Study On Bond-slip Performance Of Self-stressing Steel Slag Concrete-filled Circular Steel Tube Column

The significant problem in ordinary concrete-filled steel tube is the dehydration shrinkage in concrete hardening.It greatly weakens the bond behaviour between concrete and steel tube.Meanwhile,it reduces the reliability and durability of composite structure of concrete-filled steel tube.Steel slag has the characteristic of certain activity and hardness.Steel slag concrete,which is made of steel slag instead of ordinary aggregate,is excellent in compressive strength and durability.The steel slag contains high magnesium oxide and free calcium oxide,which can cause expansion of cement paste.This not only compensates for the drying shrinkage of concrete,but also makes the steel slag concrete and steel tube to be self-stressing,effectively improving the bond behaviour of the composite component.Thus,the experimental study and theoretical analysis of the bond behavior of self-stressing steel slag concrete-filled steel tube columns(self-stressing SSCFST columns)are carried out in this paper:The experimental study on bond-slip performance of self-stressing SSCFST columns are carried out,the effects of diameter-thick ratio,confining parameter,steel ratio and expanding rate of steel slag concrete on bond behaviour of self-stressing SSCFST columns such as damage form,load-slip curve,bond strength,strain and load-strain curv are analyzed,the research result indicates that the damage form of push-out specimens are interfacial shear failure.The process of stress and relative slip of the interface between steel slag concrete and steel tube is divided into three stages.At loading stage,the relative slip between steel slag concrete and steel tube is small,with the increment of diameter-thick ratio and confining parameter,the bond strength increases first and then decreases,with the increment of steel ratio and expanding rate of steel slag concrete,the growth of bond strength increases gradually.At load-descending stage,the relative slip between steel slag concrete and steel tube increase,while the full interface slip has not yet occurred,the bond strength increases with the growth of diameter-thick ratio,decreases with the growth of confining parameter,steel ratio and expanding rate of steel slag concrete.At load-stabilizing stage,the bonding force tends to be stable,while the relative slip between steel slag concrete and steel tube continues to increase,the full interface slip of steel slag concrete in steel tube occurs.The experimental study on mechanical properties of self-stressing SSCFST columns are carried out,the influence of steel slag concrete expanding rate,confining parameter,loading mode,damage in specimen(concrete has occurred full interface slip in steel tube)on mechanical properties of self-stressing SSCFST column such as failure form,bearing capacity,stiffness,strain,load-displacement relation and stress-strain relation are analyzed,the research result indicates that the damage form of self-stressing SSCFST columns under axial compression is compressive flexion failure,with the increment of expanding rate of steel slag concrete and confining parameter,the growth range of bearing capacity increases gradually,the stiffness degradation is small.The ultimate bearing capacity of specimens with different loading modes are quite similar,the stiffness degradation of steel slag concrete to be loaded first(type A)is slow and the elastic stage is longest,the development of stiffness of steel slag concrete to be loaded first(type B)is most unstable and the elastic stage is shortest,the stiffness degradation of steel tube and concrete to be simultaneously loaded(type C)is slow,the length of elastic stage is between type A and type B.The damage form of pre-damaged specimen is compression flexion failure,the bonding force of pre-damaged specimens are small and the load on concrete transmited from tube is small,the bearing capacity and the stiffness of pre-damaged specimen are small at the early stage.Compared with undamaged specimen,the bearing capacity of pre-damaged specimen is small,the degradation of stiffness is faster and the length of elastic stage is shorter,but the ultimate bearing capacity of both are equal,which indicates that bonding force has no effect on the ultimate bearing capacity of self-stressing SSCFST columns.Based on the test research,the force mechanism of bond-slip of self-stressing SSCFST column is revealed.The effect of self-stressing between the tube and the steel slag concrete is considered and the calculation model of bond strength is established.Regression analysis of test data,through regression analysis of experimental data,the simplified calculation formula for the bond strength is proposed.The effect of self-stressing on the bond-slip performance is considered,the bond-slip constitutive relation is deduced,with the three-stage bond-slip model,the simplified calculation formula for the bond-slip constitutive relation is proposed.The effect of self-stressing between the steel tube and the steel slag concrete is considered,the influence coefficient self-stressing is introduced,using the limit equilibrium theory,the formula of ultimate bearing capacity of self-stressing SSCFST column is deduced,the stiffness formula of self-stressing SSCFST column is put forward and the stress-strain relationship model of self-stressing SSCFST column is established,the effect of bonding force on mechanical properties of self-stressing SSCFST column is discussed.Reasonably selecting the stress-strain relationship of steel and steel slag concrete,through using the finite element software,the model of the self-stressing SSCFST column is established,using nonlinear spring element to simulate the change regulation between bonding force and slip,the simulation of three different loading modes in axial compression are conducted,the simulation of three different loading modes in axial compression test are conducted,the influence of length-diameter ratio and self-stressing of steel slag concrete on the bond behavior and axial compression performance of self-stressing SSCFST column are discussed,the bond slip mechanism and axial compression force mechanism of self-stressing SSCFST columns are revealed.