Author Topic: Concrete houses  (Read 1614 times)

Offline calura

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Concrete houses
« on: February 26, 2014, 04:53:12 AM »
I've just been listening to episode 16, and it needs to be said:
3D printing: definitely cool
Concrete houses: not cool

OK, the biggest thing, the thing that made me whimper in dismay, was Jacob suggesting that concrete is more environmentally friendly than cutting wood.  By any measure you can think of, that's not true.  Aggregate may be reasonably easy to come by, but cement takes immense amounts of energy to create.  Worldwide carbon emissions from cement production is more than double that from the aviation industry.  For the most part, concrete is recyclable.  But in order to use it again as concrete, you need a fresh batch of cement to hydrate, and therefore a fresh batch of energy to make it.  In the homes of the future, we need to use less concrete, not more.

This particular concrete is going to be more like a high strength cement slurry.  If you had large volumes of friendly aggregate, you wouldnít be able to extrude it so accurately.  It will have very high cement content, and the sand mix that it uses will have to be very tightly controlled, that is, not something you can dig up from the average local quarry.  Timber, on the other hand, re-grows itself, and is a carbon sink.  As long as itís harvested sensibly, itís just about the best building material you can get.

Looking at the videos, the houses this builds are going to be the same construction as brick houses.  Nearly all houses built in Australia (where I am) are brick, and you know what, it sucks.  Bricks are terrible at resisting lateral forces, they fail without warning (two people were killed in Melbourne last year when a wind gust blew a brick wall down on top of them), they need steel lintels over every door and window, they're not even that great in compression, and they crack at the slightest provocation.  Houses are built with brick because they're cheap, low-tech, and because that's the way it's always been done.  But there two advantages to brick that this new method takes away:
 - The low tech part; it takes a lot of labour to lay bricks, but you learn to do it quite quickly.
 - Most bricks are made from clay, and are less energy intensive to produce than concrete (this obviously doesn't apply to concrete bricks)
So really what they're doing is taking a construction method which is already quite limited, and removing a few of it's good points. 

The only advance I can think of from this method is the accuracy with which they'll be able to place the walls, and maybe the cool smoothness of the curves.  But these aren't big problems that need fixing right now.  They say they'll be able to build vast quantities of housing really fast.  Hmmm, maybe.  But apart from a housing shortage, you know what else developing countries often have?  A large population of under-employed under-educated citizens, that could do with some paying work, and they could easily be taught how to build brick houses.  I would suggest that bringing in a massively impressive and expensive machine run by just a few highly trained workers to build their houses would not be of great value to society.

The huge question, which isn't answered on their web page, is how they're going to stop the concrete from flowing away.  It's all very well to say 'fast drying concrete,' but they're going to need that stuff to remain workable enough to flow through the pipes, and then hydrate within seconds of coming out of the nozzle.  Cement usually takes about 12 hours to hydrate sufficiently and gain any appreciable strength, and until it does that it will flow.  That's why you need formwork.  If it's viscous enough to stay put as soon as it's printed, then by the time the next layer comes along, it will be so well set that the layers will bond very poorly.  Once again, just like bricks.

I don't know about the earthquake thing.... I've never had to invest much time in designing to resist earthquakes, what with Australia being so reliably calm and flat.  But if I were having a house built in an earthquake prone area, I certainly wouldn't use brick, and I wouldn't use unreinforced concrete either.  Surely it can withstand earthquakes if you build it thick enough, but it's not the right material to use if you want to do it efficiently.



I feel kinda bad here; I've often wished you guys would talk more about things that stay still, and when you finally do I'm getting stuck into it... but hey, you were right on with all the drag factors etc ;).  Please do talk more about the stationary things, even if most people don't find them so exciting. I also would love to hear stories about nanotechnology, medical engineering etc.  If they come up. 

Signed, the lone structural engineer...

Offline scikopas

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Re: Concrete houses
« Reply #1 on: February 26, 2014, 08:31:23 AM »
A++++++ review, would read again.  I'm not a structural engineer, but I know enough to know that cement is not a great material for this kind of thing, and I like your analysis.  My only question is what about tensioned concrete? isnt that often useful in cases where you might have a lot of forces in other directions, like earthquakes? I know its used for bridges, but i know nothing about actually building a building...


regarding the environmental impact of portland cement, the manufacturing process is highly energy consuming (like calura said). It involves mining lime, silica, and alumina, grinding them very very small, then mixing and heating them together in a mega-huge rotating furnace that has to reach about 2000C!  in here the calcium reacts with the aluminum, and the whole thing reacts into little rocks called clinker.

Then the clinker are really hot, and have to be cooled quickly, so they cart them up to a very tall tower that has cold air blowing up a center shaft, and drop them from the top of the tower. so they cool on their way down (they need to be cool enough to touch).  Then it has to be crushed a second time to actually come out as cement.

the rule of thumb is (was?) that 1 kg of concrete (thats after its mixed with the sand/pebble aggregate) required production of about 1 kg of carbon emissions.


semi-relevant: I uploaded a video to youtube that looks into a small porthole at a cement factory my class took a field trip to back in 2010. (I love that engineering and science classes still take field trips in university...)
<a href="https://www.youtube.com/v/l4EOtczi2mk" target="_blank" class="new_win">https://www.youtube.com/v/l4EOtczi2mk</a>

PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
my (materials) science podcast: LASER (Let's Agree Science and Engineering are Rad!) twitter @scikopas

Offline calura

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Re: Concrete houses
« Reply #2 on: February 27, 2014, 07:49:14 AM »
Hehe, thank you sir.  It was a lot longer than I thought, so I'd be impressed if anyone did read it again!  You're spot on with the cement production, it's the really hot furnace that just destroys any environmental credentials it can ever have.

Tensioned concrete (prestressed concrete) is good at long spans, like bridges.  That's when they cast ducts into the concrete beams, and once they're in place, they feed great big tendons through the ducts, tension them up really tight, and grout them in place.  That means the whole cross section of the concrete is in compression, which is really efficient.  It's pretty cool, and if they shape the ducts right, the tension actually pulls the beams up flat as well (instead of sagging under their own weight).  Normal reinforced concrete is different; you cast in bars without any tension on them.  Under bending loading the concrete takes all the compression side, and the steel takes the tension side.  Not so cool.

Everyone should do field trips, for their whole lives ;)
« Last Edit: July 20, 2014, 09:19:11 PM by calura »

Offline Ed Lolington

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Re: Concrete houses
« Reply #3 on: April 26, 2014, 12:16:50 PM »
Good thread. I learned a lot!