Further evidence of interfacial adhesive bond strength enhancement through fiber reinforcement in repairs

[Display omitted] •Adding steel or PVA fibers to a repair increases cohesive bond to the substrate.•With 0.5% & 1% fiber volume fractions, cohesion strength is increased of up to 100%.•Fibers’ enhancement of substrate-repair bond requires proper interface roughness. While intensive structural and non-structural repair is required worldwide to compensate the current infrastructure deficit, repair effectiveness is jeopardized by poor durability, compatibility, and bond. Benefits of fiber reinforcement in concrete repairs are remarkable as durability is enhanced and the effect of poor compatibility can be mitigated. Furthermore, fibers’ potential to improve concrete-concrete bond, a crucial property in repair applications, has been demonstrated. Only a few studies, however, are available on the effect of fibers on substrate-repair bond and additional analysis form different bond tests, fiber reinforcements, and substrate treatments are required to fully utilize such benefits in repair applications. In this study, substrate-repair shear bond strength in fiber reinforced repair mortars is investigated. Based on previous encouraging results on 8 mm long Poly-Vinyl-Alcohol (PVA) fibers, PVA fibers with different lengths (8 and 12 mm) and 13 mm long steel fibers are compared. Two fiber volume fractions, equal to 0.5% and 1% are applied beyond the control condition (plain mortar). While results currently available are focused on roughened substrates, sandblasted substrates and substrates left as-cast are considered in these experiments. Substrate-repair bond strength is assessed through Modified Slant Shear Cylinder (MSSC) test with different bond plane inclinations, corresponding to different normal-shear stress ratios. Adhesion strength and friction coefficient, two parameters inherently characterizing substrate-repair bond, are assessed. The bond enhancing mechanisms offered by the different types of fibers and their correlation to surface treatment are discussed. Variations of indirectly determined coefficients are statistically validated through a permutation technique applied to the 170 samples tested overall.