Study On Axial Compression Performance Of CFRP-confined Steel Tube CaO Expansive Concrete Member

Concrete-filled steel tubular(CFST)structure is widely used in modern architecture because of its excellent mechanical properties.Under the restriction of the outer wall of the steel tube,the concrete is in a state of three-dimensional compression,which improves its strength and elastic modulus.With the increase of the binding force on the core concrete,the performance of concrete is further improved.The supporting effect of concrete delays the buckling of the steel tube,improves the stability of the steel tube,and makes full use of the restraint effect.The cooperative work of steel tube and concrete is an important guarantee to play the advantages of concrete-filled steel tube structure,which requires that the steel tube and concrete should always have a good bond during the loading period to ensure that they work together.However,the difference of Poisson’s ratio between the steel tube and the concrete makes the transverse deformation of the steel tube larger than that of the concrete in the initial elastic stage,which leads to the decrease of the bond between the outer wall of the steel tube and the concrete,and even leads to the debonding between the two materials,which is not conducive to the overall performance of the specimen.The addition of expansive agent can enhance the interfacial bonding strength between steel tube and concrete,but can not change the deformation trend of steel tube and concrete.In CFST columns,the core concrete can prevent the inward buckling deformation of the steel tube,but can not prevent the outward buckling deformation of the inelastic steel tube.Based on this,a CFRP-confined Ca O-filled expansive concrete filled circular steel tube structure is proposed in this paper.In this paper,48 unconfined concrete specimens and 16 concrete-filled steel tubular(CFST)columns are tested to study the deformation of the specimens aged 91 days and 28 days and the influence of different factors on the expansion of concrete.The mechanical properties and workability of 16CFRP-confined concrete filled steel tubular columns and 32 CFRP-confined concrete filled steel tubular columns were studied under axial compression with the addition of expansion agent,concrete strength and the ratio of steel tube diameter to thickness.The experimental phenomena,the load-strain curves and the failure modes of the specimens were compared and analyzed.The whole failure process of concrete filled steel tubular columns and CFRP-confined concrete filled steel tubular columns was analyzed.The experimental results show that:1.Calcium oxide expansion agent can produce good expansion effect,which can fill and compact the internal voids of concrete,but too much calcium oxide will expand the internal voids of concrete and reduce the overall performance of concrete.2.The addition of expansion agents can reduce the elastic modulus,Poisson’s ratio,and strength of concrete,and can improve the fluidity and bulk density of concrete.With the increase of the content of expansion agent,the expansion strain of both concrete specimens and concrete filled steel tubular specimens increases.The thicker the steel tube is,the more the CFRP layers are,the stronger the constraint capacity to the core concrete is,and the smaller the deformation of the specimen is.3.Increasing the thickness of the steel tube and the CFRP restraint coefficient can increase the ultimate bearing capacity and deformation capacity of the specimen,and can reduce the generation and development of local buckling of the specimen.The greater the restraint coefficient of the specimen and the more expansive agent added to the concrete filled steel tubular column,the more obvious the improvement of its ultimate bearing capacity.After reaching the ultimate bearing capacity,the magnitude of the decrease in the bearing capacity of the specimen is also greater.