Cob: RV: Cob codes

[Shannon C. Dealy]

On Mon, 13 Mar 2000, Alejandro Ascanio wrote:

[snip]
> 1) What is the compression resistance of cob per cm2 (or inches)? Is
> datas available from lab tests ?

I have seen only a few numbers for this, and it was not for a traditional
cob mix so I don't have much to offer here, except to say that it wouldn't
mean much even if good numbers were available.  Every soil composition is
different, and how much sand or clay (if any) and straw you use in
your mix will SIGNIFICANTLY alter the strength of the cob.  The 
compressive strength of the mix I am using on my experimental structure is
easily less than half that of many of the other mixes I have used, and I
would say the compressive strength is going to be primarily a function of
the sand in your mix - percent of mix, coarsness of the sand, and shape of
the sand particles.  Round beach sand's (a poor choice) behavior in cob is
significantly different from the irregularly shaped sand particles found
in most soils.

> 2) What is the maximum length of straignt walls (no curves) made of cob
> when constructed without columns or pilaster? Or, at what interval in
> meters, is it necessary to strenghten or reinforce a straight cob wall
> with a verticle support such as columns, pilasters, wooden posts or
> other structural member ?

I am not sure I understand the question.  Cob needs no vertical structural
supports regardless of the length or shape of a vertical wall, the concern
with long straight walls is that heavy winds, earthquakes, etc. could
knock a cob wall over, which would be instantly fatal to anyone it fell
on.  Using sufficient curve in the walls makes it almost impossible to
knock it over, but if you want long straight walls without any
intersecting wall to provide lateral support, buttresses and/or a thicker
wall can be used to provide adequate safety for any length of wall, but
what is needed depends on your circumstances and what hazzards need to be
designed for.  To give a completely silly example, a wall which is 8 feet
high and 10 feet thick could be built a couple miles long with complete 
safety, if any wind storm or earthquake were to occur which was strong
enough to "knock it over", you've got much bigger worries than the hazzard
presented by one cob wall.

> 3) How tall could be a cob wall without a floor or roof for structural
> support be built?

This again is going to be a function of other aspects of the design -
curve and thickness of the walls and use of buttressing all would make a
significant difference.

> 4) Do we need below structure roof a "bond beam" to tie the cob walls
> together ?

Generally no bond beam is necessary, but I have no doubt someone could
manage to design a cob structure which would need one.  You don't
generally need to "tie" the walls together, because (normally) they are
all one continuous piece.  If you insist on using alot of square corners,
and thinner walls, a bond beam might be a good idea, since corners tend to
concentrate structural stresses all in one place, where a round building
will distribute the stress much more evenly.

> 5) Has anyone added cement (low portion) to cob mixture looking for
> drying walls faster and to improve its strength?  Actually, a lot
> of people add cement to adobe blocks. They are stronger than
> traditional adobe, however they don't use straw. What would happened to
> straw if we add cement to the mix for cob? Does anyone know?
[snip]

Never tried it.  I have heard of people using straw with cement, but I
don't recall what if any problems occured.  You could try searching the
archives at:

   http://www.deatech.com/natural/coblist/

if no one else has a better answer.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
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