| As an important building material,concrete is widely used in residential building,road,bridge,dam and other civil engineering projects,play an important role in the field of civil engineering.In the field of civil engineering structure,the failure of concrete structure does often appear the phenomenon of low stress damage,but not lie in reaching the maximum compressive strength of materials.Beside static loads,concrete structures also sustain complex dynamic and repeated loads,causing damage and fracture failure of the structures,inducing a problem of reduction of bearing capacity and durability.Therefore,this study shows that the fracture behavior of concrete depends largely on the characteristics of its fracture process zone(FPZ).So,it is of great significant to study the properties of FPZ and the mechanism of crack propagation of concrete under cyclic loading,to investigating the law of crack development during loading and unloading process,and establishing the corresponding mechanisms of crack propagation,stiffness degradation and dissipated energy.Three-point bending tests were performed on pre-notched beams to investigate the fracture properties and crack propagation of concrete under cyclic loading.The electronic speckle pattern interferometry(ESPI)technique and digital image correlation(DIC)technique were used to measure the full-fields displacement of the beams.In addition,the clip gauge was installed at the mouth of the pre-notch to measure the crack mouth opening displacement(CMOD)under cyclic loading.LVDTs were installed on both sides of the pre-notch to obtain the mid-span deflection of beam.This study has obtained the load-displacement curves of concrete beams,the full-field horizonal displacement and crack propagation,crack opening displacement(COD)and crack length,the critical crack mouth opening displacement(CMOD_c)and the critical crack tip opening displacement(CTOD_c),fracture energy and dissipation energy.The influence of loading history on the fracture properties of concrete were analyzed.The results indicate that:compared to the results by clip gauge and LVDTs,ESPI technique and DIC technique can provide accurate deformation measurement,verifying the reliability of two technique in fracture test of concrete under cyclic loading.The load-displacement curves of concrete beams under monotonic loading are nearly an envelope of that under cyclic loading.The load-displacement curves of each loading cycle can be divided into five stages.In both loading and unloading process,elastic deformation responses earlier than inelastic deformation and irreversible deformation occurs due to existence of the FPZ.The CMOD_c increase with the cyclic loading history while the CTOD_c is almost constant which can be used as a fracture characteristic of the material.The cumulative dissipative energy tends to be constant as the loading cycles increase and can be used as a fracture parameter under cyclic loading.In addition,this study also carried out three-point bending tests of high-strength(C80)concrete under cyclic loading.ESPI technique was used to measure the full-fields displacement of the specimens.The crack propagation in FPZ of high-strength concrete was analyzed.Compared with the fracture properties of ordinary concrete,influence of material strength on nominal stiffness and dissipative energy of concrete were analyzed.The analysis indicates that:The nominal maximum crack opening displacement w_c of high-strength concrete is about 13.7?m.The crack tip is no significant change at the same load during the loading-unloading stage.The strength of concrete has obviously effects on the initial stiffness.However,as the increase of the number of cyclic loading,the strength is no longer the main factor of its nominal stiffness.The fracture energy of high-strength concrete is about 130.3 N/m,and the cumulated dissipated energy reaches a plateau about 43.2 N/m.The fracture and dissipated energy of high-strength concrete are both higher than that of ordinary concrete. |
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