Large concrete dams like Hoover are almost invariably arch dams which are shaped so that the entire dam goes into compression as the water load comes on, so the

Large concrete dams like Hoover are almost invariably arch dams which are shaped so that the entire dam goes into compression as the water load comes on, so there is no real need for tensile steel. These dams can take decades to cure and just get stronger over time. The connections to the rock are usually reinforced - steel that's encased in concrete is generally thought to be relatively immune to most corrosion effects for the design life of any particular structure (plus a lot longer if it's maintained).