Effect of Temperature and Heating–Cooling Cycles on the Mode I, Mode II and the Mixed-Mode I-II Fracture Toughness of concrete

In this research, the effects of temperature and number of heating–cooling cycles on the mode I, mode II and the effective value of the mixed-mode I-II fracture toughness of concrete were investigated. In the first series, the effect of temperature was studied in a heating–cooling cycle at ambient temperature (25°C) and 60, 150, 200, 300, 500, and 700°C. The highest and lowest mode I, mode II and the effective value of the mixed-mode I-II fracture toughness were, respectively, observed at 150 and 700°C. In the second series of tests, the effect of number of heating–cooling cycles was investigated on the mode I, mode II and the effective value of the mixed-mode I-II fracture toughness of concrete specimens at 150°C and a crack inclination angle of 45°. According to the results, the mode I, mode II and the effective value of the mixed-mode I-II fracture toughness increased in the first cycle and decreased with increasing the number of heating–cooling cycles. As the crack inclination increased, the effective value of the mixed-mode I-II fracture toughness of the concrete specimens increased. The mode II fracture toughness increased up to a crack inclination angle of 45° and then decreased. Moreover, with increasing the crack inclination angle, the mode I fracture at the inclination angle of 0° was changed into the mixed-mode (tension–shear) fracture at inclination angles smaller than 28.8°. The mixed-mode tension–shear fracture was changed into the mixed-mode compressive–shear fracture at crack inclination angles larger than 28.8°.