Axial behaviour of steel tubes filled with concrete incorporating high-volume rubber

Concrete-filled steel tube (CFST) columns are gradually becoming more popular in composite construction. The steel tube not only sustains partial load but also provides confinement to the concrete-filled. Numerous studies on CFST columns are reported in the literature, but none of them stated the use of rubber powder as a partial replacement of cement. In this study, sixteen circular concrete-filled steel tubular (CFST) columns were tested under axial compression to investigate the effect of replacing cement with rubber on the axial behaviour of CFST columns. Concrete mixtures with replacement levels of 0, 15, 25 and 35% were prepared. Density and compressive strengths were also determined for all the concrete mixes. CFST columns with the same diameter-to-thickness (D/t) ratio of 45.84 and length-to-diameter (L/D) ratios of 2.9 (series 1) and 4.0 (series 2) were tested. Results of the experimental investigation indicated that increasing the replacement level of cement with rubber decreased the compressive strength and density of concrete. The axial capacity of the CFST columns was also reduced as the quantity of rubber in the concrete mix was increased. In series 1 specimens, cement replacement by 15%, 25% and 35% of rubber reduced the axial load-carrying capacity by 15.2%, 24.6% and 41.1%, respectively. In series 2 specimens, cement replacement by 15%, 25% and 35% of rubber reduced the axial load-carrying capacity by 15%, 29.4% and 45%. A reduction in the ultimate strength of CFSTs in axial compression was less significant than the loss in the concrete compressive strength. The stiffness of the CFSTs reduces, while the axial deformation corresponding to ultimate load increases as the replacement levels of cement by rubber are increased. A comparison is also made between experimental axial capacities and those obtained using the provisions of the American Concrete Institute (ACI), Eurocode 4 (EC 4), Chinese code (DL/T), and Architectural Institute of Japan (AIJ). The comparison revealed that ACI provisions provided over conservative evaluations of the ultimate axial loads for all the CFST columns. EC4, AIJ, and DL/T provisions slightly overestimated the load-carrying capacity of series 2 columns with a 35% replacement level.