| The reinforced concrete buildings in the coastal areas often suffer from the combined action of chloride corrosion and load.At present,many bridges have been damaged by steel corrosion,concrete cracking and protective layer spalling,leading to some potential safety hazards,so it is urgent to reinforce and repair them.Fiber reinforced polymer(FRP)sheets are widely used in reinforced concrete structures due to light weight,corrosion resistance and convenient construction.It is worth pointing out that the reinforcement performance of FRP external bonding technology depends on the bonding properties of FRP-concrete interface,which is not only the key part of transferring stress,but also the key point.Therefore,the mechanical behavior of FRP-concrete interface has become one of the hot research topics in the engineering reinforcement.However,the short-term mechanical properties of FRP-concrete interface have been focused on,but the research on durability of FRP-reinforced concrete interface is still insufficient.Furthermore,impact of existing damage of reinforced concrete is lack of consideration on the the durability of FRP-concrete interface reports,which is inconsistent with the actual working conditions of the reinforced objects in the project,and greatly limits the application of the reinforcement technology in coastal areas.For these reasons,this study intends to consider the influence of existing corrosion damage of reinforced concrete and take the overlapped members with different damage degree reinforced by FRP as the research object to dicusse the durability of bond interface of reinforced concrete in chloride salt environment.By combining experimental research with theoretical analysis,the following main research contents are carried out:(1)FRP-concrete interface materials were studied in the laboratory for accelerated durability in a chloride salt environment(40~oC,5%NaCl).The accelerated corrosion tests in artificial simulated marine environment about four types of FRP sheets(three types of CFRP and BFRP),two types of epoxy adhesives with epoxy values of 0.53 and 0.40,and concrete were carried out.The effects of chloride environment on the mechanical properties of reinforced materials were analyzed and the results showed that:(1)The modulus of elasticity of CFRP and BFRP sheets does not change significantly with the corrosion time.Due to the hydrolysis of impregnated resin,the interface between fiber and resin is damaged,which results in the decrease of tensile strength and ultimate strain of FRP sheets.(2)With the increase of corrosion time,the tensile strength,modulus of elasticity and ultimate strain of two epoxy adhesives(E0.53 and E0.40)increased due to curing reaction,but then decreased due to hydrolysis reaction.It was found that the durability of E0.53 was superior E0.40.(3)With the increase of exposure time,the compressive strength and elastic modulus of concrete tend to increase.The main reason is that the hydration reaction of concrete occurs,which leads to the increase of compressive strength and elastic modulus.(2)Experimental study on artificial accelerated corrosion of reinforced concrete before FRP reinforcement.Deponded on the basic principle of electrochemistry,an artificial electrochemical corrosion device is designed to accelerate the corrosion of steel bars in reinforced concrete to promote the damage of reinforced concrete and produce along-bar cracks,which can simulate the corrosion damage of reinforced concrete in chloride environment.The results show that the reference electrode of reinforced concrete is less than-350mV after accelerated corrosion with constant current which can be predicted that the probability of corrosion of steel bar is 95%.In addition,the potential of reference electrode decreases firstly and then decreases with the time of corrosion.After electrochemical corrosion,there are five crack development modes in reinforced concrete,among which the crack along the reinforcement at the top of the specimens is mainly caused by excessive electric current.By comparing the relationship between crack width and actual corrosion rate of steel bar,it can be found that crack width can better predict the actual corrosion rate of reinforcing bar.(3)The single-shear test method was used to study the interface durability of damaged RC structures strengthened by FRP.The effects of different corrosion grades on the ultimate bearing capacity,failure mode,load-slip relationship and the evolution law of strain and stress of FRP-concrete interface were studied.The results showed that the performance of FRP reinforcement material can be fully exploited by grooving and bonding with FRP and the ultimate bearing capacity is increased.Besides the failure mode is FRP peeling while the concrete protective layer is debonding by direct bonding with FRP.Compared with the specimens reinforced FRP,the ultimate bearing capacity of the member after the corrosion was decreased.For the FRP reinforcement component with less steel corrosion,the failure mode changes from the concrete debonding to the adhesive-concrete interface after the corrosion.According to the analysis of strain and stress evolution during loading process,FRP stress gradually transfers from loading end to free end and the specimen showed obviously brittle failure.(4)A bond-slip model of FRP-concrete interface in chloride environment was established.By analyzing the existing bond-slip model,this study test results are fitted based on the Popovics model.By analyzing the regression coefficients of the bond-slip models of different corrosion grades,an empirical bond-slip model,considering the corrosion rate of steel and the environmental impact of chloride salt,was established.The results showed that the fracture energy of member with large degree of corrosion of the steel can be improved by the FRP reinforcement after crack treatment.Compared with the FRP-reinforced corrosion-damaged members,the fracture energy of the members after corrosion by FRP is significantly reduced(from2.788 N/mm~2 to 0.268 N/mm~2).(5)A theoretical analysis of the FRP-concrete interface strength model in a chloride salt environment was studied.According to the two failure modes of FRP fracture and FRP interfacial separation,experimental results and the Chen and Teng classical formula,the interface strength model considering the environmental impact of chloride salt was established and verified by the existing experimental data.The results showed that the interface strength model proposed in this study can better predict the FRP-concrete interface debonding in the chloride salt environment. |
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