Cob cob recipe

[Will Firstbrook]

Hi Graeme,

The amount of shrinkage in cob would depend on how much clay was added to
the mix and how much water and the quality of the sand. Clay expands when
wet and contracts when dry. If too much clay is used the cob will crack,
severity depends on how much extra clay was used and how wet it was as the
cob structure was being built. The sand should be very angular, not smooth,
so as the clay dries it shrinks in the cavities between course grains of
sand it pulls tight and locks them together. Once a wall is built and dries
it's size shouldnt change all that much.

I'm not sure how critical shrinkage is in a cob structure. It will more
likely develop cracks (that can easily be repaired) than shrink if non-ideal
mixes are made. Since the walls are very thick the bad combinations are
buried in the center of the wall. If a crack occurs in the center of the
wall the wall has more insulation value.

Regards,
Will
   

> How about shrinkage - surely assessing this is far more critical in
>most cases than strength - what shrinkage limits are you looking for?
>
>Graeme
>
>