[xoxoxoBruce]

An example of rusting rebar can be seen where they are tearing up the bridge to widen I-94 for the Waukegan Toll Plaza. Notice some of the rebar hooks, that were encased in the now demolished concrete deck, are rusted enough there is no traces of concrete adhered to them. They should have a green epoxy paint coating like the straight rebar sticking up on the left.

The bottom picture shows missing concrete and rusty rebar. It's easy to assume the concrete broke off and the rebar rusted from exposure to the elements. This is not true, however. Actually, water, probably salt brine reinforced water, worked it's way down through cracks in the concrete and rusted the rebar. When iron/steel rusts it increases in volume, creates pressure like freezing ice, and blows the concrete apart. That process causes spalling of the concrete and appears like in the bottom picture.

Now think of how the steel can react when there's no concrete to protect it at all. Unlike 99% of the thousands of bridges, the big, high profile bridges (George Washington, Golden Gate) are being painted continuously. More importantly, their decks and underpinnings are being inspected and maintained constantly.

You've probably heard rumblings about the states selling the rights to large portions of the interstates to private corporations for cash. My prophesy is, then the politicians piss away the money like Cheney with a War Budget, and when a major catastrophe comes along the corporation declares bankruptcy, the high ups bail with golden parachutes and the taxpayers get the mess dropped in their laps.
At least the ones that weren't killed in the catastrophe.