Experimental and numerical study on the eccentric compressive performance of RAC-encased RACFST composite columns

Recycled aggregate concrete (RAC) was mainly limited to the nonstructural uses in practice due to its inferior properties. To expand the range of application of RAC, a novel structure type of concrete-encased concrete-filled steel tube (CFST) has attracted great interest due to its excellent performance. Since little literature about the behavior of the RAC-encased RACFST, it is imperative to conduct research about the new type of member for practical application to meet the requirement of the large market and sustainable development. This paper presents experimental and analytical studies on the mechanical behavior of RAC-encased, RAC-filled steel tube (RACFST) composite columns under combined compression and bending. In the experiments, sixteen specimens were tested under axial or eccentric compression to investigate the effects in the recycled coarse aggregate (RCA) replacement ratio (0%, 25%, 50%, 75%, 100%), eccentricity ratio (e/h = 0, 0.2, 0.4, 0.6), steel ratio (2.72%, 2.11%), and slenderness (l/h = 3, 6, 9) on their mechanical performance. The results indicated that the replacement ratio of RCA had little effect on the failure mode of the RAC-encased RACFST composite columns. Compared with the specimens under axial compression, reductions of 23.1%, 63.9%, and 72% were observed in the compressive strength for specimens with eccentricities of 0.2h, 0.4h, 0.6h, respectively. Both the increase in the steel ratio and the reduction in the slenderness significantly improved the compressive strength of the specimens. A finite-element model of the specimens was developed to predict the load-deformation relationship and failure mode. The ultimate bearing capacity was predicted using existing design codes and the test result were compared. The values predicted by the design codes EC4, AISC360 PSD, and CECS188 showed deviations < 15% (relative to the experimental values), which could be considered overconservative for the bearing capacity of RAC-encased RACFST composite columns.