Investigation On High-strength Stirrups Reinforced Concrete-filled UHPC Tube Under Axial Compressive Loading

Based on the existing problems of reinforced concrete structure and the excellent performance of UHPC,a new type of hybrid structure,high-strength stirrups reinforced concrete-filled UHPC tube is proposed.UHPC thin-walled tubes are prefabricated and maintained through mature production lines in the factory to ensure that the material reaches its expected high performance and meets the requirements of the development of building industrialization;UHPC prefabricated tubes can be directly used as construction templates during the construction phase;When in use,the prefabricated UHPC tube contributes to the compressive strength of the hybrid column,and provide a certain lateral constraint to inhibit the lateral deformation of the internal concrete,due to the ultra-high compressive strength and good deformation ability of UHPC;Furthermore,high-strength stirrups are used to reinforce the UHPC tube in order to improve the workability,tensile strength and ductility of the column.Based on the existing research of the research group,this paper studies the axial compressive performance of high-strength stirrups reinforced concrete-filled UHPC tube.The main contents and conclusions are as follows:（1）The axial compressive performance test of high-strength steel stirrups reinforced concrete-filled UHPC tubes was carried out.Taking prefabricated UHPC tubes and pitch as design parameters,the exterior part of the column is prefabricated UHPC tube reinforced with high-strength steel stirrups,and the core of the column is precast normal strength concrete.Based on the UHPC preparation experience,the technique of UHPC tube preparation process is further optimized.The axial compressive test was carried out on the hybrid column.Studies have shown that: UHPC tubes can effectively increase the axial compressive strength of the hybrid column and increase the early stiffness of the hybrid column.After the specimen is damaged,the UHPC protective layer can remain integral with the specimen;on the other hand,smaller stirrup pitch can effectively improve the compressive strength and ductility of the hybrid column.（2）The axial compressive performance test of FRP stirrups reinforced concrete-filled UHPC tubes was carried out.FRP have the characteristics of high strength and high durability,and have better compatibility with UHPC in corrosive environments such as the ocean projects.Therefore,FRP stirrups are used to strengthen the hybrid column to study its mechanical properties.First,test on axial tensile mechanical properties of FRP stirrups was carried out.Then,on the basis of the axial compressive test of steel stirrups reinforced UHPC hybrid columns,taking stirrup type and stirrup pitch as design parameters,axial compressive performance test on the FRP stirrups reinforced hybrid column was carried out.The results show that the type of stirrup has little effect on the compressive strength and early stiffness of the hybrid column,but it has a greater impact on the ductility of the specimen;reducing the stirrup spacing has less effect on the compressive strength of the hybrid column reinforced by FRP reinforcement,but can effectively improve the ductility of the hybrid column.（3）Finite element simulation of high-strength FRP stirrups reinforced concrete-filled UHPC tubes was carried out.Using ANSYS/LS-DYNA software to perform fine finite element modeling of UHPC hybrid columns and explicitly solve them,and compare the simulation results with the test results.The results show that the finite element model established in this paper can predict the axial compressive behavior of the hybrid column accurately.The finite element model can also realize the simulation of the crack development of the hybrid column and the stress change of the steel fiber.Based on the finite element model,the influence of UHPC tube thickness,stirrup pitch,stirrup strength and stirrup elastic modulus on the axial compressive performance of the hybrid column is further analyzed.（4）The study on the calculation method of the high-strength stirrups reinforced concretefilled UHPC tubes under axial loading is carried out.Based on the effective confining theory of the Mander model,the constraint effect of the hybrid column is analyzed.And the constitutive models of normal strength concrete and UHPC tube constrained by high-strength stirrups are established respectively.Furthermore,The corresponding compressive strength is calculated and superimposed with the existing unconstrained UHPC model part and the compressive strength of the longitudinal reinforcement to obtain the load-strain curve calculation model of the hybrid column.The results show that the high-strength stirrups are not necessarily yielded under the peak load of the hybrid column,and the actual stress of the stirrups should be used for calculation when considering the confining effect.The load-strain calculation model of the hybrid column is compared with the test and simulation results.The calculation model proposed in this paper can accurately predict the compressive strength and ductility of the hybrid column.