Experimental Study On The Bond Behavior Between Corroded Reinforcement And Concrete At Elevated Temperatures

The bonding properties of reinforcing bars and concrete significantly influences the deformation capacity and bearing capacity of reinforced concrete(RC)structures,so it is significant to investigate the bonding properties for structural design and safety evaluation.To accurately evaluate the bearing capacity of corroded RC structures under fires,the research on bond degradation between corroded reinforcing bars and concrete at elevated temperatures is needed to be carried out.This study manufactures two size eccentric pull-out specimens to investigate the influence of temperature,corrosion degree,concrete cover thickness,and rebar position on the bond behavior at elevated temperatures.The corroded specimen was obtained by accelerating corrosion methods,and were heated to target temperature in an electric furnace and then subjected to pull-out test.The difference of bonding properties under different temperature states was studied by the law of change of bond behavior under high temperature steady state.First,the concrete compressive test at elevated temperature and post-fire was conducted,and then the eccentric pull-out test was conducted at elevated temperatures and post-fire,respectively.It has analyzed the influence of temperature conditions(at elevated temperature and after high-temperature exposure),corrosion degree(2%,5%,10%,and 20%),specimen size(150mm×150mm×150mm and150mm×150mm×300mm)and stirrup quantity(single stirrup and double stirrups)on the bond behavior.Finally,the two-phase Hill and Lorentz models were used for regression analysis.The bond strength formula and continuous bond-slip relationship were obtained;reliable suggestions for fire-resistant structural design were proposed based on the different points of bond behavior under high temperatures and after high-temperature exposure.The main research work of this study is as follows:(1)To study the difference in the degradation mechanism of corroded rebar and concrete after high-temperature exposure,this paper designed three groups(23 specimens)at the side corners and eight groups(65 specimens)of corroded specimens with reinforcements at the middle of the side edges.Considering the effects of different temperatures,corrosion degrees,concrete cover thickness,and reinforcement positions on the bond behavior,the interface between corroded reinforcement and concrete,the failure mode and the change regular of bond strength of corroded RC structures after high-temperature exposure are analyzed.According to test data of the τ-s(bond-slip)curve,the bond-slip constitutive relationship considering temperature and corrosion is proposed.A sectional result provides a theoretical basis for the finite element analysis of RC structures post-fire.(2)84 concrete compressive and 110 eccentric pull-out specimens were made to investigate the bond degradation between corroded reinforcement and concrete at elevated and after high temperatures.The high-temperature pull-out test analyses the influence of temperature conditions,corrosion degrees(2%,5%,10% and 20%),specimen size and stirrup quantity on bond behavior.The calculation formula of structural bearing capacity considering bond slip under fire is established by analyzing the bond strength of different temperature conditions.Finally,the calculation method of bond strength for corroded reinforced concrete at elevated temperatures is obtained.Segmented and continuous bond-slip constitutive models are proposed.The advantages and disadvantages of the constitutive relationship are compared and analyzed to obtain a more accurate constitutive equation describing the bond-slip relationship.(3)Based on the damage theory,the bond strength calculation model considering the size effect is established,and the bond-slip constitutive model is obtained using the nonlinear relationship.The stress characteristics of beam pullout specimens and cube pullout specimens are analyzed,and the calculation method is provided for the calculation of load capacity under fire considering the bond-slip relationship.A reliable model of the bond-slip constitutive relationship brought into the structural calculation is established based on the wedge action theory.(4)Based on the test results,the existing ultimate bearing capacity formula of RC beams is modified to obtain the ultimate load bearing capacity formula considering bond under corroded beam fire.In order to apply the pull-out test results to the structural calculation formula,the size effect and bonding coefficient on the structural bearing capacity are determined,and finally the existing concrete bearing capacity calculation formula is supplemented by comparing the theoretical,test,and simulation values.(5)Considering the influence of corrosion on bond-slip behavior,three-dimensional spring elements are set up to simulate the bond-slip behavior between reinforcement and concrete.First,the temperature field of the corroded beam is simulated by the finite element method.Then the maximum temperature of each dot element is extracted to simulate the deflection of the corroded beam under fire.The accuracy of the bond-slip constitutive model in this study is verified by ABAQUS simulation,which provides a reliable basis for the fire resistance design of structures.