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Why was Roman concrete so durable? (news.mit.edu)
627 points by geox 8 months ago | hide | past | favorite | 270 comments

Here is a nice picture of the Pantheon, mentioned in the article as a prime example of these incredibly durable Roman concrete structures:


Not bad for something built nearly 2,000 years ago.

Uses pumice in aggregate as aerated mass to lighten the concrete and also varies concrete thickness by height in the dome to reduce load

And the concrete self-heals with water

I wonder what percentage of the original stones are left (given that it was repaired and restored over the centuries)

Unlike many ancient structures, the Pantheon has mostly been left intact across the millennia. The Dome is entirely original and so is most of the remaining structure. From what I know, the majority of the restoration and renovation work has been to the interior ground-level walls and some of the exterior along the ground level, but even these are mostly minor renovations. In this case, what you see truly is for the most part what always was, and especially for the main awe-inspiring part, the great dome.

I don't believe the dome structure was radically altered across time. It's possible I misunderstood but my belief was it was only cosmetic changes to the dome itself. Other structural changes were made to the portico and doorways.

The Pantheon is very notable among Roman monuments for having little done over the years other than cosmetic upgrades.

of the original Pantheon from 27 B.C. very little remained.

It was rebuilt after the fires of 80 A.C. and 110 A.C. damaged the original structure.

It's been mostly unchanged after that.

> Not bad for something built nearly 2,000 years ago.

According to the Pyramids, Stonehenge, Newgrange, Watson Brake, et al., it's a little premature to suggest that. Let's check back in 3,000 years.

The pyramids are essentially just piles of rocks. The impressive part about them is not really how they stood out the test of time, in fact they lost most of their finer details, but how an ancient civilization managed to make piles of rocks that big.

The Pantheon is not a pile of rocks, it is made of engineered materials (concrete), with domes and overhangs. Unlike "pile of rocks" architecture that essentially can't fail, if it wasn't well engineered, it would have collapsed into something unrecognizable.

stone != concrete

edit: we have a > 2,000 years old pyramid in Rome too, and it's in perfect shape, because stone != concrete


Funny, its perimeter vs. height approximates pi, rather than the originals' 2pi.

Hehe, that's funny. But the OP clearly meant it relative to other stuff built 2000 years ago, most of which has vanished without leaving a trace, maybe a couple of foundation stones buried under a meter of new topsoil.

Have an upvote, it's sad to see you grayed out.

It doesn't matter how many times I see it, every time I go to Rome and find myself in front of the Pantheon, I get overwhelmed by it.

With zero sarcasm, when I read “every time I go to Rome” I feel that I took a wrong turn in life somewhere.

Fucking awesome mate.

Worth remembering that “Going to Rome” has a very different feel to it if you live in Terni vs if you live in Whichita, Kansas. One is a day trip to shop, the other might be an adventure of a lifetime.

I was born in an East European country and I worked a few years in New York. Many people back at home react to this fact as if I made it. As if shoveling software on Lafayette street makes you a better software engineer than doing the same on the Rákóczi Körút. On the other hand for many people living and working in New York is nothing special. They were born there, it is their normal.

I for one think where you were is not as important as what you made of your time there.

Well, I lived in Italy for 10 years. Going to Rome from time to time wasn't that hard.

Now I live in Spain and I continue to fly to Rome now and then.

In Europe, distances are easy to overcome.

In Europe, 100km is a long way; in the US, 100 years is a long time.

Unless you fly ryanair for 10$

If you fly Ryanair, 20 minutes is like 100 years.

Do you mean 1000km? The thread is about Rome being easy to reach.

It's just a saying, having the same number on both ends (but measuring different things) is the part that is supposed to make you think it's clever ;)

It's not the distance but the congestion on the highways. We need someone like Elon Musk to give us the HyperLoop

Italy is wonderful, you should go, only problem is that they speak Italian.

Well, it's also wonderful they speak Italian!

But they drive Italian too. Ho well.

Drive confidently but attentively and you fit in beautifully.

Source: tried it myself.

Can't confirm. Drive unsafely like the worst drivers in LA and you'll fit right in but still be miserable.


Probably practice is needed, but your holidays are too short for that.

There's objective aspects of safe driving like leaving enough distance between vehicles (depending on speed) that are wildly ignored. You might get used to that in practice but that doesn't make it safer.

I was thinking about chaotic Italian road crossings. Or, to give another example, l'Arc de Triomphe in Paris. People tend to communicate with their cars where they want to drive and I understand that foreigners find this aggressive.

I was not thinking about excessive speeds or tailgating.

In Southern Italy, when you are stuck in a traffic jam in the city, you just drive nearer to other cars, and this is only mildly dangerous because you don't have high speeds.

If you don't drive tight enough, you just can't proceed because some other car will take your place.

This is not tailgating or driving recklessly.

I was more thinking about me being tailgated at 2 ft distance and then overtaken on an extremely narrow mountain road near Arco. People were honking while doing this so people in the oncoming lane around the bend could start breaking (I was told by a local). Happened all the time in the cumulative month I spent there, never got used to it.

Lots of crosses remembering dead drivers on the side of the roads too.

Spanish and Italian sound great to me, even though I don't speak them. They sound, well, human. Can't say the same about English or German

I think this is meant to be funny but it upsets me a little bit to be honest. Learning at least some basics of the country's language is travelling 101 to me. Going abroad and expecting the locals to speak my language is a very rude thing and a display of disrespect in my opinion.

Relevant thread: https://twitter.com/cleptok/status/1611622181665923075

Ok. Let's do a short thread on Roman cement for the 3 people out there that might be interested. You often hear that the Romans had some sort of superior and arcane knowledge on superior cement mixes that has been now lost. And still today we cannot replicate it. This is complete nonsense. Modern day cement (and any other building material, really) is far, far superior than anything from the past. There is simply no comparison. Above everything we understand the chemical composition, interaction and quality control for consistent results.

Romans were making mortars based on volcanic ashes. There was no set recipe. Every builder had their own. Volcanic ash from Pozzuoli, crushed brick, slaked like, animal fat, hair etc. Some basic rules existed (as recorded by Vitruvius on the 10 books on Architecture).

Millions of structures were constructed these way. With the exception of a handful, most of these do not exist anymore. But why the ones that exist are still standing strong?

Usually via a simple Darwinian process. Most of these times those builders got it wrong. But in a few cases, out of luck, they got the chemistry just right. This would result in a ok-ish material, with decent properties. Not only that. Unreinforced (without steel) concrete will not suffer any serious deterioration, with few exceptions. (Pantheon in Rome) Also the very coarse pozzolan they were using, will chemically react very very slowly producing low initial strengths Vs any modern material.

In the long term (centuries) though, it will continue gaining strength. Totally useless for the original purpose of the builders, but quite good for the few surviving buildings.

So what the latest MIT research told us? That lime in very small quantities will be "self-healing'. And this somehow is something that the Romans knew and used to create a superior cement.

This is simply not true. It is well known that lime expands (the pompous "self-healing" tag), so in very small and precise quantities it will counteract the shrinkage and cracking behaviour of cement binders. But you have to be pretty lucky to get it just right.

So not intentional. Just survivor bias in the remaining Roman cement structures. Beyond this, lime is a definite no-no for any concrete with steel in it -it corrodes it. The reason we use concrete today is precisely the embedded steel. It allows to build all modern structures.

And steel is the reason modern concrete deteriorates. It corrodes, expands and cracks the surrounding concrete. An unreinforced concrete (like the ones Romans used) with modern materials and understanding would last forever. But you wouldn't build anything worthwhile.

Final. Remember. Cement = the glue, the binder that holds everything together, activated by water. Concrete= the actual building material with binder, water, sand and stones. Mortar= only binder, sand and water (maybe small stones)

Honestly, I think a much bigger factor than random chance is the designed lifespan of these buildings. Some of them took decades or even centuries to build. So they must not only last throughout the construction process, they must also last long enough to make the construction process be worth it.

Spending a few decades on experimentally determining an optimal cement mixture is also worth it.

The by far biggest problem is that the vast majority of buildings get demolished and replaced by something else so making them long lasting is not very useful.

> You often hear that the Romans had some sort of superior and arcane knowledge on superior cement mixes that has been now lost. And still today we cannot replicate it. This is complete nonsense.

That’s what I heard. And that we found it recently but it wasn’t useable economically because it required an extremely long time to dry instead of modern ciment and it’s ability to dry (and so raise a building) a lot faster.

And the real deal is modern https://en.m.wikipedia.org/wiki/Textile-reinforced_concrete.

Especially the combination of carbon fiber and concrete is a very promising solution, which is researched in Germany: https://de.m.wikipedia.org/wiki/Carbonbeton

This stuff is superior to anything the romans did in every aspect.

That reads as somewhat misguided and dismissive. You don't have to understand the chemistry to improve recipes by trial and error, over multiple generations, and learn from the still-standing structures.

And there is still value in studying today those that survived in order to improve ours. The way lime healed cracks is new to me at least, but it's taken as a given here.

> you wouldn't build anything worthwhile

You just need more cement, it could be replaced with fibers. We still use unreinforced concrete in some cases.

The thread seems to have been written by a Greek, a bit of Roman-bashing is par for the course :)

Cannot recommend enough "the new science of strong materials: or, why you don't fall through the floor" by JE Gordon. Old, but amazing.

The formatting is a little off, but a fully text PDF is here: https://edisciplinas.usp.br/pluginfile.php/3975708/mod_resou...

Thanks for the suggestion

the article talks about evidence which has always been present in evaluations of Roman concrete which was "disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials," and this kind of thing REALLY infuriates me, as a layman.

if you don't know how to make Roman concrete, don't assume anything about what you see in Roman concrete while you're trying to figure out how it was made.

The modern-day belief that we are superior to humans thousands of years ago is just an absolutely insane idea, to me. we are more technologically advanced because we have more shoulders of giants to stand on, and we are NOT smarter or more clever than humans of 5000-10000 years ago. we just aren't.

as soon as you believe that we are superior to our ancestors, you dismiss the evidence you are looking for as "sloppy mixing practices, or poor-quality raw materials" and you prove modern-day humans as inferior, or at least you prove your own efforts as careless and incomplete.

leave no stone unturned when you are trying to understand something you do not understand; your lack of understanding could be due to your false assumptions.

> modern-day belief that we are superior to humans thousands of years ago is just an absolutely insane idea, to me

totally agree that kneejerk response when we see something that doesn't match our current methods is "those people didn't know what they were doing" is both potentially incorrect and certainly unhelpful to the advancement of science

I agree with the spirit of what you're saying but you go too far, ironically asserting without any supporting logic that the intelligence of homo sapiens has remained static for the last 2,000 years. But I suppose the disagreement hinges on what is meant by intelligence.

Here's a thought experiment: if you removed all technology (tools, writing, vast stores of knowledge) from a human child and raised them in this vacuum, would they be more or less intelligent? Would they understand their place in the solar system and the structure of the cosmos? Would they know how to protect themselves from sickness (germ theory)? Would their vocabulary and ability to express themselves be less or more?

Of course these questions don't have definitive answers and you can play the game the other way, arguing that modern humans would die in a couple weeks if transported back to caveman times.

But it's a poor model to view intelligence as some static property independent of technology, environment, gene mutation, sexual selection and all the other strings in the web of existence.

"intelligence," as I used it, is the ability to learn, to use what you've learned to solve problems, and to accurately predict the future outcomes of today's decisions.

my knowledge of the horsehead nebula has no positive or negative effect on my ability to learn how to make concrete or to use that knowledge to actually make thae concrete. intelligence is not simple memorization of fact, it is a collection of understanding, and the ability to both grow that understanding and to use it to accomplish things that you don't yet know how to do.

we are continuously surprised by what we learn about ancient cultures, because we assume they were intellectually inferior to us.

the Lycurgus Cup is a great example of something that some people still view as a lucky accident. Anyone who has actually learned to make physical things and worked to refine their skill in making things will know in their soul that the colors in that cup are clearly intentional and the product of intelligence and time.

our belief that only contemporary humans have control over and understanding of the their world is just simply wrong, I think.

we discover new evidence from ancient civilizations which surprise us all the time. is there enough evidence to prove that we aren't smarter than people 100-200 generations ago? I don't know.

there certainly is a lot of evidence that ancient civilizations knew more than we believe they knew, and that is significant when you think about how long that evidence needs to last to be discovered today, and that those things which do last long enough are recognized for what they are before they are discarded because of a bad assumption. how much evidence has been lost? we only see a tiny fraction of the evidence that would have existed at the time.

these discoveries happen frequently enough for me to have no problem believing that there is a great deal about ancient civilizations that we think we know which we actually do not know.

that's a rambly comment but it's all I have time for before the edit window closes.

That depends on the what you mean by smarter. At least some component of what we consider intelligence is cultural.

Just one example is that the ability to write/draw allows you to solve problems that are impossible without it.

Being literate definitely means some faculties get less practice. Practically nobody today can recite epic poetry like the Odyssey, for example. That lack of rote memorization skill may have unexpected side-effects.


Thats a really good, succinct, statement that triggers thought - where ironically - being literate is required to reply to via the most advanced machine I have ever used...

But there are so many skills lost due to the privilege of technologically enabled sedentary life-styles...

can you imagine attemting to found a town/village/city in flat/cold/resource-constrained climates without electricity/supply-lines?

Those people before us had balls and conviction of steel - and even more-so for the women. You would not exist without a woman (mother) and women are stronger than men.

That’s possible, but unlikely considering that numerous studies of literate vs illiterate people have shown that literacy correlates with improved working memory not the inverse.

Studies of using mnemonic techniques to memorize long sequences (similar to what oral performers were doing) have also not shown much promise in increasing cognitive abilities.

From the study:

> In addition to the features described above, aggregate-scale relict lime clasts, also referred to as remnant lime or lime lumps, are a ubiquitous and conspicuous feature of both architectural and maritime Roman concretes. The presence of these distinctive bright white features has been previously attributed to several scenarios including incomplete or over-burning during the calcining of lime (20), carbonation before concrete preparation (30), incomplete dissolution during setting (12), or insufficient mixing of the mortar (14).

20: https://www.sciencedirect.com/science/article/abs/pii/S00088...

30: https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1475-4754....

12: https://www.amazon.com/Building-Eternity-Technology-Concrete...

14: https://www.cambridge.org/core/journals/journal-of-roman-arc...

> if you don't know how to make Roman concrete, don't assume anything

That's not useful guidance for doing research at all. It's similar to some 19th-century physicists/philosophers saying 'if you can't see atoms, you can't possibly know anything about them or whether they even exist'. We also can't make supernovae or supermassive black holes, yet we can still try to learn about them through a combination of available data and educated 'assumptions' (ie theories).

What this research has done is replace one theory with another, and that theory might be replaced by another one later, that's just how science works.

eh, I disagree.

hypothesizing that atoms exist allows and encourages you to design experiments which can prove or disprove their existence.

assuming that they exist, and never experimentally verifying that is not research and it is not science.

assuming that some feature of a product that you do not understand exists for any reason is at most an hypothesis, and dismissing the hypothesis without experimental evidence disproving it is simply bad science.

Roman concrete is not something far away that we can not interact with. we can verify by experiment. for years we chose not to validate the assumption that the white specs were accidental, and that delayed our understanding of Roman concrete.

I just want scientists and historians that take the time to recognize their assumptions and to prove them correct or incorrect, if possible, instead of just relying on them as immutable facts; assumptions are often wrong.

I write software for a living, and the number of times that my assumptions about the cause of any particular problem have been correct approximately 0% of the time. whether it's a logic bug or a performance problem or a usability problem. despite decades of experience in this field, I am still almost always wrong about the cause of any particular problem I encounter, unless I have seen that exact same issue previously, which is rare.

historians, some of them at least, would probably benefit quite a bit from making fewer assumptions and by experimentally proving the assumptions they do make. if it isn't possible to prove or disprove your assumption, then you must never forget that it is an assumption and not proven fact.

I've seen so many scientists and historians say they know things which they simply do not know. things like "we know dark matter exists." no, we don't. we know that the currently accepted theories of our observations require dark matter to exist in order for our observations to make sense. that is not proof of dark matter.

dark matter may exist! I don't know, but for a scientist to say that it exists is incorrect because we've never observed it directly, and have devised no tests which can prove that it does exist.

our theories which require it in order for our observations to make sense could be wrong. our observations may not be as accurate as we assume. we may be observing things that we aren't aware of which look like something that we are aware of. etc.

I'm saying that professing that you know something without proof is folly. just like the white specs in the concrete were assumed to be accidental impurity without that hypothesis being proven is folly. these things set us back as a scientific community that wants to understand its environment.

An extremely good example of Chesterton’s fence?

There is a Greek Hellenistic breakwater or port ruins in Sant Martí near L'escala in Catalonia, Spain that is sitting in the sea. It is still standing. Always amazes me that it's still there though I'm sure some minor restoration work has probably been done.

Some day in the very far future, our descendants will ask, why is the code manually made in the 2020s with Rust so durable, while our modern PHP20457 code that is spit out by Tardpilot100 is so fragile? Our modern operating systems need to reboot 1000 times per hour just to function while ancient operating systems could stay on for months at a time without rebooting!

Nah. Rust evangelists of the future will still be fervently rewriting everything in Rust, and they’ll be researching why ancient code worked without being written in Rust.


Why Ancient Roman Concrete Outlasts Our Own (2017) - https://news.ycombinator.com/item?id=29366911 - Nov 2021 (67 comments)

2,050-year-old Roman tomb offers insights on ancient concrete resilience - https://news.ycombinator.com/item?id=28833525 - Oct 2021 (101 comments)

Why Roman concrete is stronger than it ever was, while modern concrete decays - https://news.ycombinator.com/item?id=25690803 - Jan 2021 (7 comments)

A chemical reaction in ancient Roman concrete makes it stronger over time (2017) - https://news.ycombinator.com/item?id=22580920 - March 2020 (64 comments)

How Ancient Rome’s Concrete Has Survived 2,000 Years (2017) - https://news.ycombinator.com/item?id=20482050 - July 2019 (81 comments)

How Did the Romans Make Concrete That Lasts Longer Than Modern Concrete? - https://news.ycombinator.com/item?id=15544128 - Oct 2017 (3 comments)

The Rock Solid History of Concrete - https://news.ycombinator.com/item?id=15480165 - Oct 2017 (12 comments)

Why Roman concrete still stands strong while modern version decays - https://news.ycombinator.com/item?id=14699652 - July 2017 (1 comment)

Ancient Romans made world’s ‘most durable’ concrete - https://news.ycombinator.com/item?id=14695876 - July 2017 (4 comments)

New studies of ancient concrete could teach us to do as the Romans did - https://news.ycombinator.com/item?id=14690329 - July 2017 (72 comments)

Ancient Roman Concrete Is About to Revolutionize Modern Architecture - https://news.ycombinator.com/item?id=5883443 - June 2013 (23 comments)

How the pantheon has lasted 2000 years without steel in its concrete - https://news.ycombinator.com/item?id=1852000 - Oct 2010 (35 comments)


The problem with reinforced concrete (2016) - https://news.ycombinator.com/item?id=27282927 - May 2021 (186 comments)

The problem with reinforced concrete - https://news.ycombinator.com/item?id=11975695 - June 2016 (147 comments)

our obsession with Roman concrete.

I was thinking as I read this, "Wait, don't we already know this?" Now I know why.

One method to reduce cement’s carbon footprint (which accounts for up to 8% of total global greenhouse gas emissions), is to improve the longevity of concrete through the incorporation of self-healing functionalities. The resulting extended use life, combined with a reduction in the need for extensive repair, could thus reduce the environmental impact and improve the economic life cycle of modern cementitious constructs.

I wonder:

How much of an environmental impact this would make given the number of structures that are not useful before the concrete requires repair?

How much of Roman concrete's reputation for durability comes from survivorship bias--the structures that lasted were just lucky?

Survivor bias comes up every time this topic is mentioned, but it really has less explanatory power than most people think. If you meet a man born in 1932 who is still running marathons and doesn't need glasses, it's probably luck, although he might have some valuable diet tips to share. If you meet a man born in 1732 running the same marathon, it's time to rule out luck, and start collecting blood samples because he might actually be some kind of vampire.

Furthermore, the "survivor bias" idea, as an attempt to explain the longevity of Roman concrete as a building material, implies that the buildings that survived were stronger than the ones that did not, so we're just seeing the upper end of structural strength. But that's not even necessarily the case. Many more were lost because they were deliberately demolished for other reasons (honoring the wrong gods, etc). And it's not like these buildings only survived in ideal environments for preservation; as the article mentions, they have survived in many different climates across a vast empire, standing in seawater, through earthquakes, and so on.

Right. Romans experimented with the formula for concrete for centuries, and learned a lot about how centuries-old concrete structures deteriorated. They didn't have modern chemistry, but they had observational skills and were serious about their work.

It would not be surprising to find that most structures weren't built to last any longer than ours, but that some were meant for the ages. An important difference today is we don't build the latter sort, excepting maybe the Sydney Opera House. Many people act personally offended that it doesn't have steel inside.

I wonder if generational reputations and transmission of methods from master to apprentice is sufficient.

"Secundus was an apprentice to Marius who was an apprentice to Darius, who built that awesome aquaduct; Marcus was an apprentice to Felix who who was an apprentice to Marcellus, whose shit bridge fell down after twenty years. We're going to hire Secundus instead of Marcus."

Real Roman engineering theory came from Marcus Vitruviouss Pollio's De Archetectura. Considered to be the ultimate reference text for building and engineering projects in Rome in 32bce, It's ten volumes cover how to build everything from temples, to aqueducts, to water mills to machines used for construction. A fascinating read, if you're curious how Romans built things.

De Architectura provides the sort of long-term perspective required to build and engineer projects that were meant to last for centuries, based on experience gathered from centuries of recorded experience.

I'm not sure if De Architectura is the only book that Roman Architects used. But it was certainly the most important.

Hundreds of pages of De Architectura list dimensions of older ancient buildings, the dimension and spacing of pillars, and failures encountered during or after construction. Something along the following lines (not actual text):


The second Temple at Delos was built in 432bce. It consisted of 13 x 29 doric columns, 6' in diameter, 22' high, spaced on 12' intervals, with two rows in the pro-cella. The cella measured ..... The entrance to the cella was 16' high and 12' wide with a granite lintel measuring 16' x 3' x 2'.

The lintel above the entrance cracked and had to be replaced after an earthquake in 430bce. The front-right architrave cracked in 329bce, but was not repaired. The temple was subsequently abandoned and collapsed completely after an earthquake in 327bce.

The temple of Apollo at Thebes ....

The main architrave cracked while it was being mounted, and further construction was abandoned.


Given a hundred or so such examples, a Roman architect of a new temple could make somewhat informed decisions about how high, how wide, and how thick he would need to build a new temple, and how afraid he should be when he was commissioned to build a new temple that was higher, wider, or taller.

as far as i know, vitruvius wasn’t original: he aped the meticulous and more successful greeks. the ten volumes regularly refer to greek knowledge or practice to cement its authority. the method of construction that is uniquely roman is mixing concrete and bricks, usually to produce aqueducts. and here the simple theory was: everything in excess. in fact, the romans may have been unique in their construction of military camps. for more on that see de re militari

That's a combination of 'Risks Digest' and the 'Engineering handbook' all in one. I'll have to go read this, thank you so much for the pointer.

The cathedral of st. john the divine in new york city has yet to be completed.

we just don't build a lot of them.

We build dams to be long lasting.

More of them, anyway, than of other types. Some have not lasted, with typically regretted consequences.

> start collecting blood samples because he might actually be some kind of vampire.

In which case your first worry would be to make sure he doesn't collect your blood samples.

meta comment: I'd love to see the stats about all bias and fallacies mentionned in conversations in the last 20 years. Something tells me wikipedia got deep into people's head.

> One method to refuce cement‘s carbon footprint…

…is to simply use less concrete by making it much stronger.

Very promising in that respect is https://en.wikipedia.org/wiki/Textile-reinforced_concrete#Su...

Especially carbon concrete can be up to 4 times stronger than steel concrete, so one can reduce the amount of material necessary by 4 times.

Don't worry the engineers will use five times less concrete and sacrifice longevity.

if we built these aquaducts now using common concrete, without steel reinforcement, they'd be in shambles in a few years.

Our dams don't appear to be that fragile.

very exciting! and I imagine there will be follow-on work delivering even greater results.

There's also an interesting finance project here, to integrate the impact of this material into projects budgets, so that designers and investors can justify this material over cheaper alternatives. In particular, different construction projects have very different expected lifespans.

I wonder if it could have relevance for nuclear waste installations.

> following all of the detailed recipes that had been optimized over the course of many centuries

i wonder how all those detailed recipes were passed around for many centuries and now all is gone. Was it all passed down verbally?

Some of it was "stone from this volcano" and then it was mined out. They didn't know how to choose a substitute.

In the 6th century, big volcanic eruptions, low temperatures and plagues bringed a dark age in Europe and other parts of the world. A large part of the population died. Maybe those recipes were lost during this period.

I don't think the knowledge is gone, we could make that or maybe even better. The point is nobody wants it, most things we build need to be torn down after at most a few decades anyway.

The knowledge is actually gone, like Damascus Steel (what we have today is just a visual reproduction) and Greek Fire.

For a more recent example: we already "forgot" how to make the spaceships and rockets that went to the moon. Even if you had the full plans - and apparently we don't, a lot of the materials and processes are not around anymore and would have to be 'reinvented'.

We have a pretty good idea of Damascus steel at this point:

- We know that early Damascus examples were made from imported Wootz ingots, which we absolutely can recreated.

- The high quality of the blades were due to naturally occuring impurities of molybdenum, vanadium, and chromium, all of which are used in various modern alloys.

- Forging and heat treating processes varied greatly across historical blades called "Damascus" and we have sequences that create both visually and metallurgically similar results.

Not quite.

For Damascus steel and Greek Fire we almost certainly still have the knowledge. We just lack a document to tell us that "Greek Fire" matches formula #44 of napalm specifically, or what specific process an ancient smith actually followed.

For the spaceships, no, we have the plans, but they're long obsolete. They're made for ancient, manual equipment nobody uses anymore, and old materials that have since been improved on. We technically could dig up and restore ancient manual milling machines, recruit retired engineers and special order obsolete materials and parts, but what for? Unless you're engaging in experimental archaeology what you want is a rocket, not the rocket that has a nostalgic feel to it, and it'd make far more sense to make one with modern methods.

For the spaceship example, I’m fairly certain that we do know the theory behind how all those materials and processes are created though. It isn’t that we lost the knowledge, but the infrastructure. Recreating the infrastructure to create the materials to create the rockets is possible, it’s just that recreating entire processes from scratch when they had already been phased out of our modern structures would be outrageously expensive, especially just to construct rockets.

I was referring to the knowledge of how to make long lasting concrete, not necessarily specifically how the Romans did it, but I guess I could've been clearer.

The answer is still no, we don't know how to make long lasting concrete with modern techniques either apparently. Hence the research.

Especially steel-reinforced concrete, the lifespan is only 50-100 years. The only exceptions are massive structures like dams that will probably last a couple millennia.

Right but the thing is neither did the Romans. The only reason roman concrete lasted so long is it didn't use steel reinforcement. All of the structures we have from them are carefully designed to only use compression.

So why didn't the researchers just ask one of the people who knows how it's made, instead of wasting all this time conducting tests and doing spectroscopy?

For people who don't know the difference between cement, mortar and concrete:

Cement is a substance used for producing mortar and concrete. It's never used on its own. It is a "binding agent". Note that the term "cement" is often not used correctly in everyday speech, with people talking about gluing bricks together with cement, when bricks are actually glued together with mortar.

Mortar is the glue between bricks. You can see it between the bricks in any brick wall. It is produced by mixing cement with sand. This results in a paste which hardens after being applied to a brick.

Concrete is a substance that can be put into any shape, and then hardens. The bricks in a brick wall can be made from it. (More examples?) It is produced by mixing cement with sand and gravel (which are larger rocks).

There is also reinforced concrete, which the Romans didn't have. While concrete by itself can in principle be moulded into any shape, this can sometimes fall apart after hardening. Reinforcement refers to putting the concrete around metal wiring, which allows for more versatile shapes while maintaining stability.

Feel free to provide corrections.

Reinforced concrete in part is designed to provide structural strength under tension forces. Concrete is stable in compression but has weak resistance to tension. Pretensioned reinforced concrete permits longer unsupported spans but even vertical concrete can benefit from reinforced bars, to prevent spalling. (I believe). Not all reinforcing steel has to be tensioned to be useful, that's a technique for long spans.

Reinforced steel bars hence rebar.

>Reinforced steel bars hence rebar.

More like reinforcing bars, rather than reinforced bars. And technically the overwhelming majority of what is sold today are actually deformed reinforcing bars, since the intentional addition of ridges on the outside of the bar helps the concrete adhere as compared to smooth mild steel.

> but even vertical concrete can benefit from reinforced bars, to prevent spalling. (I believe).

I liked your comment but this bears correcting. (Corrosion of) rebar is actually the biggest cause of spalling in vertical concrete structures.

>There is also reinforced concrete, which the Romans didn't have. While concrete by itself can in principle be moulded into any shape, this can sometimes fall apart, even after hardening. Reinforcement refers to putting a concrete around metal wiring, providing more versatility.

Not a correction per se, but some further explanation.

The reason for reinforcement is because concrete is strong in compression and weak in tension - in fact for design purposes we disregard its tensile capacity entirely. What reinforcement does is handle the tensile stresses in a structural member. When a beam, column, or slab bends it often (based on the load placed and structural design) creates tension on one side and compression on the other, so we put the rebar in the structural member close to the tension side, or on both sides if there may be tension on both sides.

In slabs reinforcing may also be used in some capacity just to limit cracking in an otherwise non structural capacity. In certain kinds of designs it may also provide confinement to the concrete which can be important for structural analysis reasons that are too technical to get into here (plastic hinging especially in earthquake design, etc).

We have different kinds of reinforcement, depending on the need of the project, but for the most part we use steel because its reasonably durable, has similar temperature expansion properties to concrete (imagine if your concrete got cold and shrunk more than things embedded in it, or grew so much the things embedded in it weren't attached anymore), its behaviour is well understood, and it is reasonably priced.

The design of steel reinforcing in concrete is also done in a way that reduces the likelihood of sudden failures, so that if something does happen, it happens slowly and with plenty of warning.

There are alternative reinforcing materials that may be appropriate in some very specific situations, but civil engineering moves very slowly and adoption is slow because risk is high. Fiber Reinforced Polymer (FRP) and Glass Reinforced Plastic (GRP) are examples of these. They may have much higher tensile stress capacity per unit of area, and also are less susceptible to corrosion, but they cost a lot, their failure modes are sudden, greater deflections under load, and each may have other tradeoffs like worse compressive behaviour, or worse fire resistance, etc.

There's a small trend towards stainless steel rebar. It's expensive, but is now used for concrete structures near salt water.

Epoxy coated rebar turned out to be a mistake. One scrape in the epoxy and it starts rusting. Exposure to UV prior to installation can damage the epoxy coating. Currently banned in Quebec.[1]

[1] https://news.ycombinator.com/item?id=27676447

epoxy coated rebar has not been banned in quebec, rather the transportation agency has decided not to allow it in their projects. You are free to use it if you'd like on your own projects. Other provinces still allow it in bridge decks, and you will find the requirements for the materials and material handling/repair in their standard specifications.

Though most concrete is reinforced with steel rebar, it can be reinforced with all sorts of other materials to strengthen it under different conditions & loads: glass rods, plastic meshes, fibres, carbon nanotubes, etc.

There are different types of cement: Portland and CSA (Calcium Sulfoaluminate).


Portand is the cheap stuff- like the $6 Quikrete bag at Home Depot. CSA is more expensive, but also available at Home Depot- look for the "Rapid Set" brand (Rapid Set Cement All is like $28 a bag).

It's kind of awesome, it's so fast you can sculpt vertically with it. You use more water with it than Portland, it starts out more mud-like. It tends to be preferred for repairs.



There are many, many, types of cement. Another common type of cement that people are probably familiar with is asphalt (a.k.a bitumen).

There are many types of even just portland cement, or portland cement with additives. Early strength development, sulfate resistance, air entrainment (trapping little bubbles of air in the concrete so that it cracks less in freeze thaw cycles), different heats of hydration, etc.

Well, technically, you can use cement instead of mortar (wet, but without sand). As far as I understand, sand is more like a neutral filler, to save money on cement. Various other things can be used to fill as well, like gravel. Sometimes even empty plastic bottles (reduces material and weight) are suitable (just make sure they don't float up). If one got very smooth bricks (e.g. precisely cut aerated concrete), then it might make sense, as there's very tiny amount of mortar (or even glue) is needed anyway.

If the sand is properly coarse, it can add strength. However, finding coarse sand is increasingly difficult, resulting in misuse of smooth sand (from error or being defrauded).

Tangent: coarse sand is also important for certain types of septic systems to function properly.

> The bricks in a brick wall can be made from it.

I think "brick wall" usually refers to fired clay blocks:


But yeah, you can make concrete blocks and build stuff with those. They are usually larger than clay bricks. In my country, the stuff used for building is mostly concrete bricks filled with small air bubbles air so that it is lighter and also insulates better.

This type is called autoclaved aerated concrete.


It's pretty good but it's also brittle. You have to use special screws and wall plugs if you want to attach something.

Breaking down cement further, the raw materials are limestone, clay, and touch of iron ore (“red earth”).

The baths of Caracalla (sp?) had concrete reinforced with copper!

>To prove that this was indeed the mechanism responsible for the durability of the Roman concrete, the team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Sure enough: Within two weeks the cracks had completely healed and the water could no longer flow.

So this method wouldn't necessarily work in a dry climate without much rain?

on the other end of the spectrum, did you know concrete can be used to demolish concrete?

dexpan is a special demolition concrete mix that, when poured into holes, expands at 18kpsi. it gets used when jackhammers and explosives cant be used, for example, in refineries and explosive atmospheres.

It's also useful if you want to do concrete demolition yourself using only hand tools. You drill holes, pour, wait for cracks, and then use a pick and hammer to remove the remaining chunks. Often sold as "expansive grout".

If you can drill holes you can use stone splitting wedges.

A lot of these high dollar modern miracles of chemical engineering are purely for use in situations where it's cheaper to just buy the $100/tube stuff than it is to try and get the approval to do something sensible. Basically you're paying to get around process tech debt.

Not in a refinery, or you have to buy weird/expensive non-sparking hammers and wedges. Aluminum bronze wedges are commercially available but the liability insurance is incredible so you're better off cutting your own (its a wedge, not a rocket engine...). It ends up being cheaper to use the expansive grout.

This is exactly what I mean by "paying to get around process tech debt"

Facility has a "no sparks" in buildings A, B and C rule. Rule obviously doesn't apply to contractors doing a ton of welding during a holiday shutdown. Non-contractors in maintenance dept want to fix some other crap at the same time. Sure they could try and get approval to use a jackhammer concurrent with the other work but in reality there's a 75% chance the safety department will intentionally stonewall until after the shutdown rather than set the precedent of a sensible exemption and the maintenance manager knows it so they buy the expensive grout.

Murphys Law implies the contractors will not be legally permitted onsite during operation so who cares what they do, but if you let the full timers play with jackhammers its guaranteed some idiot will try to use the jackhammer after operation resumes. So you'd need to do something like have the safety officer personally responsible for removing the jackhammer from the premises. Or renting it, perhaps.

Presumably dexpan has lower tensile strength so it can be disposed of easily? Or is the piece excavated and hauled away intact?

It expands in the holes you make in the to-be-destroyed concrete, cracking it. You can haul it off in chunks at that point.

The destroyed concrete, yes, but what about the dexpan? <Peregrine Took>

The dexpan is not a huge concrete structure, it's just a cylinder the size of the hole it was poured in.

Oh, I see, just a bore hole and hydraulic concrete. Sort of like the metal shims you see used in pre-industrial stoneworking.

I was picturing pouring it into a void for some reason.

The dexpan never existed as big pieces.

Won't it expand out of the holes you poured it into?

In some cases (particulary with modern, faster, formulations) it can, actually projecting (powder/smoke) out of the holes but it is not usually an issue.

When it happens is because the expansion is too fast (and consumes all the water available), making the grout itself fragile, with slower formulas, after the initial setting of the grout, you should water/keep the surface wet.

It is surprisingly difficult to find actual videos of it working (most are just a set of photos) because it is very, very slow (between 6 and 48 hours, usually), here is one from one of the many producers:


Not if it hardens before it expands.

I've used that. Drill holes. Fill holes with gunk. Come back in the morning and the concrete is all cracked up. Silent and easy. Works great.

How does this concrete compare to a state-of-the-art mix -- not just the stuff they use to pour my driveway or the highway, but the best we know how to make?

Practical Engineering can answer all your questions with their series on concrete. It’s extremely well done, including a video on “Was Roman Concrete Better?”:


Disappointed that this article didn't at least give a "tip of the hat" to the dogged research done by Dr. Marie Jackson, et.al., over her career at multiple universities.

Here is an excellent read on this subject from 2013: https://cedar.wwu.edu/geology_facpubs/75/

Says a lot doesn't it.

Marie Jackson makes it into the citations twelve times.

Otherwise not mentioned by name.


You thought capitalists were greedy for mere money? Come meet academics.

The lead author of this study, Linda M. Seymour, worked with Jackson on reference 29, on the mortar of the Tomb of Caecilia Metella, which is referenced repeatedly and the samples from which form a significant part of the hypothesis.

That study: https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.18...

Story on that study: https://als.lbl.gov/unexpected-transformations-reinforce-rom...

Jackson is quoted, if briefly, by Science's article on the new paper. https://www.science.org/content/article/scientists-may-have-...

Jackson's work is focused on pyroclastic volcanism. Seymour's work has focused on Roman water infrastructure.

Jackson works for the University of Utah, Seymour works for MIT. The linked source is MIT's PR department.

Thank you for pointing this out.

A common thread, and no doubt mutual fascination behind Jackson's and Seymour's research, is the use of specific pozzolans found in and around Campi Flegrei volcano west of Naples.

That's true, I'm not giving any credit at all to the fact the article isn't written by the paper's authors, and is at best a summary.

Agreed. But is this not an example of a fundamental attribution error? The structure and incentives of academia creates all the credit grabbing and back stabbing (IMO).


Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts (and in doing so expands, damaging more concrete and hastening further water inclusion)?

The rusting and getting wet is cyclical in nature and part of what accelerates that is the formation of cracks in the first place. Part of the maintenance of concrete is to fill any cracks that you can find to prevent it from getting worse. The cracks typically start small, and they might form in places that are hard to get to as well as being too small to see, so if the concrete is self-healing it will prevent some of these small cracks from getting worse on their own which should reduce the effect of expanding water (ice) as well as expanding rust.

The reinforcement material (rebar) already has a very close thermal coefficient of expansion to concrete and this can be thought of as kind of a lucky coincidence.

> expanding rust

The alkaline environment inside concrete "passivates" the reinforcing bar, greatly reducing the rate of oxidation and changing its form.

The problem exists only in concrete that is badly made with poorly chosen aggregate, and in corrosive environments.

1. https://en.wikipedia.org/wiki/Passivation_(chemistry)

Another method is to connect all the rebar and add a https://en.wikipedia.org/wiki/Galvanic_anode

> as well as expanding rust.

Ideally they’d seal before the rebar is exposed and can rust. As long as the rebar is encased, unless you fucked up dramatically it should be safe.

The rust is essential to bind it to the cement. Without, it adds little strength.

Modern concrete isn't expected or designed to last more than a century. It could have been made to last longer, if they wanted, but that would cost a little more. Making no provision for when it will predictably fall apart is a modern failing. That major construction with concrete is about a century old should worry everyone.

We have a very great deal of infrastructure that will fail on a predictable schedule, with nothing budgeted to replace it all.

On the plus side it makes for some wicked post-apocalyptic ruins. Remove maintenance, add 20 years, and you get the most photogenic wasteland you could ask for.

> The rust is essential to bind it to the cement

What? This is total nonsense.

> Modern concrete isn't expected or designed to last more than a century. It could have been made to last longer, if they wanted, but that would cost a little more.

Is hacker news a conspiracy theory website now?

> We have a very great deal of infrastructure that will fail on a predictable schedule

Wood rots in a few years under certain conditions and can last hundreds in others. Concrete works similarly.

The 3rd pantheon is currently standing, but it is a huge mix of repairs across centuries. The Romans built many structures that are similar but now are in ruins because the Catholics didn’t maintain the roofs on bathhouses.

It is a conspiracy theory that engineering construction is optimized for cost against requirements? Maybe ask literally any civil engineer how much cost matters.

> The rust is essential to bind it to the cement. Without, it adds little strength.

Do you have a citation for this? As far as I know this is entirely not true.

Rust is an expansionary product - when steel rusts its volume increases something like 10x, which reduces the connection between the concrete and the steel and causes issues like cracking and spalling. Minimizing rust improves the long term performance of rebar.

They leave the rebar out to rust a bit, on purpose, before they pour. Surface rust is good, but more is bad. Like many things in life.

I read your comment as I finished eating my third slice of pie and wondering why I cut myself 3 slices.

You could have cut one big slice instead.

3 is the limit. More than 3 is bad.

One would think the limit for pie is closer to 3.14159265359

It is, but it's not easy to cut pi slices of pie without being unfair to others who want to have equal slices of pie. Plus, the effort to be so exact may not be worth the utility. You may hit the true pi limit by random luck too.

Perhaps you’re just mathematically inclined.

Rust is perfect. We don’t criticize rust on HN.

So the Hoover dam will fail in 10 years?

Concrete lasts a really long time when it's under pure compression. You only run into issues when you try to build structures that need to survive in tension. That's when you start having to add rebar and lower weight and you run into issues.

Fortunately, Hoover Dam is under compressive load.

Only if the Hoover dam was made with standard concrete using standard building methods, I suppose.

Yes although it is true what the grandparent comment said about sealing - that keeping things dry, using coatings or topper materials, dewatering, etc. will all help extend the longevity and reduce maintenance requirements a bit.

Steel gives concrete tensile strength. Unfortunately, defects in the epoxy coating become a focal point of oxidation, often pitting that single point more than otherwise uncoated steel. The result is, the pitted area fails quickly and the integrity of the entire member is compromised.

this isn't really possible. For the rebar to take up meaningful tensile stresses, the concrete nearby must have cracked.

Reinforced concrete ALWAYS cracks.

If you want better corrosion resistance, you do other things - lower water ratios, increased concrete cover, sulfate resistant concrete with lower permeability, different reinforcing or epoxy coated reinforcing, etc.

Not always, if rebars are pre-tensioned before concrete solidifies. So the rebar is always under tension. Pretty common in prefab parts, like bridge bars.

pre-stressed concrete is great, and has many advantages over regular reinforced concrete and is more efficient in terms of cross section so cracks less. It still cracks, though, and in different locations like on the ends horizontally instead of at high moment/deflection areas vertically. Pre-stressed does experience fewer shrinkage cracks, which is nice. It's more expensive to build and repair.

And while you absolutely have pretensioned girders on bridges commonly, the bridge decks generally aren't, although they're almost always completely in compression and the steel is there for shrinkage cracks etc.

For someone totally outside this space I had assumed concrete is rigid and brittle and rebar reinforcement allows for it to bend and flex without breaking. But your comment makes it feel like it keeps the concrete in tension with compression forces to make it sturdier. Can you ELI5 what the rebar does to help?

You're going to love this playlist from Practical Engineering [1]. Grady explains all of this much better than I could. The second video explains reinforcement and the fifth explains pre-stressing.

[1] https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKf...

you have it backwards. The reinforcement handles tensile stress, keeping more of the concrete section in compression, where its strong. In concrete where the reinforcing isn't tensioned, the concrete below the reinforcing will crack as it is very weak in tension. In concrete that is pretensioned, the tension in the rebar is used to apply compression on the concrete so that it might never get into a tensile load and crack, which also reduces deflection under load.

The rebar handles tensile loads, the concrete handles compressive loads. You put the rebar where tension will exist so that it can handle that. Some amount of cracking on non-pretensioned concrete is normal and unavoidable.

I'm just going to throw it out there to spread the word; GFRP Rebar (Glass Fiber Reinforced Polymer) is a thing, and you can buy it today at your local hardware store. I really wish I would have known about it on my last project, probably the last time I will buy steel rebar to put in concrete.

I'm not an ME, but my understanding is that it is a comp in terms of structural reinforcement. I'd be really interested to do a deep dive on how these technologies stack up against each other, it seems like a win for construction longevity, it would be pretty cool if the ecological impact of production penciled out as a win too. My understanding is that there are some trade off when being used in large projects.

My cursory research has indicated that this stuff first hit production in the 80's and was used in a bridge in the US in 96. Still haven't figured out why it hasn't taken over.[2] Is it just for a lack of evangelists?


[2] https://www.fiberglassrebar.us/gfrp-rebar-from-the-beginning...

I recommend Tyler Ley's youtube channel[0] entirely devoted to concrete. He has an episode[1] addressing GFRP that might interest you.

[0] https://www.youtube.com/@TylerLey

[1] https://www.youtube.com/watch?v=thUZImUTZn0

The main reason is stiffness—off the top of my head, it’s roughly a quarter as stiff as typical steel, so it needs significant engineering and often much more reinforcement to keep the concrete within allowable deformation limits. Basalt and carbon fiber are more promising replacements, but are significantly more expensive.

Mike Holmes, a home reno personality in Canada, had been touting its benefits in about the mid-2000s for home foundation re-pours.

Any comparison in tensile strength?

And being driven over by semi trucks full of heavy cargo 24/7.

> Isn't the particularly short lifespan of modern concrete construction mostly a combination of its use in areas with regular freeze-thaw cycles and using a reinforcement material that rusts

No. It's a consequence of designing to requirements of minimum cost and a 50-year lifespan. It'd be easy to make concrete with much longer lifespan, but it costs more.

It's a nice thought that all these depressive brutalistic/modernist builduings will all soon crumbly by the same mechanism that spawned them!

Most brutalist buildings are already 60+ ish years old and doing great (actual brutalism, rather than “concrete buildings I don’t like,” was mostly a style of the 1950s and 60s in the west, though it lasted quite a bit longer in Eastern Europe). Even the Boston government services center, everyone’s most hated brutalist building in the US is already past 50.

for many people, myself included, actual brutalism is "concrete buildings I don’t like"

Eh, there’s no accounting for taste, but there have always been shitty buildings in every vernacular. We tend to not see the worst buildings in older styles because they’ve long since been torn down.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.” - Unknown

So how difficult is it to destroy Roman concrete? If we start using it to build things are we going to be stuck with those things for 2000 years?

Roman concrete was durable mainly because they didn't yet hit upon that silly idea of adding steel rebar.

Reminds me of the Wootz Steelmaking (and much else) in classical India which was lost during the period of Anglo-Islamic colonization. Roman empire, Egypt and Greece, IMO is, contrary to current myths, is far closer in spirit to India than to anything of the 'modern West'. Indeed, the Greeks ruled parts of India for many centuries, and Rome had a great amount of trade with it.

The loss of so much rich tradition across the world due to birth of religion and its obsession with tyrannical power over everything is such a tragedy on humankind... this was recognized by the thinkers of Europe during the wee end of the 'renaissance', but in light of post-WW2 plebification of intellectuals, is being rolled back because of the current crop's moronic obsession with another religion: Marxism.

Such tragedy. tck. Such tragedy.

What an atheist meme. These things were ‘lost’ because of breakdowns in trade networks caused by breakdowns of central authority and invasions by outside powers. If anything, the Church is what’s kept the Classics alive through its usage of Latin in the clergy and its preservation of Roman structures as churches.

The Pantheon is a perfect example. It has survived for 2000 years because it was repurposed as a church.

And lots of other Roman buildings didn’t survive because they were destroyed so the blocks could be repurposed into churches in other locations.

The Parthenon is one (1) building that survived basically as-is while countless others were actively demolished.

One thing that surprised me when visiting Rome is how little original buildings existed. But when you get increasingly far from the city of Rome, you’ll find quite a few well preserved Roman buildings that were out of reach of the long tentacles of the Catholic Church.

The Pantheon is just an example, but many other instances can be found in Rome: the Church of Santa Maria degli Angeli, the Basilica of Saints Cosmas and Damian, etc. (These are just examples you find in Rome, there are plenty of other examples here in Italy.)

The OP suggested that the loss of architectural art is mainly due to a clash of different cultures and to religion. While there were cases where this was surely true (the many churches destroyed during French Revolution!), in most of the cases the reasons were much more practical and did not involve conflicting cultures or the «long tentacles of the Catholic Church».

Before the concept of «historical preservation» was invented, what should have been the point to keep a temple to Venus in the crowded center of a city, when nobody worship her any longer and that very space could have been used for something more useful?

These acts of destruction happen all the time even within the same cultural context. For instance, the old Vatican Basilica, built in the IV century, was destroyed and rebuilt in the XVI century by the Pope himself! And the same applies to countless of other important ancient churches and public buildings (My favourite example: the astonishing church of St. Clemente [1], well worth a visit if you're in Rome! The XI-century building was erected over the old VI-century church.) Not only churches, but frescoes in churches have been continuously covered by newer frescoes, once the taste of the church goers changed. A few days ago I visited the beautiful church of Sant'Abbondio in Como [2]: the church dates back to the XI century, but the old frescoes were covered by newer ones in the second half of the XIII century.

Even the Romans did the same for many of their buildings: once they realized that they were no longer useful or usable, they either repurposed them or destroyed and built something else on the rubble. The Colosseum was not built on pristine land! And Nero destroyed a lot of houses to build his Domus Aurea, which was later destroyed by Vespasian and Titus and covered by newer constructions, with the intent of make people forget Nero. (No, as far as I know neither Nero nor Vespasian or Titus were catholic.)

It's a pity for scholars, but before historical preservation was a thing, it was often the best thing to do, as the older buildings did no longer fit their purpose. These acts of destruction made the world lose important pieces of art, but they were meant as a way to repurpose the spaces of the city according to the evolved needs of its inhabitants. This is particularly true for cities like Rome, which have been populated continuously for ~2500 years.

[1] https://en.wikipedia.org/wiki/San_Clemente_al_Laterano

[2] https://it.wikipedia.org/wiki/Basilica_di_Sant%27Abbondio

Exactly my point as well. The ancients didn’t think to not tear down buildings so we could discover them 2000 years later. It’s a miracle anything of that time is left at all in terms of physical structures, and the best preserved have been buildings repurposed by the church.

This isn’t even touching on the subject of the Latin corpus which is ONLY extant because of the Church.

Wootz steel was never lost. Just the contaminated with vanadium ores they used disappeared. We know quite well what happens to a carbon steel with vanadium. Most high performance steels today are made with it in the alloy.

they used saltwater and added volcano ash. thats the secret sauce

Chemistry might be important, but we don't necessarily need that deep of an analysis.

Romans didn't mind over-engineering with massive, inefficient structures. They didn't worry about complex/thin shapes, or any tensile elements. No rebar, no "piping" cement, etc. And they could put up with extremely long cure times.

The end result is very, very dense concrete.

We could probably recreate the durability of their concrete if we were actually willing to put up with the compromises they represent.


Sounds like the chemistry is important

> Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability.

> As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material.


> ... evidence that the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime, to create an environment where high surface area aggregate-scale lime clasts are retained within the mortar matrix. Inspired by these findings, we propose that these macroscopic inclusions might serve as critical sources of reactive calcium for long-term pore and crack-filling or post-pozzolanic reactivity within the cementitious constructs. The subsequent development and testing of modern lime clast–containing cementitious mixtures demonstrate their self-healing potential ...

Or, this study finds that Roman concrete regenerated using water-reactive calcium carbonate introduced through hot mixing–transformed lime clasts. This included observations of Roman concrete used in structures that weren't characteristically massive or inefficient, and reproducible in reformulated modern concrete.

> In the present study, we demonstrate the calcium enrichment of matrix phases adjacent to the lime clasts, supporting the hypothesis that the lime clasts are a source of calcium for leaching and recrystallization within the pore space of the mortars. Microcrack filling by calcite has been recently observed in ancient Roman mortars from the tomb of Caecilia Metella (29), and the self-healing tests carried out on our modern samples described in the present study further support this hypothesis.

"Chemistry might be important"

Conc is nearly all about chemistry. You mention curing rather than drying so you surely have some idea about what is going on. The Romans did not have electron microscopes so could not watch the way the matrix develops around the aggregate etc etc from the get go. To be fair, I fried my first sample in the beam at college by over focussing.

They didn't use rebar because they generally built in compression - arches etc. Rebar and pre-stressing enables members to cope much better under tension. A PS beam or slab is a modern marvel and even protects the steel from fire - bonus.

Cure times: weather/environment. In hot climes you need to stop the water buggering off the surface and leaving cracked and spalling surfaces before it even cures. It may also over heat (exothermic reaction). Cold - slower curing, too cold - no cure and frozen water (total disaster). Underwater - watch your fines wandering off and going for a smoke behind a reef and your structure collapsing, unless you keep them in place.

Concrete curing is not like smoking a joint of meat! A quick or slow cure may be indicated depending on conditions. We have admixtures and all sorts to fiddle with parameters. Then you have things like grading and composing your agg. and other exciting stuff.

I could go on at some length - it is really, really complicated. Roman engineers managed to invent an ancient wonder of chemistry and physics that was concrete. Their approach to its use and improvements was obviously rather better than what passed for science and medicine at the time! They do seem to have experimented and gone with what worked best but it was never deliberately developed from first principles. The self healing thing was probably a happy accident.

A Roman engineer had some rather different design constraints than a modern Civil. For starters, failure could quite legitimately be blamed on the god's displeasure. "When that bridge was commissioned, we made all the correct sacrifices and anyway, you are holding it wrong, I mean walking on it incorrectly".


In Ancient Rome blaming gods was never an excuse. For example, there an architect or a chief builder had to spent several nights under the bridge after its opening. In general they followed a rule similar to one in ancient Babylon which explicitly required to put the architect to dearth if a building collapse would kill the owner. And if the collapse would kill a son of the owner, then the son of the architect would be put to death.

That puts rather different perspective on the quality and longevity.

I worked on a campus where they were building a building that had to be incredibly stable - they had to coordinate arrivals of like 30 trucks making 4-5 trips a day. Each truck had like 180 seconds to unload.

Because of the coordination requirements, they embedded sensors in the structure and adjusted the water for curing and would slow down or speed up the flow of trucks based on how the process went.

One of the engineers told me that a thousand years from now, someone would wonder why this slab is here. I’m sure he was mostly kidding, but it seemed believable.

> Roman engineers managed to invent an ancient wonder of chemistry and physics that was concrete.

Technically, the ancient Egyptians used concrete in construction long before the Roman engineers were a thing. The difference is they used it for mundane things like wine cellars while the Romans integrated it into their whole architecture system.

Once in a while that art history degree comes in handy…

We do need a deep analysis. Roman concrete is self-healing, and this article explains why. And the reason is exactly the opposite of the one you postulate.

"Anyone can create a bridge that will hold up under a maximum load. It takes a real engineer to create a bridge that barely holds up under a maximum load."

It takes an accountant with a tight budget, you mean. Engineers will tend to make the most durable thing ever, if given enough time and money.

Interestingly, the article founds out that their concrete cured quickly.

Anyway, no, our usual concrete completely breaks down in a few centuries. No matter how it's structured, it does not match the durability of that concrete without ongoing maintenance.

>>A few centuries

Sounds good to me?

Although, many centuries old sound structures also sounds better to me...

Like the Pantheon[1]? It's still got the world's largest unreinforced concrete dome and it's in active use as a church not to mention major tourist attraction.

[1] https://en.wikipedia.org/wiki/Pantheon,_Rome

> massive, inefficient structures

How does a building, an aqueduct, or a bridge still in use after 2000 years manage to be inefficient?

Yeah, I can't imagine a municipality letting their concrete roads cure for a couple years before letting cars on them.

This is from the article:

> Masic wondered: “Was it possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?”

And this is from Wikipedia, with the source dated 2011:

> Gypsum and quicklime were used as binders. Volcanic dusts, called pozzolana or "pit sand", were favored where they could be obtained

Is it just me or are popular science articles really so simplified that they often completely misrepresent the point of the research?

Gell-Mann Amnesia is real.

Now onward to replacing the roads and bridges on freeways and highways with these new concrete.

Modern roads are designed to take serious abuse. 50 years of load cycling 20 ton semi at highway speeds causes mechanical damage which Roman concrete wouldn’t be effective in preventing. In ancient sites you can sometimes see the walls in high traffic areas have gotten smoothed and indented from hundreds of years worth of people gently dragging their fingertips across the surface as they walk by, tires are less gentile.

Also, people vastly underestimate how cheap road surfaces need to be. Try and calculate the volume of material making up the US road system.

A fun exercise is to calcualte the cost of some road in a neighboorhood, and compare that to the total collected property tax of all the homes along that road.

Then multiple that by 20 (the lifetime of the road surface) and factor in that the resurfacing is significantly cheaper than the initial build and you quickly find roads are easily subsidized by property taxes.

There are two sides to this coin. The roads need repair continually and resurfacing eventually, yes. What happens when property values and the local economy begin to deteriorate instead of prosper? All those utilities and public rights of way still need those repairs, but there's no appetite to raise property taxes, and the tax base is dwindling, and there's little reason to keep paying the mortgage on a depreciating house, especially once 2/3rds of your street is gone.

How sustainable is a system that requires perpetual good times?

Calling it a ponzi scheme is a stretch, but this guy seems to be the only one talking about this: https://www.strongtowns.org/journal/2020/8/28/the-growth-pon...

You abandon it. Detroit did this for example.

Also easy expansion is more a function of interest rates than anything. Municipal bonds buy everything and there’s been very little interest on them. This means fewer taxes to buy things. And new bonds can be sold down the road as the town increases tax base and interest rates fall.

NotJustBikes covered this too: https://www.youtube.com/watch?v=7IsMeKl-Sv0

Even in rural areas? Even if we add in the cost of maintaining infrastructure that runs under the road surface (water, sewer, and gas)?

I live in a very rural area and I had a few minutes so I calculated it.

I live at the end of a deadend road that is 0.9 miles off another very rural road.

The road is chip sealed, which according to google in 2022 is $25,000 to $42,000 per mile.

There are 7 parcels that collect $6396 per year in property taxes.

At the low end of the chip seal price, it would take 3.53 years to pay it off and 5.9 years at the high end.

This assumes that no sales or gas taxes are used to fund any of the road construction, which isn't true.

I live in a semi-rural area. There's no utility gas near me; we're too sparse for that to make sense. I'd guess most of the houses have big propane tanks (i've got two 250 gallon tanks) and there's several different companies that will come out and fill the tanks. In the nearby urban area, there's utility gas, but then they're trying to outlaw that for indoor air quality reasons.

I don't have utility water; it's available on my street, but it doesn't run by my house, it stops at a much lower elevation, our well water is fine (although we had to replace the well pump recently which was expensive). No utility sewer either, that's available in parts of my area, but mostly places with density or poor site conditions for septic or both.

Electricity is mostly overhead, with a little bit of underground telephone has a lot of undergrounding, but some overhead, cable and municipal fiber are almost all overhead. But when those are undergrounded here, they're on the sides of the road (directly under the overhead path) not under the road. That way it's easier to get to them for maintenance when needed.

> (water, sewer, and gas)?

Over their lifetime, easily. The pipes for those will last 100 years, easily. The maintenance of them are paid for by the delivery costs you pay in your bill.

As for rural I think there's 2 distinctions. 1 is we need rural communities because they produce our food and other things. Secondly, rural areas are between 2 or more populated areas. We need to connect populated areas so by necessity there are roads. Rural communities are built off of those.

The other distinction where you may be closer is exuburban communities built into rural areas. These may or may not be fully covered. Not initially anyways, but you'd assume the property taxes would cover in time.

> pipes for those will last 100 years, easily

My wife served on a sanitary district board. Decent underground sewer mains have an expected service life of 70 years. Some will last longer, some won't hit the expectation. It depends on site conditions, materials, etc, etc. Many will probably last to 100 years, but I wouldn't expect it to be easy. Especially if you had any of the not decent materials (Orangeburg pipe was common in some areas and is basically wood pulp/fibers mixed with hot tar; service life could vary between 10 and 50 years; the major manufacturer went out of business in 1974, as PVC and ABS pipes rapidly replaced Orangeburg in the materials markerplace).

Actually there was a huge nationwide boom in sewer building post WWII, especially in the 50s, and you can expect that infrastructure to need some largescale replacement over the next 20 years or so. Depending on system design and how housing and industry developed, some systems will be able to just inspect periodically and replace as needed, and some systems will probably take the opportunity to do a more modern redesign (older cities tend to have combined sanitary sewers and storm/runoff drainage; if you're tearing up all the streets to replace the sanitary sewer, it might be a good time to put in a parallel storm drain system)

I'm not sure about water mains, I'd guess they need more frequent replacement since they operate at pressure.

They are not subsidized, they are an infrastructure common good that is financed with taxes.

It depends. Where I live the roads in our neighborhoods are paid for by the town (property taxes) and many roads that run through the town are paid for by the county (income taxes) and then the highways are paid for by the state in part via tolls and other ones by the Federal Government via taxes.

A relative of mine who's otherwise a smart guy was lamenting rural broadband as an Obama boondoggle since it's never going to pay for itself to run fiber to all of the remote communities in our state... he didn't want to hear about the "P&L" for the road capital/maintenance costs.

I live in Montreal; many of the concrete structures in my city are crumbling and decaying due to the constant freeze/thaw temperature cycling that they're exposed to. I wonder if the self-healing property of this new advanced concrete would help to avoid some of the cracking & flaking we're seeing in constructions from the 60s & 70s.

I live in Montreal too, and while all what you say is true ... the same is not true for the xUSSR cold cities I lived before immigration. So quality makes difference. Also planing and designing for less maintenance. Concrete structures from the same era across xUSSR are in way better shape.

Montreal has infrastructure problems because of systemic corruption with the local Mafia.

To think that the USSR had better infrastructure (less corruption) than modern day corrupt places like Montreal is a brutal shock.

Only if you take out the rebar. Concrete is porous, water penetrates to the rebar inside, the rebar rusts and expands, cracking the concrete. But if you remove the rebar, you remove much of the tensile strength of the structure, which means you need to use way more concrete to compensate (via the sheer compressive force of its own weight).

What about those new materials for rebar? Like fiberglass? What do you think of them?

Not the parent. My understanding is metal rebar is the cheapest. You need a lot of rebar in modern concrete construction.

I worked at a geo-polymer start-up. One huge hurdle is sourcing the right kind of ash, and other ingredients, in sufficient quantities and with the right level of quality. Building a kiln or a launch pad is one thing, but building a road is orders of magnitude more expensive. Like, it's humanly possible, but it's not economically possible.

Only some bridge designs can use un-reinforced concrete. Many (most?) modern bridges need reinforced concrete to allow the concrete to bear tensile loads. Reinforcing concrete introduces new ways that concrete can degrade such as rusting of the reinforcing elements. I'm not really sure Roman concrete would be much help here.

The quick cure may be very useful for most construction. And the self-healing may be better enough for use on pavement.

But most bridges and highways are quite demanding and can't simply adopt a new concrete chemistry based on an improvement in a single dimension.

modern roads and bridges are designed to decay at the rate they decay, because building in longevity is far more expensive than simply replacing the road surface or replacing the bridge at the end of its life.

what we don't do today is follow through on the maintenance and replacement schedules, in an attempt to save money. we try to repeat our tradeoff after the infrastructure is built, again trading away longevity in favor of money. well, infrastructure decays if you don't maintain it or build it to last longer.

skimping on maintenance after you skimped on longevity is trying to eat the cake, and still have it after you've eaten it. it doesn't work that way.

cobblestones for the roads

for slow neighborhood roads I see no reason why brick isn't used more often. my hometown had brick roads everywhere and once every 3-5 years a few bricks would get replaced here and there. in the 1990s I'd look around and see a lot of the bricks on those roads that were made in the 1930s, and there were probably older bricks to be found if I'd looked harder.

the rough ride kept speeding down a lot more than the police could, because I watched people scream through my neighborhood all the time after they paved over the bricks.

plowing snow is harder on brick roads, but it isn't so hard that it is an unsolvable problem.

Speaking as someone who grew up in a neighborhood with brick streets,...

They're insanely slippery when wet. A freeze-thaw cycle tends to make them move, eventually leading to weird dips and ridges. They're expensive to install and more expensive to maintain.

they were not slippery that I noticed in my town. not sure why.

my town used dirt as the "mortar" between bricks, so there was no solid joint to degrade over time. I do remember crews going around and spreading small amounts of dirt on those roads, to refill what little rinsed away after spring thaw. they'd just have a guy following behind the truck with a push broom moving the dirt so it fell between bricks.

dirt is dirt cheap.

It's not the plowing that gets bricks in winter, or not plowing exclusively, it's the freeze-thaw cycling that kills anything with joints. Every grout joint / polymeric sand joint is an opportunity for moisture intrusion and in any climate zone that gets cold enough to freeze, those joints are going to degrade incredibly quickly. Bricks may make sense in the South, but definitely not in most of the US.

I’m not sure how this applies. Most joints are meant to flex, reducing the breakage of actual paving elements significantly. It also means that an area with significant heaving can be repaired more easily by only needing to service the affected area, and the repair has minimal impact to the surface quality, unlike concrete patching or pothole filling.

That is... not a smooth ride.

Sounds like a good way to make people slow down.

The problem with making people slow down is that it slows people down.

Sometimes that's what's needed.


But usually, it slows people down.

> In 2012, Paul Lecroart, of the institute of planning and development of the Île-de-France, wrote that "Despite initial fears, the removal of main roads does not cause deterioration of traffic conditions beyond the starting adjustments. The traffic transfer are limited and below expectations".[4] He also notes that some private vehicle trips (and related economic activity) are not transferred to public transport and simply disappear ("evaporate").[4]

> The same phenomenon was also observed when road closing was not part of an urban project but the consequence of an accident. In 2012 in Rouen, a bridge was destroyed by fire. Over the next two years, other bridges were used more, but the total number of cars crossing bridges was reduced.[4]

It seems a lot like the real-world examples of the phenomenon look like close cousins of "if you remove the smallest values in a set, the average goes up".

it's not so bad; where I grew up in France many streets were cobblestone and it was fine. It's pretty rare in the US though except I've noticed it in streets where they have trams running--presumably to reduce the need for maintenance on those streets.

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