A lot of Roman concrete has lasted to this day. The aqueducts are still impressive, the colosseum is only 2/3 standing because of an earthquake hundreds of years ago (and is otherwise a fine museum), and the pantheon is as shockingly gorgeous today as when it was built 2000 years ago.
Totally agree, but just wanted to expand on the colosseum for a moment. While traveling there, I spent a full day at the Roman Colosseum. Its outer travertine stones were all without mortar and originally held together with numerous iron clamps. Since the fall of Rome, the clamps were slowly harvested, leaving the surface of nearly every outer stone with huge holes. In the opinion of those I spoke with, had the structure been left intact with it's clamps, it would have endured in much better shape.
