I have some thoughts about the building. First, it's not uncommon for unscrupulous contractors to use sand in their concrete that wasn't washed of all the salt. Concrete with salt in it is going to erode the steel rebar and could take 40 years to weaken it enough to cause this damage. I'm not saying that's what happened here, just that I know, from living in Mississippi 40 years ago along the coast, Biloxi, that it wasn't unheard of for contractors to use beach sand.
Also, when I had my current house built, the footing was dug for the entire perimeter. Steel rebar framing was placed in the space before concrete was poured. But before the concrete could be poured, the rebar required an inspector to sign off. However, it had rained the week prior. The footing filled with water an then receded by the time the inspector showed up. He rejected the rebar because it was not 'clean'. I asked what that meant. He told me that it had mud on it and it had rust. Neither were acceptable. He said the rebar must be rust free with no contaminating materials. The contractor had to pull all that rebar and reinstall before the inspector would sign off. I asked the inspector why some muddy water film and some very light surface rust mattered. He told me the concrete would not stick to the rebar and that it was likely to continue to corrode with the moisture inherent with concrete once the oxidation has begun.
I'm not a contractor or engineer, just relating my own experience with concrete and construction.
