The problem with reinforced concrete (2016)

https://theconversation.com/the-problem-with-reinforced-concrete-56078

by hrl • 5 years ago

166 186 5 years ago

theconversation.com

dmckeon• 5 years ago

No mention of epoxy coatings for steel rebar? Look for green-coated rebar as you pass highway construction sites.

quickthrowman5• 5 years ago

There’s also hot-dip galvanized and stainless steel rebar.

rsync2• 5 years ago

... with stainless being a much better (but much more expensive) choice.

HD Galvanized is a coating whereas stainless steel is a different material - it is stainless all the way through ...

ddkto• 5 years ago

Epoxy coating is falling out of favour, as it has two downsides: 1) is has a weaker bond to the concrete, so you need longer bars to transfer the force, and 2) if it gets damaged (scratched or cut), the corrosion will concentrate at that location, and you are more likely to end up with a fully rusted-through bar, as opposed to a small amount of corrosion spread over the whole bar.

Reinforced concrete is much like clothing: a stitch in time saves nine. With regular cleaning and maintenance, it can last much longer than if you just let it deteriorate.

chairmanwow1• 5 years ago

For some reason I stumbled across a wild YouTube channel by a guy named Tyler Ley that is “crazy for concrete”. If I remember correctly he’s a civil engineering professor and has a lot of fascinating videos about concrete.

In particular one about epoxy coated rebar that gives interesting notes about why it has its problems: https://youtu.be/xVDy84rR5Z8

I had a great couple of days learning all about the complexities of concrete through his videos.

anitil• 5 years ago

He is fantastic! I was really struggling to find his name again.

Xamayon• 5 years ago

That certainly helps, but in some cases such as when the rebar is under tension the epoxy coating can result in unexpected failure if there is any damage to it. The small areas lacking coating are corroded more aggressively, resulting in deep pitting which can weaken the bar to the point of failure.

brohee• 5 years ago

It's pretty dependent on skilled people using it, every ding must be properly repainted before the concrete is poured, so extra care must be taken transporting and laying it.

https://www.ijser.org/researchpaper/Epoxy-Coated-Versus-Galv... has a lot more, but clearly not a panacea...

brutusborn2• 5 years ago

This should be an economics piece, not an environmental piece. The author states that "one of iron’s unalterable properties is that it rusts" yet further on acknowledges the existence of stainless steel.

There's nothing wrong with reinforced concrete, but the incentives to produce long lasting buildings are not there. The cheapest bidder will generally win and their building will last the "design life" of the building, but often not much more. The simplest way to change this is to extend the design life, which would result in stainless steels or another more expensive material being used in this application.

minikites• 5 years ago

Is it a good thing for society to directly incentivize the construction of longer lasting buildings?

sesuximo• 5 years ago

IMO yes; if the materials cost a lot of time/carbon/resources to produce, then we should make them last!

Taek• 5 years ago

Maybe sufficient to require that construction put down enough money to cover deconstruction and cleanup when building.

lurquer2• 5 years ago

Good point.

What hubris for a landowner to assume there will be a need for a building 1000 years hence.

Buildings aren’t usually demolished and replaced because they are dilapidated; rather, it’s because the new owner has a different need (and a different aesthetic.)

A building that takes 1000 to crumble is just as a much a blight — maybe more — as a plastic bottle that takes 10,000 years to crumble.

brandmeyer2• 5 years ago

One reason to prefer shorter-lasting buildings is to encourage higher density over time.

matheusmoreira2• 5 years ago

How can I know if a building was designed to last a long time? Is it the materials, the method of construction?

ajcp2• 5 years ago

Yes.

WalterBright4• 5 years ago

One way is to see how the building protects itself from water. Water is the great destroyer of buildings.

A flat roof, for example, is very prone to leaking, which when not constantly taken care of will wreck the building. Another is if the roof keeps water off of the walls (how big the overhang is). Many buildings have eaves that are an inch or two. The exterior walls of these buildings won't last.

Any building on a flat area near a river is going to flood. Any building without proper drainage around it is going to rot away.

Wood shingles need constant maintenance or goodbye to the building.

wahern• 5 years ago

The author states that "one of iron’s unalterable properties is that it rusts" yet further on acknowledges the existence of stainless

Even stainless steel rusts, just more slowly. Roughly 10-100x more slowly, judging by https://www.nrc.gov/docs/ML1124/ML112490377.pdf and https://www.mdpi.com/2076-3417/10/23/8705/pdf.

peteradio2• 5 years ago

That doesn't smell right. I've got 15 year old knives that have never once shown signs of rust. A pair of old scissors left in the rain rusted in under a day. That puts the factor to closer to >10000 than 10 or 100...

mrfusion2• 5 years ago

What are car mufflers made of that they resist rust so well?

strken• 5 years ago

I went and googled the most obvious solutions I could think of for protecting rebar from rust.

- dipping rebar in epoxy is sometimes done, but a single nick in the coating causes all the erosion to concentrate in that one spot, so it can be more dangerous than just uncoated rebar

- galvanised rebar works much better than epoxy, and resists corrosion at lower pH levels than normal iron, but may result in more metal loss under some conditions

- sacrificial anodes (as per the article) can and are used, but exactly how is quite complicated: if they're embedded in the concrete, the zinc breaks down into substances that can weaken it

- concrete is naturally alkaline, with cement being manufactured partly from lime, and this protects the rebar, but too high a pH causes other problems in the concrete itself, so you can't just dump alkaline substances into the mixture forever

- you can apparently use fibreglass as rebar, but I have no idea if it's any good, or what happens to fibreglass if you leave it embedded in concrete for a century

brutusborn2• 5 years ago

You are correct for certain definitions of 'stainless steel.' However as other comments have alluded to, some grades of 'stainless steel' will rust so slowly that they will survive orders of magnitude longer than recorded history. For example, some "Hyper-duplex" stainless steels are designed to resist corrosion in seawater that is above boiling point. This steel is probably not economical for building construction, but if you wanted it to last for millenia, it may be the optimal choice.

quickthrowman5• 5 years ago

The cheapest bidder will generally win and their building will last the "design life" of the building, but often not much more. The simplest way to change this is to extend the design life, which would result in stainless steels or another more expensive material being used in this application.

Just FYI, on a ‘plan and spec’ construction project, all material is specified by the architect and engineers. If the project specs say you have to use stainless steel rebar, then even the low bidder will have it included.

singlow• 5 years ago

But on big projects, doesn't the architect often work for the bidder?

mdgrech23• 5 years ago

I'm not in traditional engineering but I always felt like "specs" only tell part of the story. In your example they may use stainless steel but cut corners by using less of it than they really should or maybe just use really cheap stainless steel which leads to other problems.

I've noticed manufacturing companies like big auto will try to solve for this by creating more specs for parts provided by suppliers but that's a losing battle as its always a race to the bottom. Plus now you need large testing teams to verify parts meet all these different specs. Maybe some percentage of the parts do - what do you in that case? The whole process can be a mess.

kortex3• 5 years ago

There's also basalt fiber reinforced concrete. Stronger than rebar and doesn't rust.

readflaggedcomm• 5 years ago

But steel's more ductile, so basalt-reinforcement failures are less gradual, providing less margin of safety or ability to patch repair (all things being equal). It's more similar to glass reinforcement (both available in bars, mats, and fibers), which is seeing increased use, but better than glass.

Animats• 5 years ago

Stainless steel rebar is quite real, and becoming more common for bridges. It's essential for concrete exposed to salt water, which includes bridged de-iced with sale. Order now.[1][2]

There's epoxy-coated rebar, but that's on the way out. Quebec has already banned it. One scratch, water gets in, and corrosion starts. Also, the epoxy can be damaged by UV, like when there's a stack of rebar out in the sun.

[1] https://www.outokumpu.com/en/products/long-products/rebar

[2] https://stainlessrebar.com/

triggercut• 5 years ago

To build on your very important last point. Material handling procedure is something that I haven't seen mentioned here that is a very real issue too. Similar to agriculture or pharma where entire stocks can be thrown out if stored incorrectly, in many construction and manufacturing contexts your steel can be thrown out for not being stored correctly too (i.e. left at the port in the open for extended periods, or mixed with other metals). Not just steel, but lots of building products (like membranes) will usually be specd with how they should be stored before use.

rsa40462• 5 years ago

"This means that concrete structures, for all their stone-like superficial qualities, are actually made of the skeletons of sea creatures ground up with rock. It takes millions upon millions of years for these sea creatures to live, die and form into limestone. This timescale contrasts starkly with the life spans of contemporary buildings."

This description is fairly accurate. The CaCO3 (used as a source of calcium in the cement component of concrete) is completely decarbonated in a 1450°C kiln in the process of cement manufacture, combined with silica (from shale) +/- SO4 (from gypsum) and sintered to form an anhydrous calcium silicate (clinker: e.g. tricalcium silicate, Ca3SiO5, ‘alite’), then powdered (e.g. ordinary Portland cement, OPC). The skeletal limestone is long gone — and the above decarbonation step is the reason cement manufacturing process is a significant GHG source (in addition to fuel consumption by the kiln itself).

Mixing water with the powdered clinker generates a very rapid, exothermic, partial dissolution of the primary silicate. The rapid release of silica results in nucleation and growth of calcium silicate hydrate (CSH) plus Ca(OH)2. CSH binds the remaining unreacted solid mass together, giving cement its durability and strength.

rsa40462• 5 years ago

Sorry, typo: fairly inaccurate

lamontcg• 5 years ago

Yeah that section seemed like creative writing and stretching for a metaphor. Kind of detracted from the overall point about rusting infrastructure.

opwieurposiu• 5 years ago

Reinforced concrete has achieved regulatory capture via the building codes. If you try to build with anything non-standard, prepare to deal with a mountain of red tape.

rsync2• 5 years ago

I don't believe you would encounter any red tape if you used hot-dipped galvanized rebar - I believe it is code-compliant everywhere in North America.

It is, however, more expensive ...

rdiddly2• 5 years ago

Yep and even that zinc coating has a lifespan.

Jweb_Guru• 5 years ago

Given past "innovations" in the space of building materials, I think this is just about the last place people should be complaining about needing to comply with regulations...

leoc• 5 years ago

And in fact it seems that one attempted fix to the corrosion problem, epoxy-coated rebar, has already proven much worse than uncoated rebar at dealing with rust in production: https://www.youtube.com/watch?v=xVDy84rR5Z8

(That, BTW, is from a channel devoted to concrete made by a civil-engineering academic who specialises in the stuff https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ )

alcover• 5 years ago

  There are also interesting new methods to monitor corrosion

Then what ? Can you repair or replace parts of a building ?

ska• 5 years ago

They fairly regularly do localized repair on some structures (e.g. bridge/overpass).

quickthrowman5• 5 years ago

Yes, why wouldn’t you be able to repair replace parts of a building? You cut out the bad parts while reinforcing structural elements temporarily while the replacement work is performed, then you remove the temp supports.

New footings, beams, walls, etc are constructed or installed to replace old ones all the time.

Say you have a bad concrete footing for a post. You use jacks to support the beam on both sides of the post that sits on the footing, remove the post, remove the footing, pour a new footing, and then put a new post in and remove the beam jacks.

fnord77• 5 years ago

I saw this done in downtown SF to the old macy's men's building. they cut off the facade, cut the concrete back a bit but left the old rebar protruding some and grafted new concrete/rebar on to it.

idoh2• 5 years ago

I don't know if it is really a problem, more like a tradeoff. Reinforced concrete costs less and enables shapes that are impossible without it, with the downside that the buildings last 50 years instead of 100+ years. The present value of a building that lasts 50 years is not that much different that the same one that lasts 100 years.

With that in mind, it makes perfect sense to make an office building out of reinforced concrete.

nerdponx• 5 years ago

The present value of a building that lasts 50 years is not that much different that the same one that lasts 100 years.

That's a problem in and of itself, IMO. Construction is tremendously resource-intensive. We should not be building "throwaway" buildings.

cle• 5 years ago

Not so straightforward. Sometimes we throw things away to make room for better things.

idoh2• 5 years ago

A - The concept of present value isn't a problem, it's more like a fact, derived from the time value of money. It's like saying gravity is a problem.

B - Construction is resource intensive, no doubt about it. Without this technique the costs and resources would go up, double?, more? Many structures we take for granted, like freeway overpasses, would be impossibly expensive.

renewiltord3• 5 years ago

The classic problem is similar to that of that other famous future-proof thing: wifi in hotels. All the fancy hotels that got high-end wifi in the 2000s had shitty Internet for a long time because it was too hard to redo.

Sometimes, building to throw away is the best model. If something is so resource intensive in a way where the externalities are not appropriately mitigated, the right way is to tax the externalities, not to go after specific things.

If these builds were too expensive to build, they wouldn't be built.

hackeraccount• 5 years ago

That's a bit like the one horse shay. It's not how long it lasts but what are the costs associated with it lasting any given length of time. Nothing lasts forever but say buildings lasted 500 years - Suddenly 500 years from now all buildings have to be replaced? And what's the cost of this 500 year building? Is it 10 times as much? Or is it 5 times as much?

Saying we shouldn't have buildings that only last 50 years but rather they should last 500 is like saying they shouldn't last 50 years but instead 5. Maybe. Maybe 5 makes sense.

My assumption would be - shocker - it's probably a complicated trade off that's best adjudicated by the people with the most skin in the particular game.

majormajor• 5 years ago

That's a problem in and of itself, IMO. Construction is tremendously resource-intensive. We should not be building "throwaway" buildings.

If population levels change, up or down, we are going to have to be continually adjusting our usage of space to account for this. Making it easier to modify and/or tear-down-and-rebuild would make things a lot more efficient there. You'd need some policy changes too to fix the problems of, say, homeless people sleeping outside empty office buildings, but getting construction costs down would be a huge part of this.

We shouldn't be so arrogant to assume we are planning the right construction to serve us well for hundreds of years.

dylan6042• 5 years ago

If the buildings last a shorter amount of time, then more construction is needed which provides jobs. /s

dwighttk• 5 years ago

“This is going to last 1000 years!” … “It only lasted 50” … “I don’t know if it is really a problem…”

raylad3• 5 years ago

It will become a very big problem if/when buildings start collapsing with people in them.

I grew up partly in an 18 story reinforced concrete building built in the 1920s. The apartment I lived in was recently sold for several million dollars.

Once, when there was a leak and the plaster came off, the underlying concrete was exposed and it scraped away like very weak sandstone.

How strong is the building and when will it collapse? Does anyone know? Is anyone testing?

I think the answer to both of those questions is "no". Everyone seems to assume they will stand forever. They won't.

diegocg3• 5 years ago

Unless you are living in a developing country, the answer to the second question should be "yes".

robocat2• 5 years ago

Does anyone know? Is anyone testing?

In first-world countries/states with earthquakes, the answer to this is often yes and yes.

A good article from 2000 in Christchurch discusses the issues: https://www.canterbury.ac.nz/media/documents/event/Hopkins-L...

The article is relevant because Christchurch had a major earthquake in 2011. I know of quite a few older buildings that were retrofitted that did not even need to be demolished (most buildings are designed to just survive a major earthquake, but often they need to be demolished due to damage, similar to writing off cars after accidents).

Christchurch did have regulatory failures because many older buildings were known to be unsafe (e.g. only meeting 10% of current code/regulations), but owners could defer fixing them up to code almost indefinitely. But that regulatory failure is being addressed in other parts of the country e.g. Wellington.

The South Island of New Zealand is overdue for a magnitude 8.2 Earthquake which will devastate many towns on the West Coast, and will affect the whole country indirectly. https://www.stuff.co.nz/the-press/news/90364889/magnitude82-...

You can sometimes see where concrete of a building has been tested for example a circular hole about 10cm across is left where a sample was taken.

If interested, next time you meet a civil engineer or someone working in the relevant department that deals with the building codes will often know relevant details about your location.

foolmeonce• 5 years ago

Having 1 in 25 buildings being completely rebuilt at all times and another 1 in ~5 getting renovations is extremely annoying.

I think buildings that are too regularly under construction should carry some tax penalties, instead renovating non-durable buildings to tastelessness is a way to save on property taxes, get tax deductions and try to pressure tenants out to get the latest upscale rates.

woah• 5 years ago

Huh?

GoToRO2• 5 years ago

I spoke with a builder, and the concrete starts to decay in 50 years. Until then it's like new. It lasts much longer than that, 100+ years is not a problem.

wonder_er• 5 years ago

I suggest the builder may have suffered from some motivated reasoning.

Concrete not starting to decay until 50 years has passed is the exception, not the rule.

barrkel• 5 years ago

The problem, as per the article, isn't concrete. It's rusting steel inside reinforced concrete.

oxfeed65261• 5 years ago

To be clear, the article is discussing reinforced concrete—concrete made with steel rebar, which rusts.

Unreinforced concrete can and does last for many hundred years. Reinforced concrete, not so much.

FTA:

“Early 20th-century engineers thought reinforced concrete structures would last a very long time – perhaps 1,000 years. In reality, their life span is more like 50-100 years, and sometimes less. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years.”

rsj_hn• 5 years ago

With 30 year fixed rate mortgages at 3%, I'd say the present value of a building that lasts 100 years is 23% more than the present value of a building that lasts 50 years. It's certainly worth it to spend more for a longer lasting building.

tonylemesmer• 5 years ago

Tangentially related. This company using graphene laced concrete to reduce the amount of cement required. And eliminating steel. New product called "Concretene".

[0]https://twitter.com/Paul_Denney/status/1397132479144812544

[1]https://www.punchline-gloucester.com/articles/aanews/glouces...

Jerry22• 5 years ago

graphene

Graphene has its own set of problems. Namely, it can be toxic to humans. [1] And who knows what massive quantities of graphene in concrete will do to an environment 10-20 years after the building's construction. Even demolition with explosives will probably be problematic due to potentially massive clouds of nanoparticles it could create.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5039077/

XorNot• 5 years ago

Graphene isn't persistent though - it burns.

kortex3• 5 years ago

Namely, it can be toxic to humans

Uhhhh the rest of the stuff in Concrete isn't great for human health, either, or as my cousin likes to say, "it sure ain't vitamins."

Quartz dust and silica sand are really bad for the lungs, likely carcinogens. Lime is caustic. Dust in general is bad to breathe.

These are well-known hazards in demolition and mitigation techniques already exist.

joshuaheard• 5 years ago

The contractor I used on a remodel spoke of carbon fiber reinforced concrete, which I googled, and it's called "carboconcrete". It is described as, "a highly stressable lightweight composite construction that combines special fine grain ultra high-strength concrete and carbon fibers."

throwawaysea• 5 years ago

An interesting read, albeit with information that is likely familiar to many. Given the issues surrounding concrete with greenhouse gases, recycling, and landfill contribution, it seems that timber (https://arstechnica.com/information-technology/2012/05/wood-...) may actually be a more viable alternative. We have techniques to grow it sustainably, it can be a carbon sink within a city, and it is mostly natural wood so it can decompose.

As an economics exercise, it may also be interesting to price in the cost of dismantling/disposing of construction materials into the initial construction cost. I wonder if doing so will steer materials development away from composites that are difficult to recycle towards something new.

rektide• 5 years ago

This is the only post I found mentioning recycling, and I think that's a key factor in this all. My generation is going to be the first saddled with a lot of old buildings we have to deal with. We are going to have to get good at recycling buildings.

My understanding that the economics have already really pushed us massively forwards in the past decade or two, that we are far more aggressive about recycling construction aggregate[1].

The article has a nod midway through to these concerns,

The many alternative materials for concrete reinforcement – such as stainless steel, aluminium bronze and fibre-polymer composites – are not yet widely used. The affordability of plain steel reinforcement is attractive to developers. But many planners and developers fail to consider the extended costs of maintenance, repair or replacement.

This definitely seems like a huge societal blind-side to me. As much as it's an issue of planners and developers, I feel like there's a consumer lack of understanding. The invisible hand can't push effectively here, can't reward the builders doing it right adequately. In part because society is not aware, doesn't know what to ask for, doesn't have standards, doesn't view & comprehend the role of maintenance & ultimately recycling. These are far off things.

As my generation starts to see the limits of sustainability, see where so very many many creations begin to become risks & hazards & losses rather than values, we may develop some sense, but switching over into a fear-based emotional reaction isn't necessarily a great fix. Trying to give us all a picture of the life-cycle, the costs, the trade-offs; that seems like the necessary task. Regulating our ability to see & ascertain.

Hopefully we just get better & better about recycling. It'd be so interesting to see how reinforcements are extracted from construction aggregate today. Stainless steel reinforcement isn't expensive... if you can safely view it not as a sunk construction cost, but as a semi-long term loan for a building. Where-as more advanced materials like fiber-polymer, I tend to imagine, may have wonderful characteristics in use, I also tend to imagine them as likely having less recoverability. Steel: material we know how to re-cast.

[1] https://en.wikipedia.org/wiki/Concrete_recycling

samatman2• 5 years ago

As a point of interest, recycled concrete is referred to as "urbanite".

https://www.concretenetwork.com/concrete/demolition/urbanite...

It's definitely a case of downcycling, though: there's a lot less we can do with urbanite than with cement and aggregate in their original forms.

ClumsyPilot4• 5 years ago

"I feel like there's a consumer lack of understanding. The invisible hand can't push effectively here"

When I asked the real estate agent what's the expected lifespan of a house, she looked at me like she's just seen an alien. In fact, half the time, they don't even get the floor space right and every floor plan comes with a disclaimer "maybe this is wrong"

jaza• 5 years ago

Yes, I've heard about engineered timber - https://blog.i1machines.com/four-reasons-why-engineered-timb... - I'm no expert, but it sounds like one of the better alternatives to reinforced concrete. Among its many benefits, it theoretically has a much longer lifespan than reinforced concrete.

I work right next to a seven-storey office building in Sydney, that's built almost entirely with engineered timber - https://architectureau.com/articles/australias-first-commerc... - ever bigger and taller such buildings are going up, bit by bit, around the world.

ajcp2• 5 years ago

I'm not sure this article does a good job of highlighting "the problem with reinforced concrete" than it does "the better attributes of material x with y over concrete". Reinforced concrete seems to do exactly what it's intended to do for the designed life of that intent, with some very well known trade-offs coupled with some brilliant strengths.

Sure, compared to other materials it might not be as: long-lasting, cheap, sustainable, but as in all things it seems one can only pick two.

teh_klev• 5 years ago

Reinforced concrete seems to do exactly what it's intended to do for the designed life of that intent

Until there's apparently "no money" to replace the structure after its design life time. Thinking in decades of life span for many of these structures is very short sighted. I think the article mostly gets that across i.e. re-enforced concrete is hard to recycle and their life span is often within that of a human life. We should be able to do better and create large scale structures that can not only serve a purpose over several life times, but can be added to or enhanced rather than demolished.

The only reason we keep building these time limited structures is because building codes still allow this, which leads to easy short term profits, and it's "someone else's problem in 50-60 years time". There's no incentives.

LegitShady4• 5 years ago

Municipal and other budgets are already highly strained all over. If the cost of an interchange or large culvert goes up 2x to increase its lifespan, you can do less work now. Your level of service goes down now.

There are trade offs to everything. Build something much more durable, pay twice as much, do half the work in a season and get more complaints. We already aim to build infrastructure based on estimates of future loading - how much traffic, what kind of trucks at what weight, etc.

We'd all be much better off increasing maintenance budgets to extend the lifespan of existing structures without completely re-doing them at multiples of the current price.

ggcdn5• 5 years ago

I think many commentators, including yourself, have an erroneous assumption about structural design which is roughly summarized as: the inputs are known with certainty. This is really not the case especially when considering natural hazards like wind and earthquake. What happens to your 500 year lifespan design when we discover new information about earthquakes at the site. This is happening regularly with every building code update. Our design loads change To reflect the latest research on things like seismicity. When you factor in this consideration, it may actually make a lot of sense to only design for shorter lifespans.

0xbadcafebee3• 5 years ago

Once you know about spalling, you see it everywhere. There's a train bridge straddling a high-traffic road in Philadelphia that is crumbling year after year, and I think the freight company that owns it still hasn't done anything about it. We just have to hope there's no traffic when it eventually fails.

ConcernedCoder• 5 years ago

Tangentially related. The romans figured out that volcanic ash and salty sea water made cement that actually gets stronger with age instead of breaking-down after 50 years: https://www.sciencemag.org/news/2017/07/why-modern-mortar-cr...

samatman2• 5 years ago

I'm not sure, but this probably fits the facts better: the Romans figured out how to make cement with volcanic ash and salty sea water, and it turns out that it gets stronger with age.

It's not clear to me that the Romans knew that, or that it informed their choice of that building material. I suspect it's more that they built with what they had, and got lucky that it turns out to be incredibly durable stuff.

courtf• 5 years ago

The Roman scholar Pliny the Elder described underwater concrete structures that become "a single stone mass, impregnable to the waves and every day stronger."

https://www.nature.com/news/seawater-is-the-secret-to-long-l...

Sounds like they knew.

KMag• 5 years ago

At least those samples to be mixed just right to survive over 1,000 years had a mixture that got stronger with age. There's a big element of survivorship bias.

Also, due to a lack of reinforcement, Roman concrete structures, at least those that survived, avoided putting concrete in tension. Roman concrete won't last very long in areas of buildings that are under tension. Edit: the implication being that using Roman concrete would make many modern building designs unworkable, particularly taller thinner designs that sway a bit in the wind.

gamblor9562• 5 years ago

Left out is the reason we don't use the Roman mix today: it takes years for the concrete to set; Roman concrete is weaker for the first decade compared to modern cement mixes.

Moreover, back in Roman times, Roman concrete was not as strong back as it is today. As the article you linked points out, "Because both minerals take centuries to strengthen concrete, modern scientists are still working on recreating a modern version of Roman cement."

ggcdn5• 5 years ago

Very good point. Most building projects actually use admixtures to get as rapid strength increase as possible, even if it means less strength increase long term, because the early strength determines when you can strip formwork and start building the next floor. Builders want concrete reaching ~18MPA in like <2 days.

alliao• 5 years ago

imagine a world where new houses are cheaper and as it ages and surrounding develops houses become more desirable and stronger

rdiddly2• 5 years ago

Guessing at "the problem" before reading the piece, I figured it would be that it's weak in tension. Not quite what they said, but that does happen to be the reason you need rebar in the first place.

Concrete however does an excellent job of externalizing its costs: into the atmosphere, forward through time, and onto other people. So it will remain popular so long as those remain the expressed values of society.

Edit: Awww, boo hoo!

gonzo41• 5 years ago

Surely this could be the moment for glass fiber reinforced concrete to take center stage. It really should just last forever.

scythe2• 5 years ago

My understanding is that alkali-resistant glass is required, but the 18% ZrO2 used means that it isn't much cheaper than using carbon fiber, which seems to have slowly taken over. But GFRC is bright white, while carbon fiber makes a greyish product, so the former is preferred for decoration.

diegocg3• 5 years ago

Given the survival of ancient structures,

Survivorship bias...

scythe2• 5 years ago

Fiber-reinforced cement composites are an interesting alternative:

https://onlinelibrary.wiley.com/doi/pdf/10.1002/suco.2017001...

Some German researchers seem to think that carbon fiber-reinforced polymer "rebar" could be more durable than steel bars with similar cost and lower weight:

https://www.aboutcivil.org/carbon-reinforced-concrete-buildi...

Unfortunately, the terminology is not well standardized, and when you look up "carbon fiber reinforced concrete", you never know if you're going to get a fiber-reinforced cement composite (incorporating carbon fibers directly into the cement matrix) or a CFRP rebar system. Regardless, there do seem to be some improvements on the horizon.

djmips• 5 years ago

Lightweight video. https://www.youtube.com/watch?v=nB9ViglDMmg

pkaye• 5 years ago

I took an engineering course on concrete 30 year ago. There were a lot of additives and techniques described which improve various properties but it seems like 30 years later cost is the biggest consideration.

lmilcin• 5 years ago

Rusting is not the only reason for reinforced concrete to crack and fail.

The concrete and steel have different thermal expansion meaning that, over time, the concrete is bound to develop cracks if there are any changes in temperature.

Another reason why modern structures crack and disintegrate is because they tend to be built from rather large blocks. Whereas old structures were composed from single bricks or stones. The way these were built meant that the structure would crack in multiple places in a way that would allow the movement to be absorbed and distributed throughout the structure and be more easily repaired. See this guy explain it much better than I could: https://www.youtube.com/watch?v=p5qVxAoKwbE

brudgers• 5 years ago

Every human is valuable. Every human has opinions. But every human’s opinions are not equally valuable.

Their are two reasons for steel reinforcing. The steel can withstand tension and concrete can’t. Even worse concrete in tension fails catastrophically, the steel insures it fails ductally.

Masonry had exactly the same problems and all modern masonry structures are reinforced masonry...reinforced with steel, Just like concrete.

Aluminum and other potential reinforcement materials are less suited than steel because steel and concrete have similar coefficients of thermal expansion. Yes steel can corrode and expand and spall concrete. Other materials will spall concrete simply from seasonal and diurnal changes in temperature.

Concrete designs are engineered and like all engineered systems, there are trade offs between cost and performance. And like any design, use may exceed or deviate from specified design parameters.

toss1• 5 years ago

The article makes only a glossed-over mention of fiberglass rebar, which is about 1/7th the weight of steel, faster to install, creates no EM issues, does not rust and costs less. Here's some examples, admittedly from those selling it [1][2], and from other news/ industry sources [3]. also, other fibers such as basalt provide even better properties [4].

All it takes is for architects and engineers to get their heads around new technologies and start specifying better materials. This wouldn't fully solve the CO2 problem of concrete, but it would reduce it by making each ton of concrete last a lot longer.

[1] https://www.owenscorning.com/en-us/composites/pinkbar-vs-ste...

[2] https://www.tuf-bar.com/5-reasons-why-you-should-use-fibergl...

[3] https://www.globalspec.com/learnmore/building_construction/b...

[4] https://www.forconstructionpros.com/concrete/equipment-produ...

Bumgardner• 5 years ago

I wrote a concrete corrosion simulation as part of my graduate research. You can use it at corrosion.smcf.io

pgrddy• 5 years ago

It's also important to note that not all medieval and historic structures survived the test of time. A case of Survival-Bias. As the utility requirements of the structure may change over time,there is no incentive for engineers and clients to design a structure that would last more than 50-100 years. As far as the current building design provisions and policies go by, rebar does it's job.

What needs intervention is to find a suitable replacement for concrete/cement citing it's increasing contribution to global warming.

JimTheMan• 5 years ago

If owners were willing to pay for it, or the government wanted it to happen via the Standards then contractors would build it.

Doesn't change the building process at all, just where they procure from.

The problem with reinforced concrete (2016)

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