I’ve been watching documentaries since I was kid that do the same thing every few years. At least we’ve finally given up the were 10 years away from living forever.
Interesting. It's not just the absence of rebar, and it's not survivorship bias. It's a hot-mixing method that results in both rapid curing times, and self-healing of cracks. A lost technology, rediscovered.
Is it reasonable to build concrete structures today without using rebar? And is it true that rebar actually makes concrete structures less durable over longer time scales?
I've long been attracted to the idea of building a building with Roman concrete and no rebar that would last centuries... Guess it's a sort of vanity project. :)
You can't build sky-scrapers without rebar (i.e. with un-reinforced concrete), but you can build some pretty large structures if you use curves, arches, widening bases, buttresses, etc. The Pantheon is pretty big, built from un-reinforced concrete, and nearly 2000 years old.
You have to adapt your building style to the material you're working with and tall, thin structures depend on the tensile strength of steel; concrete doesn't have much tensile strength, but does have tremendous compressive strength, so your structure will have to be wider at the bottom, although not necessarily wider than it's tall. It's all about directing the vectors of forces in a way that they stay inside the material of the structure, so no flying slabs, upper floors have to have arches or domes supporting them from below (or lots of pillars that widen into a small arch at the ends).
Here is an idea for a technique may be useful for building with un-reinforced concrete: instead of pouring whole walls into a mold, pour "lego"-style interlocking (large) blocks, layer by layer. Between layers you paint the surface with a thin layer of weak but flexible mortar or glue before pouring the next layer. This way you keep enough room for the structure to shift and settle without cracking and you can use the angle of contact between blocks to deflect the vectors of force back into the material. The article mentions that the Roman-style concrete hardens much faster, so that'll work well with this idea (you don't have to wait too long between pours).
As for rebar making concrete structures less durable; yes, that's certainly true for steel rebar. The reason being that it will rust, very slowly at first, but once it starts the expansion of the rusted part causes cracks in the concrete which allow more humidity and oxygen to reach the steel, thus rusting faster. This is often called "concrete cancer", and limits the useful lifetime of most modern reinforced concrete structures to between 50 and 250 years (depending on the environment they are in, the forces they are exposed to, and the quality of the concrete they were constructed with).
Concrete cancer can be reduced or even eliminated by using rebar material that rusts more slowly (stainless steel) or not at all (carbon fiber), but these are much more expensive of course. There is room for research on other reinforcing materials, but basically nothing with good tensile strength is going to be cheaper than steel and considering the quantities of rebar we use, cost is definitely a major issue.
The self-healing nature of Roman concrete might also help here, but the chemistry of concrete and rust formation on embedded steel is complex, and without extensive experimentation right now we don't know if steel embedded in Roman concrete rusts faster or more slowly than in modern concrete (before considering cracks).
I think the problem is the kinds of structures we want to build. The Romans built everything supported by arches, where the loads are all distributed in compression. But to make a glass-walled multistory apartment building with overhanging balconies you definitely need rebar to handle those tensile loads.
Modern concrete is also self-healing to a degree, while being vastly stronger. It's just subjected to incomparably higher stress. We also never really "lost" the method of making concrete self-healing, t's just never been a priority.
There might be something patented in the modern production process. Or, alternatively, they could release the recipe as something "open source" and also potentially provide "pre-mixed concrete as a service"...
Think. Blacksmiths folded metal. Makes sense they’d try the same with concrete, which would result in this auto healing we see. This seems the most likely method I would have invented but no one ever thinks of.
Previous and related:
How Ancient Roman Concrete Was Able to Last Thousands of Years - https://news.ycombinator.com/item?id=39212710 - Feb 2024 (5 comments)
Why was Roman concrete so durable? - https://news.ycombinator.com/item?id=34280239 - Jan 2023 (277 comments)
Mystery of Roman Concrete Unraveled - https://news.ycombinator.com/item?id=34353330 - Jan 2023 (1 comment)
The Mystery Finally Solved: Why Has Roman Concrete Been So Durable? - https://news.ycombinator.com/item?id=34456323 - Jan 2023 (1 comment)
Mechanistic insights into the durability of ancient Roman concrete - https://news.ycombinator.com/item?id=36842712 - July 2023 (1 comment)
Why Ancient Roman Concrete Outlasts Our Own (2017) - https://news.ycombinator.com/item?id=29366911 - Nov 2021 (67 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 - Mar 2020 (64 comments)
How Did the Romans Make Concrete That Lasts Longer Than Modern Concrete? - https://news.ycombinator.com/item?id=15544128 - Oct 2017 (3 comments)
New studies of ancient concrete could teach us to do as the Romans did - https://news.ycombinator.com/item?id=14690329 - July 2017 (74 comments)
Ancient Roman Concrete Is About to Revolutionize Modern Architecture - https://news.ycombinator.com/item?id=5883443 - Jan 2013 (23 comments)
I’ve been watching documentaries since I was kid that do the same thing every few years. At least we’ve finally given up the were 10 years away from living forever.
Interesting. It's not just the absence of rebar, and it's not survivorship bias. It's a hot-mixing method that results in both rapid curing times, and self-healing of cracks. A lost technology, rediscovered.
Is it reasonable to build concrete structures today without using rebar? And is it true that rebar actually makes concrete structures less durable over longer time scales?
I've long been attracted to the idea of building a building with Roman concrete and no rebar that would last centuries... Guess it's a sort of vanity project. :)
You can't build sky-scrapers without rebar (i.e. with un-reinforced concrete), but you can build some pretty large structures if you use curves, arches, widening bases, buttresses, etc. The Pantheon is pretty big, built from un-reinforced concrete, and nearly 2000 years old.
You have to adapt your building style to the material you're working with and tall, thin structures depend on the tensile strength of steel; concrete doesn't have much tensile strength, but does have tremendous compressive strength, so your structure will have to be wider at the bottom, although not necessarily wider than it's tall. It's all about directing the vectors of forces in a way that they stay inside the material of the structure, so no flying slabs, upper floors have to have arches or domes supporting them from below (or lots of pillars that widen into a small arch at the ends).
Here is an idea for a technique may be useful for building with un-reinforced concrete: instead of pouring whole walls into a mold, pour "lego"-style interlocking (large) blocks, layer by layer. Between layers you paint the surface with a thin layer of weak but flexible mortar or glue before pouring the next layer. This way you keep enough room for the structure to shift and settle without cracking and you can use the angle of contact between blocks to deflect the vectors of force back into the material. The article mentions that the Roman-style concrete hardens much faster, so that'll work well with this idea (you don't have to wait too long between pours).
As for rebar making concrete structures less durable; yes, that's certainly true for steel rebar. The reason being that it will rust, very slowly at first, but once it starts the expansion of the rusted part causes cracks in the concrete which allow more humidity and oxygen to reach the steel, thus rusting faster. This is often called "concrete cancer", and limits the useful lifetime of most modern reinforced concrete structures to between 50 and 250 years (depending on the environment they are in, the forces they are exposed to, and the quality of the concrete they were constructed with).
Concrete cancer can be reduced or even eliminated by using rebar material that rusts more slowly (stainless steel) or not at all (carbon fiber), but these are much more expensive of course. There is room for research on other reinforcing materials, but basically nothing with good tensile strength is going to be cheaper than steel and considering the quantities of rebar we use, cost is definitely a major issue.
The self-healing nature of Roman concrete might also help here, but the chemistry of concrete and rust formation on embedded steel is complex, and without extensive experimentation right now we don't know if steel embedded in Roman concrete rusts faster or more slowly than in modern concrete (before considering cracks).
I think the problem is the kinds of structures we want to build. The Romans built everything supported by arches, where the loads are all distributed in compression. But to make a glass-walled multistory apartment building with overhanging balconies you definitely need rebar to handle those tensile loads.
Modern concrete is also self-healing to a degree, while being vastly stronger. It's just subjected to incomparably higher stress. We also never really "lost" the method of making concrete self-healing, t's just never been a priority.
None of the above is true.
> The team is now working on commercializing their concrete as a more environmentally friendly alternative to current concretes.
That may be rough, given they won't be able to patent it due to prior art...
There might be something patented in the modern production process. Or, alternatively, they could release the recipe as something "open source" and also potentially provide "pre-mixed concrete as a service"...
Think. Blacksmiths folded metal. Makes sense they’d try the same with concrete, which would result in this auto healing we see. This seems the most likely method I would have invented but no one ever thinks of.