Study On Mechanical Behavior Of PVC-CFRP Confined Concrete Column-reinforced Concrete Ring Beam Joint Under Eccentric Compression

Carbon fiber reinforced polymer(CFRP)is a kind of high performance structural material which has many good performances,such as light weight,high strength and good durability.PVC-CFRP confined concrete column is made by poring concrete into the PVC tube that winded by CFRP strips with certain hoop spacing.Research on the PVC-CFRP confined concrete column indicated that the bearing capacity and ductility of the column increased obviously because of the confining effect provided by PVC-CFRP tube.There were a lot of publications related to the static performance,seismic performance and durability of PVC-CFRP confined concrete column.However,the investigation on the PVC-CFRP confined concrete column-reinforced concrete ring beam joint was little.Considering that the eccentric loads are carried by joints in the most actual projects,the experimental research and theoretical analysis of PVC-CFRP confined concrete column-reinforced concrete ring beam joints under eccentric compression are studied in this paper.According to the design principle of "strong join weak columnt" and "strong column weak joint",the specimens in this paper were divided into two groups(group F and group S).1 PVC confined concrete strong joint weak column and 12PVC-CFRP confined concrete strong joint weak columns were performed in group F.Different parameters were considered in the analysis,such as the width,height and reinforcement ratio of ring beam,the CFRP strips spacing and eccentricity.The influences of the above parameters on the bearing capacity,ductility,stiffness,deflection,curvature and strain of PVC-CFRP confined concrete columns were conducted.The results demonstrated that the failure modes of specimens were the compressive bulging of PVC tube,the longitudinal bars yielded,the crushing of concrete and the pulling of CFRP strips near the side of axial force,and the cracking of concrete and the pulling of PVC tubes away from the side of axial force.Besides,the ultimate carrying capacity of PVC-CFRP confined concrete column decreased as the CFRP strips spacing and eccentricity increased,the reinforcement ratio,width and height of ring beam had little influence on it.Additionally,with the increase of the height of ring beam,the CFRP strips spacing and eccentricity,the ductility of the column decreased,the stiffness degradation,deflection and curvature of column increased,the reinforcement ratio and width of ring beam had no effect on them.11 PVC-CFRP confined concrete weak joint strong columns were performed in group S.The influences of the reinforcement ratio,width,height of ring beam and eccentricity on the ultimate bearing capacity,ductility,stiffness,deflection,curvature and strain of specimens were analyzed.The experimental results indicated that the failure modes of the specimens were as follows: ring bars in joint area and longitudinal bars of the column yielded,a number of vertical cracks appeared on the side of the joint and multiple radial cracks appeared on the upper surface of the joint near the side of axial force.Additionally,annular cracks appeared on the upper surface of joint and microscopic transverse cracks appeared on the middle side of some joints away from the side of axial force.Besides,as the reinforcement ratio and the width of ring beam increased as well as the height of ring beam and eccentricity decreased,the ultimate carrying capacity of joints increased,the deflection,curvature and stiffness degradation of PVC-CFRP confined concrete column decreased.As the height of ring beam and eccentricity increased,the ductility of PVC-CFRP confined concrete column decreased,the reinforcement ratio and the width of ring beam had no effects on the ductility of the column.On the basis of experimental results,the unified theory of concrete-filled steel tube was used,and the bearing capacity formula of PVC confined concrete column was derived by introducing the influence coefficient of eccentricity.According to the limit equilibrium method,the influence of eccentricity on the confining stress was considered,and the bearing capacity formula of PVC-CFRP confined concrete column was proposed.Considering the influence of eccentricity on the stress-strain relationship of the column,the equivalent confining coefficient of PVC-CFRP tubes on the core concrete is proposed,and the stress-strain relationship model of the concrete column of PVC-CFRP confined concrete column was established.According to the equilibrium equations of section strain,the influence of confine stress provided by PVC-CFRP tubes on the stiffness of column was considered,and the stiffness formula of the PVC-CFRP confined concrete column was proposed.Based on the theory of local bearing capacity of concrete,the principle of equivalence was used,the confine stress of ring beam on the core concrete of joints was equivalent to the lateral confining effect,and the influence of eccentricity on the axial compressive stress of the core concrete was considered,then the bearing capacity formula of the ring beam joint was deduced.The constitutive relationship,element type and grid size of materials were selected reasonably,the contact types between entities were defined,the boundary constraints were placed,and the finite element model of the ring beam joints was established.The finite element model was verified by comparing the calculated results with the experimental results.Then the model was used to analyze the influence of concrete strength,eccentricity,the CFRP strips spacing and the reinforcement ratio of ring beam on the performance of specimen under eccentric compression,then the working mechanism of specimen was revealed.