Axial Compressive Behavior Of CFST Columns Confined With FRP Strips

Concrete-filled steel tube(CFST)columns possess excellent mechanical properties(e.g.a high load-capacity and a good ductility).However,there is still room for performance improvement of CFST columns because some problems(e.g.,the inconsistency of Poisson ratio of steel tube and concrete result in smaller confinement provided by steel tube during the incomplete development crack stage of concrete;thin-walled steel tubes are prone to local buckling which causes excessive reduction of load capacity)are existed.In addition,the use of CFST columns face the problem of insufficient load-capacity due to adverse factors(e.g.,corrosion)and changes of structural function.The above problems need to be solved by strengthening/reinforcing CFST columns.Previous studies have shown that fiber-reinforced polymer(FRP)can significantly restore or enhance the strength and deformation capacity of concrete,so it has been widely used in structural reinforcement in recent years.For the convenience of architectural design or construction in practical engineering,partial components need to be strengthened with FRP strips.Thus,this thesis presents the results of axial compression tests on CFST columns confined with FRP strips.The key parameters investigated in the present thesis include effects of FRP types(Carbon FRP(CFRP)and Poly Ethylene Terephthalate FRP(PET FRP)),FRP layers(0,2,3 and 4)and spacing of adjacent FRP strips(0,40,80 and 120 mm)on the axial compressive behavior(including failure mode,load-strain curve,hoop strain-axial strain curve and confined concrete stress-strain curve,etc.)as well as the strain distribution(including the axial compression strain distribution and the hoop strain distribution of the specimen along the height of specimens).The results showed that:(1)The FRP has the most obvious increase in the ultimate load capacity and the corresponding ultimate axial strain of CFST columns confined with FRP strips(the maximum lift of CFRP specimens can reach 31% and 57% while PET FRP specimens can reach 43% and 607%,respectively).For CFRP specimens,the ultimate load capacity increases approximately linearly with the increase of the number of FRP layers,and increases slowly and then abruptly as the ratio of strip spacing / strip width(s/b)decreases;the ultimate axial strain increases with the increase of the number of FRP layers,and increases first and then might decreases slightly as s/b decreases.With the same FRP volume rate,the influence of the number of FRP layers on the ultimate load capacity and ultimate axial strain is more obvious than the FRP spaces.The influence of s / b on the ultimate load capacity and corresponding axial strain of PET FRP specimen is similar to that of the CFRP specimen,except that PET FRP can achieve greater ultimate load capacity and ultimate axial strain.The effects of different variables on the ultimate stress and the corresponding ultimate axial strain of the confined concrete are consistent with that of the specimen.(2)The test found that the Poisson’s ratio of CFST columns confined with FRP strips is smaller than that of CFST columns,indicating that FRP can effectively improve the steelconcrete interface at the initial stage.Further,the increase of the number of FRP layers can effectively limit the expansion deformation speed of the specimens,and the increase or decrease of s/b cannot effectively limit the expansion deformation speed of the specimens.The difference between the PET FRP specimens and the CFRP specimens is that CFRP can reduce the lateral expansion speed while PET FRP can achieve greater expansion deformation.(3)DIC technology(Digital Image correlation technique)is used to obtain the strain distribution along the height of CFST columns partially confined with FRP strips.This technology is in good agreement with the load-strain curve measured at the key measurement points by conventional measurement.The percentage of ultimate strain error is within ± 15%,which proves that the DIC technique is feasible in this study.The hoop strain and axial strain distribution curves of the specimens along the height obtained from the DIC technique found that the strain distribution along the height direction of the specimens was uneven and asymmetric,indicating that the confinement of specimens under axial compression is nonuniformity along the column height.(4)Based on experimental data,the applicability of the existing models to the prediction of the ultimate state of CFST columns partially confined with FRP strips is verified,and a new design model based on the model of Lam and Teng is proposed.The new model is more accurate,safe and applicable to predict the test value,which can provide a technical reference for the future strengthening and reconstruction project of CFST columns partially confined with FRP strips.