Cob: Couple Of Questions

[Shannon C. Dealy]

On Wed, 17 Jul 2002 snakedancer at gulftel.com wrote:

[snip]
> wasging the fence. Anyway he was wondering how long can the wall you are
> working on sit like between building. What if it dries before you egt back to
> putting more cob on it. Is this something that once you start you must saty on
> it or can it go two or three days between applications of cob. Will it make it
> weak. Like pouring concrete letting one section dry and pouring the next. It
> becomes weak at that point.
[snip]

If the wall dries before you get back to it for the next layer, the bond
between the old and new layers will be significantly weaker because you
will not be able to get as good a mechanical bond since the layers will
not be intermixed.  To improve the quality of the bonding, whenever you
are done for the day, it is best to put a lot of holes in the top using
your thumbs or a stick of similar diameter.  By doing this, you provide a
means for interlocking new cob with the old cob even if it dries.  It
won't be as good as if the wall were built continuously, but for most
cases should be more than adequate.  If the wall has dried before you go
to put on the next layer, it is important to thoroughly wet the surface
where you will be adding new cob (preferably repeated soakings over at
least an hour or two), this won't soften the old cob to any significant
extent (unless it is a really poor quality high clay mix), but it will
significantly improve the bond and reduce problems with cracking along the
joint between the old and new layers.

Having said all the above, cob does take a long time to dry except in
hot/dry weather, so two or three days between layers is generally not a
problem, in cooler weather you can just come back in a few days and pick
up where you left off, and in warmer weather just put some nylon tarps
over the fresh cob and weight them down, this will trap the moisture in
the wall for quite some time.  You will probably need to experiment to
determine the appropriate handling (when/how to use tarps) for your
climate since both temperature and humidity will significantly affect
drying time.  If you use tarps for mixing cob, once they are rather worn
out, they are great for this purpose, since a new tarp being
essentially water proof will almost completely stop drying, where the worn
ones are more breathable and will just slow the drying which is really
what you want unless you are not going to get back to the job for a long
time.

NOTE: When you put all those holes in the top of the cob, it increases the
exposed surface area and significantly accelerates the drying of the cob
if it isn't covered, so while it is always recommended to put the holes in
the top, you should be aware it has a side effect (which is useful in cold
weather where your cob is drying to slowly).

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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