Study On Mechanical Properties Of CFRP Confined Self-stressing Concrete-encased-steel Columns Under Axial Compression

FRP tube confined concrete column can effectively combine the tensile performance of FRP with the compressive performance of concrete,and improve the axial compressive performance of concrete.At the same time,the section steel can effectively reduce the section area of the column and improve the seismic performance.However,a large number of experiments showed that the FRP has a significant hysteresis for the confinement of concrete.FRP will only exert its confinement effect when the concrete is subjected to sufficient expansion deformation or cracks develop to a certain extent.In this paper,the hysteresis is eliminated by placing expansive concrete in the middle of steel and CFRP tube.The experimental study and theoretical analysis on the a xial compressive behavior of CFRP confined self-stressing concrete-encased-steel columns are carried out.The main research work is as follows:(1)Experimental research and theoretical analysis on axial compressive behavior of CFRP confined self-stressing concrete-encased-steel circular columnsIn this study,the axial compressive behavior of 24 CFRP confined self-stressing concrete-encased-steel circular columns was examined.Key parameters include CFRP layers,cross sectional area of steel reinforcement and the level of prestress(with or without prestress).The experimental results showed that the normalized ultimate load,intercept load and inflection load of self-stressing specimens were improved compared with non-prestressed test specimens.With the application of prestress,the utilization of CFRP increased as well.The normalized ultimate load,normalized intercept load,normalized inflection load and deformation capacity of the composite columns increased as the number of CFRP layer increased.Finally,by comparing experimental data with the existing model,the peak stress matches very well.Meanwhile,based on the existing theoretical models,taking into account the effect of initial prestress,the calculation of inflection load was in good agreement with the experimental results.According to the commonly used equations,taking into account the influence of steel,the calculated ultimate axial strain value agrees well with the experimental results.(2)Experimental research and theoretical analysis on axial compressive behavior of CFRP confined self-stressing concrete-encased-steel square columnsA total number of 24 CFRP confined self-stressing concrete-encased-steel square columns were casted and tested under axial compression.The variables studied included the number of CFRP layers,the area of the section steel and the level of prestress.The results show that CFRP has a good confinement effect on core steel reinforced concrete columns,and compared with non-self-stressing specimens,the self-stressing effect made the tube specimens had higher strength enhancement ratio.As the number of CFRP layer increased,the ultimate load capacity and ultimate axial strain increased,and the slope of the second linear segment of the load-strain curve increased.The models of ultimate stress of confined concrete were formulated to account for both FRP-induced passive confinement and expansive concrete-generated active confinement and were validated by the experimental data.The experimental values agree well with the calculated values.(3)Experimental research and theoretical analysis on axial compressive behavior of CFRP confined self-stressing high-strength concrete columnsFor CFRP-confined high-strength plain concrete columns,the axial compression performance of 18 columns was examined.The parameters include the CFRP layers and the level of prestress.The results showed that the intercept stress,inflection stress and peak stress of the self-stressing specimens are all higher than those of the non-prestressed specimens.With the application of prestress,the utilization ratio of CFRP increases as well.In the theoretical calculation of intercept and peak stress,by modifying the existing models,the experimental results are in good agreement with the calculated resultsFor CFRP confined high-strength self-stressing concrete-encased-steel columns,the axial compression performance of 16 columns was examined.The variables include CFRP layers and prestress levels.The results showed that the axial strain and CFRP failure strain of self-stressing specimens are larger than those of non-self-stressing specimens.The confinement efficiency of CFRP for steel high-strength concrete cylinders is higher than that of steel high-strength concrete square columns.The stress and strain calculation model of CFRP confined concrete column was modified.The stress-strain relationship model was used to compare with the load-strain curve obtained by experiment.The experimental curve was in good agreement with the calculation curve.