[Cob] Coblist Digest, Vol 8, Issue 192

[Monica Proulx]

Hmmm, interesting topic and good point.  You got me thinking, if this
erosion works in any way similar to erosion on sloping soil surfaces
(and stream bed erosion) once you get channeling going on the ground,
some of the water instead of sheeting out over the surface, goes into
the channel and this increaces the velocity and cutting action of the
water.  However, it would seem that on a vertical surface the water
would still sheet out over quite a bit over the whole surface despite
channeling (especially if rain is hitting it directly), as gravity
isn't  working quite the same as it is on mainly horizontal surfaces
(even with water surface tensions taken into account).  Upshot here is
no matter how channeling starts and operates, once it starts you have
more surface area to erode from and it may happen at a quicker rate.
Anyway, fascinating topic, and I bet someone has studied the science
of strictly vertical channeling somewhere, somebody in England
probably related to cob.

Come to think of it, channelling on vertical walls may be due more to
the fact that the surface of your cob is more tightly packed than the
interior because of the way people smack cob with their hands.  Once
water cuts through this layer in some places quickly, erosion can
happen faster and this may cause channeling in the more loosely packed
cob and it might also be able to take advantage of air pockets and
straws and start wicking deeper into the cob.  Given that scenario,
seems like it's best if water sheets off rapidly from vertical cob
walls, and I like Damon's idea of burnishing or compacting the surface
of the cob, if you aren't going to plaster that is, to provide less
"tooth" for water to erode with.  Rain droplets are in and of
themselves pretty good eroders, in addition to the sheeting water, and
so size of sand grains might come into play (maybe bigger being
better, up to a point of not return?  Anybody have an idea on this?

One can see why a good "hat" on cob is important where you get lots of
rain, and maybe extra rain shelter the rainy side (fence) and do a
nice burnish job, lots of folks could get away without plastering,
particularly one story structures.  Ok, nice diversion but now back to
my plum jam making!
>

>
> Message: 2
> Date: Fri, 15 Oct 2010 17:09:55 -0400
> From: Christopher Reinhart <sandymud at gmail.com>
> Subject: Re: [Cob] no need for plaster
> Cc: coblist at deatech.com
> Message-ID:
>         <AANLkTin5fSc0q_WyVvbmbXMbm3JdgpJ5=shbB12m64gz at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> The exterior of my two cob buildings have gone for five years with no
> plaster - there is only very minor erosion on the windward side of one
> building. It does seem that now that the erosion has started, and the water
> has made small channels, that it is happening more rapidly, but I'm guessing
> it's still less than 1/2" deep. I plan on adding plaster in the future,
> mostly for aesthetic reasons.
>
> -Chris Reinhart
>
>
>
>
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> End of Coblist Digest, Vol 8, Issue 192
> ***************************************
>