Failure mode and flexural capacity of concrete beams prestressed with unbonded FRP tendons

Abstract

Due to the incompatible deformation between unbonded fiber-reinforced polymer (FRP) tendons and adjacent concrete, performance prediction of unbonded FRP prestressed concrete (FRP-UPC) beams is rather complicated. This paper presents design recommendations for FRP-UPC beams at ultimate limit state. Strain and stress conditions for three types of balanced failure modes were specified. Introducing the idea of resultant force comparison, a failure mode discrimination method applicable both in theory and design was developed. To ensure compression failure and efficient utilization of materials, amount limits of bonded and unbonded FRP reinforcements were provided. In addition, after quantitatively describing strains in bonded and unbonded reinforcements, a unified flexural capacity model for FRP-UPC beams with various types of auxiliary bonded bars was proposed. Comparing the predictions with measured results, the proposed model demonstrated sufficient accuracy for beams with both steel and FRP auxiliary bonded bars. A parametric analysis was conducted to reveal the influence of reinforcement amount on the overall performance of the beams. It was observed that for over-reinforced beams with the same reinforcement ratio, decreasing the amount of bonded FRP bars while increasing that of unbonded prestressed FRP tendons improved the flexural capacity, serviceability and ductility, but offered no benefit to the deformability.