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?
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!