Research On Mechanical Properties Of High Pre-cracked Concrete Columns Strengthened With CFRP Strips Under Eccentric Compression

Fiber Reinforced Polymer(FRP)has the advantages of light weight,high strength,high temperature resistance,corrosion resistance and radiation resistance,and is widely used in many fields such as buildings,bridges and so on.In the existing research on FRP reinforced concrete columns,the main focus is on the mechanical properties of the uncracked concrete columns,while the reinforced concrete columns generally have different degrees of damage in practical engineering.Therefore,this paper carries out experimental research and theoretical analysis on the eccentric compression performance of high pre-cracked concrete columns strengthened by CFRP strips.This experiment is divided into two groups(group A and group B).Through five small eccentric compression specimens in group A and seven large eccentric compression specimens in group B,the eccentric compression performance of concrete columns strengthened by CFRP strips is tested.The influences of pre-cracked degree CFRP strips spacing and eccentricity on ultimate bearing capacity,ductility,load-deflection,moment-curvature,stiffness and strain of group A specimens are mainly considered.The failure mode of the columns in Group A is that the longitudinal bars near the axial force yield,and then the concrete in the compression zone is crushed,but the longitudinal steel bars far away from the axial force do not yield.The failure mode of the columns strengthened with small eccentric pressure is the yield of the longitudinal compression steel bars,and then the concrete in the compression zone is crushed.However,the longitudinal tension bars did not yield and the transverse CFRP strips are not broken.The results show that with the increase of pre-cracked degree and eccentricity,the ultimate bearing capacity of the specimens decreases,the deflection increases,the curvature decreases,the ductility decreases,and the stiffness degenerates rapidly in the later stage.The strain growth rate of longitudinal steel bars,concrete in compression zone and transverse CFRP strips are accelerated.The strain growth rate of longitudinal steel bars,concrete in compression zone and transverse CFRP strips are accelerated.With the increase of CFRP strips spacing,the strain growth rate of ductility,longitudinal reinforcement,concrete in compression zone and transverse CFRP strips decreases.However,the influence of CFRP strips spacing on ultimate bearing capacity,deflection,curvature and stiffness of specimens is not obvious.The influence of pre-cracked degree,CFRPs strip spacing and eccentricity on ultimate bearing capacity,ductility,load-deflection,moment-curvature,stiffness and strain of group B specimens is mainly considered.The failure mode of group B large eccentric compression contrast column is that the longitudinal tension steel bars yields,then the longitudinal compression steel bars yields,and finally the concrete in the compression zone is crushed.The failure mode of columns strengthened with large eccentric compression is that the longitudinal tension steel bars yield,then the longitudinal compression steel bars yield,and finally the longitudinal CFRP strips are pulled off,and the concrete in the compression zone is crushed,but the transverse CFRP strips are not pulled off.The results show that with the increase of pre-cracked degree and eccentricity,the ultimate bearing capacity of specimens decreases,the deflection increases,the curvature decreases,the ductility decreases,and the stiffness degenerates rapidly in the later stage.The strain growth rate of longitudinal steel bars,concrete in compression zone,transverse CFRP strips and longitudinal CFRP strips are accelerated.With the increase of CFRP strips spacing,the strain growth rate of ductility,longitudinal steel bars reinforcement,concrete in compression zone,transverse CFRP strips and longitudinal CFRP strips are increases.However,the influence of CFRP strips spacing on ultimate bearing capacity,deflection,curvature and stiffness of specimens is not obvious.On the basis of experimental study,The working mechanism of CFRP strips reinforced concrete eccentrically loaded columns is proposed.According to the strain relation of the section and the equilibrium condition of the strain,considering the section shape,the circular restraint stress of the rectangular section strengthened by CFRP strips are proposed.Considering the factors such as pre-cracked degree,CFRP strips spacing and eccentricity,the pre-cracked degree coefficient and CFRP strips strain distribution coefficient are introduced,and the formulas for calculating the bearing capacity of small and large pre-cracked degree concrete columns strengthened with CFRP strips are proposed respectively.On this basis,considering the influence of eccentricity and pre-cracked degree and introducing eccentricity increasing coefficient,the stress-strain relationship model of pre-cracked degree eccentric column strengthened by CFRP strips are established.On the basis of experimental study,the effects of pre-cracked degree,CFRP strips spacing and eccentricity on crack width and crack spacing of A and B groups of specimens are mainly considered.With the increase of pre-cracked degree,the crack width of group A specimen increases rapidly,the crack width increases gradually,and the average crack spacing decreases gradually.CFRP strips spacing has little effect on crack width and average crack spacing of group A specimens.With the increase of pre-cracked degree,the cracks of group B specimens develop rapidly,especially the high pre-cracked degree specimens,which are still in the working state with cracks.The crack width increases obviously in the later stage,and the average crack spacing decreases gradually.With the increase of CFRP strips spacing,the crack width and average crack spacing of group B specimens are basically the same.On the basis of theoretical research,the working mechanism of crack formation in high pre-cracked degree concrete columns strengthened by CFRP strips are proposed.According to the stress condition of crack element model,the formula for calculating average crack spacing of CFRP strips reinforced concrete eccentric compression column is deduced.Based on the calculation formula of average crack spacing,considering the factors of pre-cracked degree and eccentricity,the influence coefficient of pre-cracked degree on crack width is introduced,and the formula of crack width of pre-cracke degree concrete eccentric compression column strengthened with CFRP strips are proposed.According to the strain balance relationship of the specimen section,considering the influence of pre-cracked degree,CFRP strips spacing and eccentricity,the influence coefficient of pre-cracked degree on stiffness is introduced,and the stiffness calculation formula of pre-cracked degree concrete eccentric compression column strengthened by CFRP strips are deduced.On the basis of material stress-strain relationship model,the stress-strain relationship and element type of steel bars,concrete and CFRP strips are reasonably selected.Contact between CFRP strips and concrete,and boundary constraints of reinforced concrete members are defined,set up analysis steps and create load and mesh elements.At the same time,the influence of concrete strength,reinforcement ratio and CFRP reinforcement layer number on load-deflection curve is analyzed.The calculated results of the model are in good agreement with the test results.