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How Has Roman Concrete Lasted for Millennia? 1,900-Year-Old Latrine Offers Clues
(www.smithsonianmag.com)
That's cool and all, but the main reasons modern concrete structures don't last are either:
- They're full of steel reinforcement (because it would be unacceptably expensive to build structures relying entirely on compression the way the Romans did) and the steel deteriorates, or
- They become functionally obsolete regardless of the continuing stability of the structure.
There are precious few things we build these days, aside from deliberate monuments, that we even want to last 100+ years to begin with. Even huge things like the Hoover Dam aren't meant to be permanent on that kind of timescale.