Response and modeling of axially-loaded concrete-filled steel columns with recycled coarse and fine aggregate

Recycled aggregate concrete-filled steel tube (RACFST) could be a practical structural application of recycled aggregate concrete (RAC) with fine recycled aggregate (FRA). Despite its great potential, limited attention has been devoted to the compressive response of RACFST with FRA. This study investigates the effects of FRA on the structural response of RACFST with different coarse recycled aggregate (CRA) replacement ratios (r(C)), and how FRA influences the confinement effect of RACFST. Fifty-four RACFST specimens were tested to failure with different FRA and CRA replacement ratios (r(F) and r(C)), concrete strengths (f(cm)) and effective steel ratios (alpha). Experimental results show that FRA and CRA have a significant compounding effect (i.e., FRA effect is different at varying r(C)) on the strain at the maximum load (epsilon(0)) of RACFST. Unlike CRA which has an insignificant influence on the confinement effect, the use of FRA significantly reduces the confinement of RACFST. The test results were used to modify the existing mathematical expressions for normal concrete-filled steel tube (CFST) to predict the stress-strain relationships for RACFST. The predicted results matches well with the available test data.