alt Hacker NewsConcrete is not very water proof on its own. It's also quite hard to prevent cracks from forming, even with reinforcement.
There is a lot you could do, like neoprene barriers on the skin of the fuselage, sacrificial anodes, spray sealant on the poured concrete. But at that point, you might as well not even have the fuselage and just build a regular, ol' concrete bunker.
Concrete is used as a moisture barrier inside walls if it is specifically built for that purpose. Without it, soil moisture damages the walls. Yet concrete is still electrically conductive and can cause galvanic corrosion, although not as bad as direct contact with soil.
It depends on how it is built and whether the constructors are aware of the problem.