Study On Mechanical Behavior Of PVC-CFRP Confined Concrete Column-reinforced Concrete Beam Joints Strengthened With Core Concrete Filled Steel Tube Under Axial Compression

Recently,carbon fiber reinforced polymer(CFRP)has been widely used in civil engineering due to its high strength,light weight,corrosion resistance,and fatigue resistance.PVC-CFRP confined concrete structure,which is composed of PVC tube winded by CFRP strips and concrete infilling,has high bearing capacity and good ductility.For the application of PVC-CFRP confined concrete structure,it is necessary to develop the design formulas for the joint between PVC-CFRP confined concrete column and reinforced concrete beam.Hence,the purpose of this study is to investigate the mechanical behavior of PVC-CFRP confined concrete column-reinforced concrete beam joints strengthened with core steel tube under axial compression.The experiment of ten PVC-CFRP confined concrete column-reinforced concrete beam joints(PCBJ)and one PVC confined concrete column-reinforced concrete beam joint(PBJ)subjected to axial compression was performed.All these specimens were designed according to the principle of strong joint and weak column.As expected,the failure mode of the PBJ was the yielding of longitudinal reinforcement and stirrup in column and buckling of PVC tube.The PCBJ failed by the fracture of CFRP strips at the top column and cracking of PVC tube.The effects of joint height,length of core steel tube,stirrup ratio of joint,steel proportion of core steel tube,and spacing of CFRP strips on the bearing capacity,deformation,crack development,and load-deformation curve of columns and joints were investigated.(1)The ultimate bearing capacity and yield bearing capacity of PVC-CFRP confined concrete column increased as the spacing of CFRP strip decreased.Additionally,the axial and circumferential ultimate strain of PVC-CFRP confined concrete column increased as the spacing of CFRP strip decreased,and the stiffness of PVC-CFRP confined concrete column decreased as the spacing of CFRP strip decreased.(2)The axial and circumferential ultimate strain of joint decreased as the length of the core steel tube,stirrup ratio of join,or the steel proportion of core steel tube increased.The axial and circumferential ultimate strains of joint increased as the spacing of CFRP strip or the joint height decreased.In contrast,the stiffness of joint decreased as spacing of CFRP strip or the joint height decreased.The experiment of ten PCBJ was performed which were designed according to the theory of strong column and weak joint.The PCBJC failed by the crushing of joints,yielding of longitudinal reinforcement and stirrups in joints and columns,and yielding of inner core steel tubes in joints.In addition,four or five vertical and oblique cracks on the side of the core area of joints and concrete shedding on the surface of the core area of joints.The effects of joint height,length of core steel tube,stirrup ratio,and steel proportion of core steel tube on the bearing capacity,deformation,crack development and load-deformation curve of columns and joints were investigated.(1)The axial and circumferential ultimate strains of PVC-CFRP confined concrete columns increased with the decrease of joint height.Additionally,The axial and circumferential ultimate strains of columns increased with the decrease of stirrup ratio and steel content of core steel tube.(2)with the increase the stirrup ratio of annular stirrups and steel content of core steel tube,the ultimate bearing capacity and yield bearing capacity of joints increased,the axial ultimate strain in the core area of joints increased,the circumferential ultimate strain of joints decreased,and the stiffness degradation of joints slowed down.A coefficient of joint height,?_h,for considering the influence of joint height on the ultimate bearing capacity of PVC-CFRP confined concrete columns under axial load was proposed.According to the condition of static equilibrium at the ultimate state,an accurate model for predicting the ultimate bearing capacity of PVC-CFRP confined concrete columns under axial load was derived.A coefficient of joint height,?_h,for considering the influence of joint height on the stress-strain relationship of PVC-CFRP confined concrete columns under axial load was proposed.An accurate model for estimating the stress-strain relationship of PVC-CFRP confined concrete columns was suggested.A coefficient of confining effect of beam,?,for considering the influence of the restraint effect of beam on the ultimate bearing capacity of joints was proposed.And the other coefficient of steel tube,?,for considering the influence of the reduction about the bearing capacity of core concrete filled steel tube was also proposed.According to the limit equilibrium method and the confined concrete theory,an accurate model for predicting the ultimate bearing capacity of joint under axial compression was derived.Another coefficient of the confining effect of beam,?_b,for considering the influence of the restraint effect of beam on the ultimate strain of joints was proposed.An accurate model for estimating the stress-strain relationship of joints was suggested.After the constitutive relation and element type of each material were reasonably selected,the analysis steps were set up,the contact and constraint between entities were defined,the load and boundary conditions were created,and the mesh were divided,a finite element method for estimating mechanical performance of PCBJ under axial compression,which correlated well with the results of experiment,was developed using ABAQUS.