Study Of The Compressive Performance Of Ultra-High Performance Concrete Columns Confined By High-Strength Transverse Reinforcement

Ultra-high performance concrete(Ultra-high performance concrete,UHPC)is a new type of cement-based composite material,which has excellent characteristics such as ultra-high strength,high toughness,high durability and strong self-healing ability of micro-cracks.Although the addition of steel fiber in UHPC greatly improves the strength and toughness of the material,the ductility of UHPC compression members still needs to be improved.High-strength steel bars(High strength steel,HSS)have the characteristics of ultra-high yield strength,which is conducive to the development of lightweight,high-strength and high-performance structures.Using high-strength steel bars as stirrups to constrain UHPC can not only give full play to the high stress advantages of high-strength steel bars,but also improve the bearing capacity of UHPC members and greatly improve their ductility,realizing the complementary advantages of the two high-strength materials.At present,there are few researches on the mechanical properties of UHPC columns confined by high-strength stirrups,so this paper makes a systematic study on the mechanical properties of UHPC columns confined by high-strength stirrups under axial compression and eccentric compression through experimental research,numerical simulation and theoretical analysis.The main contents and conclusions are as follows:(1)The compression tests of 12 UHPC columns confined by stirrups were carried out.The effects of stirrup spacing on the axial compression performance of UHPC columns were analyzed and discussed in 3 columns,and the effects of stirrup strength,stirrup spacing,stirrup form,steel fiber content and eccentricity on the eccentric compression performance of 9eccentrically loaded columns were discussed.The test results show that compared with ordinary stirrups,high-strength stirrups can improve the eccentric bearing capacity and deformation capacity of confined UHPC columns more effectively.Reducing the distance between stirrups can improve the bearing capacity,ductility and toughness of axially compressed columns,as well as the bearing capacity and deformation capacity of eccentrically loaded columns.The use of composite stirrups in UHPC columns with small section size(200×200 mm)can improve the deformation capacity of eccentrically loaded columns after peak load,but has no obvious effect on peak bearing capacity.Properly increasing the content of steel fiber in UHPC can significantly improve the bearing capacity and deformation capacity of eccentrically loaded columns.(2)The fine finite element model of high strength stirrup confined UHPC column including UHPC core area,UHPC cover,stirrups,longitudinal reinforcement and steel fiber is established by using finite element software.The failure mode,load-strain curve and load-deflection curve obtained by simulation are in good agreement with the experimental results.The results of parametric analysis show that the bearing capacity of the specimen decreases but the deformation increases with the increase of eccentricity.Increasing the strength of longitudinal bars can improve the bearing capacity and deformation capacity of columns under large eccentric compression,but the lifting effect is obviously reduced when the strength of longitudinal bars is larger.With the increase of slenderness ratio,the stiffness and bearing capacity of the specimen decrease,but the deformation increases,and the larger the eccentricity,the more obvious the deformation.(3)Uniaxial stress-strain curves of UHPC columns confined by high strength stirrups are studied.In the compression section of the constitutive model,based on the Mander model confined concrete theory,considering whether the high strength stirrups yield at the peak load point,the stirrup confined UHPC constitutive model is established.In the tensile section of the constitutive model,based on the previous studies on the drawing load of single fiber,considering the influence of fiber distribution,inclination and embedded depth,the theoretical calculation formula of UHPC peak tensile stress and its tensile constitutive model are proposed.By comparing with the experimental curve,the rationality of the constitutive model is verified.(4)The N_u-M_u curve of UHPC column confined by high strength stirrups is obtained by integrating the cross section stress of the column using the proposed constitutive model,which can better predict the bearing capacity of the specimen.In order to simplify the calculation,the stress composition of the section of UHPC columns confined by high strength stirrups is analyzed,and the simplified formulas for calculating the bearing capacity of large and small eccentric compression considering UHPC tensile strength and stirrups are put forward.The calculation results predict the test results more accurately.