[Cob] RE: Susan's foundation

[Barbara Roemer]

Mary Lou,

I don't know a lot about the properties of concrete, and while I agree with
you about the use of rebar in cob being more prone to rust than that in
concrete, I think there are some problems with your description of what
happens in concrete.  Concrete is vapor permeable, and vapor is airborne
moisture, so I would think both "air" and moisture are present in concrete.
It's oxygen that causes the rust, not just air, and the O in H2O is, well,
O. Having bashed up some reinforced concrete, I've seen enough rusty rebar
to know that rust happens, perhaps more slowly than in cob, but it happens.
It's a major problem in dams throughout the west, which do have a lifespan.
Many are reaching that limit in the next twenty years or so, which has, in
part, prompted discussion about decommissioning them.

That being said, those of us in seismic zones still need a means of tying
the top plate and roof-bearing assembly to the foundation, and at this
point, the best solution I've seen is rebar, though it's far from perfect.
The cob will still fall away from the rebar, unlike concrete, but perhaps
enough of it will stay together that you'll remain safe or have time to get
out of the building.  Until we have more seismic testing, as has been done
with straw bale, we just don't know enough about how cob will behave in
various types of earthquakes for me to feel safe with cob alone.  As a
component in a hybrid system, yes.

Friends who are building here in the Sierra Foothills in seismic zone 4 have
a rubble trench foundation, a bond beam, lots of large (~ 1 ton) stones set
into the bond beam, cob above, and a top plate, all of which is tied
together with rebar or all-thread.  This solution means a lot of concrete
has gone into the wide foundation, even with the rubble.  We'd love to hear
from anyone with better solutions.  John Fordice has spoken at some length
about the need to tie the plate and the foundation together, and about his
reservations about combining cob and other methods/materials (dissimilar
properties, unpredictable results).