Interface bond strength of concrete repair systems: Influence of nanosilica and recycled PET fibers - A systematic review

Old-to-new concrete bond performance is a critical factor governing the durability and effectiveness of repaired structures; however, the roles of nanosilica and recycled polyethylene terephthalate (PET) fibers in improving repair interfaces remain insufficiently synthesized. This study systematically reviews the individual contributions of nanosilica and recycled PET fibers and identifies knowledge gaps regarding their potential combined application. Following the PRISMA 2020 framework, 2060 records from Google Scholar and Scopus were screened, resulting in 99 eligible studies comprising 44 direct-interface studies, 26 nanosilica studies, and 29 recycled PET-fiber studies. The methodological quality was assessed using an eight-item checklist, and the findings were synthesized through structured narrative analysis. The results indicate that old-to-new concrete bond strength is governed by multiple factors, including surface preparation, substrate condition, material compatibility, water-to-binder ratio, curing regime, testing age, loading configuration, and failure mode. Nanosilica contents of approximately 1.5–3.0% generally enhance matrix densification and interfacial transition zone refinement through additional C–S–H formation and pore refinement, with one direct-interface study reporting slant-shear, splitting-bond, and flexural-bond strengths of 14.19, 2.87, and 4.39 MPa, respectively, at 2.5% nanosilica. Meanwhile, recycled PET fibers at approximately 0.3–1.0% primarily improve crack bridging, toughness, and post-cracking integrity, although direct evidence of enhanced old-to-new concrete adhesion remains limited. This review demonstrates that nanosilica and recycled PET fibers provide distinct but potentially complementary mechanisms; however, their combined effect on concrete interfaces has not yet been experimentally validated and requires controlled future investigation.