3 ms·

This article seems to imply that only Roman marine concrete is more lasting because of the ongoing reaction with the sea water, but I was under the impression t

by dre85 9y ago

This article seems to imply that only Roman marine concrete is more lasting because of the ongoing reaction with the sea water, but I was under the impression that all Roman concrete was more lasting... Is it just the presence of rebar that makes today's concrete so short lived or is there something else as well?

MrTonyD 9y ago
I'm no expert, but I've read that their use of volcanic rock is a major factor - since "pumice" has a swiss-cheese like surface, and that tends to add strength to concrete. Also, they used incredibly little water in their mixtures - there are descriptions of them pounding in the concrete (as opposed to the way we "flow" it.) I understand that studies have shown that concrete mixed with very little water is much stronger. I also read that fast-setting concrete has been found to be very weak - and was never actually tested in any significant way before it was approved. So very long curing times result in much stronger concrete. Obviously, all these discoveries are being fought by the building industry - so none of these things are reflected in current code. Instead, they just argue about rebar while ignoring the other discoveries.
- peterwwillis 9y ago
  Just because it's stronger doesn't mean it's more durable, which the article points out. Portland concrete has higher compressive strength but doesn't last as long.
  - MrTonyD 9y ago
    Well, old concrete, even pre-"quick set" has been found to be much more "durable" (colloquially speaking - not really an engineering term.) And in most applications it isn't concrete's compressive strength which causes failure - for either old concrete or new concrete. So I would hesitate to defend current building practices based on the lone metric of compressive strength. But, again, I'm not a structural engineer (though I do know one who works for the state, and he isn't allowed to express his real opinions about the danger of our current infrastructure - many building failed in earthquakes which they should have survived easily.)
  - snovv_crash 9y ago
    From my experience with epoxy, thicker viscosity and slower-curing is well known to result in a stronger end material. I'm not saying this is also true with concrete, but there is a parallel precedent.
johan_larson 9y ago
I seem to recall reading something about concrete density. Roman concrete when wet was about as malleable as Silly-Putty. It took a lot of work to shape it into its forms and eliminate air pockets. Our concrete is looser and pours when wet, so it's a lot easier to work with, but it's also weaker.
- toyg 9y ago
  Easier to work with -> faster and cheaper. The Romans optimised for long-term durability; modern builders optimise for cost. It's like any manufacturing industry: building expensive stuff that lasts forever will naturally diminish your business in the long run. It's much better to make "cheap but good enough" stuff that you throw away and re-make every X years. Such is the way of the private profit.
  - Spooky23 9y ago
    Also, union construction guys are more expensive than slaves. Remember that manhours were cheap in the Roman era.
jaclaz 9y ago
There is a big difference in the "base" cement, even modern Pozzolanic cement is more lasting than Portland. Pozzolan has been considered for years an "inferior" cement (IMHO wrongly) because it is much slower in setting/curing. The typical compresssion test for concrete (it depends on countries/standards) is carried on samples of 28 days of age. A concrete made with Portland cement normally is very near, (let's say conventionally 85-90%) to the maximum strength it will ever reach (the maximum is reached usually within months). A concrete made with Pozzolanic cement, tested in the same manner is more likely to be in the 50-60% range, Pozzolan will normally continue to harden (for years), often overtaking the Portland. Additionally there is a "mass effect" particularly if the environment has a high humidity, that makes the samples being "less representative" of the actual concrete mass in the case of Pozzolanic (a "huge" thickness of Pozzolanic cement has generally speaking more compression resistance than what the test sample may prove). Now if you have to build multi-storey buildings or - say -high raise pillars/columns one of the key factors is time, the soon you can remove formworks and scaffolding and go to the next storey or level is essential and a much faster setting Portland cement has been preferred for all the 20th century, before durability was even thought of. Nowadays there is a trend towards "blended" cements, see: https://en.wikipedia.org/wiki/Pozzolan https://en.wikipedia.org/wiki/Pozzolan https://en.wikipedia.org/wiki/Pozzolan#Use https://en.wikipedia.org/wiki/Pozzolan#Use

by dre85 9y ago

MrTonyD 9y ago

I'm no expert, but I've read that their use of volcanic rock is a major factor - since "pumice" has a swiss-cheese like surface, and that tends to add strength to concrete. Also, they used incredibly little water in their mixtures - there are descriptions of them pounding in the concrete (as opposed to the way we "flow" it.) I understand that studies have shown that concrete mixed with very little water is much stronger. I also read that fast-setting concrete has been found to be very weak - and was never actually tested in any significant way before it was approved. So very long curing times result in much stronger concrete. Obviously, all these discoveries are being fought by the building industry - so none of these things are reflected in current code. Instead, they just argue about rebar while ignoring the other discoveries.

peterwwillis 9y ago
Just because it's stronger doesn't mean it's more durable, which the article points out. Portland concrete has higher compressive strength but doesn't last as long.
- MrTonyD 9y ago
  Well, old concrete, even pre-"quick set" has been found to be much more "durable" (colloquially speaking - not really an engineering term.) And in most applications it isn't concrete's compressive strength which causes failure - for either old concrete or new concrete. So I would hesitate to defend current building practices based on the lone metric of compressive strength. But, again, I'm not a structural engineer (though I do know one who works for the state, and he isn't allowed to express his real opinions about the danger of our current infrastructure - many building failed in earthquakes which they should have survived easily.)
- snovv_crash 9y ago
  From my experience with epoxy, thicker viscosity and slower-curing is well known to result in a stronger end material. I'm not saying this is also true with concrete, but there is a parallel precedent.

johan_larson 9y ago

I seem to recall reading something about concrete density. Roman concrete when wet was about as malleable as Silly-Putty. It took a lot of work to shape it into its forms and eliminate air pockets. Our concrete is looser and pours when wet, so it's a lot easier to work with, but it's also weaker.

toyg 9y ago
Easier to work with -> faster and cheaper. The Romans optimised for long-term durability; modern builders optimise for cost. It's like any manufacturing industry: building expensive stuff that lasts forever will naturally diminish your business in the long run. It's much better to make "cheap but good enough" stuff that you throw away and re-make every X years. Such is the way of the private profit.
- Spooky23 9y ago
  Also, union construction guys are more expensive than slaves. Remember that manhours were cheap in the Roman era.

jaclaz 9y ago

There is a big difference in the "base" cement, even modern Pozzolanic cement is more lasting than Portland. Pozzolan has been considered for years an "inferior" cement (IMHO wrongly) because it is much slower in setting/curing. The typical compresssion test for concrete (it depends on countries/standards) is carried on samples of 28 days of age. A concrete made with Portland cement normally is very near, (let's say conventionally 85-90%) to the maximum strength it will ever reach (the maximum is reached usually within months). A concrete made with Pozzolanic cement, tested in the same manner is more likely to be in the 50-60% range, Pozzolan will normally continue to harden (for years), often overtaking the Portland. Additionally there is a "mass effect" particularly if the environment has a high humidity, that makes the samples being "less representative" of the actual concrete mass in the case of Pozzolanic (a "huge" thickness of Pozzolanic cement has generally speaking more compression resistance than what the test sample may prove). Now if you have to build multi-storey buildings or - say -high raise pillars/columns one of the key factors is time, the soon you can remove formworks and scaffolding and go to the next storey or level is essential and a much faster setting Portland cement has been preferred for all the 20th century, before durability was even thought of. Nowadays there is a trend towards "blended" cements, see: https://en.wikipedia.org/wiki/Pozzolan https://en.wikipedia.org/wiki/Pozzolan https://en.wikipedia.org/wiki/Pozzolan#Use https://en.wikipedia.org/wiki/Pozzolan#Use