Flexural Behavior of Concrete-Filled Steel Tubes (CFSTs) using Recycled Aggregate Concrete (RAC)

Abstract

Concrete-filled steel tubes (CFSTs) can be considered as possible and promising alternative to conventional reinforced concrete members and steel structures since they have superior structural properties and economic advantages. In recent years, researchers have investigated the replacement of natural aggregate concrete (NAC) infill with recycled aggregate concrete (RAC) and utilizing the confinement provided by the steel tube in order to improve the mechanical properties of recycled aggregate concrete (RACFST). However, the research on RACFSTs has been limited to their behaviour under compression. The main goal of this research is to investigate the flexural response of RACFSTs experimentally and theoretically considering circular and rectangular cross-sections. For this purpose, a total of 12 circular and 8 rectangular CFST beams with different Diameter-to-thickness (D/t) and depth-to-thickness (h/t) ratios were tested under four-point bending. Concrete compressive strengths of 30 and 50 MPa and recycled coarse aggregate replacement percentages of 50 and 100% were used in the experimental investigation. The test results revealed promising outcomes on the feasibility of using RAC in CFST systems under flexure. The flexural behaviour of RACFSTs was found to be very similar to NACFSTs, and the change in the concrete compressive strength and the recycled coarse aggregate replacement percentage slightly affected the flexural behaviour of RACFSTs. In addition, the experimental flexural capacity of RACFST beams were compared to the theoretical nominal moments predicted by well-known design codes and methods including the AISC-LRFD, the Architectural Institute of Japan (AIJ), EuroCode4, the British Standard (BS) and Han’s method. The AIJ design code was the most accurate to predict the flexural capacity of RACFSTs followed by the EuroCode4 as they underestimated the flexural capacity of RACFST beams by an average of 5% and 12%, while the British Standard significantly underestimated the flexural capacity of the tested RACFST beams by an average of 34%.