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This study investigated the fracture properties and FPZ evolution of concrete/rock bi-material interface after low-level fatigue loading. Firstly, the fatigue loading tests were performed under the frequency of 1.0 Hz with respect to two upper limits less than the initial cracking load, i.e. 60 % and 70 % of the peak loads. After 80,000 loading cycles, the quasi-static loading tests were performed to measure the fracture behavior of concrete/rock bi-material interface. Thereafter, the DIC tests were conducted to observe the crack extension process and analyze the FPZ evolution. According to the comparisons of fracture behaviors between the fatigue specimen and control specimen, the effects of the low-level fatigue loading on the fracture properties and FPZ evolution of concrete/rock bi-material interface were discussed. The results indicate that the unloading paths follow the loading paths coincidentally for each loading cycle, and this means that the mechanical response of the concrete/rock bi-material interface under low-level fatigue loading is elastically dominated. After the low-level fatigue loading, the initial cracking load and initial fracture toughness exhibit obvious increasing tendency, whereas the unstable fracture toughness decreases obviously, which is induced by the significant decrease of the critical crack length. In addition, the fully-developed FPZ length and the corresponding crack length of fatigue specimen are less than that of control specimen, indicating that the brittleness of concrete/rock bi-material interface becomes larger after low-level fatigue loading. [ABSTRACT FROM AUTHOR]

This study proposes an effective method to determine the stress intensity factors (SIFs) of the interface crack considering the viscoelasticities of the bilateral materials and verifies the feasibility of the initial fracture toughness-based fracture criterion for the concrete-rock interface under sustained loading. Firstly, low-level sustained loading tests were conducted on the composite concrete-rock specimens with respect to two interface roughness degrees and two sustained load levels less than the initial cracking load. Then, an implicit creep equation was employed to characterise the viscoelasticities of bilateral materials and then calculate the mechanical responses of the concrete-rock interface in the low-level sustained loading test. The SIFs of the interface crack under sustained loading were determined by using the elastic strain energy density ahead of the crack tip. By averaging the SIFs over the radius of the core region of the crack tip, the representative SIFs under sustained loading were obtained. The test results indicated that the SIFs showed decreasing tendencies under sustained loading, and the initial fracture toughness can be considered as a material property of the concrete-rock interface. The initial fracture toughness-based fracture criterion was verified to be feasible to determine the initial cracking status of the concrete-rock interface under sustained loading. In addition, due to the reductions of the SIFs under sustained loading, additional loads were required to compensate the losses of the SIFs, leading to the enhancement of the initial cracking loads after experiencing the sustained loading. [ABSTRACT FROM AUTHOR]

• Analytical expressions of the interfacial stress intensity factor and the critical crack length were derived. • With the increase of the interfacial roughness, the full fracture process length decreased. • The ratio of the fracture energy to the tensile strength for interface approximately remained constant. • With the increase of the interfacial roughness, the interfacial brittleness became larger. To investigate fracture properties and find analytical solutions of fracture parameters at rock-concrete interface, three-point bending tests were performed on the composite rock-concrete specimens with different degrees of interface roughness. Numerical simulations were conducted to analyse the crack propagation and the fracture process zone evolution at rock-concrete interface. For convenient applications in practice, a series of analytical solutions were derived or curve-fitted to calculate the fracture parameters of rock-concrete interface. The results indicate that the roughness degree has significant effects on the fracture properties of rock-concrete interface. With the increase of the roughness degree, the fracture energy, the critical crack length and the double- K fracture parameters exhibit obvious increasing tendencies. Meanwhile, the ratio of the unstable fracture toughness of rock-concrete interface to concrete is much less than that of the initial fracture toughness, indicating the much weaker cohesive effect of rock-concrete interface compared with concrete. In addition, with the increase of the roughness degree, the length of the fully formed FPZ and the corresponding ratio of the crack propagation length to the ligament length decrease. The characteristic length of rock-concrete interface is proved to decrease with the increase of the roughness degree, indicating larger brittle behaviour with the increase of the roughness degree. Finally, a series of analytical expressions were curve-fitted or derived to calculate the fracture parameters of rock-concrete interface under three-point bending loading, including the mode-I stress intensity factor, the critical crack length and the characteristic length of rock-concrete interface. [ABSTRACT FROM AUTHOR]

• Under different exposure conditions, the initial fracture toughness of concrete increases but the unstable fracture toughness of concrete decreases with the increase of exposure duration. • The stress-free crack opening displacement decreases largely under exposure conditions, indicating the brittleness of the concrete becomes stronger. • The sulphate ion contents under dry-wet cycling are obviously higher than those under the other exposure conditions, demonstrating that dry-wet cycling has the most serious sulphate attack. The fracture properties of concrete subjected to sulphate attack are investigated. First, concrete beams were subjected to different exposure conditions, i.e., full immersion, partial immersion and dry-wet cycling, for 90, 180 and 270 days, and a subsequent three-point bending test was conducted. A softening constitutive relationship of concrete was then derived and a simplified bilinear model was proposed for different exposure conditions. Finally, a chemical titration test was conducted to obtain the profiles of sulphate ion contents along the depth of concrete. The results indicate that, when concrete is subjected to different exposure conditions, the initial fracture toughness increases but the unstable fracture toughness decreases with the exposure duration. The stress-free crack opening displacement in the simplified bilinear model decreases significantly, indicating that concrete attacked by sulphate becomes more brittle. In addition, the sulphate ion content decreases rapidly along the depth and the sulphate ion content in concrete under dry-wet cycling is obviously higher than those under the other exposure conditions, demonstrating that dry-wet cycling results in the most serious sulphate attack on concrete. [ABSTRACT FROM AUTHOR]