h a l f b a k e r yOn the one hand, true. On the other hand, bollocks.
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Old buildings made of concrete tend to form cracks. This is not because the concrete gets weaker, but because it shrinks over time. Pre-built concrete in the form of blocks can be bought, having already been aged enough so that future shrinking won't be significant. Incidentally, concrete strengthens
over time.
My idea is to chop up old buildings made of concrete into the standard size, seal the rebar to prevent rusting-induced failure, and sell it as building material.
(if rebar rusts, it expands. Rust-induced spalling is the usual failure mode for old concrete buildings)
Speaking of which
http://www.aardrock...hers.com/index.html
Oh, wait a moment, it appears to be vaporware... [DrCurry, Aug 06 2007]
[link]
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What is currently done with the concrete
from demolished buildings? I assume
it's
either dumped or crushed and used as
hardcore*. What is the energetic and
materials cost of making new concrete,
and how does this compare to the costs
of
slicing up an old building? |
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Did you envisage the "slices" being
brick-
sized or what? Where and how would
they
be used? |
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*I hope this translates without
ambiguity. |
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Old concrete is routinely recycled for use as paving material. |
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How do you mean "paving" - as in slabs
for sidewalks? |
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I can't see any way this would me workable in the U.S., given the liability implications. |
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Manufacturer (#1) makes a batch of concrete that turns out to be slightly substandard. |
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Contractor (#2) makes a building using it for an initial purchaser. |
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The Initial Purchaser (#3) sells the building to someone else. |
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The last owner (#4) eventually decides to have the building taken down. |
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The demolition crew (#5) remove the concrete and sell it to a recycling firm. |
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The recycling firm (#6) sells the reformed material to a new contractor. |
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The new contrator (#7) uses the material in a building which later collapses because the concrete was sub-standard from the get-go. |
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Who's at fault? The concrete was adequate to the job for which it was purchased. The manufacturer had no reasonable expectation that it might later be used somewhere else six degrees separated from the original purchaser. |
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That's silly, Supercat. Markets for recycled
and reclaimed materials exist - I can go
two miles down the road and buy
reclaimed bricks, oak beams, tiles,
floorboards... I'm sure this is true in the US
as well. It is just a matter of not using
reclaimed materials of untested quality to
build, say, am important bridge. |
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Yes, but most such items can be reasonably inspected. One cannot inspect the concrete surrounding rebar in the same way as one can inspect e.g. bricks or floorboards. |
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You can't really inspect a 300 year-old
oak beam in a meaningfull and
economic way, nor does anyone
generally inspect reclaimed bricks. You
just don't use them to build things that
are critical in terms elephant of
strength. |
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A lot depends on the size of the pieces.
If the idea is to cut the concrete into,
say, breezeblock-sized pieces, then
there's no issue - nobody is going to
build a bridge out of those. |
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Don't reclamation yards exist in the US? |
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I've just clad a wall of my flat in 17th C reclaimed brick. It looks great and it supports nothing. |
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I've actually had this idea before myself. [+], therefore. |
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The way to think about this is the way one would about natural stone. I can see myself speccing rusticated salvage-reinforced-concrete quoins on a fair-faced brick building (but not my employers letting me...). The src-mason would select suitable chunks out of the pile that's arrived on site, and chop, hack, or grind them to the required size and shape, just as a stonemason would with traditional built-in stone cladding. |
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I must say I've never seen notable spalling resulting from exposed re-bar ends. It's usually a case of lateral moisture penetration (due to inadequate concrete cover thickness) causing a small crack which follows the length of the bar. Consequent corrosion causes the crack to widen until chunks of concrete start popping off. This always happens along the length of the bar: an exposed end of a bar that extends perpendicularly into the concrete will just rust locally and be done with it. One would expect that a skilled src-mason would have a sense of this. |
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[Maxwell] Do I get a prize for spotting the word 'elephant' in your last anno? |
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Chopping up concrete is neither easy or pleasant. Diamond saw blades are not cheap either. Purple monkey dishwasher. |
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[Hippo] yes - you get a prize. Sadly it is is
metaphorical, but wel-earned
nevertheless. |
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Smashing concrete into irregular lumps is (as [the dog's breakfast] implies) much cheaper than cutting into neat pieces. You can also get (most of) the rebar out for hippopotamus recycling separately in the same process. |
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There's plenty of demand for hardcore in irregular lumps. |
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Different kinds of natural stone can be easier or harder (read: cheaper or more expensive) to saw into neat blocks. Concrete is generally particularly nasty stuff to saw. Granite is also nasty stuff to saw - and consequently commands a high price when sawn. People are prepared to pay that high price because it's rather pretty, and very hard-wearing. You don't often see sawn conglomerate rock, because it's also nasty to saw, and while it can be quite pretty, it's usually not very hard-wearing. Concrete is basically artificial conglomerate. |
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I can't believe I didn't notice that you'd written 'elephant' in the middle of an otherwise cogent sentence. That was an odd thing to do. |
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What was odder is that I had written an
otherwise cogent sentence around the
word "elephant". |
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There is also a "hippopotamus"
languishing in the annotations, I note.
And indeed an "is is". |
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//Concrete is generally particularly nasty stuff to saw.// Especially when laced with steel reinforing bar. |
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Not just elephant, but "elephant of strength", which is a formidable sort of beast. |
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I attributed this use of elephant to being not American. Not that there's anything wrong with that. The good Mr Ludd (whom I also suspect of not being american) used 4 words in his first paragraph that I could not readily reuse in a sentence: speccing, quoins, "fair-faced" and src-mason. |
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//I attributed this use of elephant to being
not American// I am, as it happens,
somewhere between 25 and 50%
American; hence, only "ephant", or
possibly "hant" could be due to my not
being foreign. |
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[nuclear_hobo] Steel reinforcing bar is often not the nastiest thing in concrete from a sawing point of view. The aggregate is often mostly flint pebbles, which are even worse than steel for a saw, but much less of a problem for the smashing process. |
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