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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%.

Fiber-reinforced cementitious matrix (FRCM) systems have recently emerged as an innovative technique to strengthen and retrofit reinforced concrete structures. These non-corrosive systems involve the use of high strength reinforcing textiles sandwiched between layers of cementitious mortars. This paper aims to study the effect of retrofitting newly constructed short columns with FRCM using PBO type of textiles. The experimental program consisted of testing 4 rectangular columns and 4 circular columns under concentric loading. All columns had a reinforcement ratio of 0.02 and were cast with concrete of 30 MPa compressive strength. For each type of cross-section, columns were wrapped with 1, 2 or 4 layers of PBO FRCM. Overall, the strengthened columns exhibited higher load carrying capacity than their control unwrapped counterpart with an increase ranged between 5.1% and 36%. The confining effect of FRCM layers was more pronounced in the circular columns than in the square ones. It was also noticed that all columns exhibited similar responses in terms of load -strain relationship irrespective of the column shape and the number of FRCM layers used. Furthermore, the displacement levels increased as the number of layers increased which indicated an increase in ductility of columns wrapped with FRCM.