Why Roman concrete is still stronger than RAAC (and other modern concretes)
Why Roman concrete is still stronger than RAAC (and other modern concretes)
The concrete dome of the Pantheon in Rome remains stable enough for visitors to walk beneath, and some Roman harbours have underwater concrete elements that have not been repaired for two millennia – even though they are in regions often shaken by earthquakes.
Whence this remarkable resilience of Roman concrete architecture? It’s all down to the chemistry.
This is one of the most commonly touted engineering myths that simply doesn't hold up to even a brief analysis. The first glaring problem is the inherent survivorship bias behind claiming Roman concrete was objectively better than modern concrete. As other users have already mentioned, modern concrete is actually very strong and exceeds the strength of Roman concrete when such strength is required, but where it really has an advantage is in its consistency.
If every concrete structure built in Rome was still standing and in good shape to this day, engineers would be salivating over the special blend and would be doing whatever they could to get their hands on it or replicate it. But we don't see that. We see the Roman concrete structures that have survived the test of time (so far), not the myriad structures that have not. Today's concrete on the contrary is deliberately consistent in chemistry, meaning even if it typically isn't designed to last hundreds of years, you can say with a great deal of confidence that it will last at least X years, and all of it will likely exhibit similar wear and strength degradation behaviors over that same duration.
There are other factors at play too:
- Romans didn't use steel reinforcing re-bar, instead opting for massive lump sums of concrete to build structures. These massive piles are better against wear and porosity-related degradation, especially due to the self-healing properties of the Roman concrete blend due to volcanic ash helping to stop crack propagation.
- Our modern concrete structures are much, much larger in many cases and/or are under significantly higher loads. Take roads for example—no Roman road was ever under the continued duress of having hundreds of 18 wheelers a day rumble over them.
- Our modern concrete structures do things that would have been considered witchcraft to a Roman civil engineer. Consider the width of unsupported spans on modern concrete bridges compared to the tightly packed archways of Roman aqueducts.
None of this is to detract from Roman ingenuity, but to make the claim that Roman concrete was objectively better than what we have today is farcical.
And to add a platitude: “Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”
Though it's important to note that factors of safety are always incorporated into structural designs, in case higher loads/wear/etc than expected occur
Thank you. This is a part of the whole trad mess, where they basically claim that everything was better for reasons and we should all turn back to the traditional values like bigotry and slavery. A fun mess where I lose my shit with anger - your answer is wonderful.
This article is literally about doing research to better understand the chemistry behind the self healing properties of Roman concrete to maybe use the findings to improve modern concrete. This is the aspect which I find so interesting : the chemistry. Literally no one is talking about going back to traditional values and blah blah. That's something which I personally abhor. Did you even read the article? Where did you find this in the article? Of course titles of articles tend to be over dramatic.
This article is about research on self healing properties of Roman concrete. It's not all about a one on one comparison. The chemistry behind the self healing properties is interesting and not definitively established.
I remember finding this Practical Engineering video on Roman concrete to be informative: https://youtu.be/qL0BB2PRY7k?si=5exDGyEK_LTfGNOy
Veritasium also has a chapter on ancient concrete in this video: https://youtu.be/rWVAzS5duAs?si=EJ8rPDTPHlq90kgW
My memory is fuzzy, but I think some of the details are:
- We know how to make Roman concrete, but it's not necessarily the best choice, and it might be more expensive than is appropriate for a given project.
- Ancient structures don't have rebar, so they don't degrade due to rust causing expansion. But rebar is so useful that it's often a worthwhile trade-off.
Definitely see the other comments here about survivorship bias, and higher demands on modern structures.
Is it that we don't know how to make concrete of equal/greater resilience? Or that modern concrete optimizes for something else (I'm guessing cost)? I didn't RTFA.
They are comparing roman concrete to portland cement, the most common formula. The kind of strength being emphasized is durability, because roman concrete has unique chemistry that allows small cracks to fill themselves. Modern special-purpose concrete blends can outperform roman concrete in other measures of strength, however.
Thats right. I remember the engineering dept. of my university would hold "concrete boat" competitions to highlight this point exactly. Concrete is a mixture, and different mistures are used for different purposes. Im not a civil engineer, but I wonder how well would roman concrete perform in 100+ floor buildings or expansive multi-floor complexes. Even if it performs "well", is it cost effective? I doubt it will be better than a min-maxed mix determined by project/area/budget/etc
Cool! Thanks!
The article suggests that the roman concrete gets it's properties due to using a certain kind of volcanic ash found around Naples, but not common everywhere else, so using their recipe wouldn't be sustainable with the amount of concrete we use these days.
On the other hand, these days it wouldn't be about that specific ash. You could take a look at it, say "oh, yeah, that's just florgium whateverthefuxide" and then there'd probably be readily available sources of it.
Got it.
I have heard too there are differences in available raw materials. Even our newer concrete is not as good as older.
Notice also they said common cement too. I suspect supply and demand meaning cost and obsolecense are what we design for. For that matter too cheap patio blocks are not as good as expensive ones. Sad but we do not build for even decades let alone centuries.
Keep in mind too that technology does not automatically improve. For tech to even continue at the same level we have to continually practice it.
We build for decades, literally ALL of our materials are better. We know why we make things the way we do and we choose according to the thing we are building. Now, I think we are doing some practice with concrete because it's the most used thing in the world, it's even a strong co2 contributor. There is no mystery about the concrete, no conspiracy, there is nothing but the fantasy being peddled by people who need to find fisting in their lives so it can fill that emptiness
Our newer concrete is better for the goals we are setting in construction, which does not tend to include permanence. Our goals are mostly about strength to weight ratios and other properties that allow for massive numbers of floors with and as little mass as possible with reliability measured in decades. Basically guaranteed to last reliably with minimal upkeep.
We are continuously practicing new ways to build with concrete, wtf are you talking about?
We mostly know how they made theirs, and could make our own version of it, but we optimize for different things.
The Romans optimized for "that's cement and it works well", because they didn't have anything close to the level of chemical understanding we do now.
We optimize for strength and predictability. Ours can hold a higher load and will likely need repairing about when we predict.Roman concrete can sometimes, in certain circumstances and with variable effectiveness, repair certain types of damage by chemically interacting with the environment. So maybe it crumbles in a decade or maybe it lasts a millennium.
Article basically points at some researchers who are looking to see if they can bring that healing capability to modern concrete in a predictable and more versatile fashion.
I think that plenty users here already highlighted the main points (survival bias, lack of reinforcement with steel, optimisation for other characteristics). I'll focus on the chemistry instead.
Think on a tea strainer, a chicken wire, and some chain link fence. Sure, they might be made of the same steel, and they're all meshes. But they're all linked in different ways, with different properties, and they will serve different purposes. Aluminosilicates are also like this; even if you have the exact same composition, it's perfectly possible that some are more resistant than others, based on their structure.
Studying Roman concrete might reveal something about the aluminosilicate of the surviving buildings that might become useful later on. With that knowledge, even if you believe (as I do) that modern concrete already surpassed Roman concrete in plenty attributes, we can make it even better.
This is exactly what I was thinking about!