Impact of reinforcement on performance of reinforced and recycled aggregate concrete-filled square steel tube columns under eccentric load

Tests and numerical simulations were performed to investigate the effect of reinforcement on the eccentric load behavior of Reinforced and Recycled Aggregate Concrete (RAC)-Filled Square Steel Tube (R-RACFST) columns, utilizing RAC infill composed of 100 % recycled coarse aggregate (RCA). Three test groups were designed, such as R-RACFST, Concrete-Filled Square Steel tube (CFST), and RAC-Filled Square Steel Tube (RACFST), all subjected to the same eccentric load and two replicates for each group. Based on the test results, the effects of reinforcement on failure modes, loading performance, sectional strain distribution, and ductility were analyzed. The test results confirmed that the reinforcement significantly improves the performance of an R-RACFST under eccentric load. To further investigate the improvement mechanism of reinforcement, a Finite Element Simulation (FES) model was developed, detailed confined RAC material models for R-RACFSTs were proposed, and various modeling methods were introduced and validated against the test. Using this FES model, numerical simulations were conducted on 176 model specimens, considering the coupling effect between reinforcement and steel tube wall thickness, reinforcement and eccentricity, and reinforcement and slenderness ratio. Based on test and FES results, strength prediction methods for eccentric load-bearing capacity of R-RACFST were studied and new equations were proposed and validated. After the extensive analyses, it was concluded that the reinforcement makes it possible to use RAC of 100 % RCA in R-RACFSTs without decrease of performance; reinforcement performs the same role as steel tubes, and more effective when incorporated with thinner steel tubes; there is an appropriate total steel ratio, putting the member into optimum performance, which can be recommended in the range of 5.35 % to 13.54 %; the proposed equations can accurately predict the eccentric load-bearing capacity of R-RACFST.