The Experimental Study On Time-dependent Fracture Process Of Concrete Under Long-term Sustained Loading

As a kind of multiphase composite material,concrete is widely used in engineering construction. Due to the intrinsic defects and external factors, the concrete cracks exist widely in building structure, especially in the mass concrete structure in service. The development of the cracks under long-term sustained loading is directly related to the safety of concrete structure. Therefore, the study on the development of cracks and their influences on structure under long-term sustained loading have great realistic significances and engineering values. At present, the study concerning time-inpendent fracture in concrete has achieved plentiful results, and a variety of concrete fracture theories are proposed. However, because of the complexity of the test, there is little research on time-dependent fracture in concrete. In this paper, the test object size of standard three point bending beam is L×D B=515mm×100mmx 100mm. By combining experiment with theoretical analysis, the time-dependent fracture process in concrete under long-term sustained loading was studied. The main contents and conclusions obtained from this study included:1. The regular test of time-independent fracture in concrete was carried out. Also, the maximum load Pmax and related fracture parameters of concrete were obtained.2. The test of time-dependent fracture in concrete was conducted. By applying about 0.95Pmax,0.85Pmax,0.8Pmax,0.75Pmax,0.7Pmax constant load to concrete,the data such as time-dependent crack mouth opening displacement CMOD(t) in concrete under different long-term sustained loading were obtained.3. Based on the fictitious crack model, the time-dependent crack tip opening displacement CTOD(t), time-dependent equivalent crack length a(t), and time-dependent stress intensity factor K(t) were obtained. Meanwhile the laws of them were summarized.4. The concept and calculation method of time-dependent fracture toughness in concrete KIcun(P) were proposed. According to the laws of time-dependent crack mouth opening rate CMOR(t) on solving the extreme point,the critical state time of the time-dependent fracture in concrete could be determined. Thus,a time-dependent double-K fracture criterion of concrete was established:K<KIcini, the preformed crack does not crack;K=KIcini, the preformed crack begins to crack initially;KIcini<K< KIcun(P), the preformed crack develops steadily;K=KIcun(P),the preformed crack begins to crack unsteadily;K>KIcun(P), the crack propagates unsteadily.5. The effects of load level P/Pmax on the time-dependent fracture in concrete were analyzed: With the increase of the load level, the lifetime and time-dependent fracture parameter of concrete gradually decreased all. Combined with the time-independent test,the relationship of the critical fracture parameters between the time-dependent fracture and time-independent fracture in concrete was established.6.The fitting formulas of time-dependent cohesive toughness KC(t) and KR extension resistance curve KR(t) were obtained. And a time-dependent KR extension resistance criterion of concrete was established:K(t)< KR(t), the preformed crack develops steadily;K(t)=KR(t), the preformed crack begins to crack unsteadily;K(t)>KR(t), the crack propagates unsteadily.7.The result of the stability analysis about time-dependent crack propagation shows that the time-dependent double-K fracture criterion model and KR extension resistance curve model have the same principle.