Cob RE: 'used' concrete foundation

[Robert Bolman]

 I am presently completing a 600 sq. foot (outside dimensions) straw bale
and cob house in Eugene, Oregon that I put on a foundation made from
concrete slab chunks.  I was able to get a building permit for it too.
In an urban location at least, concrete slab chunks are a very abundant
resource.  I hadn't gotten through the letter "B" in the yellow pages under
excavation contractors before I had five dumptruck loads delivered free of
charge.  They were so easy to get that I ended up regretting that I hadn't
been more selective to get chunks of a more consistent thickness to
facilitate stacking.  My foundation is three feet wide at the bottom, just
over two feet wide at the top and over three feet tall.  To get it approved,
I had to pour a reenforced concrete bond beam 8" thick on top which tied it
all together and created a level, stable surface on which to build my
structure.  So I did end up using about five yards of concrete in addition
to the cement mortar which I used between the chunks.  But I used a quarter
of the concrete that would have otherwise been necessary to create a such a
tall, robust foundation.  For future structures, I feel that the bond beam
can be substantially reduced.  I also feel that the mortar recipe can go a
little lighter on the portland cement.
Following my lead, Eric Hoel (former CCC intern in Salem) built the
foundation for his cob house out of slab chunks.  He was able to not repeat
several of my mistakes.
One problem is that it is rather labor intensive to build a foundation in
this manner.  Mine took three weeks with a lot of friends coming around.
It's definately not cost effective in THIS economy, but if concrete were
priced to reflect its TRUE cost, building a slab chunk foundation would be
quite cost effective.  I'd even like to see "concrete slab demolition
services" that go around with a slab saw cutting slabs into rectilinear
pieces that could then be reassembled into foundations or paved surfaces
elsewhere.
Robert Bolman