Cob test bricks

[Shannon C. Dealy]

On Fri, 19 Sep 1997, Benton J. Miller and Phyllis Rameriz wrote:

> Hi cobbers!!
>
> Got a question. We made several test bricks with the soil we would
> like to use to build our cob home. They turned out very strong.
> It seemed as if we were on the right track.  Then we got a heavy rain,
> and we left the bricks out in the rain.  Upon checking the bricks
> after the rain we were dissappointed to see that the bricks had become
> soft and crumbly. This was after they had dried completly after the
> rain.  We are wondering what may have caused this.
[SNIP]

Slow to respond as usual.

You don't indicate how they were lying in the rain, were they lying down 
on a flat surface where water could accumulate around the edges and soak
in over time?  The greatest danger to cob is standing water, and if this
was the case, it's to be expected.  When water is added to clay or straw,
they will swell which would tend to push the brick apart.  In response
to the immediate and obvious: "but we use water to make the cob", I
can't explain it with any certainty, but here are some possibilities to
consider:

   1) When it is soaking up the standing water, the moisture distribution
      is extremely uneven (unlike the mix that was used to make the 
      brick), so the swelling will be unevenly distributed which will
      create stresses within the brick which I would expect to be more
      damaging than if all parts of the brick were swelling evenly.

   2) Cob is generally set into place, or made into bricks using a mixture
      that has as little water as possible; just enough to completely wet
      the mixture and keep it workable.  If you use cob that has
      significantly more water in it, the cob will crack as it dries which
      depending on the size and depth of the cracks can significantly
      weaken it.  If your bricks got overly wet, their moisture level
      may have gotten so high that they had significant cracking when
      they re-dried which could have caused them to become crumbly.

It is important to note, that even on an unpainted/unprotected cob wall,
the vast majority of the rain water will run off of it, and the part that
does soak in will be extremely small compared to what will happen with
cob in standing water.

My two bits ("01" :-)  for what it's worth.

Shannon Dealy
dealy at deatech.com