Cob cob recipe

[Graeme North]

Will Firstbrook wrote:
> 
> 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
> >
> >
Dear Will,

Thanks for your reply - I understand the mechanism of clay shrinkage OK
I am an architect that specialises in earth building of many different
techniques - my concern with shrinkage is with cracking that has some
severe structural implications when designing for earthquake conditions
at least. The Standards New Zealand technical committee that I chair
could not include cob as a "standardised" form of construction in our
standards because no-one I have talked to yet who is familiar with the
medium has been able to put a figure on a sensible limit on shrinkage
for pre-construction material testing, although one Australian
practitioner suggested zero. My own fairly limited experience with cob
suggests that a good cob mixture will have shrinkage approaching about
0.1 percent, but I need further amd wider consideration of a figure such
as this - an iseal cob mixture as I see it will effectively not shrink
at all as it dries.

For your interest, Standards New Zealand has now issued for Public
Comment three Earth Building Standards covering Engineering Design,
Materials and Workmanship, and Earth Buildings not Requiring Specific
Design.

Further info on these world first comprehensive standards can be
obtained from me or from Standards New Zealand :
IANB at standards.synet.net.nz .

Cheers,

Graeme