[Cob] cob w/adobe and other materials

[Kindra Welch]

My observation as to why cob vs adobe block is that adobe block generally is
pure native soil while cob mixtures add sand to control shrinkage.  When you
do not have the option of adding sand to native clay soil, you have to
pre-shrink the mud (make it into blocks and dry it in the sun) before
putting it in the wall.

With that understanding you can mix adobe and cob if you keep in mind that
the adobe is not going to shrink with the cob: 1. lay it slowly and let cob
dry fully before moving forward 2. make a very sandy cob mix 3.make a zigzag
seam instead of a straight line.  Here is a photo of an example of a
cob-adobe corner under construction, finish plastered now - not a crack in
three years.  For our mud mortar we used local clay and added sand to stop
cracks.
http://www.claysandstraw.com/projects/mtncttge/timeline/2005/uptoOct10/cobad
obeseam.jpg

Ovens have been built with cob and/or adobe for centuries. I know several
ovens that have cracked from thermal expansion (cracks open when fired and
close when cold), but baking does not seem affected by cracks. Cement not
necessary, rain protection important.  I would especially not recommend
cement on a first oven... you'll learn alot with your first oven and
immediately have ideas how to make it better. If you use cement you are
STUCK, with mud after a couple years just recycle the old oven into a better
one informed by everything you learned the first time around.

Mixing cob with non-shrinking materials is always do-able, you just have to
anticipate the shrinkage.

Kindra


Date: Sun, 29 Jun 2008 13:55:56 -0700 (PDT)
From: Mitch Ventura <h_anpyp at yahoo.com>
Subject: [Cob] Differential adhesion qualities


There has been some discussion and comments for sometime about using cob in
conjunction with other materials (cob and cement, with wood, adobe, etc.),
which causes me to ask for one of the Forum's experts to comment upon, as
those of us with layman's or limited hands-on experience can go back and
forth discussing what we "think" is best.
?
My "research" has lead me to read books by engineers and architects?who
insist that if other materials?are used with cob, then there will be a
separation, sooner or later, due to differential expanding and contraction
qualities, which explains in part why a cement-based plaster is probably not
good for cob structures (besides inhibiting the "breathing" that cob needs).
?
However, there are also experts who advise using cement and other materials
(mainly referring to adobe).
?
In my own case, while not being able to?complete a project that I needed,
due to being out of the country, I ordered?an adobe?structure built around a
water tank to hide its ugliness. (The builder refused to use cob even though
I offered to pay for his training!!!). Although he is excellent with adobe,
he did show me structures with much cracking in the "adobe" mortar which is
why he uses a cement-based mortar. I explained to him that even though my
experience is limited, it was easy to see that the adobe was not from the
immediate area while the mortar was, accounting for two different mixtures
(there was at least a 16 km separation between both locales), requesting
that the adobe brick manufacturer supply him with almost the same clay-sand
mixture (slightly elevating the clay percentage) in order to prepare the
mortar.
?
Needless to say, upon my return I found the adobe structure constructed but
with a cement mortar (that he cleverly disguised to make it look like
adobe). Although I was not pleased with him using cement, I have to admit
that other cabins and tanks that he has made in the area using that type of
mortar shows no separation (even though some are 5 years old) as compared to
the adobe mortar that I mentioned. This does not make sense., as I would
expect the opposite to be true.
?
When it comes to mixing adobe bricks and cob (perhaps to save construction
time), I would also expect a separation... am I also wrong on that?
?
I understand that there is always some "mixing" as many cob structures are
placed on a rock, cement, or other foundation, but that is a different
scenario as the weight is (or should be) laying evenly on a different
surface... I would ot expect the cob to rise off the cement or rocks.
?
Thanks in advance.
?
Mitch