Analysis Of Mechanical Properties Of Concrete Columns With Circular Section Confined By CFRP Strips

As an important stress component,the durability of reinforced concrete columns is affected by factors such as load and external environment during service.On the other hand,when subjected to seismic action,a certain degree of damage and destruction of reinforced concrete columns will adversely affect the entire concrete structure,so the mechanical properties of pier columns are crucial.In view of the above problems,the reinforcement and repair of RC columns can meet the design requirements and engineering needs.Fiber Reinforced Polymer(FRP)has the advantages of high strength,fatigue resistance,structural size stability and good corrosion resistance.In recent years,it has been widely used to improve the compression,bending and shear properties of reinforced concrete members.Experimental and analytical studies have shown that FRP can improve the strength and ductility of concrete columns under axial compression.FRP confined concrete columns are divided into two types:FRP fully wrapped confined concrete columns and FRP strip confined concrete columns.Compared with FRP fully wrapped confined concrete columns,FRP strip confined concrete columns have the following advantages:(1)FRP strip confined concrete requires less FRP materials,saves time and labor,and saves costs;(2)FRP strip confined concrete has more design parameters(FRP strip width and FRP strip net spacing),more flexible;(3)FRP strip constraint is more suitable for reinforced concrete beamcolumn joint area or reinforced concrete column plastic hinge area.For FRP-confined concrete columns with circular or non-circular cross-sections,domestic and foreign scholars have developed a variety of stress-strain models.However,it should be noted that most of the studies are aimed at FRP fully wrapped confined concrete columns,and there are few studies on FRP strip confined concrete columns,and the corresponding theory and analysis are relatively imperfect.Therefore,the FRP strip confined concrete force is carried out.In this paper,the uniaxial compression test of 26 specimens of FRP strip confined circular section concrete columns was carried out by using 300 T pressure testing machine.Based on the test results,the failure mode,stress-strain curve,characteristic points(ultimate strength and ultimate strain)and expansion behavior were analyzed.The plastic parameters(friction angle,cohesion and expansion angle)of Drucker-Prager model were modified,and the model verification and parametric analysis were carried out through the secondary development of ABAQUS.The arching effect of FRP strip confined concrete columns was studied.In this paper,the following results are obtained by combining experimental analysis,numerical simulation and theoretical research:(1)Based on the analysis of the test results,the influence factors of friction angle are determined,and the friction angle is defined as the function of plastic strain.The whole process of cohesion is analyzed,the cohesion model is established by piecewise function,and the expansion angle model considering only the constraint stiffness ratio is established.Based on ABAQUS,the friction angle model,cohesion model and dilation angle model are customized by USDFLD subroutine and GETVRM subroutine.The results show that the modified Drucker-Prager model can well predict the stress-strain response of FRP confined concrete columns.(2)Based on the finite element numerical simulation,the influence of design parameters such as the compressive strength of unconstrained concrete,the width of FRP strips,the net spacing of FRP strips and the thickness of FRP strips on the arching effect of FRP strips confined concrete was analyzed.The results show that the net spacing of FRP strips and confining pressure have great influence on arching.At the same time,the stress state of FRP strip confined concrete is analyzed,and the analytical solution model and arch angle model of effective constraint coefficient are derived.(3)Based on the effective confinement coefficient,the ultimate strength model and ultimate strain model of FRP strip confined concrete are proposed through the test samples in the existing literature.Three evaluation indexes are used to compare the existing strength model and strain model.The proposed ultimate strength model and ultimate strain model have good prediction accuracy.