How ancient Rome's concrete has survived that long can probably be attributed to one main factor: no inclusion of steel rebar.
> Modern cement mixtures tend to erode, particularly in the presence of seawater, but the Roman recipe of volcanic ash, lime, seawater and a mineral called aluminium tobermorite actually reinforces the concrete and prevents cracks from expanding, researchers found.
But the advantage rebar gives you is a gradual failure mode, where the steel holds the block together while the concrete cracks.
Without rebar, the failure of concrete is usually catastrophic (instant).
(Think of how hard you'd have to shake the Great Pyramid of Giza to make it collapse. That's not concrete obviously, but it too is under compression almost entirely, except for the corbel and lintel ceilings. The proper arches used in Roman concrete construction show that with the right engineering you can push unreinforced concrete pretty damn far. The Pantheon is probably the most impressive example of it. Compare how well it's withstood earthquakes with the Colosseum.)
The reason why the Collosseum fell is that everybody removed it's lead rebar because it was worth money when Rome fell--without that lead rebar in the joints, every earthquake knocked a bit more off the building.
The Romans lived in an active fault zone and understood building to survive earthquakes.
If that steel bridge can be kept from rusting, it can keep a long time.
Incidentally, this fatigue vs. approaching the strength limit issue is why airplanes have limited flights. The fuselage/hull is made to last long enough, but not necessarily do well longer than that.