Interfacial Bond–Slip Behavior of Carbonated Recycled Aggregate Concrete-Filled Flat Steel Tubes: An Experimental Study

The recycling of construction and demolition waste and the reduction of carbon emissions are important issues in sustainable construction. Recycled aggregate concrete (RAC) is promising for structural use, but the weak old mortar and multiple interfacial transition zones in recycled coarse aggregate (RCA) may reduce the load-transfer capacity at the steel–concrete interface. To address this problem, this study developed carbonated recycled aggregate concrete-filled flat steel tube (FST-CRAC) members and investigated their interfacial bond–slip behavior through material strength tests and push-out tests on nine specimens. The effects of RCA replacement ratio, carbonation treatment, section aspect ratio, and width-to-thickness ratio were examined. RCA was carbonated at 0.5 MPa for 24 h. The 28-day compressive strength increased from 32.6 to 44.3 MPa in the uncarbonated P series and from 36.2 to 46.2 MPa in the carbonated T series. However, because the two series were developed through separate preliminary mix-design trials, these differences should be interpreted as being jointly associated with carbonation treatment and mix-proportion adjustments rather than as evidence of an isolated causal effect of carbonation. Push-out failure was governed by interfacial debonding, local crushing near the corners, and post-peak frictional slip, with damage consistently concentrated at the short sides and corners of the flat section. Carbonation treatment increased the peak bond load by 2.85–26.23%, with the largest benefit observed at a moderate replacement ratio, while increasing the RCA replacement ratio from 50% to 100% increased the peak load by 27.90% for uncarbonated specimens but only 4.21% for carbonated specimens, indicating that carbonation reduces the sensitivity of bond capacity to replacement ratio. A moderate increase in section aspect ratio increased the peak load by 22.30–25.86%, and reducing the width-to-thickness ratio increased the peak load by 5.95–40.92%. A four-linear bond–slip constitutive model was proposed to describe the full interfacial response, from initial bonding through peak degradation to residual friction. These findings provide experimental support for the use of carbonated recycled aggregates in steel tube-confined composite members and a basis for subsequent nonlinear analysis.