Study On Mechanical Properties Of Frp-confined Steel Fiber Reinforced Concrete Columns Under Axial Compression

The present research shows that because of the high brittleness of high-strength concrete and ultra high strength concrete,the lateral expansion property is worse than that of ordinary concrete,The FRP confined high strength and super high strength concrete subjected to axial compression has temporary softening behavior(i.e.softening after initial peak value),which affected the overall effect of the confining to some extent.Adding steel fiber into high-strength and super high strength concrete can effectively improve the brittleness and enhance the deformation performance of concrete.Therefore,FRP confined steel fiber reinforced concrete columns can achieve good confinement effect,and it is an effective way to overcome the softening behavior.At present,there is a lack of research on FRP confined steel fiber reinforced concrete with low restrained stiffness.In this paper,concrete mix test without steel fiber is carried out.,to obtain the benchmark mix proportion,and then mixed with steel fiber on the basis of the mixture ratio,experimental research on CFRP confined steel fiber reinforced concrete with low confining stiffness is carried out.The main work is as follows:(1)The mix ratio design and preparation of steel fiber reinforced concrete.The use of common portland cement,silica fume,quartz powder,quartz sand,gradation of high effective carboxylic acid water reducing agent and defoaming agent and other materials,set stirring process,water binder ratio,sand ratio,water glass model and the age of the five parameters,in room temperature water conservation.The test indexes include the cube compressive strength,slump and slump expansion.The best mix ratio is cement: Silica Fume: quartz powder: quartz sand: water: water reducing agent: defoaming agent =1:0.25:0.32:1.787:0.251:0.031:0.029.(2)Axial compression test of steel fiber reinforced concrete columns confined by CFRP.The axial compression tests of 21 confined specimens and 15 un confined specimens were carried out.The best mix proportion of the second chapter was steel fiber reinforced concrete.The test parameters include CFRP layer number(1-3 level),steel fiber content(0-2.5%)and concrete strength(70-140MPa).The test contents include axial stress,axial displacement and circumferential strain.The effects of the parameters on the peak stress,the axial strain,the axial stress strain curve,the hoop strain-axial strain curve,the hoop strain distribution and the softening behavior after the initial peak are analyzed.The test results show that the higher the strength of the concrete is,the more obvious the softening phenomenon is,the softening phenomenon can be improved by adding the steel fiber content and the thickness of CFRP,while the steel fiber can improve the effective strain of the ring direction,while the influence of the thickness of the CFRP on the circumferential strain is not obvious.(3)Axially stress-strain model of CFRP confined steel fiber reinforced concrete columns.The FRP confined concrete axial stress and strain models of existing literatures are compared with the experimental data in this paper.The results show that all these models have different errors.In this paper,the axial stress-strain curves are divided into three stages: elastic section,transition section and straight line section.The peak stress,axial ultimate strain formula and axial stress-strain model of FRP confined steel fiber reinforced concrete are fitted by experimental data.Through the relationship between the slope of straight line segment and the ratio of confined stiffness,the limit value of the ratio between the middle and the weak type is determined to be 4.3.Considering the influence of steel fiber content,the effective strain reduction factor of FRP confined steel fiber reinforced concrete is fitted.(4)Finite element analysis of CFRP confined steel fiber reinforced concrete columns under axial compression The results show that CFRP select Hashin criteria,S4 R element,concrete damage plastic model and C3D8 R element,which can be used for ABAQUS analysis of CFRP confined steel fiber reinforced concrete columns.The axial stress-strain curves and failure modes of the finite element simulation agree well with the experimental results.The numerical simulation results show that the softening after the initial peak is reduced by increasing the constraint stiffness ratio.Under the steel fiber content of 2.5%,the constraint stiffness of CFRP has no softening when the ratio is greater than 19.8,and the overall performance of the high tensile strength type FRP is better than that of the elastic modulus and high ductility.