Influence Of Sustained Load On Fracture Properties Of Concrete

Due to the low tensile strength of concrete,cracks always occur in concrete structures during construction or in service.These cracks are likely to propagate over time and eventually lead to the failure of the structure when subjected to sustained loads.For quasi-brittle material such as concrete,crack propagation is closely related to the development of the fracture process zone（FPZ）.However,it exhibits time-dependent when concrete structures are subjected to sustained loads.There are two sources for the time-dependent behavior:the viscoelasticity of the bulk material within the structure outside the crack zone and the relaxation of softening behavior within the FPZ.Under this circumstance,the results from static tests cannot be available comprehensively for analyzing the fracture behavior of the structures under the sustained loads.Therefore,it is significant to investigate the effects of creep on the fracture properties,the tension softening law and the FPZ of concrete.For this reason,the thesis carries out the research as follows:（1）The influence of sustained load on fracture properties of concrete:Experimental studyFirstly,flexural creep tests were conducted on pre-notched concrete specimens.Two sustained load levels（30%of the peak load and the initial cracking load）and three different sizes（500 mm×100 mm×100 mm,900 mm×200 mm×100 mm and 1300 mm×300 mm×100mm）were considered in the creep tests.Experimental results showed that for specimens under30%of the peak load,crack did not propagate and the creep displacement increased with the size of specimen.However,micro-crack formed and propagated in specimens under the initial cracking load.After the creep tests,three-point bending（TPB）tests were conducted immediately to investigate fracture properties of specimens.Results showed that for the low load level（30%of the maximum load）,the influences of creep on initial fracture toughness K _ICⁱⁿⁱ and unstable fracture toughness K_IC^un was small for small size beams,but there was an increase of K_ICⁱⁿⁱ and K_IC^un for medium and large size beams.For the initial cracking load level,both the critical equivalent crack length a_c and K_IC^un increased.In addition,the influence of sustained load on fracture energy was insignificant and the value of K_ICⁱⁿⁱ was almost the same when the stress relaxation effect was considered.（2）Influence of sustained load on the tension softening constitutive relationshipFor comparison specimens and specimens under 30%of the maximum load,based on the load-deformation（P-δ）curves measured in the TPB tests,the energy dissipation and energy balance during the crack propagation process can be established through modified J-integral method.Hence,the tension softening constitutive law can be obtained.For specimens under the initial cracking load,micro-crack propagated in creep stage.Thus,in order to accurately calculate the tension softening constitutive law in this case,a stress-relaxation effect in the creep-induced micro-crack zone was considered.Based on the obtained exponential tension softening law,a bilinear expression was proposed for simplification.The results showed that the value of w_s and w₀ were not affected by creep for specimens under 30%of the maximum load,while w_s increased and w₀ decreased for specimens under the sustained initial cracking load.（3）Experimental study on the crack propagation under three point bending tests by using the digital image correlation methodThe digital image correlation method（DIC）was used to observe the development of FPZ in the TPB tests.For specimens under the initial cracking load,the residual deformation was considered when determining the crack opening displacement within the FPZ.The results showed that 30%of the maximum load had an insignificant effect on the crack length and the FPZ development.However,for specimens under the initial cracking load,before the full FPZ,crack length was large（about 6.5%）compared to comparison ones.After the full FPZ,crack length was almost the same as comparison ones,but the length of FPZ was comparatively small（about 7.1%）.（4）Numerical study on the crack propagation of specimens after creepBy using the tension softening law obtained in the TPB tests and considering the stress-relaxation effect in the creep-induced micro-crack zone,the crack propagation process was simulated by introducing an initial fracture toughness-based crack propagation criterion.The predicted load versus crack mouth opening displacement（P-CMOD）curves showed a reasonable agreement with the experimental ones,indicating the validity of the tension softening law and the numerical method used in this study.