Investigating the influence of UHPC layer length on the flexural response of prestressed concrete I-beams

Abstract

This study presents an experimental investigation into the flexural behavior of prestressed reinforced concrete (PRC) I-shape beams strengthened with cast-in-situ ultra-high-performance concrete (UHPC) layers reinforced with Glass Fiber Reinforced Polymer (GFRP) bars. Five composite beams were subjected to four-point bending to determine the optimal length of UHPC layers on the tension side for enhancing flexural capacity. The varying lengths of the UHPC layers allowed for a comprehensive analysis of their effects on ultimate load, energy absorption, ductility index, stiffness, bonding behavior, and failure modes. The external UHPC reinforcement layers significantly delayed crack initiation and substantially increased the load-carrying capacity by 58% to 159%. Furthermore, the incorporation of UHPC layers on the tensile side of the PRC beams resulted in a varied response in energy absorption, ranging from a 23.54% reduction to a substantial 750.87% gain with increasing reinforcement layer length. While this strengthening approach led to a change in the ductility index, ranging from a 15.89% decrease to a 285.64% increase, it consistently enhanced the initial stiffness by 54.18% to 88.85% as the UHPC layer length increased. The findings underscore the significant influence of UHPC layer length on the beam's overall response and failure characteristics. A perfect bond was observed between the UHPC layer and the PRC beam, with no separation at the interface. Crucially, this robust bond, combined with decreasing UHPC layer lengths, shifted the failure mode from a ductile flexural failure to a sudden failure attributed to concrete cover separation. This research uniquely contributes by providing the first comprehensive experimental data on the effect of varying in-situ cast UHPC layer lengths on the combined flexural performance and distinct failure mechanisms of PRC I-beams reinforced with GFRP. Findings offer critical insights into optimizing strengthening designs and understanding the complex interplay between layer length and failure mode transition for such advanced composite systems.

Keywords

Prestressed beam / UHPC / Strengthening / GFRP / I-shaped beam

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