6 ms·

> It turns out that another chemical reaction, known as carbonation, might also contribute to Roman concrete’s longevity. Roman concrete was made lime cement (

by bombela 2mo ago

> 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.

deleted 2mo ago
[deleted]
litoE 2mo ago
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?
- aw1621107 2mo ago
  Just FYI, I think you're looking for "(in) tension" instead of "(on) traction".
- mikepurvis 2mo ago
  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. [1]: https://www.theartnewbie.com/blog/rome/roman-arch https://www.theartnewbie.com/blog/rome/roman-arch
  - cm2187 2mo ago
    They could do large arches too, see https://en.wikipedia.org/wiki/Basilica_of_Maxentius https://en.wikipedia.org/wiki/Basilica_of_Maxentius
  - Zardoz84 2mo ago
    or go and revive Gothic architecture
  - Modified3019 2mo ago
    The “it’s built so everything is under compression” is great thing to bring up, but fyi your link is AI generated, filled with senseless repetition and “it’s not just x, it’s y”. Looking it up on archive.org shows it was generated this year.
    - mikepurvis 2mo ago
      Blah, so you're right. I was just looking for a link with some representative reference pictures, but I should have done more diligence. Thanks for pointing it out.
  - throw0101d 2mo ago
    > 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. Until an earthquake occurs, and then all of those mostly-down forces turn into side-to-side forces.
- deleted 2mo ago
  [deleted]
- cma 2mo ago
  There's also stainless steel rebar
  - 21asdffdsa12 2mo ago
    https://en.wikipedia.org/wiki/Galvanic_anode https://en.wikipedia.org/wiki/Galvanic_anode
  - mikepurvis 2mo ago
    Even without the rebar rusting, I think you can still have issues with the steel and concrete having difference thermal expansion properties, particularly for outdoor strucutures where a freeze/thaw cycle is in play. Basically the rebar works itself loose over time and creates micro-fractures in the concrete that then get moisture in them and can cause expansion damage— all without the rebar itself ever rusting or "failing".
    - cma 2mo ago
      Maybe so, OP mentioned they were very similar and that still seems to hold for stainless. How about the aggregates within? The composite makeup can result in different CoTE than the individual aggregates, I think that's one part of concrete sidewalks cracking though, sometimes near shaded/unshaded boundaries. Roman concrete supposedly had some self healing properties before the cracks grew, from the lime inclusions.
- deepsun 2mo ago
  > exactly the opposite No, steel is better in both ways, ten times over. It's just more expensive, concrete is a "filler" to cheap out construction. If you think about it, all engineering is about cheaping out things. It's pretty easy to build awesome projects having unlimited budget.
  - selcuka 2mo ago
    As one of my professors said back in time: "Steel is one of the best materials for a number of applications. We don't use it only because it's abundant. "Abundant" and "good" are not always mutually exclusive."
- pinkmuffinere 2mo ago
  > Wouldn't your beams start cracking at the bottom, where they are subject to traction? You have a few mistakes here. I’m not trying to demean you, but I’m going to number them just for clarity, as it can get confusing when there are many misunderstandings. 1. You are intending to ask about tension (which the rebar helps with), not traction (the force your tires exert against a road). 2. Tension is not only experienced at the bottom of beams, the location with the most tension will depend on the geometry. For a vertical beam, I think tension will probably be pretty even through the whole beam in most “normal” designs and loading configurations. But it will really depend on the geometry and on the loads being applied. 3. I think when you say concrete beams you’re meaning columns (apologies if I’m wrong about this). Concrete columns are remarkably good at holding up without rebar, because they experience almost exclusively compression! And indeed, ancient Roman designs did not use rebar at all :). It’s certainly possible.
  - kergonath 2mo ago
    > 1. You are intending to ask about tension (which the rebar helps with), not traction (the force your tires exert against a road). I don’t know if it is the case here, but it is a common mistake for some non-native English speakers. In some languages traction is a false friend.
    - projektfu 2mo ago
      And in English, traction also means pulling, but it got somewhat misapplied to the friction of tires, causing confusion in the language. For example, a tractor is something that pulls, and when a broken leg is put in traction, it's suspended to pull it apart and keep the muscles from pulling it together. Tension, as you say, is indeed the term used in physics and engineering for the force on an object pulling it from either end.
  - litoE 2mo ago
    1.- My apologies for the terminology - I took one overview class on the subject in Spanish, and haven't looked at it in the last 60 years. 2.- Agreed - that's why rebar on horizontal beams (and slabs) sometimes goes on the top edge and shifts to the bottom edge. 3.- By beams I do mean horizontal beams. This also applies to horizontal slabs. However, vertical columns also need rebar, especially in earthquake-prone areas. If the ground moves horizontally in an earthquake, the (vertical) columns have to transmit the motion to the upper parts of the structure. This create huge tension stresses on one side of the columns so they need vertical rebar rods on their perimeter.
Animats 2mo ago
Stainless steel rebar is a thing. It's expensive, but structures built with it last much longer. Washington State now has a policy that bridges over salt water will be built with stainless steel rebar. Stainless steel costs much more, but total project cost goes up less than 10%. Epoxy coated rebar looked promising for a while, but it's on the way out. Water gets in at cuts and joints. So all field joints have to have field patching. There is a great comparison. Two piers were built in the 1940s, side by side, one with carbon steel rebar, one with stainless steel rebar.[1] Go look. [1] https://worldstainless.org/wp-content/uploads/2025/02/ref19_case_study_of_progreso_pier.pdf https://worldstainless.org/wp-content/uploads/2025/02/ref19_... [1] https://worldstainless.org/wp-content/uploads/2025/02/ref19_case_study_of_progreso_pier.pdf https://worldstainless.org/wp-content/uploads/2025/02/ref19_...
- ErroneousBosh 2mo ago
  > Stainless steel costs much more, but total project cost goes up less than 10%. I think people are often surprised how little materials cost affects the total cost of a job. I'm having this argument at work, where I want some more expensive cabling installed in a bunch of offices across Scotland so it doesn't need to be done again in another ten year's time, but "that stuff is so expensive, do we really need it?" is what I'm running up against. So as it turns out today I'm giving them a breakdown of the cost of the job. Guess what the expensive bits are? Did you guess "wages for two guys, hotel rooms for two guys, 800 miles of diesel, and a couple of ferry tickets?" Well, you're way ahead of today's crowd then.
  - iamflimflam1 2mo ago
    I hope you’re prepared for a huge amount of bike shedding… but if we pick this color, we can save a couple of dollars…
    - ErroneousBosh 2mo ago
      It's going in a suspended ceiling so it has to be purple.
  - 2mo ago
Izmaki 2mo ago
> but that is more costly and and less efficient. Maybe this is the clue as to why our concrete crumbles after 100 years: it's not economically efficient to make it last longer?
- aYsY4dDQ2NrcNzA 2mo ago
  I’m passing my latrine down to our great grandchildren.
  - Saline9515 2mo ago
    You don't really own a latrine, but just look for it for the next generation.
- rkangel 2mo ago
  A broader view of this point would be "the economic structures in some countries do not allow for investments with a long payback".
  - PunchyHamster 2mo ago
    Or simpler: people don't exactly want a house layout that was popular 100 years ago. And definitely not the level of insulation it provided. Nor the fact putting any new wiring and piping thru concrete is PITA Tastes change and so do other requirements. but, if we actually built for longevity in use, we could build a very durable shell of a house and then use more perishable but easier to modify methods for inside. Build a nice durable brick and concrete shell then use wood and plasterboard for room walls and floors
    - rkangel 2mo ago
      There are some downsides, but most people on the UK would prefer houses that were 100+ years old. In the village I live in the houses immediately around me are "only" 50 years old. Most of the ones on the main street are well over 100 years old. Housing tastes don't really change that much. Yes over the years we've had to fit things like double glazing and better insulation but that's a whole lot cheaper (and better for the environment) than building a whole new building.

by bombela 2mo ago

deleted 2mo ago

[deleted]

litoE 2mo ago

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?

aw1621107 2mo ago
Just FYI, I think you're looking for "(in) tension" instead of "(on) traction".
mikepurvis 2mo ago
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. [1]: https://www.theartnewbie.com/blog/rome/roman-arch https://www.theartnewbie.com/blog/rome/roman-arch
- cm2187 2mo ago
  They could do large arches too, see https://en.wikipedia.org/wiki/Basilica_of_Maxentius https://en.wikipedia.org/wiki/Basilica_of_Maxentius
- Zardoz84 2mo ago
  or go and revive Gothic architecture
- Modified3019 2mo ago
  The “it’s built so everything is under compression” is great thing to bring up, but fyi your link is AI generated, filled with senseless repetition and “it’s not just x, it’s y”. Looking it up on archive.org shows it was generated this year.
  - mikepurvis 2mo ago
    Blah, so you're right. I was just looking for a link with some representative reference pictures, but I should have done more diligence. Thanks for pointing it out.
- throw0101d 2mo ago
  > 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. Until an earthquake occurs, and then all of those mostly-down forces turn into side-to-side forces.
deleted 2mo ago
[deleted]
cma 2mo ago
There's also stainless steel rebar
- 21asdffdsa12 2mo ago
  https://en.wikipedia.org/wiki/Galvanic_anode https://en.wikipedia.org/wiki/Galvanic_anode
- mikepurvis 2mo ago
  Even without the rebar rusting, I think you can still have issues with the steel and concrete having difference thermal expansion properties, particularly for outdoor strucutures where a freeze/thaw cycle is in play. Basically the rebar works itself loose over time and creates micro-fractures in the concrete that then get moisture in them and can cause expansion damage— all without the rebar itself ever rusting or "failing".
  - cma 2mo ago
    Maybe so, OP mentioned they were very similar and that still seems to hold for stainless. How about the aggregates within? The composite makeup can result in different CoTE than the individual aggregates, I think that's one part of concrete sidewalks cracking though, sometimes near shaded/unshaded boundaries. Roman concrete supposedly had some self healing properties before the cracks grew, from the lime inclusions.
deepsun 2mo ago
> exactly the opposite No, steel is better in both ways, ten times over. It's just more expensive, concrete is a "filler" to cheap out construction. If you think about it, all engineering is about cheaping out things. It's pretty easy to build awesome projects having unlimited budget.
- selcuka 2mo ago
  As one of my professors said back in time: "Steel is one of the best materials for a number of applications. We don't use it only because it's abundant. "Abundant" and "good" are not always mutually exclusive."
pinkmuffinere 2mo ago
> Wouldn't your beams start cracking at the bottom, where they are subject to traction? You have a few mistakes here. I’m not trying to demean you, but I’m going to number them just for clarity, as it can get confusing when there are many misunderstandings. 1. You are intending to ask about tension (which the rebar helps with), not traction (the force your tires exert against a road). 2. Tension is not only experienced at the bottom of beams, the location with the most tension will depend on the geometry. For a vertical beam, I think tension will probably be pretty even through the whole beam in most “normal” designs and loading configurations. But it will really depend on the geometry and on the loads being applied. 3. I think when you say concrete beams you’re meaning columns (apologies if I’m wrong about this). Concrete columns are remarkably good at holding up without rebar, because they experience almost exclusively compression! And indeed, ancient Roman designs did not use rebar at all :). It’s certainly possible.
- kergonath 2mo ago
  > 1. You are intending to ask about tension (which the rebar helps with), not traction (the force your tires exert against a road). I don’t know if it is the case here, but it is a common mistake for some non-native English speakers. In some languages traction is a false friend.
  - projektfu 2mo ago
    And in English, traction also means pulling, but it got somewhat misapplied to the friction of tires, causing confusion in the language. For example, a tractor is something that pulls, and when a broken leg is put in traction, it's suspended to pull it apart and keep the muscles from pulling it together. Tension, as you say, is indeed the term used in physics and engineering for the force on an object pulling it from either end.
- litoE 2mo ago
  1.- My apologies for the terminology - I took one overview class on the subject in Spanish, and haven't looked at it in the last 60 years. 2.- Agreed - that's why rebar on horizontal beams (and slabs) sometimes goes on the top edge and shifts to the bottom edge. 3.- By beams I do mean horizontal beams. This also applies to horizontal slabs. However, vertical columns also need rebar, especially in earthquake-prone areas. If the ground moves horizontally in an earthquake, the (vertical) columns have to transmit the motion to the upper parts of the structure. This create huge tension stresses on one side of the columns so they need vertical rebar rods on their perimeter.

Animats 2mo ago

Stainless steel rebar is a thing. It's expensive, but structures built with it last much longer. Washington State now has a policy that bridges over salt water will be built with stainless steel rebar. Stainless steel costs much more, but total project cost goes up less than 10%. Epoxy coated rebar looked promising for a while, but it's on the way out. Water gets in at cuts and joints. So all field joints have to have field patching. There is a great comparison. Two piers were built in the 1940s, side by side, one with carbon steel rebar, one with stainless steel rebar.[1] Go look. [1] https://worldstainless.org/wp-content/uploads/2025/02/ref19_case_study_of_progreso_pier.pdf https://worldstainless.org/wp-content/uploads/2025/02/ref19_... [1] https://worldstainless.org/wp-content/uploads/2025/02/ref19_case_study_of_progreso_pier.pdf https://worldstainless.org/wp-content/uploads/2025/02/ref19_...

ErroneousBosh 2mo ago
> Stainless steel costs much more, but total project cost goes up less than 10%. I think people are often surprised how little materials cost affects the total cost of a job. I'm having this argument at work, where I want some more expensive cabling installed in a bunch of offices across Scotland so it doesn't need to be done again in another ten year's time, but "that stuff is so expensive, do we really need it?" is what I'm running up against. So as it turns out today I'm giving them a breakdown of the cost of the job. Guess what the expensive bits are? Did you guess "wages for two guys, hotel rooms for two guys, 800 miles of diesel, and a couple of ferry tickets?" Well, you're way ahead of today's crowd then.
- iamflimflam1 2mo ago
  I hope you’re prepared for a huge amount of bike shedding… but if we pick this color, we can save a couple of dollars…
  - ErroneousBosh 2mo ago
    It's going in a suspended ceiling so it has to be purple.
- 2mo ago

Izmaki 2mo ago

> but that is more costly and and less efficient. Maybe this is the clue as to why our concrete crumbles after 100 years: it's not economically efficient to make it last longer?

aYsY4dDQ2NrcNzA 2mo ago
I’m passing my latrine down to our great grandchildren.
- Saline9515 2mo ago
  You don't really own a latrine, but just look for it for the next generation.
rkangel 2mo ago
A broader view of this point would be "the economic structures in some countries do not allow for investments with a long payback".
- PunchyHamster 2mo ago
  Or simpler: people don't exactly want a house layout that was popular 100 years ago. And definitely not the level of insulation it provided. Nor the fact putting any new wiring and piping thru concrete is PITA Tastes change and so do other requirements. but, if we actually built for longevity in use, we could build a very durable shell of a house and then use more perishable but easier to modify methods for inside. Build a nice durable brick and concrete shell then use wood and plasterboard for room walls and floors
  - rkangel 2mo ago
    There are some downsides, but most people on the UK would prefer houses that were 100+ years old. In the village I live in the houses immediately around me are "only" 50 years old. Most of the ones on the main street are well over 100 years old. Housing tastes don't really change that much. Yes over the years we've had to fit things like double glazing and better insulation but that's a whole lot cheaper (and better for the environment) than building a whole new building.