Bond Behavior Between Steel Bar And Concrete Under Sustained Load Coupled With Chloride Ion Erosion

Reinforced concrete structures are inevitably subjected to sustained load in the service process.For the reinforced concrete structures in coastal areas,they are often suffered from the chloride ion erosion.As the foundation of reinforcement and concrete work together——the bond behavior between steel bar and concrete will be deteriorated by the coupled effects of sustained load and chloride ion erosion.The concrete in reinforced concrete members usually cracks due to shrinkage,thermal effect and loading.The formation of cracks makes chloride ion,oxygen and moisture ingress into concrete more convenient and diffusion rate of chloride ion to the steel bar also increases,thereby accelerating the deterioration of concrete properties and the corrosion of the steel bars,which in turn has a significant impact on the bond behavior between the concrete and steel bar and the service life of the structures.A total of 32 reinforced concrete beam specimens are tested for a period of 120 days to investigate the individual and coupled effects of sustained load and chloride ion erosion on the bond behavior between steel bar and concrete.The main research contents are as follows:(1)The immediate slip between steel bar and concrete and the time-dependent slip during120 days of sustained loading were recorded.The time-dependent slip curve of reinforced concrete was obtained and its variation law was analyzed.The effect of sustained load level on immediate slip and time-dependent slip were discussed.Creep occurs between steel bar and concrete under sustained load,and the higher the grade of sustained load,the greater the slip.(2)Based on the relationship of slip and time between steel bar and concrete under the action of sustained load proposed by Franke,and considering the effect of chloride ion erosion,the computational formula of time-dependent slip between steel bar and concrete under the combined action of sustained load and chloride ion erosion was given.(3)The bond strength and bond stress strain of reinforced concrete beam specimens after120 days sustained load and chloride ion erosion was measured by flexural-bond test.The failure modes and crack characteristics of the beam specimens were observed,and the effects of sustained load and chloride ion erosion on the bond strength were analyzed.The results show that the sustained load and chloride ion erosion significantly reduce the bond strength of the beam specimens,and the higher the sustained load grade,the lower the bond strength.Thedamage caused by the coupling of sustained load and chloride ion erosion is greater than that caused by single factor.(4)The microstructure of the concrete at the interface was analyzed using a scanning electron microscope(SEM).In addition,considering that the reinforced concrete beam type specimen with cracks is used in this test,and the stirrup is provided in the test specimen.Therefore,the effects of initial cracks and stirrups on the bond behavior between steel bar and concrete were studied by12 reinforced concrete beam specimens.The failure mode of reinforced concrete beam specimens during the test was observed,and the effects of stirrups and initial cracks on bond strength and bond stress-slip curve were analyzed.The results show that the stirrup can effectively limit the development of cracks,improve the ductility of specimens,increase the friction force between steel bars and concrete and increase the mechanical bite force,thus enhancing the bond strength between steel bars and concrete.Moreover,the initial crack reduces the bearing capacity of the specimen,reduces the bearing capacity of the concrete to the steel bar,and thus greatly reduces the bond strength of the specimen.