[Cob] concrete mixer cob

[jane at kirstinelund.dk]

It sounds like people either use machines for the whole process, or no
machines at all. We use a combination of machines and footwork.

We use a concrete mixer (200 liters) for mixing the clay, water and sand.
Our local clay is relatively pure and very hard, and think it would be
nearly impossible to do this by foot. Even when kept in buckets with water
for weeks, it still stays in lumps.

When we have a nice smooth and rater wet clay mix we put layers of straw
and clay (lasagna principle) on a tarp. We tread it and turn it a couple
of times, which takes max. 10 minutes. I can mix twice the amound I'm able
to take away in a wheelbarrow in one go.

People seem to consider foot mixing the most time consuming part of
cobbing, but I think that for us the mixing takes up about 10 percenet of
the time. Filling the concrete mixer takes some time too, of couse, and we
are builing double walls of 20-30 cm, which might mean that applying the
cob takes somewhat longer that with the usual two feet walls. But still I
think our  method is relatively quick.

Usually we are only two people working on the house, and often only one
person is cobbing. I have a feeling that tractor cobbing might be too much
trouble in this situation, even if it is rational with large cobbing
crews.

Moreover I think we are able to mix in more straw this way. We use about
25 percent straw by volume, and when tractor mixing is describes, it says
5-10 percent. I have a clear feeling than straw rich cob will be both
stronger (as long as the mix sticks together properly) and more
insulating.