Experimental Study On The Bond Behavior Of Corroded Reinforced Concrete Under Monotonic Or Repeated Loading

The bond resistance in RC structures represents the complex interaction between concrete and reinforcing steel bar.It directly affects the crack width,crack spacing,stress distribution,deflection and hysteretic behavior under seismic action of RC structures.Adequate bond between concrete and steel bar is essential for the performance and durability of RC structures.However,during the long term service life of RC structures the steel bar embedded in the concrete can corrode due to the chloride contamination or carbonation.The corrosion products have higher volume than the steel and can cause damage of concrete cover,and influence the bond properties.Once the concrete cover is cracked due to the expansive corrosion products,the bond properties become seriously deteriorated.The deterioration of bond resistance,which is the fundamental property of reinforced concrete,can directly lead to the increase of deflection and the reduction of load capacity of RC structures.To better evaluate the mechanical properties and the service performance of corroded RC structures,profound knowledge regarding the bond deterioration is needed.It is therefore of great significance to cast deep sight on the bond deterioration related problems.The present study has systematically investigated the bond behavior between normal concrete and deformed steel bars.The main research work of the present study can be listed as follows:(1)A kind of beam-end specimen was designed,and systematic bond tests were performed on the specimens with different concrete cover thickness and stirrup spacing to investigate the bond behavior between normal concrete and deformed steel bars without corrosion.Based on the systematic analysis of the measured bond stress-slip curves and the test results reported in the literature,the bond mechanism of the ascending branch and the descending branch of the bond stress-slip curves was analyzed in detail.Moreover,a parameter K was proposed to consider the overall confining ability provided by the concrete cover and the stirrups.With the confinement parameter K the present study has developed a method to predict the possible bond failure mode and established two empirical equations for the relative slip corresponding to the peak bond stress.A local bond stress-slip relationship model determined by parameter K was further proposed.(2)A kind of beam specimen was designed,and a series of bond tests were performed on the beam specimens with tensile steel bars of varying corrosion levels and stirrup spacing.The influence of concrete cover,stirrups and corrosion rate on the bond deterioration was systematically analyzed,and based on the large amount test results in the literature a bond degradation model considering the confinements and the corrosion rate was proposed.The correlation between the bond deterioration and the deterioration of the mechanical performance of the beam specimens was also discussed.(3)Through a series of eccentric pullout specimens with corroded stirrups and tensile steel bars,the concrete cover cracking and the bond deterioration were investigated.After comparing the test results of the present paper with the test results of specimens with only corrosion of tensile steel bars,the influence of the corrosion of stirrups on the bond deterioration and the concrete cover cracking was introduced.Based on the experimental results,a mathematical model for the prediction of lateral crack width was established and a reduction factor was proposed to describe the bond deterioration induced by the corrosion of stirrups.A bond degradation model considering the influence of both the corrosion of longitudinal steel bars and the corrosion of stirrups was finally developed.(4)Through a series of eccentric pullout specimens with non-corroded stirrups and corroded tensile steel bars,the influence of concrete cover and stirrups on the bond deterioration and the propagation of the surface crack width was investigated.According to the experimental test results,the potential correlations between the mass loss,the surface crack width and the bond strength were revealed.Based on the aforementioned empirical model for the prediction of lateral crack width and the developed reduction factor reflecting the influence of the corrosion of stirrups on the bond deterioration,the present paper further developed a bond degradation model with the lateral surface crack width and the longitudinal crack width as the governing parameters.(5)Through a series of eccentric pullout specimens with non-corroded stirrups and corroded tensile steel bars,the present paper investigated the influence of corrosion on the bond strength and the bond stress-slip curves under monotonic loading and repeated loading.The variation of the slip increase due to repeated loading with corrosion levels,applied stress levels and the number of loading cycles was also introduced.Two empirical models for the residual slip which were available for both corroded and non-corroded specimens were proposed.Based on the empirical models for the residual slip,the local bond stress-slip relationship models and the bond fatigue model for the corroded and non-corroded specimens were furtherer developed.Through the bond fatigue model,it was found that the corrosion of tensile steel bars can lead to the reduction of the bond fatigue life.