## Concrete

From: "andrew cooke" <andrew@...>

Date: Thu, 6 Jan 2005 15:20:18 -0300 (CLST)

Nice article about concrete -
http://www.sciencenews.org/articles/20050101/bob9.asp

The translucent stuff is super-cool.  We're waiting for a couple of
concrete coffee tables to be made for our living room.  They're not
translucent, unfortunately, but still pretty cool.  The legs and "support"
are made of iron; on top rests a concrete slab, cast in two colours - a
dark inky centre that "leaks" out into a white surround.  Looks more like
plastic than concrete.

A couple of things that articles doesn't make clear.  First, basic
concrete is only strong under compression, not tension.  Just like stone.
Which is why stone buildings are built the way they are, with arches to
transmit the load through the structure.  Reinforced concrete includes
steel rods to take the tensile stresses, but these stretch under load and
so put an upper limit on the strain (if they stretch too much the concrete
cracks open, water can enter, and things corrode).  Hence the fibres in
Ductal, as described in the article, which replace the steel rods.

The fibres are important for another reason too - a crack grows at the
tip, where stresses are concentrated.  When a crack meets a fibre that is
lying across its path, the stresses are dispersed significantly.  This
stops/slows the growth of the crack, making the material safer.  For
similar reasons sharp corners are avoided in steel structures (the sharp
corner acts as a crack).

One last thing omitted - titanium dioxide is also a common white pigment.
So the "self cleaning" concrete doesn't look shiny and white just because
it's extra-clean, but also because it has white colouring...

From BoingBoing -
http://www.boingboing.net/2005/01/06/nextgeneration_concr.html

(Chilean concrete tables from these people -
http://www.nuevodiseno.cl/ndweb_publi_gatica.htm - although they seem to
have just removed any info from their web site...)

Andrew

PS I'm going to try adding more comments to my posts if I have the time
and something useful to say, because I've realised that otehrs may not
realise why I think things are interesting.  I'm also looking at putting
these posts on a blog.

--
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