Behavior Of Composite Columns Of Concrete-filled UHPC Tube (CFUT) Under Axial Compression And Seismic Loading

Ultra-high performance concrete(UHPC)is an emerging cement-based material,with ultra-high compressive strength,excellent durability,and good toughness.With the use of UHPC to replace conventional concrete in structural members,the mechanical properties of these members can be greatly improved,and also the poor durability of ordinary reinforced concrete(RC)structures can be solved from the material level.The UHPC-concrete composite members are formed by the partial substitution of concrete by UHPC in the important parts of the structural members,which can not only make full use of the ultra-high compressive strength and excellent durability of the UHPC materials but also be beneficial to reduce the cost of UHPC materials.Therefore,a new type of composite column,named CFUT(Concrete-Filled UHPC Tube)is proposed in this paper.The UHPC tube in the composite column can not only be used as the permanent formwork of the post-cast concrete in the construction stage but also can be used to bear the load and act as the high-durability protective jacket of the steel bar and the core concrete in the service stage.Therefore,the advantages of UHPC material can be fully utilized.In this paper,by the means of experimental research,numerical simulation,and theoretical analysis,the properties of UHPC material,the interfacial bond between UHPC and concrete,and the axial compressive and seismic behavior of CFUT composite columns were systematically investigated:(1)A series of material properties tests,including the slump test,compression test,and flexure test,were carried out to clarify the effects of mixture proportion on the workability and mechanical properties of UHPC.The compressive behavior of unconfined UHPC cylinders,steel stirrup,and FRP-confined UHPC cylinders was studied by finite element(FE)analysis.The FE model was verified to properly simulate the volumetric expansion of UHPC under compression and the enhancement of strength and ductility of UHPC under passive confinement.(2)Based on the study of damage evolution of UHPC-concrete interfacial bond strength,and by the combination of the bilinear cohesion model and the Coulomb friction model,a cohesive-friction interfacial model was proposed to simulate the bond behavior of UHPCconcrete interface,and the model was implemented into a FE software.Based on the proposed model,the influence of interfacial bond strength between UHPC and concrete on the mechanical properties of UHPC-concrete composite members was studied.The results show that a great error could be obtained if the full bond was assumed,while a good agreement between the experimental and FE results could be obtained if the developed model was used in the FE model.(3)The axial compressive performance of CFUT composite columns was experimentally studied,and the failure mode and the effect of the UHPC tube on the compressive behavior of CFUT columns were clarified.Compared with RC columns,the yield and peak strength of the CFUT columns were greatly increased,and the ductility of CFUT columns remained at a high level.Based on the FE analysis,the strengthening effect of the UHPC tube was revealed.The parameter analysis was carried out based on the verified FE model,from which the effects of key parameters,such as the thickness of UHPC,the yield strength,and the configuration of stirrups on the compressive behavior of CFUT columns were investigated.A model to predict the axial load versus axial strain relationships of the CFUT column was proposed,and the accuracy of the model was verified by comparing it with the test results.(4)The hysteretic behavior of CFUT columns was studied by the quasi-static test,from which the failure mode,the hysteretic curves,and the skeleton curves were obtained.Test results showed that the reinforced UHPC tube can work well with the inner concrete.Compared with the RC column,the CFUT column has a higher load-bearing capacity and deformation capacity.The three-dimensional FE model was developed to investigate the failure mechanism of composite columns.The fiber-based FE model was developed to analyze the effects of column parameters such as axial compression ratio,UHPC thickness,concrete strength,diameter,and yield strength of steel bars on the seismic behavior of CFUT columns.(5)Based on the above research on the axial compressive and seismic behavior of CFUT columns,a design approach of CFUT composite columns is proposed to calculate the loadbearing capacity of the composite columns.The design approach was verified by comparing it with the test results.Several issues in establishing the design approach,such as the use of the rectangular stress distribution method,the influence of UHPC tensile strength,and the definition of limit state were discussed.Combined with the experimental research and theoretical analysis in this paper,the relevant design recommendations for CFUT columns were given.