Study On The Bond Behavior Between GFRP Bar And Marine Concrete Under Sustained Load

Reinforced concrete structures serving in the marine environment will inevitably be corroded by corrosive medium environment such as chloride ion corrosion,causing corrosion of the internal steel bars,leading to cracking and peeling of the concrete protective layer,and ultimately affecting the bearing performance of reinforced concrete structures.As a new type of material,GFRP bar has the advantages of high strength,low weight and corrosion resistance.Therefore,replacing reinforced concrete with high durability and corrosion resistance marine concrete in the marine environment can avoid the corrosion of reinforcement caused by chloride ion corrosion.Good bond performance is the basis for the reinforcement and concrete to give full play to their respective properties.However,due to the differences in material properties and surface morphology between GFRP bar and ordinary ribbed reinforcement,it is necessary to study the bond performance between GFRP bar and concrete;In addition,reinforced concrete structures will inevitably bear sustained load in service.In this paper,16 GFRP bars concrete beam specimens with different load levels were tested for 120 days.The main research work is as follows:(1)Through 120 d load holding test,the instantaneous slip and time-varying slip of GFRP bar and marine concrete beam specimen during load application and 120-day load holding were tested,the relationship between the slip between GFRP bar and marine concrete and the time of load holding was studied,and the effect of sustained load on the instantaneous slip and time-varying slip between GFRP bar and marine concrete was analyzed,The magnitude and variation of the time-varying slip of GFRP bar and marine concrete and ordinary reinforced concrete under sustained load are compared and analyzed.The test results show that the time-varying slip value increases nonlinearly with time,increases rapidly in the early stage,and tends to be stable in the later stage;Instantaneous slip and time-varying slip increase with the increase of continuous load value;Due to the uneven distribution of bond stress in the direction of bond length,the slip value at the free end is always less than that at the loaded end;The time-varying slip between GFRP bar and concrete is similar to that between reinforcement and concrete,but the slip value between GFRP bar and concrete is greater than that between ordinary reinforcement and concrete,about seven times of that between reinforcement and concrete.Referring to the time-varying slip model between reinforcement and concrete under continuous load,and considering the characteristics of GFRP bar,a time-varying slip model between GFRP bar and marine concrete under continuous load is established.(2)Through flexural bond tests,the failure mode,slip value,and bond strength of GFRP bar concrete beam specimens subjected to 120 days of sustained load were obtained.The bond slip curve of GFRP bar concrete beam specimens was plotted,and the effect of sustained load levels on the bond strength between GFRP bar concrete and marine concrete was analyzed.The results show that all of the specimens are pull-out failures,and the bond strength between GFRP bars and marine concrete decreases significantly after sustained load.The higher the sustained load level,the smaller the bond strength.With reference to BPE model,continuous curve model and Hao Qingduo model,the bond stress-strain constitutive relationship between GFRP bar and marine concrete under continuous load is established.(3)The flexural bond tests were carried out on 4 GFRP bar and marine concrete beam specimens without stirrups,the failure modes of the specimens were observed,and the bond stress and slip values of the beam specimens were measured;Through the comparison and analysis of the bending bond test results of the GFRP bar and marine concrete beam specimen with stirrups,the following conclusions are obtained: the failure mode of the GFRP bar and marine concrete beam specimen without stirrups is pull-out failure,and the pull-out failure only occurs on the half beam on one side of the beam specimen;In the process of applying external load,the sliding occurs at the loading end of the specimen first,and the sliding at the free end of the specimen is relatively lagged;As the load continues to be applied,the slip value between the loaded end and the free end gradually approaches;Stirrups have little influence on the bond strength between GFRP bars and marine concrete.