Research On The Mechanical Properties Of Concrete-filled Steel Tube Columns With FRP-UHPC Core Columns Subjected To Axial Compression

Concrete-filled steel tube columns with FRP-UHPC core columns(SCF-UHPC column)is a composite column of consisted of inner FRP-UHPC core column and concrete-filled steel tube.The ordinary concrete filled steel tube and UHPC filled with FRP are three-dimensional stress under axial load,and the inner FRP improve the fill effection of steel tube,the bearing capacity and ductility of SCF-UHPC column has been improved.Firstly,UHPC mixture was researched,and then UHPC was used in SCF-UHPC composite column,and the axial mechanical properties of SCF-UHPC column was researched,the main work is as follows:(1)This paper reviews the theoretical basis of active powder concrete(RPC)mix design,and summarizes the typical design methods of RPC mix ratio,summarizes the suitable range of RPC design parameters,and evaluates the typical semi-empirical semi-theoretical formula analysis.Based on the theory of the most compact,the effect of water-cement ratio,sand-to-plastic ratio,defoamer,silica fume and quartz powder content on UHPC was systematically studied.The results show that the hot water curing time is 3 days(3d);when the water-cement ratio is increased,the compressive strength of UHPC decreases,the fluidity increases,the water-cement ratio should not exceed 0.18;the compressive strength is the highest when the sand ratio is 1.14,the workability is also better;the optimal dosage of defoamer is 0.8%;the compressive strength of UHPC is more than 160 MPa.(2)The prepared UHPC was applied to the SCF-UHPC composite column.The effect of thickness of the FRP,the thickness of the steel pipe and the diameter of the core UHPC column were studied systematically by 12 composite columns.The experimental results show that the failure modes of SCF-UHPC composite columns are FRP faults,and there are many convex curves in the middle of steel tubes.Because of the double confinement of steel pipe and FRP,the failure of composite columns shows ductile failure characteristics.The load-displacement curve of the composite column is mainly divided into elastic phase,elastic-plastic stage,plastic flow stage(strengthening stage)and failure stage.The load-strain relation relation have bilinear characteristics,and can be divided into ascending,transition and damage stage.(3)Based on the experimental study,the influence of FRP and steel tube thickness on the performance of the composite column is analyzed.When the thickness of the steel tube is constant,the bearing capacity increases and the ductility increases with the increase of the FRP thickness.When the FRP thickness is constant,with the increase of the thickness of the steel tube,the bearing capacity is greatly improved,and the ductility is also improved.When the FRP thickness is small,with the increase of the thickness of the steel pipe,the bearing capacity of the composite column increases and the improvement of ductility is obvious.(4)The core UHPC column diameter has a great influence on the mechanical characteristics of the composite column.When the diameter of the core UHPC column is small,the load-displacement curve of the composite column has a long strengthening stage;On the contrary,after the outer steel tube yielded,the load began to decline,and soon FRP fracture,load drop.(5)Based on test,the stress state of the composite column and the confinement of the steel tube and FRP are analyzed.The formula of the axial load bearing capacity of the SCF-UHPC column is deduced based on the double shear uniform strength theory and the limit equilibrium method.The formula calculation results of this paper and the selected literature was compared with the experimental results,the calculation accuracy is good.In addition,the influence of the lateral pressure coefficient on the calculation accuracy is also discussed,and the reasonable value is given.