Axial capacity of non-circular high-content recycled aggregate concrete with active confinement: a unified approach

This study presents a new semi-empirical confinement model for actively confined non-circular recycled aggregate concrete (RAC) cubes. The model explicitly considers the RAC replacement ratio and section shape factor, which are ignored in existing formulations. 150 mm cubic specimens without longitudinal reinforcement were cast with RAC at replacement levels of 0%, 50%, and 100% (by coarse aggregate mass). The cubes were actively confined using various confinement ratios (ρv = 0.11, 0.20, 0.34, and 0.67) provided by post-tensioned metal straps (PTMS). Different corner radii (r = 10, 15, 20, and 25 mm) were adopted to assess confinement efficiency. Results from experiments and finite element analyses (FEA) in ABAQUS® showed the influence of corner radius and section shape on the stress-strain response beyond the available test results. The FEA predictions closely matched the experimental results (accuracy < 10%), confirming the reliability of the simulation approach. The findings show that RAC benefits more from active confinement than NAC, with normalised strength enhancement increasing by +23%, +67%, and +103% for 0%, 50%, and 100% RAC at a confinement ratio of ρv = 0.34, respectively. Corner rounding significantly improves confinement effectiveness, particularly at higher confinement ratios. Based on the results, a new confinement model is proposed by extending a baseline PTMS confinement equation. Comparative evaluation against ACI 318 and fib Model Code 2020 shows that the proposed model leads to the lowest statistical error (5%) and highest predictive accuracy. This work provides a practical design-oriented tool for sustainable structural applications involving actively confined non-circular RAC members.