Cob Woodshavings

[Shannon C. Dealy]

On Tue, 4 May 1999 robden at mweb.co.za wrote:

> Hi
> Its is a great mailing list to be part of !
> Could somebody out there please advise would woodshavings serve the same
> purpose as straw.
> Mixture: 6 wheelburrows of soil
>              2 wheelburrows of sand
>              1 wheelburrow    of woodshavings
> 
> Are we doing the right thing.
> Thanks in advance
> Rob and Niece

The answer is maybe (never give an answer anyone can hold you to :-)
It would depend on the the size/shape of the shavings, the particular
wood, and possibly other factors.  The shavings are taking the place of
the straw, so they need to provide tensile strength.  This means they need
to be long enough for the sand/clay mix to be able to "grip" each
shaving and tie the cob together.  They also need to be strong enough to
perform this function as well.  Personally, I think I would want the
shavings to be a minimum of four to six inches long for use in structural
cob (as opposed to a plaster or floor mix which could use shorter fibers).

Ultimately, the only real way to determine if the shavings will work, as
well as determine if the mixture you mention above is adequate, is to test
it.  Make test bricks from small batches of cob using the ingredients in
varying proportions, once they bricks have cured, inspect them and
test them.  Check them for shrinkage cracks (to much clay and/or water in
the mix).  Test to see if the edges of the brick crumble easily (to little
clay).  Finally try to break them, if the brick breaks cleanly in two, my
personal take would be that you either don't have enough shavings, or they
are to brittle and are probably not up to doing the job.  When you break a
brick with adequate good quality fiber (like straw), the initial break
will go through the sand/clay, leaving much of the fiber still holding the
two ends of the brick together.  For my test breaks, I generally grab each
end of the brick and slam the center of it against the lip of a 55 gallon
drum, or the edge of a large rock (it may require several blows to break a
brick with a reasonably good mixture).  Of course if you are in the habit
of breaking your bricks with an axe or other such implements, you may get
slightly different results :-)


Shannon C. Dealy      |                    DeaTech Research Inc.
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