Mechanical and composite fracture behavior of polypropylene, polyvinyl alcohol, and hybrid fiber-reinforced mortar overlay systems for structural retrofitting

The development of high-performance, durable retrofitting materials is crucial for extending the service life of aging concrete infrastructure. This study investigates the mechanical and fracture behavior of fiber-reinforced mortar (FRM) overlay systems incorporating polypropylene (PP), polyvinyl alcohol (PVA), and a hybrid PP-PVA blend for structural repair applications. An integrated methodology combining compressive and flexural testing with digital image correlation (DIC) and fracture characterization was employed. PVA–FRM achieved the highest compressive and flexural strengths. Although hybrid and PP mortars exhibited lower peak strengths, they significantly improved post-cracking ductility, toughness, and fracture energy, with PP-FRM demonstrating the highest fracture energy. Inverse analysis yielded piecewise-linear 𝜎 – 𝑤 constitutive models for all four mixtures, capturing the hardening–softening behavior of fiber-reinforced systems. The constitutive fracture energy 𝐺 𝜎 - 𝑤 𝐹 was found to exceed the RILEM work-of-fracture for all FRC specimens, indicating that the RILEM work-of-fracture method underestimates the true energy dissipation capacity of fiber-reinforced systems. Accordingly, 𝐺 𝜎 - 𝑤 𝐹 is adopted as the primary fracture energy metric throughout this study. Among 16 two-layer configurations, substrate fiber type proved more critical than overlay selection, with PP-substrated systems consistently outperforming C-substrated systems regardless of overlay material. DIC analysis confirmed the transition from single-crack brittle fracture to distributed quasi-ductile damage in fiber-reinforced systems. The study concludes that while PVA fibers deliver superior mechanical performance, the hybrid PP-PVA system offers a balanced and cost-effective alternative for retrofitting applications, providing a compelling synergy of strength, ductility, and fracture resistance.