Flexural behavior of FRP-HSC-steel composite beams

• Eight FRP-concrete–steel composite beams are tested under four-point bending.
• Effects of sectional shapes of inner steel reinforcement and external FRP tube are studied.
• Use of mechanical connectors to improve bond between concrete and inner steel is investigated.
• The composite beams are shown to exhibit a very ductile flexural behavior.
• Slips at concrete–steel reinforcement interface can be eliminated through mechanical connectors.

This paper reports on an experimental study on the flexural behavior of fiber reinforced polymer (FRP)-high-strength concrete (HSC)-steel composite beams. Seven double-skin tubular beam (DSTBs) and a concrete-filled FRP tube (CFFT) with an internal steel I-beam were tested as simply supported beams in four-point bending. The main parameters of the experimental study included the cross-sectional shapes of inner steel reinforcement and external FRP tube, concrete strength, presence (or absence) of concrete filling inside the steel tube, and effects of the use of mechanical connectors on the inner steel tube. The results indicate that DSTBs are capable of developing very high inelastic flexural deformations. However, the results also indicate that slip between the concrete and the steel tube of the DSTB can be relatively large, unless the bond between concrete and steel tube is enhanced through the use of mechanical connectors. The results of the beam tests illustrate that the flexural behavior of DSTBs is influenced significantly by the diameter and thickness of the inner steel tube. Concrete-filling the inner steel tube and increasing the concrete strength increase the flexural capacity of DSTBs without affecting their overall ductility. Furthermore, the shape of the inner steel tube influences both the flexural capacity of DSTBs and the occurrence of slippage between the concrete and the inner steel tube. It is shown that the bond slip between the concrete and inner steel tube can be prevented through the use of mechanical connectors. These results are presented together with a discussion on the influence of the main parameters on the flexural behavior of DSTBs.