Research On Compressive Mechanical Behavior Of Concrete Columns Confined By GFRP Composite Spiral Stirrups

FRP bars are considered to be able to solve the problem of poor durability of marine engineering structures because of its excellent corrosion resistance.The concrete column confined by FRP stirrups can solve the problem that the outermost steel stirrups are more susceptible to corrosion and affect the durability of the structure.FRP spirals provide strong confinement effect on the core concrete,but cannot confine the concrete at the corners of the square section column.Although FRP stirrups can be used for square section columns,their confinement effect on the core concrete is relatively weak.The composite spiral stirrup form composed of FRP spirals and square stirrups can not only be used for square section concrete columns,but also can improve the load capacity and ductility of the concrete column by increasing the effective lateral confining pressure of the stirrups on the core concrete and solve the problem of brittle failure of concrete columns reinforced with FRP bars.Therefore,research on the compression characteristics of concrete columns confined by FRP composite spiral stirrups is of profound significance for the promotion and application of concrete columns reinforced with FRP bars and the improvement of the durability of marine engineering facilities.The research work mainly includes the following three parts:Based on the test results of the basic mechanical properties of GFRP bars,this paper involved investigating the axial compressive behavior of 9 concrete columns confined by GFRP composite spiral stirrups.The influence of critical parameters including the stirrup spacing,stirrup diameter,stirrup form and longitudinal reinforcement material type was focused in the experiments.The effect of the above parameters on the load capacity and deformability of concrete columns was analyzed.The crack shape and final failure mode of the axial compression specimen were summarized.The experimental results showed that the axial compression process of concrete columns confined by GFRP composite spiral stirrups could be divided into elastic phase,elastoplastic phase,peak plateau phase and failure phase.There were roughly three types of cracks in the specimens: "H" type cracks,"N" type cracks and shear cracks.Reducing the stirrup spacing and increasing the stirrup diameter could significantly improve the strength of the core concrete and ductility of the specimen.When the stirrup spacing was reduced to 50 mm,the load-axial strain curve appeared "secondary peak" phenomenon.The confinement effect of GFRP composite spiral stirrups on core concrete was better than that of composite diamond stirrups and ordinary square stirrups.The load carried by the GFRP bars was far smaller than the load carried by the steel bars at the peak load.This paper involved investigating the eccentric compressive behavior of 7concrete columns confined by GFRP composite spiral stirrups.The influence of critical parameters including eccentricity,stirrup spacing and stirrup form was focused in the experiments.The effect of the above parameters on the load capacity and ductility of concrete columns under eccentric loading was analyzed.The experimental results showed that failures of concrete columns under eccentric loading were all initiated by crushing of concrete in compression.The distribution of the strains in the column satisfied the assumption of plane section.The load capacity of the specimen decreased with the increase of eccentricity.When the load was constant,the greater the eccentricity,the greater the corresponding lateral deformation.Reducing the spacing of stirrups could improve the load capacity and ductility of the concrete columns.GFRP composite spiral stirrups could keep the concrete columns high load capacity and improve their deformation performance.The external square stirrups confined the concrete between the spirals and the square stirrups and at the same time produced "double-confinement" on the core concrete together with the GFRP spirals.Based on the regression analysis of the test data,the calculation formulas for the maximum confined concrete strength,concrete strain corresponding to confined concrete strength and maximum concrete strain of the square concrete column confined by GFRP composite spiral stirrups were proposed.Three typical confined concrete models,including Mander model,Afifi model and Guo Zhenhai-Zhang Xiuqin model,were introduced.Based on the existing constitutive relationship model for reference,the constitutive model of concrete confined by GFRP composite spiral stirrups was established through regression analysis of test data.