Cob cob recipe

[Graeme North]

Graeme North wrote:
> 
> 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