I was a bit shocked when I learned that reinforced concrete only has a lifespan of about 100 years. That means that if you buy a condo in a building built in the 1970s, the building will become unsafe and need to be demolished in your life time. I don't know if people are pricing this consideration.
It also means there will be pretty much nothing left of our architecture, all those appartments, offices, museums, bridges, will all need to be destroyed and rebuilt. Not that modern architecture is really worth preserving. But it feels the only thing our era will leave behind is financial debt and bits in the cloud!
The oldest buildings in the world[0] are as old as they are because they use materials with lifespans of much less than 100 years. Instead they make them modular. Make it so that you can take out parts of the building to replace them without collapsing the whole thing. Instead we build our buildings in such a way that makes maintenance very hard and costly. We prefer to pretend like our buildings will last forever rather than actually plan for the inevitable degradation
There's small scale movements towards changing this. Some cities are specifically building buildings in a way that makes them modular and reusable[1]
The ancient Roman concrete was very different from modern: it was more thick, could not ne poured etc. But more importantly, it had no rebar, which is the main thing that corrodes over time. Ancient buildings could use molten lead to join together pieces of stone, but steel could not be made in the volume needed for reinforced structures.
Thing is we can use rebar in concrete for indefinately long use if we plan ahead, use sacrificial annodes to stave off corrosion or use stainless steel or use carbonfiber rebar as they won't rust. Hell using a polymer coating on rebar can prevent moisture contacting the surface of steel and rusting but that would require added cost and care when preping to prevent the polymer from being damaged prior to being embedded in concrete.
Also in Japan, there has been an increasing use of aluminum (!) in construction, since it will generally be melted and re-used after the structure is no longer useful.
What's the problem with aluminium? I would always build structures out of aluminium if aluminium would be strong enough. It has extreme corrosion resistance, it's cheap, it's easy to handle, you can easily make almost any imaginable shape with it, you can cheaply reuse it, it's very light.
Aluminum is generally much more expensive than other construction materials, but less obviously, it has practically no fatigue limit, meaning a structure made of aluminum will eventually fail under repeated loads.
Where did you hear that reinforced concrete has a lifespan of 100 years?
As a structural engineer, I can tell you this is not correct as a general rule, but is true for structures subject to chlorides, especially marine structures like bridge piers.
One of my now-retired professors at the University of South Florida studied concrete durability for FDOT. He told me FDOT is now using a 100-year design basis for bridges. The concrete materials and additives have gotten quite good over the past 2 decades. We are learning a lot and still improving our concrete.
The key is to make a tortuous path for the chloride ions so it takes them decades to build up enough to overcome the passive film at the steel-concrete interface. The high pH of the concrete matrix causes this passive film, and it takes either acidification or chlorides to defeat it. Concrete bridge decks and roads in cold regions that use deicing salts are also damaged by chlorides.
Reinforced concrete protected from the weather in buildings would not have a 100-year lifespan forecast. If the building is properly maintained, the concrete should last much, much longer than that. I say "properly maintained", because of the Surfside Condo collapse in Florida. There a leaking plaza deck, lack of maintenance, and design flaws (columns too skinny) led to a tragic collapse of a building.
What baffles me (as someone working in Advanced Composites) is why not simply get rid of the steel, the steel-concrete interface, the chloride ion and corrosion problem altogether — by instead using rebar made of composites?
Rebar of fiberglass, carbon fiber, basalt, and other combinations is all redily available and has known properties.
The key issue is that steel will corrode, then expand and put the concrete in tension (which concrete sucks at reacting), causing the concrete to crack, then spall off. None of the composites do this.
Yet, despite composites being available for years, and even being cheaper than steel [0], they are being picked up at remarkably slow rates.
It seem blindingly obvious to me that everyone should have just switched some time ago. Yet, this has not happened. Why?
Alternative reinforcement is an area of ongoing research. There are issues with cost and stiffness of the more exotic types. Research is probably slower because of the need to at all times be confident our structures are safe. We cannot freely experiment with exotic reinforcement in the built environment. It needs to be proven, first analytically then in the laboratory, then in pilot projects, and then in the legal and political forum before adoption in life safety critical applications like bridges and buildings.
Yes, I certainly agree that validation research needs to be done, and done absolutely solidly before wholesale switching .
Yet in this case, the existing tech is known bad (although TBF, the how-bad is well-characterized), and the new technologies are already qualified to fix this bad tech, e.g., carbon-fiber re-wrap of disintegrating steel-rebar bridge columns [0,1,2].
Certainly seems that applications like road-bed construction that require rebar, where the worst-case is a part of the road gets potholes prematurely, vs abridge or building collapsing, should already be mandated to use composite rebar. That would significantly increase the data set that can be used for real-world aging studies, with minimal risk?
Worth noting that 100 years is not exactly the hard deadline, but a typical (default) *target* for the calculations made by the engineers planning the construction.
What I mean is that this number is defined by design, although they are of course also constrained by what is possible within the set budgets.
Reinforced concrete lifespan depends on a lot of factors, including some that can be controlled, such as isolation of the reinforcement, materials used, various qualities of the concrete itself. When the new structures are designed, engineers do the calculations to achieve a specific goal in terms of lifespan, construction cost and maintenance cost.
I don't know how this works in practice, but I would hope that various factors such as the total cost and environmental impact (yes, I can hear my inner self laughing at my naivety here) are taken into account to arrive to the optimal target.
CFRP rebar has been developed with a much longer lifespan. However, it is currently considered non-economical, possibly because design lifespans are not long. Aramid FRP has been considered for rebar as well; glass fibers are attacked by cement alkali; basalt fibers have good performance dependent on the source of basalt (rocks with varying composition).
One issue with a long-lived building is that you never know exactly when it will fail. 700 years down the line, it starts to crack: now what? The evolution of biological systems has strongly preferred regular replacement to longevity.
Considering the astounding cost of building anything now, I guess being in the mitigation industry (retrofits, shoring-up and general can-kicking) will be rather profitable.
But I can see at least some buildings being profitable even then. A modest London property can easily cost 500k, most of which is the land itself. If it lasts 100 years, that's "only" 5k per year, and you keep the land. Obviously the calculation is different if you're buying a 95-year-old concrete property, rather than building a new one. I imagine mortgage providers are paying attention, they already strongly dislike lending against steel-framed houses.
I don't think we'll be seeing mass demolitions of 100 year old properties and infrastructure, except probably quite a bit of useless office space if RTO continues to not really happen over the next few decades. But we will see some very expensive and disruptive repairs-in-place to infrastructure when it can't be ignored any more.
Stainless steel contains a necessary 12% (often more, usually 18%) chromium, which is relatively scarce from a building materials perspective. Already 85% of available chromium is used for stainless steel; many other metals are primarily supplied as byproducts of chromium extraction because of the extensive efforts [1, 2] made to locate and exploit deposits. In fact, total rebar production is roughly the same scale (if not larger) as total stainless steel production, meaning that practically the entire world supply of stainless steel would have to be diverted to rebar in order to use it for this.
In general only the 12 or so most common elements can be used for construction. Copper is the rarest material used in major applications, but this is relatively less total mass consumed than rebar.
> Copper is the rarest material used in major applications, but this is relatively less total mass consumed than rebar.
At 43:51 in the video, he claims that an outer layer of stainless steel rebar is sufficient to protect the interior, so the goal is not 100% stainless.
I occasionally buy stainless steel strut and a 10’ stick of 12ga 1-5/8” deep SS 316 strut is around $250. The same strut made of galvanized steel is around $40.
> That means that if you buy a condo in a building built in the 1970s, the building will become unsafe and need to be demolished in your life time. I don't know if people are pricing this consideration.
Isn't possible to do some repairs to make it safe again, instead of completely demolish it? I live in a big city (South America) with a lot of old buildings. People treats property as something that will last forever, but if this is the case, a lot of people are going to lose assets - and I am also scare of the gov to check that these buildings are still safe.
> Isn't possible to do some repairs to make it safe again
This ends up being a complex question that's unique to every building, and needs to take into account the architecture, the soil (reinforced concrete is used in footers/pilings) and a ton of other factors.
But what makes it extra scary? In most countries there is always a company that will tell you "Yes, it can be done, and for less than the cost of replacing the building". Even in the USA this happens and then we have deadly condo building collapses.
I say "most countries" because I hope that there's some country where they have tight enough regulations to prevent it. Like maybe Norway or Luxembourg or something.
In the US the accounting treatment of a building (depreciation) is, IIRC 40 years. So a century is considered longer than the typical lifespan.
Looking around Palo Alto, there are a fair number of wood frame building ps of about a century in age, and a LOT built in the 1950s. I dont know about survivorship bias, but a century seems like a pretty reasonable assumption.
(I was built in the 1960s so appreciate the thought that I might still be going strong in the 2070s, thanks!)
European acquaintances often poke fun at our North American wood frame construction, noting their concrete houses are stronger and "will last for centuries." Are they wrong? I know there are a lot of stone buildings that had excellent longevity but those are made of large stone blocks instead of reinforced concrete. And that's just walls, floors in old houses are usually spanned by wood beams (there goes the wood again).
We have wooden houses here in Norway that are considerably more than a hundred years old and wooden churches from Viking times nine hundred years ago. My house is timber frame, built in 1952, and there is no reason why it should not last until the end of this century. But many US timber frame houses are built on the assumption that they will be demolished after only forty or fifty years and built accordingly.
I think a lot of houses are made of bricks, not necessarily reinforced concrete. Appartement blocks on the other hand is all reinforced concrete I believe.
Good point - yes the walls not necessarily, but floors. There used to be a common community activity similar to "barn raising" - "building the plate" where after the homeowner DIYd the molds and rebar, the neighbors would come together and mix and pour concrete over the span of one day. Fun times.
> Appartement blocks on the other hand is all reinforced concrete I believe.
Five-over-one is probably the most common new residential apartment building variant in the US today, it’s a single story of concrete (commercial space or common areas) with 4-5 floor of stick built (wood-framed) apartment housing on top. Developers only build apartment towers when the land is expensive enough to force them to.
In the US four to five stories of wood framed apartments over a primarily wood podium garage is a common construction technique. Type IIIA construction.
On the west coast of America, I would much rather be living in a wood frame house when an earthquake hits. Depending on construction, the house may not survive, but I probably will. Cement, stone, or brick all seem much riskier to me in an earthquake zone.
Properly built concrete will protect you much better. But that is assuming that it is built in accordance with the regulations regarding earthquake resistance.
Probably a lot of buildings will hit the end of the lifespan just as we're going through the other hard bumps of climate change and mass population ageing, compounding the problems.
The majority of our debt, and the majority of our bits, are not really aligned with especially meaningful goals. Why would the future keep them around?
While it's interesting to explore ancient concrete, actually scalable improvements of concrete production will be made elsewhere.
For instance HeidelbergCement has successfully piloted and is currently implementing technologies that bring the CO2 emissions from cement production close to zero. The OxyCal hybrid process that they are deploying in Bulgaria and Belgium will capture >97% of all emissions from the cement production process.
Their CFO gave a talk this summer that I went to. He noted that green energy does not solve their sustainability goals, as about a third of the CO2 emissions are process related. He talked about their capture plans but noted that they are unlikely to be big enough to capture ongoing emissions, essentially admitting that this looks good on slides but would work at scale.
It's their name for combining oxyfuel combustion with amine capture.
Oxyfuel is where you feed the combustion process using pure oxygen mixed with exhaust gas, instead of air. Since you're not pulling in all that nitrogen gas from the air, the exhaust is much richer in CO2, which is then easier to catch.
Amine capture is where you use an amine solvent liquid that is sprayed as droplets into a large column, where the droplets fall down and absorb CO2 into the liquid, while the exhaust gas flows up and leaves the column free from CO2. Then the liquid that collects at the bottom is moved to a stripper cylinder, where the liquid is heated up and the CO2 is released as pure gas.
Specifically for the cement process, using both of these technologies together, it turns out you can do some cool process integration where the byproduct of one is the requirement of the other, and vice versa, so you can make the whole setup a lot cheaper in both CAPEX and OPEX.
Does it turn into rusted out rubble in 100 years like the reinforced concrete we use today?
I remember reading an article about the longevity of Roman concrete. The best they could come up with was the use of volcanic ash and much slower curing in salt water.
You are correct that reinforced concrete will corrode, generally the service life of reinforced concrete is around 50 years if no special measures are taken. This is because the initial alkalinity of the concrete, which protects the steel, will be neutralized over time. This is the same for modern and for Roman concrete.
But many methods are available both to extend life, and to monitor the health of reinforced concrete. The failures that do occur are typically due to negligent maintenance.
The Romans didn't use steel reinforcement, of course, they just used massive concrete structures. We can do the same today, but it would mean no skyscrapers, no long span bridges, etc.
To be able to build the Pantheon, at 43 meters height, they used walls with a thickness going from 6.4 meters at the base down to just 1.2 meters at the top. In addition they reduced the density of the concrete by almost 50% towards the top, by using very light aggregates.
In comparison the Burj Khalifa at 828 meters heigt has less than a tenth of the wall thickness at the base, just 0.6 meters of reinforced concrete wall thickness in the hexagonal frame.
I'm more interested in using concrete for normally sized structures with greater longevity. The Pantheon has stood for 1,900 years. I doubt Burj Khalifa will last much more than 100 years.
Roman concrete is far more durable than modern concrete, reinforced or otherwise. Roman concrete self-heals cracks.
The theory is that the Romans hot-mixed their concrete with quicklime. As soon as tiny cracks start to form within the concrete, the cracks travel to lime clasts. The lime then reacts with water, creating a calcium-saturated solution. That solution then recrystallizes as calcium carbonate, sealing the cracks and strengthening the concrete.
The concrete is cheap. They are going to commercialize a modern formulation. One to watch.
The proposed approach (stacking irregular rocks) is much more labor and probably time intensive, one of the reasons it’s not done any more. Another is the difficulty of estimating the load bearing strength when you don’t have predictable uniformity in the structure. And transporting irregular large rocks is less efficient than gravel.
The good news is these problems can be solved down the road! On the labor side, I imagine robots can be used to do the stacking and transporting. Probably 20 years down the road but definitely within the realm of serious possibility. As for the strength, you can watch and assay each large rock as you place it, and use some test + statistical methods to estimate structural integrity (the current model of pouring slurry into a wooden form is just a simplified version of this anyway, one that can be computed with a slide rule). As a bonus the result could be stronger due to its amorphous structure.
> "For centuries, builders tried to cut costs by limiting how much cement they used and incorporating recycled waste materials into their concrete," says Salvatore Aprea, head of the Acm research group. "The challenge now is to revive these old methods—not for financial reasons but for the sake of our planet."
I put it to you that if it was financially viable and simple ("ancient methods"), it would already be done.
Financial viability is just (ahem) a question of pricing in externalities correctly (i.e. tax CO2). It must happen, when exactly and on what schedule is a very good question.
These old techniques come from a time with completely constraints. Modern techniques are generally all about creating uniformity as far upstream as possible to them deploy cheaper labour/logistics systems/automation downstream. Think about modern masonry. The "bricks" are uniform, the mason man knows exactly how they can be shipped, moved around the site, and individually placed. Concrete mixes are standardized as much as possible and the optimized so that the man with the trowel can work as efficiently as possible.
Working with off cuts/minimally processed stone is harder. You need to be able to manipulate a greater variety of shapes and sizes. Stuff doesn't stack. It takes way more brain and finesse on the work site to make things work well.
These ancient methods are in many ways much more complicated (not simple) than modern methods. But they are complicated in ways that we're just starting to be able automate well (computer vision systems to sort/group/place irregular blocks for example).
I find it a bit hard to imagine you’re the first person to think of this, let alone those people working on it who I’m sure are a bit familiar with concrete then you ?
> Masic and his colleagues were trying to re-create an ancient Roman technique for making concrete, a mix of cement, gravel, sand and water. The researchers suspected that the key was a process called “hot mixing,” in which dry granules of calcium oxide, also called quicklime, are mixed with volcanic ash to make the cement. Then water is added.
> Hot mixing, they thought, would ultimately produce a cement that wasn’t completely smooth and mixed, but instead contained small calcium-rich rocks. Those little rocks, ubiquitous in the walls of the Romans’ concrete buildings, might be the key to why those structures have withstood the ravages of time.
A big reason the Romans stopped using concrete is because they no longer wanted to build projects that needed concrete. r/AskHistorians has a good post on this...
Why Was "Roman Concrete" Not Used for Centuries After the Fall of the Empire?
C'mon it was right there! From the article -- consisting of experts from EPFL's Archives of Modern Construction (Acm) research group,
Archives of Modern Construction Engineering (ACME) :-)
I am glad that this is being investigated. One of the effects of climate warming is going to be a requirement for a lot of walls and being able to make those walls without aggravating climate warming as much as concrete use does now, is good for us. (it sucks the walls will be needed but alas that ship appears to have sailed).
It also means there will be pretty much nothing left of our architecture, all those appartments, offices, museums, bridges, will all need to be destroyed and rebuilt. Not that modern architecture is really worth preserving. But it feels the only thing our era will leave behind is financial debt and bits in the cloud!