Cob: Re: Rosemary Lyndall Wemm - Cob wall mixtures - British

[Christopher Greenslade]

Cob walls are normally built off a stone plinth which can vary in height
from 450mm above ground level up to first floor joist level in some domestic
buildings.

Correct mixing of the material is as important as the actual construction
process.  The soil is first broken down to a fairly fine tilth, all large
stones greater than 50mm diameter being removed in the process.  It is then
spread out into a bed some 100mm in depth on a hard, pre-wetted surface on
top of a thin layer of straw.  Water is then added and a second, thicker
layer of straw is spread evenly on top (about 25kg of straw per cubic metre
of soil - 1.5% to 2% by weight - is considered adequate).  The straw is then
trodden into the soil which is turned several times, more water being added
as required.  Thorough treading of the mix (traditionally by either men or
animals) is vital because it ensures even distribution of the clay and
renders the material to a consistency and state of cohesion suitable for
building.  The quantity of water used will vary according to the soil type
but is usually in a range of 10% to 12% by weight.  If too little water is
added the necessary blending, in other words efficient distribution of the
clay fraction throughout the soil, will be difficult to achieve; the
material will also be difficult to compact when it is placed on the wall.
On the other hand, excessive amounts of water reduce the soil to a soft,
plastic state, making building virtually impossible.  In this case the
material may need to be left for several days to dry out before using.

A typical serviceable cob mix would contain clay and aggregates in the
following proportions:

Stones and gravel (over 5mm diameter) 30%-40%
Fine and coarse sands 25%-30%
Silt 10%-20%
Clay 10%-25%

As the wall dries out shrinkage takes place (the first 3 to 4 weeks are the
most critical) and the straw has the beneficial effect of distributing
shrinkage cracks throughout the wall, which prevents the formation of large
fissures thereby reducing the risk of subsequent structural failure.

The height of a lift or "raise" would vary according to soil type and
consistency from 300mm up to 900mm!

Best regards
Chris

----- Original Message -----
From: "Rosemary Lyndall Wemm" <lyndall at neurognostics.com.au>
To: "'marksaich'" <marksaich at zoom.co.uk>; <coblist at deatech.com>
Sent: Monday, May 22, 2000 1:38 PM
Subject: Cob: RE: White crystals


>
> >It is almost certainly effervescence. This is the leaching of salt out of
> >the wall. It gets in there from the sand (particularly beach sand) and
> >sometimes from the water. .....
>
> This area is part of the coastal sand plain. The bore water is full of
> minerals which stink and stain everything red [like the colour of my clay
> from the hills].  The water I am using is on-line drinking water from the
> hill damns so is probably unaffected.  The sand I am using would almost
> certainly have come from the local sand plains which are no doubt highly
> salty.  So I suspect the culprit is the sand.  As you say, the crystals
> brush off very easily .. and then re-occur in the same or another spot the
> following day!  I'll keep brushing.  How long does it take to leach out
> completely?
>
> The cob wall is now about one foot high and drying out at the bottom.
It's
> taken over a week to get this high.  I can't figure out how other people
can
> do 18 inches a day!  Perhaps their weather is hotter.  It's nearly winter
> here.  Or perhaps my mixtures are too wet or too sandy.  The last mixture
> has been the best so far.  I changed the clay/sand ratio from 2:5 to 1:2
and
> used less water.  I'm still unsure about how much straw to add to the
> mixture.  Can anyone supply a rule of thumb.
>
>
> I've been staightening walls today.  I've discovered that hard cob can be
> difficult to shape and firm cob can be a problem when the straw content is
> high because the straw comes out leaving holes.  However, the holes are
> reasonably easy to fill up with some of the dampish cob which has been
> scrapped off.  I'd probably have done better if I'd wet the offending
areas,
> but I was not in my proper work clothes at the time and didn't want to get
> my usual filthy self as I had a doctor's appointment within the hour.
> Somehow I don't think it would have gone down too well if I'd arrived
> covered in red clay stains!  Oh well, I'm sure the stuff will still be
> workable tomorrow so I'll give all the new surfaces a blending "wash"
then.
>
>
> ---------------------------------------------------------------------
>  Rosemary LYNDALL WEMM,
>  B.Mus.(Inst.), T.S.T.C., B.A.(Hons), M.A.(Neuropsych.), etc.  _--_|\
>  Clinical Neuro-psychologist                             Perth/      \
>  Perth, Western Australia    lyndall at neurognostics.com.au  -->\_.--._/
>  ------------------- http://www.neurognostics.com.au ---------------v-
>
>