[lll-o-lll]

The point, is that the steel needs water to rust. The cracks allow the water, but supposedly the Roman concrete self heals and water does not continue to penetrate. One assumes, therefore, limited rust (perhaps). In practice this may not be the case, but surely someone has tested? Steel has the right properties + price; if you can keep the water out.

[mjevans]

Concrete isn't waterproof, it might resist the flow of water, but as even most rocks are, there is at least a slow migration of water through the material.