, the exact crack width of concrete has been prefabricated. In this work, the effects of steel fiber(SF) and macro polypropylene(PP) fiber on tortuosity, recovery rate and damage of concrete cracks were studied. The crack widths at different locations were measured using the digital microscope (Supereyes) and Image Pro Plus (an image processing software). In order to analyze the relationship between ultrasonic velocity and crack width as well as damage variable factors of different specimens, the ultrasonic velocity passing through the concrete disc under splitting load was also measured. The experimental results show that the crack widening can be controlled by the fiber bridging effect, and the crack recovery and tortuosity of crack can be increased simultaneously. Compared with the concrete sample with a SF content of 25 kg/m³, the crack tortuosity of the sample with a SF content of 55 kg/m³ increases by 26.9%; the hybrid use of macro PP fibers and SF indicates significant positive hybrid effect on the increasing of crack surface tortuosity. The steel fibers acrossing the cracks may improve the propagation of ultrasonic wave and decline the loss of the ultrasonic velocity. The ultrasonic velocity may decrease with the increasing of the crack width, and there is an exponential relationship between the crack width and ultrasonic velocity, which can be used to illustrate the crack development of concrete matrix.