FRACTURE TOUGHNESS OF BASALT-FIBER REINFORCED CONCRETE SUBJECTED TO CYCLIC FREEZING AND THAWING

The law of horizontal displacement and strain strange in the fractured process of concrete beams was analyzed through the real-time observation of deformation to the incision concrete in a three-point bending test by using a digital-image-correlation method. Based on the law, the crack loads could be determined. The effects of freeze-thaw cycles and the fiber content on the fracture toughness, the instability of toughness, and the critical opening displacement, were studied. The results show that for grade C30 concrete, the fracture toughness and instability toughness, which reduced by 0.6 MPa·m^1/2~0.80 MPa·m^1/2, decrease with the freeze-thaw cycles, and that to the contrast, slightly increase about 0.1 MPa·m^1/2 and 0.2 MPa·m^1/2 with the mixed basalt-fiber volume. And basalt fiber can improve the frost resistance of grade C30 concrete, but the degree of increase is limited. Basalt fiber cannot completely inhibit the damage of concrete subjected to freezing and thawing. The raise of instability toughness with basalt-fiber volume is larger than that of fracture toughness. The crack and instability toughness gain ratio, which were the largest with 2.0 kg/m³ fiber content, was increased first and then decreased. Therefore, 2.0 kg/m³ can be used as the optimal fiber content of strength grade of C30 concrete. Critical opening displacement increases with the freeze-thaw cycles, but not obviously with the fiber. Compared with the effects of the fiber, the significant factor of the deformation in the process of fracture is the damage of freeze-thaw cycles. Finally, according to the experiment, the fitting model is established for the crack and instability toughness with the fiber content and freeze-thaw cycles.