Flexural Performance of Unbonded Posttensioned Rectangular Concrete Filled FRP Tube Beams

This paper experimentally investigates the effect of using the posttensioning (PT) steel strands on the flexural performance of rectangular concrete filled fiber-reinforced polymer (FRP) tube (CFFT) beams. Eight PT beams were tested to failure in four-point bending. The main parameters are the effect of prestressed reinforcement, the glass fiber-reinforced polymer (GFRP) tube, the tube thickness, the magnitude of the prestress level, and the number of strands. The test results indicate that rectangular PT CFFT beams showed superior load–deflection behaviors and higher flexural strength compared with the non-PT CFFT designed with identical structural laminate and tube thickness. The results show that PT CFFT beams performed substantially better than PT concrete beams with the same cross-sectional dimensions and prestressing. The flexural behavior of the tested PT CFFT beams is highly dependent on the FRP tube confinement and thickness and, to a lesser extent, on the magnitude of the prestress level and the number of strands. A strain compatibility model has been developed considering the partially confined and unconfined concrete models and verified against the experimental results. The proposed model successfully predicts the flexural moment capacity of the tested beams with satisfactory accuracy on average of 1.05 ± 0.03 and 1.21 ± 0.04, respectively. In addition, it was found that neglecting concrete confinement highly underestimates the flexural strength. More investigations, however, are needed to assess the effect of a wide range of key influencing parameters to better model and understand the flexural behavior of rectangular PT CFFT beams.