Steel reinforced grout (SRG) composites, consisting of ultra-high tensile strength steel (UHTSS) cords embedded in an inorganic matrix, are an effective solution for the external strengthening of existing buildings. Zinc-coated (galvanized) UHTSS cords are the most widely adopted, with a good compromise between durability and cost. The tensile behaviour of galvanized UHTSS textiles meets the durability performance required by certification standards. Nonetheless, recent studies have demonstrated that corrosion can occur when the textile is embedded into lime-based matrices or due to salt crystallization, affecting the stress-transfer capacity. The deterioration of this latter in aggressive environments, however, has largely remained unexplored until now. Within this framework, this paper describes an investigation on the durability of galvanized SRG, with a focus on the stress-transfer mechanism, both between composite and substrate and between multiple layers at overlaps. Two textiles and two mortars—a cement-like one and a lime-based one—were tested. Shear bond tests were carried out on concrete and masonry substrates, after freezing-thawing conditioning, and wetting-drying cycles in water and in salt solution. To study the effects on the textile-to-matrix stress-transfer, lap-tensile tests were carried out, after aging in either distilled or salt water and alkaline solutions.