Mechanical Performance Of Seawater Sea-Sand Concrete Filled CFRP-Steel Composite Tube Columns Under Axial Compression

In the construction process of marine civil engineering,there are problems of shortage of building materials and shortened structural life cycle.Applying seawater sea-sand concrete directly to steel-concrete structures can effectively alleviate the problem of river sand shortage,while saving a large amount of engineering construction costs.Concrete filled steel tube（CFST）has the advantages of high bearing capacity and good ductility,but the corrosion of steel by chloride ions limits its application in marine civil engineering.Applying FRP to the inner and outer walls of steel tubes can improve its bearing performance and durability.Currently,the focus of most studies is only on the reinforcement effect of wrapping FRP on the outer wall of steel tubes,while the existing studies have less involved in the mechanism of action and the difference in reinforcement effect between inner and outer FRP,and further exploration is needed.Based on the above background,by means of experimental research,numerical simulation,and theoretical analysis in this paper,the axial compression performance of seawater sea-sand concrete filled CFRP-steel composite tube columns and action mechanism of inner and outer CFRP were studied in depth.The main work and corresponding conclusions are as follows.1.The axial compression experiment of 18 seawater sea-sand concrete filled CFRP-steel composite tube columns were completed.The whole process of apparent damage of the specimens were observed,and the displacement-load curves were obtained.The axial compression performance indicators of the specimens under different parameters were compared and analyzed.The results show that.The final failure pattern of the specimens is obviously related to the position of CFRP.The specimen without inner CFRP is in the transition state from waist drum failure to shear failure,while the specimen with inner CFRP is in shear failure.The ultimate bearing capacity of the specimen is obviously correlated with the number of layers of CFRP.With the increase of number of layers of CFRP,the average increase rate of the ultimate bearing capacity of the specimen is 7.3%,20.7%and 33.4%,respectively.Under the same number of layers,the ultimate bearing capacity of the specimen with inner CFRP is higher.The initial elastic stiffness of the specimens has little correlation with the position and number of layers of CFRP,and the effect of inner CFRP is better than that of outer CFRP in slowing down the rate of secant stiffness decline of the specimen.The ductility of the specimens based on energy dissipation decreases with the increase of the number of layers of CFRP,and has little correlation with the position of CFRP.In addition,the axial compression working mechanism of seawater sea-sand concrete filled CFRP-steel composite tube columns is summarized.2.A nonlinear finite element model of seawater sea-sand concrete filled CFRP-steel composite tube columns was established.The whole process of axial compression experiment was simulated.The failure mode,axial displacement-load curve,and ultimate bearing capacity are in good agreement with the experiment results.Based on this,19 extended parameter specimens were established.The effects of changes in parameters such as CFRP strength,number of layers of CFRP,steel yield strength,and concrete strength on the bearing capacity and displacement-load curve of seawater sea-sand concrete filled CFRP-steel composite tube columns were further analyzed,and optimization suggestions are given.3.A formula for calculating the bearing capacity of axially compressed columns considering the radial stress correction factor of inner and outer CFRP is proposed.The calculation formula for bearing capacity was verified by collecting specimens from existing literature,and the average value of N_ca/N_uwas 0.995,with an average absolute error of 8.78%.The agreement was good,verifying the accuracy and applicability of the calculation formula.The research results of this paper reveal the changes in axial compression performance and failure mechanism of seawater sea-sand concrete filled CFRP-steel composite tube columns.A bearing capacity calculation formula with good accuracy has been proposed.The relevant achievements can provide certain reference value for the application of FRP confined steel tube concrete structures in marine civil engineering.