> It turns out that another chemical reaction, known as carbonation, might also contribute to Roman concrete’s longevity.
Roman concrete was made lime cement (calcium dioxide); which cures via carbonation (hardens with carbon oxide). And adding pozzolan to lime makes it hydrolic (hardens with water). Is it surprising that it can still carbonate some? Modern concrete has steel which rust and crack concrete. You can use fiberglass rebar for longevity, or build without rebar even, but that is more costly and and less efficient.
As I understand it, concrete has excellent resistance to compression but fails easily on traction, while steel bars are exactly the opposite. That is why you put rebar in concrete: the steel handles the traction loads and the concrete handles the compression. This works well because both materials have similar coefficients of thermal expansion, so as the temperature changes they both expand and contract at the same rate. I suppose you can engineer fiberglass to have the same thermal expansion coefficient and use it to replace steel (assuming it is just as strong on traction). But how would you "build without rebar even"? Wouldn't your beams start cracking at the bottom, where they are subject to traction?
To build without tension you have to build structures that basically look like Roman structures [1]: a bunch of tightly spaces arches so that the entire thing is in compression, with no meaningful tension anywhere.
But it turns out that's pretty inconvenient; we really like doing dozens of feet of span for highway overpasses, building floors, and everything else. So we put rebar in all the concrete and just acknowledge that that means it has an absolute maximum lifespan of a century or two, and will certainly not last for millennia the way pure concrete in pure compression can.
Modern concrete has steel rebar, which is very useful, but eventually corrodes. Stainless steel rebar could be used if longevity mattered, but usually it doesn’t because the building will likely become functionally obsolete and need replacing before then.
Related, Grady Hillhouse on the myth of Roman concrete.
> The miracle of modern chemistry has given us a wide variety of admixtures like superplasticizers to improve the characteristics of concrete beyond a Roman engineer’s wildest dreams. So why does it seem that our concrete doesn’t last nearly as long as it should? It’s a complicated question, but one answer is economics. There’s a famous quote that says “Anyone can design a bridge that stands. It takes an engineer to build one that barely stands.” Just like the sculptors job is to chip away all the parts of the marble that don’t look like the subject, a structural engineer’s job is to take away all the extraneous parts of a structure that aren’t necessary to meet the design requirements. And lifespan is just one of the many criteria engineers must consider when designing concrete structures. Most infrastructure is paid for by taxes, and the cost of building to Roman standards is rarely impossible, but often beyond what the public would consider reasonable.
A large part of why Roman concrete lasted longer than ours tends to is that we suffer from a shortage of narcissistic emperors with the means to wield entire economies towards their own immortality.
Most infrastructure is paid for by taxes, and the cost of building to Roman standards is rarely impossible, but often beyond what the public would consider reasonable.
Would you pay 10x more to have something that lasts 100x or even 1000x longer? The upfront cost is higher, but the TCO is ultimately lower. IMHO it's ultimately a form of planned obsolescence. This becomes even more obvious when plenty of expense is spent just on "engineering" to deliberately reduce lifespan.
First, we can’t summon infinite money to pay for things. Paying 10X more per bridge means we can build 1/10th as many bridges or we have to start stealing from other budgets.
Second, we don’t know what the needs will be for the bridge in that location 100 or 1000 years from now. It could need to be torn down to be widened. Maybe we’re all riding around in electric vehicles that coordinate perfectly with each other and the bridge isn’t needed for cross traffic any more. We don’t know.
The thing is, we're actually pretty crappy at knowing what we'll need 50 years from now, much less 500. Doesn't make sense to overbuild for an unknown future, when hundred years from now us will likely be able to do a far better job anyway.
The whole promise of engineering is not to build a bridge that stands but to build a bridge that barely stands. It is not a good idea to build a bridge that last 500 years. You likely destroyed valuable resources to build one. Build a bridge that lasts 100 years and save those resources. In 100 years the technology to build bridges improves so much that it is lot easier to build a new one. At least in most countries like India.
Nobody is going to tear down old bridges and rebuild them at enormous costs just because technology changed. Some things can't or won't be redone and so it's worth it to build it to last and getting it right on the first try.
This is exactly what we did with houses, buildings. We had perfectly functional 100+ years old fully functional, gorgeous buildings and we replaced those with brutalist concrete/glass barely functional garbage.
Edit: if you were an expert in this field and that link saved you from typing, and you mentioned you could confirm every word, that’d make sense - I think those Flash models were tested as being as reliable as a coin flip in some hallucination test scenarios, so linking it’s like… eh do I wanna read potentially-only-plausible history?
The folk of the 1930s were entirely capable of making poor quality concrete that barely lasted 30 years (source, my father, born 1935, still alive despite having mixed many a batch of concrete and having laboured).
The reason you don't see that walking about is that poor quality 1930s concrete was replace 50+ years ago.
Roman concrete was made lime cement (calcium dioxide); which cures via carbonation (hardens with carbon oxide). And adding pozzolan to lime makes it hydrolic (hardens with water). Is it surprising that it can still carbonate some? Modern concrete has steel which rust and crack concrete. You can use fiberglass rebar for longevity, or build without rebar even, but that is more costly and and less efficient.
But it turns out that's pretty inconvenient; we really like doing dozens of feet of span for highway overpasses, building floors, and everything else. So we put rebar in all the concrete and just acknowledge that that means it has an absolute maximum lifespan of a century or two, and will certainly not last for millennia the way pure concrete in pure compression can.
[1]: https://www.theartnewbie.com/blog/rome/roman-arch
https://en.wikipedia.org/wiki/Galvanic_anode
There are also coated and non-metallic rebars.
> The miracle of modern chemistry has given us a wide variety of admixtures like superplasticizers to improve the characteristics of concrete beyond a Roman engineer’s wildest dreams. So why does it seem that our concrete doesn’t last nearly as long as it should? It’s a complicated question, but one answer is economics. There’s a famous quote that says “Anyone can design a bridge that stands. It takes an engineer to build one that barely stands.” Just like the sculptors job is to chip away all the parts of the marble that don’t look like the subject, a structural engineer’s job is to take away all the extraneous parts of a structure that aren’t necessary to meet the design requirements. And lifespan is just one of the many criteria engineers must consider when designing concrete structures. Most infrastructure is paid for by taxes, and the cost of building to Roman standards is rarely impossible, but often beyond what the public would consider reasonable.
https://practical.engineering/blog/2019/3/9/was-roman-concre...
A large part of why Roman concrete lasted longer than ours tends to is that we suffer from a shortage of narcissistic emperors with the means to wield entire economies towards their own immortality.
Would you pay 10x more to have something that lasts 100x or even 1000x longer? The upfront cost is higher, but the TCO is ultimately lower. IMHO it's ultimately a form of planned obsolescence. This becomes even more obvious when plenty of expense is spent just on "engineering" to deliberately reduce lifespan.
First, we can’t summon infinite money to pay for things. Paying 10X more per bridge means we can build 1/10th as many bridges or we have to start stealing from other budgets.
Second, we don’t know what the needs will be for the bridge in that location 100 or 1000 years from now. It could need to be torn down to be widened. Maybe we’re all riding around in electric vehicles that coordinate perfectly with each other and the bridge isn’t needed for cross traffic any more. We don’t know.
not recently
Turns out algae is hard to kill, especially when you feed the reflecting pool from a tidal basin.
This hasn't even been true for 200 years lol
From: https://www.science.org/doi/10.1126/sciadv.aeb0754
https://share.gemini.google/5g0gxGyOmAPD
Edit: if you were an expert in this field and that link saved you from typing, and you mentioned you could confirm every word, that’d make sense - I think those Flash models were tested as being as reliable as a coin flip in some hallucination test scenarios, so linking it’s like… eh do I wanna read potentially-only-plausible history?
The folk of the 1930s were entirely capable of making poor quality concrete that barely lasted 30 years (source, my father, born 1935, still alive despite having mixed many a batch of concrete and having laboured).
The reason you don't see that walking about is that poor quality 1930s concrete was replace 50+ years ago.