Axial Compressive Behavior Of Rectangular Concrete Columns Confined By BFRP And Steel Spirals

Rectangular concrete columns confined with FRP and steel spirals consist of three parts:outer fiber reinforced polymers,built-in traditional steel spirals and concrete.It combines the dual confinement characteristics of FRP and stirrups,showing higher strength and ductility.However,the researches on FRP-stirrup composite confined concrete columns mostly focus on circular section,while the researches on confined concrete rectangular section columns is relatively less at home and abroad.Besides,carbon fiber reinforced polymer(CFRP)is a common confining material.At present,a few studies have been carried out on concrete column confined by basalt fiber reinforced polymer(BFRP).Thirty-six BFRP-stirrup composite confined rectangular concrete columns are designed in this paper.Monotonic axial compression tests were carried out on rectangular concrete columns confined with BFRP and steel spirals,and stress-strain curves were plotted based on the experimental data.The effects of several parameters on the mechanical properties of rectangular concrete columns confined by BFRP-stirrups are also studied.The test results show that the double confinement mechanism of BFRP and steel spirals on core concrete can effectively improve the bearing capacity and deformation capacity of concrete columns.The axial compression performance of FRP-stirrups confined rectangular concrete columns is closely related to the number of FRP layers,stirrup spacing and the chamfering radius of specimens.Increasing the chamfering radius can improve the properties of the concrete column significantly.With the same layers of FRP and the spacing of stirrups,the bigger the chamfering radius is,the better the performance will be.Besides,increasing the number of FRP layers or enhancing stirrup spacing can effectively improve the bearing capacity and deformation capacity of the confined concrete column with identical chamfering radius.The stress-strain curves of FRP-stirrup confined rectangular concrete columns can be roughly divided into four parts:elastic deformation stage,transition stage,strengthening stage and residual stage.In the stage of elastic deformation,the stress-strain curves of specimens are basically consistent with that of plain concrete specimens.During the transition period,the outer FRP and the inner steel spirals begin to act to provide lateral confinement to the core concrete.The lateral confinement of stirrups on core concrete is basically constant in the strengthening stage,and the strain of FRP increases continuously,which provides the lateral confinement for continuous growth.After FRP breaks,it enters the residual stage.The lateral restraint force of FRP on concrete fails,and only the steel spirals remain in the continuous working state.In addition,the transverse strain comparison results of FRP-stirrup composite confined rectangular concrete column specimens show that the transverse strain at the chamfer tangent and chamfer center of the section is less than that at the middle of the sectionunder the stress concentration at the corner.The deformation in the middle of the specimen is obviously larger than that in the corner of the specimen.The existing FRP-stirrups confined rectangular concrete models are evaluated and analyzed through the database which includes the test results in this paper and the related test data of predecessors.The effective confinement factor k_e and section shape factor k_s of confined rectangular concrete are introduced to modify the calculation model of limit point and peak point of FRP-stirrup confined concrete cylinder.Furthermore,considering the reduction of confinement effect of core concrete with small chamfer radius,a new model of FRP-stirrups confined rectangular concrete columns is proposed after regression analysis of a large number of experimental data.Finally,the calculation models of ultimate stress,ultimate strain,peak stress and peak strain of rectangular concrete confined by FRP and steel spirals with wide applicability are proposed,and the errors are less than those of the existing models.By introducing the theoretical values into the general model,the stress-strain curves of rectangular concrete specimens confined by different materials can be predicted,especially the four-stage stress-strain complete curves of rectangular concrete confined by FRP and steel spirals under axial compression.Compared with the actual test curves of different specimens in the database,the proposed model in this paper performs well.It fully reflects the correlation among the parameters of confined concrete specimens under FRP,stirrups and their composition.It has wide applicability and accuracy.It can provide a basis for the research of confined rectangular concrete structures and has a good reference value for practical engineering applications.