Cob: Codes Testing

[Shannon C. Dealy]

On Tue, 7 Mar 2000, Alison Krohn wrote:

> I hope this is not addressed directly in the archives.  I breezed
> through them and couldn't find exactly what I'm looking for.
> I'm a landscape architecture professor and have been contacted by a
> native Ukranian who wants to build what I would call formed cob for a

I'm not sure what you mean by "formed cob" cob is built without forms,
there is no need for them, and the use of forms would tend to make it
much more difficult to put any curve into your walls which greatly
improves stability.

> house in Moorehead, MN.  Needless to say, the local officials want
> someone to sign off on his design.  NDSU has some architecture profs who
> are interested in this method but are leery about this project.  My
> concern is the local soil material.  It is fat clay with a high
> shrink-swell rate (plasticity 25-50, CH silty clay according to the soil
> survey).  These soils have toppled grain bins!  Am I right in believing
> this is not a good material for earth-straw walls?  Anatoly made some

This is what sand is for, no matter how much the clay you use may shrink,
if you have sufficient sand in your mix, there will not be any significant
shrinkage, because the sand can't shrink/compress.

> test bricks that our engineers will run compression tests on.  One of
> them was amended with lime and or cement.  It was very light and seemed
> very strong. I am inclined to think that this material would be better.

While people have added lime, cement, etc. to their mixes, I don't really
consider this cob, and it really isn't necessary in order to build a
strong, safe structure.

> So here are my major queries to the list:
> Do any states have codes for formed cob?

I have heard that some counties in Arizona or New Mexico have a code for
puddled or poured adobe which is another name for cob.

> Shouldn't sand be added to the mix? 70% I believe I've read

Assuming (based on the above statements) that the clay content is to high,
you would need to supplement the local soil with some coarse sand.  I
don't know anything about your particular soil mix, but I believe that in
most cases, high clay soils generally still have alot of sand in them
(a cursory look at the soil I am currently using would make one think it
was mostly clay, even though it is only about 30%-35% clay, 20% silt & 50%
fine sand).  The point is that except in very unusual cases where someone
has pure clay/clay-silt, you would not be adding 70% sand, but rather
enough sand to bring it to the appropriate level.  With regard to the 70%,
that is kind of a general purpose number and should only be considered a
starting point.  The best mix for a given soil will vary depending on the
silt content, sand content (and size/coarseness of the grains), clay
content and type of clay (different clays have radically different degrees
of "stickiness", expansion, etc.).  Depending on the type of clay, your
optimal mix will generally have between 10% and 25% clay with the rest
being sand and silt, in some cases, the optimal amount of clay may fall
outside this range, but that is fairly rare.  The only reliable way to
make the determination of the optimal mix is to make test bricks with
varying ingredients, allow them to completely dry and then test them for
strength.

> What is the difference between forming this mix and rammed earth?

Rammed earth requires heavy duty forms to hold the shape of the material
while it is "rammed" together.  Because of the need for pre-made strong
forms, it is very difficult to make rammed earth in any shape other than
rectangular blocks.  Cob is built up in layers by hand, requires no forms
and can be molded into whatever shape you desire as you are building with
it.

> Rammed earth has no straw?

Yes, rammed earth has no straw, where cob has around 10% straw.  This
gives cob good tensile strength, and helps hold it together even after the
cob has been broken (for those of you in earthquake zones).

> Which labs are approved for testing nonconventional materials in the
> upper midwest?

I have no clue.

> Is there an experienced engineer or architect who'd be interested in
> helping with this project?

A good starting point would be John Fordice, he is an architect in
Berkeley who has been working on Cob building codes, and I believe I heard
that he had recently done a permitted residential cob building.  He is
also a member of this list (though rather quiet lately), his email address
is:
      otherfish at home.com

If he can't help you, he probably knows someone who can.  You may also
wish to contact Cob Cottage Company for information as well.  They don't
have an email address, but can be reached by phone and mail at:

        Cob Cottage Company
        Box 123
        Cottage Grove, OR 97424

        Phone or FAX: (541) 942-2005

> The codes department also wants an R-value because MN requires a
> threshold of energy efficiency for all homes,

This has always been a point of great debate in cob, and based on what I
have seen of measured, computed, and speculated values, it:

  A - Probably varies quite a bit from mix to mix
  B - Is probably in most cases somewhere between 0.5 and 0.75 per inch.

In climates with severe winters, a supplemental insulation of some type
wraping the outside of the cob will be a virtual necessity, strawbale
would be my personal preference.  Cob's best point in thermal design is
that it makes good thermal mass, so it can be used to stabilize
temperatures, which can make completely passive solar designs practical in
more moderate climates, and help to keep interior temperatures more
stable and uniform in colder climates.

> and fire resistance
> although I question the official's common sense on this one.

Bring him a sample of cob to look at, and if he still wants fire testing,
bring him a couple gallons of gasoline, kerosine, dynamite, hand grenade,
magnesium powder, whatever :-) and invite him to try and make it burn.

> Any assistance/advice/web pages will be greatly appreciated.
> 
> Alison


Shannon C. Dealy      |               DeaTech Research Inc.
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