We don't make concrete like the Romans did though.

Still concrete will still last longer than most other materials.

The biggest issue with modern concrete construction is the usage of reenforced concrete using iron or steel rebar which tends to corrode quickly. Reenforced concrete seems to have a half life of ~50 years.

Will be interesting to see how carbon-fiber reinforced concrete[1] fares in comparison.

[1]: https://www.compositesworld.com/news/tprc-research-studies-v...

stainless steel rebar and polymer coated rebar are also options for reinforcing concrete that have a lot of potential, as the issues is moister induced rusting of the rust rebar expanding and cracking the concrete there are multiple solutions that i don't know that we have data on the long term durability of yet. I wonder how one would go about artificially aging something embedded within a block of concrete to test for that.

It appears that coated rebar just offers a channel for the ingress of water, and thus corrosion, if there is a break anywhere in the coating, like a wick in a lamp.

Basalt fiber rebar might be a better way to go, as it won't oxidize. Studies will be needed to find out if it actually is better in the long run.

We do actually, it's just not as useful and used in more demanding applications.

The biggest problem is rebar spalling though, not the concrete.

Correct, because the way the Romans made concrete wouldn't be suitable for 30 and 70-story buildings.

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.)

I think the time it takes for the concrete to set is why, as keeping it dry for a long time would be difficult.

Not to mention the wooden beams, under the portico, which are also original. They of course, are out of the rain; so are a solution to a lesser problem, where aging is concerned.