I have recently had my foundation concrete poured. I am using strip foundations with 1 layer A393 mesh bottom (they ground is good but the engineer specified 1 layer of mesh bottom in case there would be any weak pockets of soil). On site the contractor used C20/25 Concrete instead of the C25/30 specified. The engineer said this was fine, as the ground conditions are good and the foundations can take the loading. I wanted to ask if there would be any problem with the mesh reinforcement corroding now that a slightly weaker mix has been used?