A big part of why Rome's monuments have lasted is they didn't use reinforced concrete. Reinforcing with steel rebar allows much thinner concrete walls than without, however it also makes them susceptible to rust/expansion/crumbling when water gets into the material.
François Coignet was a French industrialist of the nineteenth century, a pioneer in the development of structural, prefabricated and reinforced concrete. Coignet was the first to use iron-reinforced concrete as a technique for constructing building structures. In 1853 Coignet built the first iron reinforced concrete structure, a four story house at 72 rue Charles Michels in the suburbs of Paris. Coignet's descriptions of reinforcing concrete suggests that he did not do it for means of adding strength to the concrete but for keeping walls in monolithic construction from overturning. [...]
Joseph Davidovits explains the chemistry behind this and much more in his books and other works over many decades. Once ridiculed, acceptance of his ideas has slowly grown. Certainly the chemistry that he describes is very repeatable, and the Pantheon and other Roman works are very … can't resist … concrete examples and proof of his ideas. Follow the links in this wiki article and do some Google searches. I read the book in 2000, and was very impressed by the time I finished it. I later met with Davidovitz, and found him to be quite reasonable, intelligent and clear thinking. The biggest obstacle was/is that it was in more people's interest to promote the "mystery of the pyramids", and established dogma in academic circles than to just repeat his experiments. http://en.wikipedia.org/wiki/Joseph_Davidovits
> This study conclusively demonstrates that there is absolutely no evidence of an alkali-aluminosilicate-based composition in the binder phases of the casing stones, nor is there any evidence of “unusual” constituents in the pristine, bulk uncontaminated interior of the casing stones to call for a “man-made” origin. Despite the detection of a man-made “coating” on the Lauer casing stone, the stone itself is determined to be nothing but a high-quality natural limestone mineralogically, texturally, and microstructurally similar to that found in the quarries at Tura-Masara. ...
> Despite its many reported ancient routes, and unquestionable potential future applications of geopolymer technology, the Egyptian pyramids, in author’s opinion, still stand as marvels and mysteries of ancient engineering technologies exercised by the Old Kingdom (2500 BC) stone masons.
As a non-geochemist, I can say nothing about it one way or the other, and I know that I can be easily swayed by good sounding but wrong statements.
Perhaps it's like Thor Heyerdahl's Kon-Tiki. He showed that it was possible to go by balsa wood raft to Polynesia. That doesn't mean his diffusionist model of cultural development was what happened. That the Egyptians could have used geopolymer technology doesn't mean they did.
Roman concrete was called opus caementicium. The Romans had two main sources of the right kind of volcanic ash in Italy, the main one in Pozzuoli (http://en.wikipedia.org/wiki/Pozzolan)
but apparently they mined other places, such as in Eifel where it is known as Trass (http://en.wikipedia.org/wiki/Trass). For example concrete was used in the very extensive aquaducts serving the Roman city that is now Cologne.
It is very similar to lime plaster (lime and sand) and so it is thought to have been discovered by simple substitution of local pozzolan for sand.
The advantage of Portland cement is that it cures very quickly. Roman cement takes something like a month to cure.
Another apparent advantage of Roman concrete is that they were able to throw up large structures with concrete vaults or domes with a large workforce in relatively short time frames, whereas the Medieval cathedrals (which were based initially on the same Roman basilica) took a smaller, highly skilled workforce long periods to cleverly piece together without concrete.
Pozzolan is used today, generally as an additive. Note that fly ash is similar (http://www.ctlgroup.com/lab-services/lab-tests/astm-c-618-co...). Again because of the long cure time the original Roman concrete is not in favor, but you may achieve the performance you need through the use of an admixture.
I should add that some of these recent articles may be linked to the general business interest in the increased use of pozzolanic admixtures, since there is a ready source in fly ash and also some parts of the world have vast amounts of pozzolanic ash ready to be exploited.
Yes, and there's more - Portland cement is made of constituent parts, each presumably extracted from different stones (using energy intensive processes) and then precisely recombined in certain ratios. Roman concrete, lime mortar, and Natural cement are made largely from just moderately heating limestone.
If you enjoy history of architecture/engineering, it's worth picking up when it's on sale, the video quality is standard def, but you can either stream the videos or download DRM free copies of each lecture.
(Note, not a CE any more, but I did Civil Engineering @ Cornell with Professors that were big into concrete, where many of the people who wrote the paper are from.)
First of all, I bet that the strength of the concrete builds pretty slowly. They're mentioning a 180 day test, where most modern concrete is considered 'fully cured' at 28 days, and generally you're looking at a few days to a week before it's got enough strength to continue construction. (Note that concrete will continue to cure indefinitely as long as it's kept moist and at a good temperature. E.g., I looked at some concrete from a floating bridge that was 40 years old and it was 3x stronger than it was supposed to be based on the original specs. There's some scatter in those, but 40 years in a cool poist environment didn't hurt.) Also note that there are a lot of interesting additives to concrete that can change the behaviour of modern concrete, to make it easier to place, faster to set, slower to set, pumpable, high strength, high stiffness, or whatever.
Second, Concrete tends to be a pretty local thing. You don't tend to ship the raw materials long distances, since they're heavy and low value. In places where there is volcanic ash, it can be used. But then you get to places like the whole US east of the Rockies, and there's none of that.
Third, we lost the recipe for Roman Cement for the better part of 2 millennia, and only recently rediscovered it, well after the discovery of Portland Cement and it's spread through the world.
Case in point, Rosendale cement[1] is another natural cement. It was popular in the 19th century because it does not require additives to produce (the limestone already contains the desirable traces) and can set underwater. Being produced along the Hudson River, one of the biggest shipping routes of the day, probably didn't hurt. However, it requires significantly longer to set and was discarded in favor of the faster setting Portland cement.
(Bonus factoid: while Rosendale cement is named or the location of the limestone deposits mined to produce it, Portland cement is named for the resemblance of the finished product to Portland stone, http://en.wikipedia.org/wiki/Portland_stone)
Concrete as used by the Romans has good vertical / compressive strength, but lacks the tensile strength you get with steel-reinforced concrete (which has shorter lifespan due to corrosion and similar effects). So you'd have to build thicker walls and use something else for floors.
Longevity of modern building techniques (or rather lack thereof) is a pet peeve of mine. Romans, Greeks, like many other ancient cultures built for millennia with much simpler means than we have available today, while the methods a building contractor is offered these days yield lifespans of 30-100 years maximum (that's in Europe where we don't typically build something that needs only last till the next Hurricane). It's a sad state of affairs, owed to the throwaway mentality we have about pretty much everything today.
How much of that is survival bias? We've only found a few timbers from the roundhouse buildings, and I don't think any of the old Germanic mead/feasting halls other structures still exist. In the American SW, adobe mud construction is worn down easily by rain, so is rebuilt often in order to persist, and neither traditional Hawaiian hale, made with thatched pili grass, nor sod houses would last 30 years without a lot of maintenance.
Certainly an igloo is a traditional architecture style where the result is meant to be thrown away.
Modern builders are operating on a much different set of constraints than the Romans. Simply include 2 additional factors in the equation (cost and harsh climate) and the Romans would not have fared nearly as well. The circumstances under which all of these ancient structures were built were more-or-less ideal for longevity. Is this a coincidence? No, it's survivorship bias[0].
How is the climate worse nowdays than in the past 2000 years? For a sensible comparison, look at the (many) wars these buildings had to endure as well. It's easy to claim survivor bias, but the durability of these buildings is simply a matter of technical analysis and specification, as with modern construction. I had to choose the material for a building facade (renovation) recently and the best option for longevity was: natural stone, the worst being the most commonly used type.
When I mention survivorship bias, I do so in order to highlight that people are being selective when talking about those buildings which have survived and ignoring the vast multitudes of Roman and other buildings which have not.
As for climate? Yes, the climate where I live (Ontario, Canada) is vastly more harsh than Rome (ancient or modern). Why do I bring this up? Because generally the people making these sorts of survivorship biased observations are also neglecting to control for other significant variables such as location.
You also didn't mention cost. Many of the great Roman structures which have survived to this day would have astronomical costs should we attempt to build them in the modern day using ancient building techniques and materials.
> You also didn't mention cost. Many of the great Roman structures which have survived to this day would have astronomical costs should we attempt to build them in the modern day using ancient building techniques and materials.
I'm not so convinced as far as materials are concerned. Techniques have improved and modern ones could be used (cranes etc.). The main issue I see here is that materials aren't even available anymore, nor are they legal w.r.t. building codes. So even if cost wasn't a factor, I don't see how anyone could build a modern building with Roman concrete, marble etc.
In fairness to today's engineers, modern buildings have a much different cost model associated with them (e.g., ensuring worker safety, environmental costs of acquiring materials, building close to other large structures, designing around high traffic areas, inclusion of non-structural beautification elements like large glass windows). Not to mention, recreating many ancient structures would cost an order of magnitude greater now, due to higher costs of living.
inclusion of non-structural beautification elements like large glass windows
This item isn't peculiar to modern architecture, particularly when it comes to large public buildings. If anything, modern architecture suffers a lack of ornamentation.
Something I've tried and failed to find out is whether the extreme durability of Roman construction was desired, or was a side-effect of building stuff at the scale they wanted. It would be nice to know whether they made it _this_ way by policy. If they'd had a cheaper construction method that they estimated would last only a hundred years, say, would they have used it?
>If they'd had a cheaper construction method that they estimated would last only a hundred years, say, would they have used it?
I believe they did have one and did use it - timber buildings. I think most ancient societies used timber but we don't see it now because it doesn't last. There's an article here on the find of "layer upon layer of Roman timber buildings, fences and yards" under London.
I know about their timber constructions, but I'm pretty sure that some of the things they built that have lasted couldn't have been built with timber, like the aqueducts, the Pantheon or the Colosseum. It's those things I am wondering about.
