Investigation on ring-steel confinement of concrete columns utilizing recycled materials

Abstract This study explores experimental and numerical full/partial confinement on circular concrete columns using recycled aggregate concrete (RAC) and bottom ash concrete (BAC). Both RAC and BAC specimens have lower compressive strength compared to natural aggregate concrete (NAC) of unconfined compressive strength 22.15Mpa. To assess the steel confinement effect, recycled aggregate concrete confined by steel pipe (RAC-SP) and bottom ash concrete confined by steel pipe (BAC-SP) specimens were encased in circular steel pipes with a diameter and thickness of 112 mm and 2.5 mm, respectively. RAC replacement ratios of 0%, 30%, 60%, and 100% and BAC replacement ratios of 0%, 10%, 20%, and 30% were considered for unconfined and fully confined specimens (NAC-SP, RAC-SP, and BAC-SP), all with 430 mm column length. Partial confinement was further examined using ring- steel wrapping 33%, 50%, and 100% of the column length for 0% and 50% RAC specimens. Experimental results demonstrated that partial confinement with 50% steel coverage provides efficient and cost-effective strengthening solution, increasing the ultimate capacity by 274% compared to unconfined RAC specimens. Conversely, fully confined RAC-SP100 and BAC-SP30 specimens exhibited maximum reductions in ultimate capacity of 17.21% and 11.4%, respectively, relative to corresponding NAC-SP specimens. AISC 360 showed conservative predictions for fully confined columns but unsafe estimates in other cases, highlighting limitations in existing design provisions. These findings demonstrate that RAC and BAC can achieve reliable structural performance when adequate confinement is provided, supporting their practical application in load-bearing composite columns and contributing to the development of improved confinement-sensitive design models.