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)-- Voice, Aug 04 2007 Speaking of which http://www.aardrock...hers.com/index.htmlOh, wait a moment, it appears to be vaporware... [DrCurry, Aug 06 2007] 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?
Did you envisage the "slices" being brick- sized or what? Where and how would they be used?
*I hope this translates without ambiguity.-- MaxwellBuchanan, Aug 04 2007 Old concrete is routinely recycled for use as paving material.-- nuclear hobo, Aug 04 2007 How do you mean "paving" - as in slabs for sidewalks?-- MaxwellBuchanan, Aug 04 2007 I can't see any way this would me workable in the U.S., given the liability implications.
Manufacturer (#1) makes a batch of concrete that turns out to be slightly substandard.
Contractor (#2) makes a building using it for an initial purchaser.
The Initial Purchaser (#3) sells the building to someone else.
The last owner (#4) eventually decides to have the building taken down.
The demolition crew (#5) remove the concrete and sell it to a recycling firm.
The recycling firm (#6) sells the reformed material to a new contractor.
The new contrator (#7) uses the material in a building which later collapses because the concrete was sub-standard from the get-go.
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.-- supercat, Aug 04 2007 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.-- MaxwellBuchanan, Aug 04 2007 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.-- supercat, Aug 04 2007 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.
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.
Don't reclamation yards exist in the US?-- MaxwellBuchanan, Aug 04 2007 I've just clad a wall of my flat in 17th C reclaimed brick. It looks great and it supports nothing.-- wagster, Aug 04 2007 I've actually had this idea before myself. [+], therefore.
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.
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.-- Ned_Ludd, Aug 06 2007 [Maxwell] Do I get a prize for spotting the word 'elephant' in your last anno?-- hippo, Aug 06 2007 Chopping up concrete is neither easy or pleasant. Diamond saw blades are not cheap either. Purple monkey dishwasher.-- the dog's breakfast, Aug 06 2007 [Hippo] yes - you get a prize. Sadly it is is metaphorical, but wel-earned nevertheless.-- MaxwellBuchanan, Aug 06 2007 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.
There's plenty of demand for hardcore in irregular lumps.
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.-- Cosh i Pi, Aug 07 2007 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.-- wagster, Aug 07 2007 What was odder is that I had written an otherwise cogent sentence around the word "elephant".
There is also a "hippopotamus" languishing in the annotations, I note. And indeed an "is is".-- MaxwellBuchanan, Aug 07 2007 //Concrete is generally particularly nasty stuff to saw.// Especially when laced with steel reinforing bar.-- nuclear hobo, Aug 08 2007 Not just elephant, but "elephant of strength", which is a formidable sort of beast.
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.-- bungston, Aug 08 2007 //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.-- MaxwellBuchanan, Aug 08 2007 [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.-- Cosh i Pi, Aug 10 2007 random, halfbakery