Insights into the Use of Ultra-High-Performance Fiber-Reinforced-Concrete Plates Reinforced with Glass Fiber-Reinforced-Polymer or Steel Bars for Flexural Upgrading of RC Beams

Reinforced concrete (RC) beams are crucial load-bearing members in multistory buildings. Due to architectural modifications, increased service loads, or construction deficiencies, these members often require flexural strengthening to restore or enhance their performance. The use of prefabricated reinforced ultra-high-performance fiber-reinforced concrete (UHPFRC) plates has recently emerged as a promising strengthening technique. When attached to the tension, compression, or both faces of RC beams, these plates provide noteworthy structural benefits. This study presents a detailed investigation—using nonlinear calibrated finite element (FE) models—into the flexural strengthening of RC beams using reinforced UHPFRC plates. A total of 18 large-scale RC beams were explored, including two control specimens and 16 strengthened beams. The control specimens comprised one beam with a tensile steel ratio close to the minimum code thresholds and another with a conventional reinforcement ratio typical of standard design. The strengthening schemes were developed to enhance the flexural capacity of the first control beam to a level comparable to the ideal reference specimen. A simplified analytical tool was developed to estimate the peak load of control and strengthened specimens for the design of upgrading schemes. The parametric study in the FE matrix examined the effects of reinforcement type within the UHPFRC plates (steel or glass fiber-reinforced-polymer (GFRP) bars), plate location (tension side, compression side, or both), bonding method (adhesive, mechanical, or combined), and end anchorage condition (with or without fiber-reinforced polymer (FRP) U-wraps). The beams’ behavior was evaluated in terms of load-deflection response, stiffness, and failure mode. The results demonstrated that combined adhesive–mechanical bonding with compression-side UHPFRC plates provided the most efficient and reliable strengthening technique.