Cob RE: Robert Bolman's comments on 'used' concrete foundation

[Don Stephens]

Good info!  Did you have to put any kind of reinforcing between slab
layers?  Did you tie it together vertically other than with the cement
mortar?  Was your top bond-beam the full two foot width of the top of
foundation?  How much steel and what size in bond-beam?  Was this solution
engineered or a seat-of-the-pants requirement from the bldg. dept.? 

I wonder if it would be less labor-intensive to start with your 3' wide
base pieces, then dry-stack the narrower slabs up from there, maintaining
a relatively vertical inside face outset about 8" from the inside line of
your base pieces and farther offsetting the top two rows about 4" more;
(have I lost everyone yet?) then run some 6" x 6" 10-10 welded steel wire
mesh (or equivalent)) up the inside face of the resessed stack from the
top of the base pieces to the top of the foundation, then form an inner
surface ~ 4" in from the inner face of the dry-stack from base pieces to
top (using salvaged plywood, pinned strawbales, cob or whatever), put a
couple of # 4 or 5 bars in that top widened space where the dry-stack is
inset an extra 4" and pour it all with a fairly wet concrete mix and
vibrate it well (manual or mechanical).

What you'd end up with would be a monolithic unit with a 3' base, ~ 4"  of
concrete bonded to and filling the gaps in the dry-stack all the way up
the inside, the poured concrete widening out to 8" for the top 8" to 10"
(two slab thicknesses) to form a bond-beam, and most of the mass and the
entire exterior face consisting of recycled concrete. 

This would greatly reduce the amount of new poured concrete over what
would be used in a conventional footing and 8" foundation wall, save the
labor (and time) of mortaring all the slab chunks together, assure better
lateral strength than a vertically un-reinforced mortared foundation wall,
probably use no more cement than the mortared approach with full width
bondbeam, give an exterior surface of broken concrete which could be
ferro-stained to give a nice, warm tan color and rugged texture to blend
with earthtone stucco above, provide less tendency to wick up moisture
from below, keep a lot of "waste" concrete rubble out of the landfill and,
hopefully, still please the building department.  

Comments/other/better alternatives?  I sure like keeping new cement use to
a minimum and making renewed value of the plenitude of "pre-owned" slab
rubble!                                  - Don Stephens, Spokane WA USA