[HN Gopher] The problem with reinforced concrete (2016)
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The problem with reinforced concrete (2016)
Author : hrl
Score : 73 points
Date : 2021-05-25 21:29 UTC (1 hours ago)
(HTM) web link (theconversation.com)
(TXT) w3m dump (theconversation.com)
| ConcernedCoder wrote:
| 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...
| idoh wrote:
| 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.
| foolmeonce wrote:
| 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.
| nerdponx wrote:
| > 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.
| idoh wrote:
| 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.
| renewiltord wrote:
| 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.
| hackeraccount wrote:
| 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 - shocker - it's probably a complicated
| trade off that's best adjudicated by the people with the most
| skin in the particular game.
| cle wrote:
| Not so straightforward. Sometimes we throw things away to
| make room for better things.
| GoToRO wrote:
| 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.
| dwighttk wrote:
| "This is going to last 1000 years!" ... "It only lasted 50" ...
| "I don't know if it is really a problem..."
| raylad wrote:
| 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.
| diegocg wrote:
| Unless you are living in a developing country, the answer to
| the second question should be "yes".
| tonylemesmer wrote:
| 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...
| joshuaheard wrote:
| 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."
| Jerry2 wrote:
| > 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 wrote:
| Graphene isn't persistent though - it burns.
| dmckeon wrote:
| No mention of epoxy coatings for steel rebar? Look for green-
| coated rebar as you pass highway construction sites.
| brohee wrote:
| 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...
| quickthrowman wrote:
| There's also hot-dip galvanized and stainless steel rebar.
| rsync wrote:
| ... 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 ...
| sgtnoodle wrote:
| It's a coating, but my understanding is that it basically
| creates a battery that keeps the nearby iron from
| deteriorating. Small gaps in the coating therefore don't
| really matter as long as there is still some zinc nearby.
| ddkto wrote:
| 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 wrote:
| 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.
| Xamayon wrote:
| 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.
| brutusborn wrote:
| 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.
| matheusmoreira wrote:
| How can I know if a building was designed to last a long time?
| Is it the materials, the method of construction?
| ajcp wrote:
| Yes.
| matheusmoreira wrote:
| Please elaborate. Which materials and methods?
| 0xbadcafebee wrote:
| There is no way to know other than to ask the architect.
| You can make educated guesses but that still won't tell
| you. Even then it's up to the contractors to have done
| everything properly.
|
| Or if it's made of stone. Stacking giant stones on top of
| each other is a sure-fire way to make a building outlive
| you.
|
| After that, the longest-lived buildings that I am aware
| of are made of wood. The catch is they've been rebuilt 50
| times, once per time they burned to the ground.
|
| After those, the longest-lived buildings are made of
| Roman concrete that we can't reproduce. (To give you an
| idea how insane Roman concrete was, you can go kayaking
| north of Naples, and kayak through a concrete Roman
| building that is _sitting on piles in the Mediterranean
| sea_ )
| minikites wrote:
| Is it a good thing for society to directly incentivize the
| construction of longer lasting buildings?
| Taek wrote:
| Maybe sufficient to require that construction put down enough
| money to cover deconstruction and cleanup when building.
| sesuximo wrote:
| IMO yes; if the materials cost a lot of time/carbon/resources
| to produce, then we should make them last!
| renewiltord wrote:
| No, time is not an externality: maybe time under
| construction yields to disruption to neighbours under
| construction and we can charge for that. Resources are
| fully internalized. If a building needs x sand and another
| needs 2x sand, the second will pay twice for sand. Carbon
| is externalized, but that's a general problem. How do we
| know that making a lasting building is better/worse than
| having the building not exist / exist and having people
| driver farther / closer?
|
| Simple, for externalities, you directly charge for the
| externality.
|
| All these stop-gap "it costs carbon, so we must make it
| last 50 years" is like placing massive `if-then-else`
| statements throughout your codebase and then being
| surprised when the emergent behaviour of your program
| somehow results in uglier, more carbon polluting, sicker
| buildings that are now 100 years old and imposing massive
| costs on society around them.
| autokad wrote:
| I agree. Its like one of the few things we can give future
| generations. 'sorry about the debt, but heres some
| buildings'
| wahern wrote:
| > 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.
| rdiddly wrote:
| 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.
| opwieurposiu wrote:
| 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.
| rsync wrote:
| 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 ...
| rdiddly wrote:
| Yep and even that zinc coating has a lifespan.
| throwawaysea wrote:
| 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.
| ajcp wrote:
| 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.
| alcover wrote:
| There are also interesting new methods to monitor corrosion
|
| Then what ? Can you repair or replace parts of a building ?
| quickthrowman wrote:
| Yes, why wouldn't you be able to replace parts of a building?
| ska wrote:
| They fairly regularly do localized repair on some structures
| (e.g. bridge/overpass).
| 0xbadcafebee wrote:
| 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.
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