Tensile behavior of fiber-reinforced cementitious matrix with ultra-high-performance hybrid fiber-reinforced concrete (UHP-FRCM) with enhanced crack width control, fracture energy, and ultimate strength

Fiber-Reinforced Cementitious Matrix (FRCM) is an effective solution for strengthening deteriorated concrete structures, valued for its lightweight design, concrete compatibility, and ease of application without formwork for underlay strengthening systems. This study aims to characterize the tensile behavior of a newly developed FRCM system by maximizing the load-bearing capacity through full utilization of Polyparaphenylene Benzobisoxazole (PBO) fabric mesh, leveraging its synergy with an eco-friendly UHPC matrix reinforced with hybrid steel and Ultra-High Molecular Weight Polyethylene (UHMW-PE) fibers. Tensile tests conducted per AC 434 evaluated the tensile response of FRCM systems incorporating PBO mesh with a UHPFRC matrix, using various combinations of steel and UHMW-PE fibers. A commercial FRCM system served as the reference. Digital Image Correlation (DIC) analysis was employed to measure crack width evolution. The proposed UHP-FRCM system fundamentally altered the collapse mechanism, achieving the full tensile capacity of the PBO mesh without compromising ductility. Compared to the reference system, the UHP-FRCM system demonstrated significant enhancements, with a threefold increase in bulk fracture energy and a twofold increase in ultimate strength, all while maintaining ductility. The results indicate that the addition of short fibers enhances the bond between the fabric mesh and the matrix through a bridging effect, which helps reduce interfacial slippage. Moreover, the hybrid use of steel and UHMW-PE fibers reduced crack widths at the serviceability stage, keeping them below the 100 μm threshold required for water impermeability. The developed UHP-FRCM system shows remarkable potential for advancing the strengthening of concrete structures, offering improved ductility, strength, and impermeability, providing an effective means to extend the service life of reinforced concrete structures.