Stay-in-Place Permanent Formwork for Concrete Structures Using FRP-Reinforced Ultrahigh-Strength Engineered Cementitious Composites

This study investigates a novel stay-in-place permanent formwork system employing fiber-reinforced polymer (FRP)–reinforced ultrahigh-strength engineered cementitious composites (UHS-ECCs) to enhance the durability and structural performance of concrete structures. The flexural behavior and failure mechanisms of reinforced concrete beams incorporating a 3-mm FRP bar–reinforced, 20-mm-thick UHS-ECC formwork were evaluated through experimental testing, digital image correlation, and theoretical analysis. For comparison, ultrahigh-performance concrete (UHPC) was used as an alternative formwork material. To improve interfacial bonding, the precast formwork was fabricated with rectangular grooves spaced at 50 or 120 mm. The results indicated that the UHS-ECC formwork effectively mitigated early crack localization, resulting insuperior load-carrying capacity and postyield stiffness. In contrast, the UHPC formwork was more susceptible to localized cracking, which induced interfacial deformation incompatibility and consequently reduced structural ductility and load-bearing capacity. These findings demonstrate the potential of FRP-reinforced UHS-ECC permanent formwork as a viable solution for improving the mechanical performance and durability of concrete structures.