Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why Ancient Roman Concrete Outlasts Our Own (2017)

sci-hubtw.hkvisa.net

Post reply on HN
Print view View original

41–50 of 69 posts

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#41

Post by masklinn »

metaphor wrote: post #24
masklinn wrote: post #7

Earlier quoted context omitted.

> Maybe cost is too high for the average customer? Well yes, and even for the non-average one. Plus the goal is generally to build things which barely stand, a building which can face the vagaries of time for thousands of years would generally be way over-engineered. Countries where pozzolanic ash is readily available do use it but that’s not usually the case. Fly ash is also being explored for that role. An other is…

> Plus the goal is generally to build things which barely stand... Since when? Design criteria baked into international building code is quite robust; not just naive static load, but wind, snow, seismic activity, fire/flood/termite resistance, etc. are all considered. That's a far stretch from something that can "barely stand".

> Since when?

Since always

> Design criteria baked into international building code is quite robust; not just naive static load, but wind, snow, seismic activity, fire/flood/termite resistance, etc. are all considered. That's a far stretch from something that can "barely stand".

“Barely stands” in this context is “Barely passes code”, as in “any idiot can build a bridge that stands, it takes an engineer to build a bridge that barely stands”.

The more you go over the more money you’re wasting.

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#42

Post by markvdb »

hef19898 wrote: post #11
eru wrote: post #9

Earlier quoted context omitted.

First, we only see the part of Roman architecture that lasted. There's an observer bias. The Romans also made plenty of ephemeral stuff, just like we are making plenty of ephemeral stuff. See also https://www.youtube.com/watch?v=qL0BB2PRY7k Second, there's more to engineering than making stuff last long. There are different trade-offs. Cost being one of them, but also different material properties. Eg re-inforced con…

Trade offs should be covered a lot more during engineering studies. I know that during my time at university, as an industrial engineer destined to work at the interface engineering and economics, that aspect wasn't covered nearly as extensive as it should have. The other question that usually get's ignored is maintenance. The Roman stuff we see lasted millennia without maintenance, we on the other hand can maintain…

> we on the other hand can maintain the stuff we want to last a long time. That we don't do it, e.g. infrastructure with the particularly bad maintained bridges in Germany, is not the materials fault, or the original designs fault.

I have to respectfully disagree on this one. Humans in general have become spectacularly bad at maintaining physical infrastructure across longer timespans. Technological advances have enabled a cheap/fast/overbuild culture. This very much includes the design phase.

Look at the evolution of design. The large majority of our infrastructure, depending on the spot on our earth, largely dates from the last 200 to 50 years. Look at infrastructure predating that. Look at evolution. The bond with local communities depending on the infra? You'll inevitably find it cut. You'll find more bloat in the design. You'll find less local involvement.

To some extent, this is progress. Unfortunately, this has an impact on maintainability.

Looking at your example of German bridges. I'll make it even more tangible and look at the Eifel region with so much of its infra recently destroyed by flooding. How do we get excellently maintained bridges, when we know this is very much against human nature?

This means questions like: - Does this bridge really need to be (re)built? To this specification? In this place? - How much does this bridge benefit the local community where it is built? Can we think of ways to increase that? - How much of this bridge absolutely needs to be built out of reinforced concrete? - Wouldn't it make sense to build some infra in now very flood-prone areas out of less durable but cheaper, more quickly replaced materials like wood? - Can we bring the design closer to the layman? Can we for example design a bridge so that it will visually degrade in step with safety degradation? - ...

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#43

Post by jaclaz »

Eelongate wrote: post #17
jbay808 wrote: post #14

Earlier quoted context omitted.

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 runnin…

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.

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

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#44

Post by Animats »

Phithagoras wrote: post #2

In a second year class I took the prof posed us the question: why don't we do this today? Why is this not part of an ASTM standard? He ended up saying it's too tough to get a hold of proper pozzlanic ash, but I suspect its more a "this is the way we've always done it" difficulty. Does anyone know more to the story?

There is an ASTM standard: "ASTM C618 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete (AASHTO M 295)."[1]

Fly ash, like volcanic ash, is a pozzlanic ash. Both work in concrete. Fly ash is pulled out of the exhaust from a coal-fired power plant using electrostatic precipitators. Just like electrostatic air cleaners, but huge, they pull particles out of gases. So fly ash is cheap if there's a coal-fired power plant nearby. Convenient when there is no volcano handy.

There are downsides. The concrete takes longer to cure, which can hold up the next stage of construction. Curing in cold weather is difficult. The Romans didn't have that problem in their Mediterranean climate. Concrete curing requires some air in the mix, and fly ash, for some reason, tends to entrain less air than Portland cement.[2]

[1] https://www.cement.org/docs/default-source/fc_concrete_techn...

[2] https://www.fhwa.dot.gov/PAVEMENT/recycling/fach03.cfm

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#45

Post by simonh »

Phithagoras wrote: post #2

In a second year class I took the prof posed us the question: why don't we do this today? Why is this not part of an ASTM standard? He ended up saying it's too tough to get a hold of proper pozzlanic ash, but I suspect its more a "this is the way we've always done it" difficulty. Does anyone know more to the story?

>but I suspect its more a "this is the way we've always done it" difficulty.

Is the construction industry really known for resisting the adoption of new materials like that?

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#46

Post by Eelongate »

jaclaz wrote: post #43
Eelongate wrote: post #17

Earlier quoted context omitted.

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.

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 pract…

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.

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#47

Post by ncmncm »

I am becoming partial to the ancient Egyptian concrete, that they might have used to mold the pyramid blocks in situ.

It takes just limestone crumble, clay (which was already in their limestone), natron, and water.

It is a fair bet the precursors to the Inka who built with the really big blocks had that, or a similar trick. Local observers report the big blocks do not show embedded marine shells at the surface, unlike native limestone. (I have not had opportunity to verify this.)

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#48

Post by he0001 »

eru wrote: post #30
he0001 wrote: post #29

Earlier quoted context omitted.

> First, we only see the part of Roman architecture that lasted. There's an observer bias. Couldn’t it also possible that the romans had some techniques that they weren’t aware of that would last longer than others?

Yes, of course. I didn't offer a guess why some things lasted longer than others. Might be sheer blind luck, might be deliberate design, might be side-effects of things done for other reasons (like you suggest), or something else. Or a mixture.

I was more referring to the observation bias you mentioned. If they were making informed decisions on which technique to use for it to last a millennia or not then I’d also think there was some sort of observation bias. But if they didn’t, there wouldn’t be any observation bias since the process is then mostly random because they were not really aware of what technique that worked, no? We’re just seeing the results because they sometimes used a technique that lasted.

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#49

Post by yourapostasy »

Eelongate wrote: post #46
jaclaz wrote: post #43

Earlier quoted context omitted.

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 pract…

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.

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

Re: Why Ancient Roman Concrete Outlasts Our Own (2017)

#50

Post by yourapostasy »

ithkuil wrote: post #39
TeeMassive wrote: post #19

Earlier quoted context omitted.

Not to mention that concrete steel reinforcement is the main reason it doesn't last due to water eventually making its way and the rust taking expansion thus cracking the concrete.

Would the use of stainless steel address this problem? What other problems would it create?

Stainless of the right grade to address the problem is project-killingly expensive. There is work on evaluating alternative kinds of reinforcement not subject to the same kinds of rust problems, but extremely few organizations today plan, build and maintain over the kinds of timescales where this comes into play. They'll give lip service to those timescales, but watch what they do with their budgets, not what they say.
Post reply on HN