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Are you essentially saying that you don't think there are important material and chemical differences between Roman and modern concrete that might be responsibl

Are you essentially saying that you don't think there are important material and chemical differences between Roman and modern concrete that might be responsible for orders-of-magnitude differences in durability, aside from the absence of rebar, and that it's basically a matter of building so many bridges, harbours, and aqueducts that some of them end up lasting for thousands of years despite being immersed in running rivers or salt water?

Eelongate 5y ago
Absence of rebar is a huge difference. From what I understand it's the primary reason most modern concrete structures are expected to last only on the order of decades to a century or two. Some modern structures have been built without much if any rebar, for instance the Hoover Dam, and I've heard that might last thousands of years.
- jaclaz 5y ago
  But beside that (whch surely is the main reason), there is also a great difference between pozzolanic and portland, nowadays (and since several years) pozzolanic cement (for whatever reasons[1]) came out of use and anything today (and since several years) is portland. I have worked with both in massive structures (mainly bridge foundations and tunnel lining) and the differences between the two is staggering, in practice portland cement based concrete is very good (compression resistance) already at 3 or 7 days, just fine at 28 days, but tops around the 60-90 days. Pozzolanic cement based concrete is barely reaching specs at 28 days, but continues to mature (increasing resistance) for years to follow, expecially in massive layers and in humid environments. If you prefere Portland is a better cememt because it is fast, but - given time - pozzolanic is way "stronger". In tunnels (which have all the best requisites for concrete to mature correctly) we had at the time (some 30-40 years ago) specs of 250 Kg/cm2 cubic resistance (at 28 days), we used pozzolanic cement for the lower arch and portland for the vault, while both reached specs, after 2-3 years we made some tests and the vault (portland) reached 300, in some cases 350, the lower arch was never below 450, in some cases 500 and even 600. [1] essentially because it cures faster, allows thinner layers/slabs and for anything where formwork is involved this makes a huge difference
  - Eelongate 5y ago
    What about non-volcanic pozzolans like fly ash? From what I understand, fly ash is sometimes substituted for some portion of the portland cement that might otherwise have been used, because works similar to volcanic ash to make the concrete stronger.
    - yourapostasy 5y ago
      Fly ash is similar but still doesn't perform just like basaltic volcanic ash [1] in admixtures. [1] https://www.mdpi.com/1996-1944/12/16/2603/pdf https://www.mdpi.com/1996-1944/12/16/2603/pdf
    - jaclaz 5y ago
      Fly ash is - generally speaking - an exceptionally good additive, its use is essentially due to the size of its particles, you can consider it as a very good filler with some added value (and it has some very useful side-effects, namely it makes concrete much easier to pump). Besides the raw resistance, you have to imagine concrete as being a sort of artificial stone, the idea is to fit into a given volume as much material as you can, and you obtain this by mixing together gravel (usually one, two or even sometimes three sizes), crushed sand (rather big in size) and (where available) natural sand (or more finely crushed sand). Then you add the cement, which is the finer "powder like" material, and water, BUT in many cases the "granulometric curve" remains "empty" in the lower part, and there are practical limits in the amount of cement you can put in the mix, so you need to add something (a filler) that is fine as or finer than cement, and this is often fly ash, which while not being as powerful as cement as a binder has anyway a pozzolanic effect, that helps in reducing the permeability of the set concrete, the matter is briefly explained in the second part of this: https://en.wikipedia.org/wiki/Pozzolan https://en.wikipedia.org/wiki/Pozzolan
eru 5y ago
No. That's not what I am saying. Both us and the Romans are and were building lots of different things for lots of different purposes with lots of different properties and trade-offs. Some of the earliest re-inforced concrete was used to make better flowerpots. https://en.wikipedia.org/wiki/Reinforced_concrete#History https://en.wikipedia.org/wiki/Reinforced_concrete#History You can bet your hat that this was a very different type of (re-inforced) concrete than what we are using to build houses these days. We use yet other kinds of concrete for tunnels or underwater. Just like the Romans used different building materials.
bawolff 5y ago
I read the person's comment as saying - Romans built a lot of stuff, some high quality and some presumably low. We only see the high quality stuff because the low quality stuff has long since disintegrated. We then assume that everything they made was high quality instead of a mix of qualities because we never see the low quality stuff. Basically survivorship bias

Eelongate 5y ago

Absence of rebar is a huge difference. From what I understand it's the primary reason most modern concrete structures are expected to last only on the order of decades to a century or two. Some modern structures have been built without much if any rebar, for instance the Hoover Dam, and I've heard that might last thousands of years.

jaclaz 5y ago
But beside that (whch surely is the main reason), there is also a great difference between pozzolanic and portland, nowadays (and since several years) pozzolanic cement (for whatever reasons[1]) came out of use and anything today (and since several years) is portland. I have worked with both in massive structures (mainly bridge foundations and tunnel lining) and the differences between the two is staggering, in practice portland cement based concrete is very good (compression resistance) already at 3 or 7 days, just fine at 28 days, but tops around the 60-90 days. Pozzolanic cement based concrete is barely reaching specs at 28 days, but continues to mature (increasing resistance) for years to follow, expecially in massive layers and in humid environments. If you prefere Portland is a better cememt because it is fast, but - given time - pozzolanic is way "stronger". In tunnels (which have all the best requisites for concrete to mature correctly) we had at the time (some 30-40 years ago) specs of 250 Kg/cm2 cubic resistance (at 28 days), we used pozzolanic cement for the lower arch and portland for the vault, while both reached specs, after 2-3 years we made some tests and the vault (portland) reached 300, in some cases 350, the lower arch was never below 450, in some cases 500 and even 600. [1] essentially because it cures faster, allows thinner layers/slabs and for anything where formwork is involved this makes a huge difference
- Eelongate 5y ago
  What about non-volcanic pozzolans like fly ash? From what I understand, fly ash is sometimes substituted for some portion of the portland cement that might otherwise have been used, because works similar to volcanic ash to make the concrete stronger.
  - yourapostasy 5y ago
    Fly ash is similar but still doesn't perform just like basaltic volcanic ash [1] in admixtures. [1] https://www.mdpi.com/1996-1944/12/16/2603/pdf https://www.mdpi.com/1996-1944/12/16/2603/pdf
  - jaclaz 5y ago
    Fly ash is - generally speaking - an exceptionally good additive, its use is essentially due to the size of its particles, you can consider it as a very good filler with some added value (and it has some very useful side-effects, namely it makes concrete much easier to pump). Besides the raw resistance, you have to imagine concrete as being a sort of artificial stone, the idea is to fit into a given volume as much material as you can, and you obtain this by mixing together gravel (usually one, two or even sometimes three sizes), crushed sand (rather big in size) and (where available) natural sand (or more finely crushed sand). Then you add the cement, which is the finer "powder like" material, and water, BUT in many cases the "granulometric curve" remains "empty" in the lower part, and there are practical limits in the amount of cement you can put in the mix, so you need to add something (a filler) that is fine as or finer than cement, and this is often fly ash, which while not being as powerful as cement as a binder has anyway a pozzolanic effect, that helps in reducing the permeability of the set concrete, the matter is briefly explained in the second part of this: https://en.wikipedia.org/wiki/Pozzolan https://en.wikipedia.org/wiki/Pozzolan

eru 5y ago

No. That's not what I am saying. Both us and the Romans are and were building lots of different things for lots of different purposes with lots of different properties and trade-offs. Some of the earliest re-inforced concrete was used to make better flowerpots. https://en.wikipedia.org/wiki/Reinforced_concrete#History https://en.wikipedia.org/wiki/Reinforced_concrete#History You can bet your hat that this was a very different type of (re-inforced) concrete than what we are using to build houses these days. We use yet other kinds of concrete for tunnels or underwater. Just like the Romans used different building materials.

bawolff 5y ago

I read the person's comment as saying - Romans built a lot of stuff, some high quality and some presumably low. We only see the high quality stuff because the low quality stuff has long since disintegrated. We then assume that everything they made was high quality instead of a mix of qualities because we never see the low quality stuff. Basically survivorship bias