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To investigate the fracture property of the rock–concrete interface after sustained loading, creep tests were conducted on composite rock–concrete beams with two mechanical grooving interfaces, 4×4 and 7×7 interfaces, under two sustained load levels, 50% and 75% of the maximum load. After 90-day sustained loading, the creep specimens were unloaded and reloaded to failure in the subsequent three-point bending tests. The viscoelastic characteristic of the rock–concrete interface was numerically simulated by employing the Bailey–Norton creep law to describe the constitutive relationships of bilateral materials. Based on the numerical results, the elastic and creep strain energies within the whole specimen and the core region near the crack tip were calculated during the loading process. The results indicated that during the creep tests, the local elastic strain energy decreased due to the stress relaxation near the crack tip, and the local creep strain energy increased due to the accumulated creep strain. It was found that, at crack initiation status, the average elastic strain energy density in the core region after sustained loading was identical to that under static loading, which indicates that the elastic energy in the core region dominated the fracture behavior. Then, an energy-based fracture criterion was proposed to judge the crack initiation of the rock–concrete interface considering viscoelastic characteristics. It was found that the average elastic strain energy density of the rock–concrete interface was not affected by the viscoelasticity, which can be regarded as a material parameter. [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]

This paper explored the viscoelasticity-induced fracture behavior of rock-concrete interface after sustaining the creep process. Creep tests were firstly conducted on the composite rock-concrete specimens under three-point bending loading at the load levels of 50% and 75% of the maximum load. After 90-day sustained loading, composite specimens were unloaded and then reloaded under quasi-static loading conditions, and the digital image correlation technique was employed to quantify the crack profiles at the rock-concrete interface. The results indicate that the pre-set crack at the rock-concrete interface would not initiate further during the creep tests if the sustained load was below the initial cracking load. After sustaining a creep process, the initial cracking load increased obviously but the elastic strain energy near the pre-crack tip remained unchanged. The creep deformations during the creep tests made the characteristic lengths of the rock-concrete interfaces decreased largely, leading to the obvious enhancement of the interfacial brittleness. As a result, the interfacial nonlinear fracture behavior from the initial cracking state to the unstable fracture state would become weaker. In addition, the fully formed fracture process zone length and the corresponding crack length increased with the increase of the sustained load level, indicating that the boundary effect of the rock-concrete interface would decreased. [ABSTRACT FROM AUTHOR]