Section-dependence of stirrup confinement in reinforced and recycled aggregate concrete-filled steel tubes

To investigate the influence of cross-sectional shape on confinement efficiency of stirrups in reinforced and recycled aggregate concrete-filled steel tube (R-RACFST) with circular and square cross-sections under 100% recycled coarse aggregate, axial compression tests were conducted on 28 stub columns. Test variables included stirrup patterns (hoops and spirals) and spacing (volumetric ratios were 0.50%, 0.54%, 1.11%, and 1.12%). Mechanical performance was analyzed based on failure mode, load transfer behavior, peak load, ductility, fracture toughness, and composite effect. Although reinforcement did not alter the global failure mode, it significantly improved the integrity of the core recycled aggregate concrete, which remained freestanding after removal of the steel tube. R-RACFST exhibited superior peak load, ductility, fracture toughness, and composite effect compared to RACFST and even conventional concrete-filled steel tube. Stirrups showed significantly higher efficiency in enhancing peak load, fracture toughness, and composite effect in square-sections than in circular-sections, particularly at a volumetric stirrup ratio of 1.11%. Circular-sections, however, demonstrated greater ductility improvement. Spiral stirrups effectively compensated for the inherent confinement deficiency of square steel tubes, and reducing stirrup spacing further enhanced all performance aspects. The volumetric stirrup ratio of 1.11% was identified as a critical threshold at which confinement efficiency transitioned from gradual to substantial. Load–bearing capacity equations for circular and square cross-section R-RACFSTs were developed based on unified theory and the superposition principle. Predictions agreed well with experimental and literature data. This study provides experimental and theoretical support for the efficient use of high-replacement-ratio RAC in structural engineering.