The way the Romans used concrete to form structures that could be plausibly carved out of raw rock. The dome of the pantheon is an impressive feat of engineering, but the general shape bears similarity to rock caverns. Which is great, because concrete is essentially rock.
Meanwhile, the kinds of beams and walls we make out of concrete wouldn't work if carved out of rock, they are only structurally sound because we put steel rebar in the concrete. But rebar expanding due to rust is how concrete usually fails. Plus using rebar prevents you from using salt water in your concrete, which is one of the ways in which Roman concrete might be better than ours.
What exactly is keeping us from making concrete with salt water, for things like sidewalks?
In the northeast US, sidewalks frequently are in horrible condition after only a few years, because of the salt used in the winter. Sidewalks do not use steel rebar; they're just simple poured concrete. So why can't they make them with a better mixture that doesn't corrode so easily? (I'm guessing the answer is: replacing them frequently makes a lot of money for some concrete company that the town mayor is friends with.)
Meanwhile, the kinds of beams and walls we make out of concrete wouldn't work if carved out of rock, they are only structurally sound because we put steel rebar in the concrete. But rebar expanding due to rust is how concrete usually fails. Plus using rebar prevents you from using salt water in your concrete, which is one of the ways in which Roman concrete might be better than ours.