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I'm hoping some structural engineering goons might be able to help me out here. I'm thinking long and hard about the costs and benefits of residential construction with insulting concrete forms. While reinforced concrete can make for a spectacularly strong structure, able to survive anything nature has to throw at it, the source of its strength is also the source of its fatal weakness: the rebar inevitably rusts and causes spalling, cracking and the demise of a once-sturdy edifice in a depressingly short amount of time. Given that reinforced concrete structures are very greenhouse-intensive, can't really be recycled, and are expensive to demolish, I find that problem hard to accept for a single-family home. This is doubly annoying because there are light-frame houses out there made of 2x4s and the like that easily survive well over a century, and failing to meet even the poor standard set by a mediocre stick build adds insult to injury. But the positive characteristics of ICFs continue to impress me, so I want to know what, if anything, can be done to extend the lifespan of a reinforced concrete building out to 125+ years. (That's satisfactory to me; I don't want to build another Pantheon.) I've heard that fly ash has excellent pozzolanic qualifiers that can help improve lifespan, and that there are various forms of galvanized, coated, stainless, non-ferrous (aluminum or silicon bars), and even non-metallic reinforcing bars (GFRP, CRRP, basalt), as well as various structural fibres that can be added directly to concrete mixes, but I have no real idea as to their current practicality or cost. I would also expect that, due to cladding, roofing, and the properties of the EPS forms, the concrete might be at least a little bit protected. A low w/c mixture might help by sucking up water for a while, too, but again I'm at a loss because I can't really predict its effects. Could someone knowledgeable help fill this rank amateur in? How solvable is this problem?.
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Apr 7, 2017 03:38
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John Smith posted:Not going to help out somebody who is so disrespectful of concrete. Brutal (  ). Do you have any better suggestions? JnnyThndrs posted:I would imagine a big part of the reason why concrete isn't used for residential construction in the US has a lot more to do with people's perception of the medium rather than it's unsuitability. Certainly, the key factor here would be the presence of chloride ions and their permeation through the concrete matrix as well as its loss of pH. Obviously a house will have a lower degree of exposure than a bridge deck, but I don't know enough to speculate as to how much less and what kind of implications that might have. I can't really speak to the lifespan of average new construction; I think, given the nature of concrete and the costs of working with it, it should be expected to last considerably longer than OSB and farmed wood and any failure to achieve that would be unacceptable. Given that new wood construction is pretty cheap and far less environmentally damaging*, I think it can get away with short lifespans. *Comparing apples to apples, anyway. Certainly even a basic ICF house will probably outperform a wood house built strictly to code minimums, but there's no reason you can't build wood to be positively frugal on energy. So I'm just talking about construction here.
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Apr 11, 2017 15:33
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P-Mack posted:You can use cathodic protection to extend the life of the rebar. But generally speaking there's no way around being on top of maintenance. Why design a building for 125 years of no one giving a poo poo about it? I'm not sure why there seems to be a common assumption that I had some sort of insane goony idea to build a house that never needed to be maintained. That's not it at all. The intention was to build a house that was in general just like a conventional house, except that it would benefit by having the structural strength provided by concrete rather than dimensional lumber. I'm no Frank Lloyd Wright; making exposed concrete look good is something that I doubt I'd ever be able to accomplish. Even if I could, I doubt I would; I'd rather build something that fits in. My concern wasn't that I wanted to find the right blend of concrete that would allow it to stand forever, unmaintained; it was to find the right blend that would prevent it from being a complete write-off in 75 years. I'm not sure what kind of maintenance could possibly be done on concrete; my understanding was that the concrete would inevitably load up with chloride ions, the rebar would just as inevitably rust, and then the whole thing would be an un-repairable write-off because remediation of in-place concrete costs many times more than simply tearing down the structure and building it again. If that's not the case - if concrete walls are somehow easily patched and fixed when the need arises - then I have been approaching this from entirely the wrong angle, and won't have any problem. But I have a hard idea understanding how the concrete/rebar bond could be broken. I certainly do hope that environmental technology will be better fifty years from now than it is today. With that said, I think that there are diminishing marginal returns. We're already able to build houses in many climates that can be passively heated by solar gains, and in any case it's fairly straightforward to build them to be net-zero energy usage with the use of solar panels. In terms of joules and watts, there simply isn't as far to go over there next 125 years as there was in the last 125. That's inviolable thermodynamics. So I'm not really sure that argument holds any water. If things have changed so much in the last fifty years that it's now totally impractical to build a house to last more than the next fifty, then concrete (barring some sort of other design need that can't be met by wood) is obviously the wrong building material. And I think we'll have lost a great deal by that change. David Corbett fucked around with this message at 04:15 on Apr 12, 2017 |
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Apr 12, 2017 04:07
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