The Mechanical Behavior Of Steel Reinforced ECC Encased CFST Columns

Concrete filled steel tube(CFST)columns have gained increasing attention in structural engineering due to their numerous structural benefits,including high strength,favorable ductility and convenient for construction.Despite its structural advantages,however,the outer steel tube of CFST susceptible to corrosion especially under chloride environment.Moreover,the fire resistance of CFST has been a concern because the outer steel,without protection by concrete cover,nearly loses its strength at 600℃.In recent years,a new design namely concrete-encased CFST column,which comprising both inner CFST component and outer reinforced concrete(RC)component,has attracted interests of structural engineers and researchers.The addition of the outer RC layer leads to higher corrosion resistance,fire resistance,load carrying capacity and buckling resistance of the CFST column.However,since the inner CFST component is ductile while the outer RC component is brittle,it was noticed that the outer concrete was crushed while the inner CFST was still in the elastic-plastic stage,which indicated a poor composite effect between the two components.Moreover,since the outer concrete is easily-crushed,the longterm durability of concrete-encased CFST columns become a major concern especially for those exposed to severe environment such as marine or freezing-thawing environments.In order to improve the ductility and durability of the composite column,it is proposed to substitute the outer concrete with engineered cementitious composite(ECC),and the ECC encased CFST columns was formed.ECC is a class of high performance cementitious composites with pseudo strain-hardening behavior and excellent crack control capacity.ECC and concrete have similar ranges of tensile strength(4-6MPa)and compressive strength(30-80MPa),but ECC shows a high tensile strain capacity of 3%-5%(about 300 times that of concrete)with a crack spacing of 3-6mm and crack width of about 100μm.In uniaxial compression,the strain at the ultimate strength is nearly two times that of conventional concrete.Substitution of concrete with ECC can largely reduce the cracking and durability problems associated with low tensile strength and brittleness of concrete.Moreover,ECC’s crack control and self-healing ability lead to its superior durability under various mechanical and environmental loading conditions such as freeze thawing and chloride exposure,which will then improve the durability of composite column.This paper investigated the mechanical behavior of ECC-encased CFST columns under uniaxial compressive loading,eccentric compressive loading and cyclic loading through experimental studies,theoretical analysis and finite element simulations.The specific contents are as follows:(1)The axial compressive performance of ECC-encased CFST columns was experimentally investigated.Four ECC-encased CFST columns and two concrete-encased CFST columns were tested.The influences of longitudinal reinforcement ratio,stirrup ratio and the thickness of steel tube on ECC-encased CFST columns were analyzed.According to the experimental research,the concrete-encased CFST column failed with serious concrete spalling and crushing,while the ECCencased CFST column generally maintained its integrity.The compressive strength for ECCencased CFST column was about 30% higher than concrete-encased CFST column,even though the outer ECC has similar compressive strength with the outer concrete,indicating that the ECC encased CFST has a better composite effect than that of concrete-encased CFST columns.Also,the ductility index and accumulated energy dissipation for specimen ECC-encased CFST were about 40% and 110% higher than that of concrete-encased CFST column,indicating that substituting concrete with ECC could increase the ductility of the composite columns.(2)The eccentric compressive performance of ECC-encased CFST columns were experimentally investigated.Seven ECC-encased CFST columns were tested and all the ECCencased CFST columns failed in a ductile mode under different eccentricity ratios.The outer ECC in both sides did not spalled and all the specimens kept its integrity.There were three types of failure modes for all specimens depending on different eccentricity ratio: compression-controlled failure mode,tension-controlled failure mode and balanced failure mode.With the increase of eccentricity ratio,the failure mode shifted from compression-controlled failure to balanced failure mode,and then tension-controlled failure.(3)The hysteretic behavior of ECC-encased CFST columns were experimentally investigated.Seven ECC encased CFST columns as well as two concrete-encased CFST columns were tested under axial forces and lateral cyclic loading.The concrete-encased CFST column failed with severe concrete spalling and crushing,while the ECC-encased CFST column generally maintained its integrity.The hysteresis loops for ECC-encased CFST columns were much fuller than that of concrete-encased CFST columns.The ECC-encased CFST columns exhibited stable ductility behavior while a clear pinching effect was observed for concrete-encased CFST columns.Compared with concrete-encased CFST column,ECC-encased CFST column has a higher ductility as well as a lower strength and stiffness deterioration during the post-peak stage.Also,the cumulative energy dissipation for ECC-encased CFST column is about twice as much as that of concrete encased CFST columns with same geometry.(4)Based on plane cross-section assumption and rational constitutive relations,the theoretical models were developed in order to predict the sectional capacity of ECC-encased CFST columns under both axial loading and eccentric loading.A good agreement was achieved between the experimental and analytical results,indicating the proposed theoretical models is suitable to evaluate the carrying capacity of ECC-encased CFST columns.(5)The mechanical behaviors of ECC-encased CFST columns under uniaxial compressive loading,eccentric compressive loading and cyclic loading were investigated with nonlinear finite element method.The appropriate constitutive models and contact element were applied in these models and good agreements were achieved between the experimental and simulation results.Based on the proposed FEA models,the parameter analyses were conducted in order to investigate the effect of different parameters on the mechanical behavior of ECC-encased CFST columns.(6)Based on the experimental and analytical research as well as the related design codes and standards,the design methods for ECC-encased CFST columns under both static and dynamic loading was proposed in this paper.Also,the suggestions for the construction and acceptance of ECC-encased CFST columns were also proposed in this paper.