Effect of different cross-sections and concrete types on the flexural behavior of CFSTs

The main goal of this research is to experimentally investigate the flexural response of concrete filled steel tubes (CFSTs) considering different cross-sections and concrete types. For this purpose, a total of 12 circular and 8 rectangular CFST beams with different Diameter (depth)-to-thickness ratios were cast using normal and recycled aggregate concrete mixes and were tested under four-point bending. Concrete compressive strengths of 30 and 50 MPa and recycled aggregate (RA) replacement percentages of 50 and 100% were used in the experimental investigation. The test results revealed promising outcomes on the feasibility of using RA in CFST systems under flexure. The flexural behavior of RACFSTs was found to be very similar to NACFSTs, and the change in the concrete compressive strength and RA percentages slightly affected the flexural behavior 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. The Architectural Institute of Japan design code was the most accurate to predict the flexural capacity of RACFSTs with an average of 5% difference only, while the British Standard significantly underestimated the flexural capacity of the tested RACFST beams by an average of 34%.