Cob: Use of Forms?

[Joe Skeesick]

Actually in the UK the terms used are cob and clay lump. (which according to
my understanding of "rammed earth" is more in keeping with what is being
described here than the later term). Clay lump is simply a more moist
mixture of cob that is shuttered, then turned out but the contents of the
mix are basically the same. The end result of both building types is
basically the same (with obviously more uniformity being the result of the
clay lump type).

My area of the country (East Anglia) has a significant number of clay lump
buildings where as the South-West took more to cob. The reality being that
the most significant difference between clay lump and cob is the term
itself. Some cob in the west (UK) was also shuttered and rammed as well.
Both are monolithic load bearing earth structures and (I think) fall easily
into the modern definition of "cob". Rammed earth is a term used much more
often with using full forms, interregnal reinforcements (rebar) and
typically a cement stabilized earth mix (and of course copious use of
powered equipment).

On the topic of chalk, to my knowledge no large amount of chalk was
implemented into the mix directly. Typically you simply dug down to the
chalk layer and built atop it as bedrock. Current building practices here
require a set depth foundations must run to however and you are often forced
to dig a significant depth into the chalk itself (at great effort for no
good return). As for what chalk is and the definition. Next time your in a
classroom pick up that little white stick resting under the blackboard and
have a look. Just like that only in big slab formations 1-3 feet down. I
have a big block of it out by the forge that I chip of a piece to mark with
as needed. But to build with it? It has similar characteristics as sand
stone.... great with compression when left in the ground but very poor at
dealing with shear forces. I am however far from the end all on questions
like this. It might well be used in 100 ways I'm unfamiliar with.

Joe

-----Original Message-----
From: owner-coblist at deatech.com [mailto:owner-coblist at deatech.com]On
Behalf Of Darel Henman
Sent: Monday, January 06, 2003 8:42 AM
To: Ray Luechtefeld
Cc: cob list
Subject: Re: Cob: Use of Forms?


Ray,

Thanks for the source information.  From reading the paper it states
that the only difference in the shuttered (formed) process and the
rammed earth (see Techniques 2 page, bottom section) is that rammed
earth uses mix with less moisture than the shuttered type and requires
more sturdy forms.  The rammed earth would be stronger then.  The
introduction section, paragraph one, states that "...Less commonly, but
still significantly, earth (or perhaps more often chalk) which was
rammed into place between shuttering can be found."  Indicates a rammed
earth, but on the the Techiques 1 page, the "shuttering process"
described seems to be a mix of cob earthen material of a considencty
between that required for a true monolithic bearing cob wall and the
light-clay mixture used for partitions or other non-bearing walls.
Since it is wetter than rammed mixtures, it would shrink more when it
dries and cures, the shrinkage extent will depend upon the amount of
straw  mixed in.  This shrinkage might be able to be fixed by a
subsequent application to fill any cracks and top of the wall.  I will
read the rest of the report.

It would certainly be easy enough to make some small little test forms
to make some test pieces in.   Maybe a test size of 1 foot long by six
inches by six inches.  Maybe a  pound cake pan?

Darel

Ray Luechtefeld wrote:
>
> Hi Darel,
>
> I don't think so, I believe the paper I'm looking at refers only to
> cob.  The page talking about forms (called "shuttering") is at
>
> http://www.ihbc.org.uk/Cob_Paper/page2.html
>
> What do you think?
>
> Ray
>