Innovative UHP-FRCM Systems for Enhanced Flexural Behavior of RC Beams: Addressing Fabric Slippage and Introducing a New Tension Stiffening Model

A key limitation of commercial fabric-reinforced cementitious matrix (FRCM) systems is the tendency for the fabric mesh to slip within the cementitious matrix, which restricts the fabric’s contribution to the overall capacity of the strengthened element. This study examined the structural performance of RC beams strengthened with a new FRCM system, termed ultrahigh-performance fabric-reinforced cementitious matrix (UHP-FRCM). The proposed system combines a polyphenylene benzobisoxazole (PBO) mesh impregnated in an ultrahigh-performance fiber-reinforced concrete matrix that incorporates either steel fibers or a hybrid of steel and ultrahigh-molecular-weight polyethylene fibers. A total of five beams were tested: two strengthened with UHP-FRCM having different matrix formulations, one strengthened with two layers of PBO mesh within the hybrid UHP-FRCM system, one strengthened with a commercial FRCM system, and one unstrengthened control beam. The beams strengthened with UHP-FRCM systems outperformed both the control beam and the beam strengthened with the commercial FRCM system at the serviceability and ultimate limit states. Digital image correlation analysis showed that beams strengthened with UHP-FRCM systems developed numerous fine cracks, in contrast to the wide, isolated cracks observed in the control beam and the beam strengthened with the commercial FRCM system. The UHP-FRCM systems also improved stiffness, load capacity, and fabric contribution. To model the effect of the UHP-FRCM systems on the serviceability limit state, a modified tension stiffening factor, adapted from Bischoff’s approach, was introduced and incorporated into the effective moment of inertia and crack width formulations. The predicted load–deflection responses and crack widths, incorporating the proposed tension stiffening factor, showed excellent agreement with the experimental results.