FRP Tube Confined Sea Sand Concrete Columns With Composite Bars: Axial Compressive Behavior

In order to solve the problems of corrosion and durability of steel bars in reinforced concrete and sea sand concrete members,and to avoid the disadvantages that GFRP bars have no obvious yield point and low elastic modulus when they are damaged,so that the stiffness of concrete structures strengthened with GFRP bars is small and sudden damage occurs in the limit state,this paper introduces steel-GFRP composite bars(hereinafter referred to as composite bars)which combine their advantages as spiral of sea sand concrete columns.A new concept of composite reinforced sea sand concrete column is put forward: the highstrength reinforcement is designed as the inner core of composite reinforcement and the outer layer is GFRP fiber,and the ductility of composite column is improved by CFRP restraint.Monotonic axial compression test is carried out with CFRP tube restrained steel-GFRP composite reinforced sea sand concrete column(hereinafter referred to as composite restrained column)as the main research object,and the failure mode and axial compression performance such as ultimate bearing capacity and ultimate axial strain of composite restrained column are studied.The spiral types,spiral spacing,CFRP tube layers and section shape are summarized.The main research contents of this paper include:(1)failure mode of 1)CFRP tube confined steel-GFRP composite reinforced sea sand concrete column.The test results show that the ultimate failure of the specimen is mainly CFRP fiber fracture near the middle height of the non-overlapping area,GFRP longitudinal reinforcement and spiral fracture mainly near the middle height,and high-strength steel spiral has no obvious visual change.(2)The axial stress-strain curve of the specimen mainly includes two stages: rising stage and falling stage.The rising stage generally has three stages: elastic stage,nonlinear transition stage and strengthening stage.For the descending section,it can be divided into two types: 1)CFRP tube and GFRP spiral are damaged at the same time.It mainly occurs in CFRP tube confined GFRP reinforced sea sand concrete cylindrical specimens.2)CFRP tube and GFRP spiral were damaged successively.There is an obvious inflection point in the descending section,which mainly occurs in the CFRP tube confined composite reinforced sea sand concrete cylinder specimen.In the axial stress-hoop strain curve,all specimens show a monotonic upward trend,and the average hoop strain of CFRP ends at about 0.8%.(3)Through monotonic axial compression tests of 16 CFRP tube-confined steel-GFRP composite reinforced sea sand concrete columns and 8 CFRP tube-confined steel-GFRP composite reinforced sea sand concrete square columns,the influences of four parameters,i.e.spiral type,spiral spacing,CFRP restraint stiffness and cross-section shape,on the axial compression performance of specimens were studied.The results show that with the increase of CFRP restraint stiffness,the bearing capacity and axial deformation capacity of concrete with composite restraint columns are improved to a certain extent;When the spiral spacing is 50 mm compared with 100 mm,the steel-GFRP composite spiral has higher efficiency in improving the axial deformation capacity of concrete columns than GFRP spiral.The axial compression performance of composite restrained cylindrical specimens is better than that of composite restrained square columns.(4)By discussing four typical existing design models of steel-GFRP confined concrete,comparing the predicted values of the design models with the experimental values(including ultimate axial stress and ultimate axial strain),the applicability of different models to confined concrete in FRP tube confined composite reinforced sea sand concrete columns is analyzed.Wei's ultimate axial stress model considers the effects of CFRP tube and internal spiral respectively,and predicts the lifting effects of two kinds of constraint materials on the ultimate axial stress with different coefficients.Under different cross-sectional shapes and spiral,the model can accurately predict the ultimate axial stress of concrete confined by composite reinforcement with sufficient constraints and insufficient constraints.Lee's ultimate axial strain model can well predict the overall trend,but generally underestimates the effect of spiral constraint on concrete.