| For concrete structures in the form of fracture failure,such as gravity dams,the deformations of the structures under sustained loading are mainly caused by viscoelastic characteristics of concrete and crack propagation,which may cause the structures to fail or their carrying-capacity to be reduced.Meanwhile,the cracks in the dams are inevitably initiated during its manufacturing and applications,due to the low tensile strength of concrete.Under low sustained loading(less than 30%peak load(Pmax)),the stress at the crack tip cannot drive crack propagation.Accordingly,the creep deformation of the structure is linear and mainly caused by the viscoelastic characteristics of concrete.Under high sustained loading(greater than initial cracking load,about 65%Pmax),the crack propagates and interacts with the viscoelastic properties of concrete,leading to the nonlinear creep deformation or the failure of the structure.Therefore,it is necessary to further study the time-dependent behavior and its effects on the fracture properties of concrete under sustained loading,so that the safety and durability of structures can be assessed comprehensively.Based on the above-mentioned considerations,the studies on the influence of sustained loading on the time-dependent fracture behavior and fracture properties of concrete were carried out in this study.The specific contents of the research work and main conclusions are summarized as follows:(1)The creep test under low(30%P max)and middle load levels(60%Pmax,initial cracking load)and the creep recovery test under middle load level were carried out on the three-point bend(TPB)specimens firstly,and then the following quasi-static loading tests were performed on the specimens to study the effects of creep and creep recovery on the fracture properties of concrete.Meanwhile,the creep,creep recovery and the following quasi-static loading tests were analyzed numerically,which could consider the stress relaxation occurred in the creep process.The results indicated that compared with the reference specimens under quasi-static loading,the initial cracking and peak loads of the creep and creep recovery specimens increased obviously,and the increments increased with the load levels and decreased with the increase of the recovery time.However,considering the effects of stress relaxation,the creep and creep recovery process had no effects on the initial and unstable fracture toughnesses.(2)To investigate the time-dependent crack propagation of concrete under sustained loading,the sustained loading tests were conducted on the concrete specimens under high load levels(90%Pmax,95%Pmax)until the specimens failed.Meanwhile,by considering the viscoelastic characteristics of the crac tip of concrete and fracture process zone,the numerical method taking initial fracture toughness as crack propagation criterion was established to analyze the time-dependent crack propagation for concrete under different load levels.By comparing with the experimental results,the numerical method was verified.The numerical results indicated that for concrete under sustained loading,there existed critical load levels.When the load levels were larger than the critical load level,the concrete would fail.The inflection points on the crack growth and CMOD rate versus time curves could be considered as important characteristics to predict the failure of concrete.(3)The sustained loading tests were conducted on the concrete beams under middle and high load levels(60%Pmax,70%Pmax,80%Pmax)at the first,and then the quasi-static loading tests were carried out on the creep specimens to study the effects of sustained loading on the residual loading capacity of concrete.Meanwhile,numerical simulations were carried out to analyze the effects of stress relaxation and crack propagation on the residual loading capacity of concrete.The results indicated that compared with the reference specimens,the residual loading capacity of creep concrete increased obviously,and the load increments increased with the load levels and decreased with creep time.Under high load levels,the crack propagated in concrete at the first,and then stabilized and the concrete specimens maintained stability eventually with a crack.The crack propagated in the creep process had no effects on the residual loading capacity of concrete.(4)The creep tests were carried out on the TPB specimens under middle(60%Pmax)and high(80%Pmax)load levels at the first,and then the digital image correlation method was employed to detect the crack propagation of creep concrete under quasi-static loading.By comparing with the reference specimens,the effects of sustained loading on the variations of crack propagation and FPZ were obtained.The results indicated that the variations of crack propagation and FPZ of concrete under the middle load level were similar to those of reference specimens,and the critical crack propagation length was approximately equaled to that of reference specimen.However,under high load level,the crack obviously propagated in the creep process,and then when the peak load was reached under quasi-static loading,the critical crack crack length was also approximately equaled to that of reference specimens.(5)Based on the numerical method of time-dependent crack propagation for concrete under sustained loading proposed in this study,the effects of low(less than 46.7 m),middle(50 m,52.5 m)and high(55 m)water levels on the time-dependent behavior of concrete gravity dams were analyzed.Meanwhile,for gravity dams maintained stability under sustained loading,the residual loading capacity was assessed.The results indicated that under low water levels,due to the stress relaxation,the stress at the crack tip decreased obviously,and the residual loading capacity after sustained loading increased obviously.Under middle water level,the crack in the dam propagated at the first and eventually maintained stability.Under high water level,the crack in the dam propagated gradually,and eventually entered into unstable propagation,leading to the failure of the dam. |
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