Axial Compressive Performance Of Fiber Reinforced Lightweight Aggregate Concrete Confined By Stirrups Using Experiment And Numerical Analysis

Lightweight aggregate concrete（LWAC）has the advantages of light weight and high strength,excellent good heat preservation,heat insulation and low cost,while it exhibits higher brittleness compared with the normal weight concrete（NWC）.The compressive strength and ductility of LWAC can be improved by the composite confinement of stirrups for the reason that LWAC columns using common stirrups could not meet aseismic demands.This paper focuses on the fiber reinforced lightweight aggregate concrete confined by stirrups,and completes the axial compression test of the fiber reinforced lightweight aggregate concrete confined by stirrups,and theoretical analysis.An analysis model of the axial compressive performance of fiber reinforced lightweight aggregate concrete confined by stirrup is established,and a constitutive model of the fiber reinforced lightweight aggregate concrete confined by stirrup is proposed.The main research content includes:（1）Completion of 13 axial compression tests of fiber reinforced lightweight aggregate concrete confined by stirrups,analyzing the influence of hoop spacing,fiber type,and fiber volume on the confinement effect of concrete,observing the failure process and morphology,and studying the relationship between hoop strain and axial strain of constrained concrete.The results show that increasing the volume of reinforcement can effectively increase the lateral confinement force and the constrained concrete area;when the volume of reinforcement is relatively low,the strength of the reinforcement is not fully utilized when the constrained concrete reaches its peak strength;the addition of fibers effectively improves the brittleness of the lightweight aggregate concrete,and the improvement effect of steel fibers is better than that of carbon fibers.（2）Numerical simulation of the axial compressive performance of fiber reinforced lightweight aggregate concrete confined by stirrups.A model of the random distribution field of aggregate is established,and the range of model parameters is defined.An equivalent mechanical constitutive model of the sub-elements of the random distribution field of lightweight aggregate concrete is proposed by a parallel mechanical model.The parameter extension analysis is completed,focusing on the diameter of hoop reinforcement,concrete strength grade,and hoop spacing,and performance improvement suggestions are put forward.It was found that it is recommended to set the maximum value range ofηat 3.5 to 7.5,and the recommended range for the spatial correlation length is 2.5 D_max to 4 D_max.Reducing the spacing between reinforcement bars is the most effective way to improve the ductility of fiber reinforced lightweight aggregate concrete confined by stirrups.（3）Study of the constitutive model of fiber reinforced lightweight aggregate concrete confined by stirrups under axial compression.The effective lateral confinement force is selected as the main calculation parameter,and the effective confinement coefficient is modified.Considering the hoop reinforcement efficiency and the actual stress of the reinforcement at the peak point,a model for calculating the peak stress and peak strain of concrete is established,and a stress-strain relationship model for axial compression of fiber reinforced lightweight aggregate concrete confined by stirrups is proposed.The results show that the fitting results of the stress-strain curve of the constrained specimen in this paper by the selected four typical constitutive models are discrepant and need further modification.Three types of ascending stage forms and four types of descending stage forms are preferred,and the key parameters and characteristic points are corrected to improve the accuracy of the constitutive model.The work completed in this thesis has important significance for in-depth understanding of the properties of fiber reinforced lightweight aggregate concrete confined by stirrups,establishing a stress-strain relationship model of fiber reinforced lightweight aggregate concrete confined by stirrups,and promoting the application of lightweight aggregate concrete in practical engineering.The research on the basic mechanical properties of confined LWAC columns is of great significance for proposing the stress-strain curves for confined LWAC under uniaxial compression and further study of the performance of the confined LWAC,which has important theoretical significance and practical value for promoting the engineering application of lightweight aggregate concrete.