Experimental Study On PET FRP-Concrete-High Strength Steel Columns Under Compression

High-strength steel is often used in combination with concrete,but the ultimate compressive strain of the two materials does not match,so that the ultimate strain of the materials cannot be reached at the same time,and the materials are not fully utilized.Moreover,steel menbers is prone to buckling under pressure,such as the outward buckling of steel tubes when concrete-filled steel tubes are compressed.A kind of PET FRP with recycled plastics as the main raw material has great deformation,and the fracture strain can reach 7%.Wrapping high-strength steel and concrete with PET FRP can improve the deformation of concrete and restrain the buckling of steel pipes.However,PET FRP has low elastic modulus,so it needs to produce large deformation before it can fully play its role.Therefore,when using PET FRP to wrap high-strength concrete-filled steel tube columns,adding a layer of concrete to the contact surface to form a composite column of PET FRP tube-concrete-high-strength steel tube is beneficial to give full play to the material properties of PET FRP,and can simultaneously improve the deformation ability of concrete,make the high-strength steel and concrete deform cooperatively and inhibit the buckling of high-strength steel tube.All three materials can be fully utilized,and a composite column with high bearing capacity,strong deformation ability and environmental protection can be obtained.In this paper,under the same steel pipe diameter,the effects of three different FRP pipe diameters（210mm,250 mm,290mm）and three FRP layers（2,3,4 layers）on the cross-section strain distribution,the overall bearing capacity of concrete,the bearing capacity of core concrete and the failure mode are investigated through monotonic axial compression test.The effects of three different slenderness ratios（1500/210,1000/210,500/210）,three different eccentricities（25mm,50 mm,75mm）and two FRP layers（3layers,4 layers）on the lateral deformation capacity,cross-sectional strain distribution and failure mode of specimens were investigated by eccentric loading test.On the basis of the test results,the variation law of steel tube stress in hybrid columns with the increase in specimen deformation is calculated.By quoting Zeng model,adding the constraint effect of steel tube on concrete and quoting Lin model.The stress-strain curve of composite column concrete as a whole and core concrete confined by FRP and steel tube are predicted and compared with the experimental results.Cross-section analysis method adopted by Jiang and Teng was used to analyze the hybrid columns under eccentric compression test and compared with the test results.The main conclusions are as follows:（1）The main reason for the decrease of bearing capacity of specimens under axial compression is that FRP suddenly breaks and loses its restraint on the specimens.The main reason for the decrease of bearing capacity of specimens under eccentric compression is the local buckling of steel tube in compression zone,and the bearing capacity of specimens does not rise after buckling,and the ultimate bearing capacity has no obvious change when the FRP layers to reach 3 or more.Both axial compression and eccentric compression specimens show excellent deformation,and the maximum peak strain of hybrid columns is 38 times that of unconstrained concrete.The buckling of steel tubes in hybrid columns is effectively restrained,and the combined effect that the bearing capacity is greater than the sum of individual bearing capacity of each material is obtained.（2）Both Zeng model and Lin model can accurately predict the ultimate stress of composite column concrete or core concrete,and the prediction accuracy of ultimate axial strain is second.（3）By considering the bearing capacity decline after the steel tube reaches the yield strain and the stress-strain relationship of concrete restrained by FRP and steel tube,the cross-section analysis method can accurately predict the initial stiffness of the load-compression zone limit point pressure strain curve,the load-mid-span lateral deflection curve,the bending moment-curvature curves and the peak bearing capacity of the specimen.