Bond Behaviour of Round-Ended Recycled Aggregate Concrete Filled Steel Tube (RE-RACFST) Columns

Round-ended recycled aggregate concrete filled steel tube (RE-RACFST) column is an innovative composite structural member, which inherits the advantage of round-ended concrete filled steel tube (RE-CFST) and promotes the application of coarse recycled aggregate (CRA) in structural members. In this study, four repeated push-out experiments on 18 RE-RACFSTs were carried to investigate the interface bond performance. The failure patterns, bond stress versus slip relationships, bond mechanisms and the strain developments were analyzed, and the influences of CRA replacements, cross-sectional dimensions and the steel tube thicknesses on the bond strengths were discussed. Moreover, the mechanism of chemical bonding, micro-interlock and macro-interlock on the bond stress was investigated. The experimental results indicated that the bond strength was negatively correlated with the cross-sectional dimension, and RAC presented greater bonding behaviour than normal concrete. The bond stress-interfacial slip (τ-S) responses manifest that the chemical bonding has been destroyed and the micro-locking declined after 1st cycle. Besides, the bond strength after the first loading cycle was mainly composed of the macro-interlock. In addition, the bond strength of the RE-RACFST members was compared to the rectangular CFST members with similar aspect ratio. Finally, simplified equations for estimating the ultimate bond stress (τu) and the bond stress versus slip model between round-ended steel tube and RAC were proposed.