Axial compression behavior of steel tube filled with steel-fiber-reinforced self-stressing recycled aggregate concrete with moderate slenderness

An innovative steel-fiber-reinforced self-stressing recycled aggregate concrete-filled steel tube (SSRCFST) is proposed for the widespread application of recycled coarse aggregates (RAs). This study aims to explore the axial compression behavior of SSRCFST columns with moderate slenderness. A total of 27 specimens were designed with various slenderness ratios, steel-tube diameter-thickness ratios, concrete strength grades, RA quality replacement rates, and steel-fiber contents. The failure modes, load-deflection relationship, strain response, ultimate bearing capacity, and deformation capacity of the specimens were investigated. The experimental results indicated that an increase in the slenderness ratio aggravated the sectional stress gradient, transforming the specimens from strength failure to buckling failure. Although self-stress improves the ultimate bearing capacity of the SSRCFST columns, the second-order effect becomes more evident, and faster strength degradation of the slender SSRCFST columns occurs. A refined finite element model of slender SSRCFST columns was proposed based on concrete constitutive law modifications and temperature-load applications. The numerical results show that self-stress contributes to the confinement effect in the initial elasticity phase. Moreover, a new prediction model that accurately captured the compressive properties of slender SSRCFST columns is proposed.