Why not simply use gold?

Gold attracts vermin.

How poignant, that the obvious solution is the worst "solution" of all, due to human nature.

Concrete, bricks and granite can also work pretty well... as the Pantheon in Rome proves.

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.

Stone and adobe, in the right conditions.

https://en.m.wikipedia.org/wiki/Ancestral_Puebloan_dwellings

In those conditions even plain, untreated wood lasts a long time. Many of those structures still have wood beams in pretty good condition.

ie, the durability of those structures is more due to where they are, than what they are made of.

Although (at least the right) stone is almost certainly a pretty good answer in general. There are plenty of historical examples and I'm skeptical that anything else exposed to the elements would do better. And if not exposed to the elements that feels like cheating.

Fair, but the sandstone and clay in that area is generally pretty awful. TBH most of the ancestral Puebloan structure are crumbling apart despite being protected by overhanging rock "roofs".

You're right about the pueblos specifically. They're still extant mostly because the climate is so dry there.

Something like granite would be a much more long-lived structural material in wetter climates. Indeed that's the primary stone used in, say, Machu Picchu.

It depends on whether they are occupied or not. I worked on an upstate New York farm with a barn that was post and beam, with a stone foundation. It was ~350 years old but had a good roof for that entire time.

Fiberglass/FRP is an interesting one. I believe the limiting factor there would be that the polymers used would slowly degrade/weaken over time due to chemical aging. Fiberglass boats last up to 50 years but that is in a harsher environment - not sure how long you could go on land. Perhaps some polymers could be formulated to last that long, I'm not sure.

They can last a lot longer than that but that’s about the duration that frp has existed for boats, probably early ‘60s is when production boats started to use frp hulls.

Frp is great for fuel tanks and exhaust systems - it’s superior to stainless for these applications (corrosion resistance being key factor). But yes as mentioned by sibling UV will definitely degrade

UV from sunlight degrades fiberglass/FRP over time. I believe that boats are painted to protect them from this, and need to be recoated periodically because the uv sunlight will degrade the paint eventually, also.

Seems unlikely that you’d have a reliable source of paint for 1000 years, which might be the limiting factor.

But there’s always some rate of chain scission, even in the absence of UV/water/whatever. I just have no idea whether that has a meaningful impact on material properties over 1000 years.

I feel like if you're really hard pressed for a UV blocker you could just use mud.

Yeah I've seen old boats turn to dust. UV utterly destroys fiberglass given a little time.

>I trust any engineer who responds with an “it depends..” over someone who has an absolute answer.

That heuristic matches on the hordes of liars who try and sound smart while hedging their claims with a liberal application of weasel words to avoid being provably wrong. And those people outnumber the "good engineers" probably by at least an order of magnitude...

"When a measure becomes a target, it ceases to be a good measure."

It is, however, far more interesting to me when people are able to split the hypothesis space well. For instance, take the question "Is drinking 1500 ml of water daily good for you?". Well, that depends on what other fluids you consume, whether or not you are on an IV as well, and whether or not you are drinking it by accidental consumption via waterboarding. It even depends on whether or not Baldur the Interstellar Water Hunter exists and will kill you for every drop over 1499 ml that you consume.

It's true, it does depend. But machines can exhaustively describe the hypothesis space now. Comprehensiveness is not the only virtue of a human.