Performance Of Recycled Aggregate Concrete-Filled High-Strength Steel Tubes Under Combined Compression,Bending And Torsion

Recycled aggregate concrete-filled high-strength steel tubes(RAC-FHST)are novel composite structures which have remarkable characteristics:on the one hand,RAC has the advantages of saving natural aggregate and realizing the recycling of construction waste;On the other hand,the application of high-strength steel tubes(f_y≥460 MPa)is in line with the development trend of high performance of building materials.Therefore,the mechanical properties of RAC can be improved and the steel local buckling can be prevented or delayed due to the excellent composite behavior between RAC and high-strength steel tubes.It is thus expected that RAC-FHST has the advantages of good mechanical performance,high bearing capacity and great efficiency in environmental terms.So far,previous research on the interfacial property of RAC-FHST,performance of RAC-FHST members under axial compression,tension,compression-bending,torsion and compression-bending-torsion,and the performance of repeated recycled concrete is still limited.This dissertation thus investigates the following four aspects of research work:1.A series of push-out tests,compressive tests and tensile tests are conducted on58 RAC-FHST specimens.Analysis on the bond mechanism and the influence of key parameters are carried out.Based on the experimental study and finite element model(FEM),the performance of composite behavior between RAC and high-strength steel tubes are investigated.The microstructure difference between different generations(1st and 2nd)of repeated recycled concrete and ordinary concrete is revealed by scanning electron microscope.Based on the analysis,simplified models for bond strength,ultimate compressive and tensile strength calculations are proposed.2.A series of 22 RAC-FHST specimens under bending and compression-bending loading states are carried out.The main parameters varied in the tests are the shear span to depth ratio,the load eccentricity and the steel yield strength.The FEM is established based on the stress-strain model of core RAC.The stress development and distribution of RAC and high-strength steel tubes,and the influence of steel strength and the confinement factor on the N/N_u-M/M_u curves are investigated.Based on the analysis,empirical formulas for ultimate strength of specimens under compression-bending load are proposed.3.A series of 12 RAC-FHST specimens under pure torsion are carried out and the FEM is established.The influence of steel strength and RAC on the failure mode and the ultimate torsional strength,the yield surface and the distribution of compressive stress and shear stress of high-strength steel are investigated.Based on the parametric analysis,simplified model for ultimate torsional strength calculation is proposed.4.A series of 32 RAC-FHST specimens under combined compression-torsion,bending-torsion and compression-bending-torsion loading states are carried out.The influence of axial load and moment level and steel strength on the performance of RAC-FHST specimens are conducted.The FEM is established and verified to investigate the mechanical mechanism,failure modes and the influence of the confinement factor on the N/N_u-M/M_u-T/T_u curves.Based on the analysis,empirical formulas for ultimate strength of specimens under compression-bending-torsion are proposed,which can provide reference for the engineering practice and the formulation of standards.