Analytical Modeland Experimental Investigation On Bond Slip Relationship Between Rebar And Concrete

As the material which is widely applied in modern civil engineering,reinforced concrete(RC)has been extensively studied since its appearance.Bond performance between rebar and concrete is one of the basic mechanical properties for RC composite structures,and it is a factor that must be considered in the RC numerical analysis.The service performance such as ductility,bearing capacity and hysteretic behavior relies on the interaction between rebar and concrete,and a clear bond stress-slip relationship is required in the structural design and the improvement of RC structure theory system.However,the variety of the factors that affect the bond performance leads to a complicated interaction mechanism between rebar and concrete.Therefore,an in-depth study for bond behavior is necessary to analyze the overall mechanical properties of RC structures.In this paper,the bond stress-slip relationship is deduced through theoretical analysis for RC structures due to splitting failure,and then the pullout test for cylindrical specimens is carried out to investigate the bond mechanism.Meanwhile,the bond stress-slip relationship for specimens due to different bond failure modes is analyzed with test data.By pasting strain gauges on the outer surface of concrete,the transmission mechanism from the interface between rebar and concrete to the interior concrete is analyzed.The main research contents and conclusions are as follows:(1)Discussing the solution method of bond stress for splitting failure.Bond performance between rebar and concrete relies on the transverse confinement capacity of concrete to rebar when RC structures undergo splitting failure.Therefore,based on the thick-walled cylinder model,the concrete is considered as a two-phase material with cracked part and uncracked part to analyze its stress state.When cracks occur,the fracture property of concrete should be studied by introducing softening relationship and assumption of circumferential deformation,and the selection for different types of them affects the form of solution,which has been illustrated in detail.The concrete confinement model can be converted into the bond stress-slip relationship through the analysis of stress transmission mechanism at the interface between rebar and concrete.Meanwhile,the solution of bond strength depends on the effective cover depth for RC structures due to splitting failure.In this paper,the application of the thick-walled cylinder model in practical engineering is presented to make the method can be used to predict the bond behavior of the key parts of RC structures.(2)Presenting the improved bond stress-slip relationship on the basis of the analysis above.The process of splitting failure can be divided into four stages through classifying the state of crack propagation: the uncracked stage,the crack propagation stage,the entirely cracked stage and the residual stage.At the crack propagation stage,a new assumption of circumferential deformation is proposed to approximate the deformation field within the cracked concrete area.Then,the applicability of concrete confinement model is evaluated by comparative analysis,whereas the bond stress-slip relationship can be evolved by the confinement model through stress-transmitting mechanism at the interface between rebar and concrete.The comparison between test data and calculated results indicates the proposed bond stress-slip relationship has a high accuracy.(3)Investigating the bond performance between rebar and concrete with the pullout specimens which are composed of axisymmetric rebar and cylindrical concrete.The 20 specimens of 10 groups,taking rebar diameter,concrete diameter,relative cover depth,bond length as variables,are prepared to study the bond failure mechanism.Through the analysis of the test phenomenon and test data,the factors that influence the concrete cracks are considered in detail,so are the factors that affect the test curves and the peak load.Then,the bond stress-slip relationship for pull-out failure is presented through data comparison.Then,the prediction of the bond strength due to splitting failure are achieved by introducing the reduction coefficient,and the whole process of the bond slip due to pull-out failure is predicted by using the dimensionless bond stress-slip relationship.(4)Pasting the strain gauges on the outer surface to display the strain transmission mechanism.Therefore,the factors that affect the peak value of strain are analyzed in detail,so is the distribution law of strain along bond length.