| The bond between steel bar and concrete is a complex interfacial action,which is the guarantee of stress transfer and deformation coordination between the two materials so that they can work together effectively.Reinforced concrete(RC)structures such as bridges,marine platforms,nuclear power plant containments are subjected to both static loads and long-term repeated loads.And they may suffer from reinforcement corrosion under the long-term environmental actions,resulting in deterioration of the bond behavior between steel bar and concrete.Based on the local bond mechanism between deformed steel bar and concrete,the bond-slip behavior has been systematically investigated by experimental investigation and theoretical analysis considering the effects of steel bar corrosion and repeated loads.The eccentric pull-out tests of uncorroded deformed steel bar were carried out under three different forms of loading,including monotonic load,repeated load and monotonic load after repeated loading.Based on the monitoring and analysis of the relative slip,concrete tensile strain and splitting crack width during the test,the failure mechanism of different bond failure modes was clearly pointed out.The pull-out failure is caused by the shear damage of concrete between the rebar ribs while the splitting failure is dominated by the tensile damage of concrete cover.The fatigue pull-out test verified that the development of fatigue bond damage under repeated loading shows as a three-stage growth law.The fatigue bond failure criterion and the degradation law of bond behavior after repeated loading were investigated by analyzing the relationships between the characteristic values of fatigue bond damage index and the characteristic parameters of static bond-slip behavior.The corroded pull-out specimens were obtained by the accelerated corrosion method with impressed current.The relationship between corrosion-induced crack width and corrosion degree were investigated,and the evolution of geometric characteristics with the corrosion degree was revealed through analysis of the 3D scanning model of corroded steel bars.With the increase of corrosion degree,the width of corrosion-induced crack increases linearly;the cross-sectional area and nominal diameter of steel bar decreases;the surface roughness of plain round steel bar increases;and for the deformed steel bar,the relative cross-rib area increases and the periodic distribution of cross-sectional area recedes.The central pull-out test of corroded plain round steel bar under monotonic load provided the basic test data to analyze the interfacial properties between corroded steel bar and concrete.The eccentric pull-out tests of corroded deformed steel bars were also conducted under different forms of loading to investigate the effects of steel bar corrosion on bond-slip behavior.Under monotonic loading,the peak slip decreases and the bond stiffness increased with the increase of corrosion degree,and the bond strength decreases significantly after corrosion-induced crack occured.Under repeated loading,the corrosion of steel bars affects the development law of fatigue bond damage,but the fatigue bond failure criterion and the degradation law of bond behavior with fatigue loading history for uncorroded steel bars are still applicable to corroded steel bars.Based on the bond mechanism between deformed steel bar and concrete,a local bond-slip model consisting of ring-ribbed steel bar and thick-walled concrete cylinder was established.The whole splitting process of concrete cover was simulated by using the finite difference method.The ultimate splitting force of concrete cover was found to be linearly and positively correlated with concrete tensile strength and concrete cover thickness.The calculating formula for the ultimate splitting force of concrete cover was proposed through parametric analysis,and it was used as the criterion of the splitting failure mode.The development process of bond action and the failure mechanism of each characteristic state were clarified through the observation test of mechanical interlocking.It was revealed that the Goto crack and splitting crack are type I cracks which are controlled by tensile stress,while the concrete wedge surface in front of the rib and the shear failure surface of concrete between the ribs are type II cracks which are controlled by shear stress.The interface interactions between the deformed steel bar and concret were analyzed according to the development process of bond action,and the whole local bond stress-slip curve of deformed steel bar was simulated.The envelope model of bond failure was proposed for determination of bond failure mode and corresponding bond strength calculation,and the transformation mechanism between different bond failure modes was discussed.The parametric analysis was carried out with the theoretical local bond-slip model.The results show that the bond strength increases in three linear stages with the increase of the concrete cover thickness,and the bond failure mode changes from splitting failure to pull-out failure.The threshold of concrete cover thickness to change the bond failure mode increases with the increase of the concrete strength.It also indicated that the cross-rib inclination angle of commonly used deformed steel bar can allow the full play to the bond behavior of deformed steel bars.The mechanism study was carried out based on the three influencing aspects of steel bar corrosion on bond-slip behavior,and a local bond-slip model of corroded steel bar was established.The corrosion product layer was introduced into the thick-walled concrete cylinder model to simulate the whole process of corrosion-induced cracking of concrete cover.The interfacial properties between corroded steel bar and concrete were analyzed based on the pull-out test results of corroded plain round steel bar.A slipvarying model of friction coefficient was proposed considering the influence of the corrosion product layer.The initial friction coefficient increases from 0.3 to 0.65 due to the increase in surface roughness of the corroded steel bar,and then decreases to about 0.45 due to the lubrication of corrosion products and then remains constant.The friction coefficient exponentially decreases with the increase of relative slip.The relative crossrib area of corroded deformed steel bar increases,but the practical interlocking area between the corrosion-damaged cross rib and concrete decreases with the increase of corrosion degree.The whole bond stress-slip curve of corroded steel bar was simulated by substituting the corroded parameters into the local bond-slip model originally for uncorroded steel bars.The reduction in the confinement capacity of concrete cover caused by corrosion-induced cracking is the most important influence factor of steel bar corrosion on bond-slip behavior.A simplified model was proposed to obtain the bond stress-slip relationship for corroded steel bars by translating and correcting of the uncorroded curve according to the corrosion degree of steel bar,which unified the bond stress-slip model for both uncorroded and corroded steel bars.Based on the result analysis of bond test under repeated loading,a fatigue bond damage model for deformed steel bars was proposed by analoging with the fatigue damage constitutive model of concrete,which unified the bond stress-slip model for deformed steel bars under both monotonic and repeated loading.Taken the relative slip as the unified fatigue bond damage index,the development law of relative slip under repeated loading was mathematically expressed.The fatigue bond failure criterion was defined when the relative slip under repeated loading reaching the corresponding slip value of the maximum bond stress in the descending branch of the static bond stressslip curve,and the prediction formula of bond fatigue life was derived.The fatigue loading history leads to an increase of bond stiffness.If the relative slip under repeated load reaches the static peak slip,the post-fatigue bond behavior deteriorates along the descending branch of the static bond stress-slip curve,otherwise,the fatigue loading history has little influence on bond strength.Finally,the effects of steel bar corrosion on the development of fatigue bond damage were analyzed,and the fatigue bond behavior of corroded steel bar was discussed.Steel bar corrosion significantly reduces bond fatigue life.When the fatigue bond failure doesn’t occur,the corrosion of steel bar is the primary cause of bond behavior deterioration,and the effects of repeated loading on bond behavior is relatively limited.This study provides a theoretical basis for numerical simulation of performance evolution of RC structures under sustained actions of loads and environments,and contributes to the development of life-cycle design and maintenance theory of RC structures. |
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