The concrete dome of the Pantheon in Rome remains stable enough for visitors to walk beneath, and some Roman harbours have underwater concrete elements that have not been repaired for two millennia – even though they are in regions often shaken by earthquakes.
Whence this remarkable resilience of Roman concrete architecture? It’s all down to the chemistry.
And to add a platitude: “Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”
Though it’s important to note that factors of safety are always incorporated into structural designs, in case higher loads/wear/etc than expected occur