Effect of confinement system and recycled aggregates on axial compressive behaviors of CFST columns

The utilization of recycled aggregate concrete (RAC) has emerged as a pivotal technology for global environmental sustainability. Extending RAC's application to Concrete-filled steel tube (CFST) columns offers an opportunity to strengthen RAC's inherently weaker attributes. This research investigates the effects of recycled aggregates (RAs), basalt fiber (BF), and various confinement systems on the mechanical performance of CFSTs. A series of experiments were conducted on CFST columns under diverse confinement scenarios, incorporating varying proportions of basalt fiber and RAs. Additionally, numerical analysis was carried out to evaluate the influence of key design parameters on the load capacity of CFSTs, and theoretical discussion was conducted based on experimental results. The findings reveal that incorporating BF into concrete at an optimal dosage of 0.2 % significantly improves compressive, tensile, and flexural strengths across various RA replacement ratios, while the addition of BF results in minimal improvements in load capacity and fracture toughness for CFSTs. Replacing NAs with RAs generally reduces concrete strengths, with the severest reductions observed at higher RA replacement ratios. However, the degradation in the load capacity of CFSTs is less pronounced than the strength decrease observed in concrete. While a 40 % RA replacement in CFSTs has a minimal impact on the stress-strain curve, a 60 % RA replacement significantly alters the stress-strain relationship, leading to an average reduction of 26.3 % in fracture toughness and 13 % in load capacity. The confinement system plays a crucial role in the stressstrain relationship of CFSTs. Increasing the steel tube thickness from 2 mm to 4 mm substantially increases fracture toughness, and the introduction of GFRP wrapping further enhances both load capacity and ductility. The order of load capacity improvement based on confinement systems is as follows: combined confinement with a 4mm-thickness steel tube and GFRP wrapping, a 2mm-thickness steel tube with GFRP wrapping, a 4mm-thickness steel tube, and a 2mm-thickness steel tube. GFRP wrapping proves to be the most effective and economical method for enhancing load capacity compared to other strategies. It is suggested that the confinement stresses from the steel tube and GFRP reinforcement on the core concrete should be estimated independently, as combining them using a single confinement coefficient may not accurately reflect their individual contributions.