A tale of two bridges:
1. Starrucca Viaduct -- built in 1848 by the Erie Railroad, out of stone.
(I've been over this with freight trains back in the Conrail days)
2. Nicholson Viaduct -- built by the Lackawanna in 1915, and for many years the largest concrete bridge in America.
Both are in Northeastern PA.
If you visit each (you can do both in the same day), and stand underneath them, something is quite apparent:
Starrucca Viaduct, almost 70 years older, is actually in much better shape.
Stand under Nicholson, and look upwards, and you'll notice lots of places where the original concrete is crumbling, pieces broken away.
Look at this photo from the top:
My own conclusion:
Concrete works good for some things, but it's not the stuff from which big bridges should be built.
For that, use steel... or stone.
Even worse, build a concrete bridge from a deliberately odd design (which appears overly "light" like the bridge in Italy), and its long-term prospects... well, they ain't so good.
What -is- concrete, anyway, if not "molded-together aggregate"?
What was that saying about building upon sand, vs. solid rock?
Or, like Paul Simon once wrote, "everything put together sooner or later falls apart".
But with concrete, it falls apart
faster....
