[Cob] supporting a one ton tank

[Shannon Dealy]

On Wed, 6 Jul 2011, Ocean Liff-Anderson wrote:

> Actually, cob has incredible compressive strength, similar to a concrete 
> wall.  I don't know the numbers - Shannon, can you pitch in some figures? 
> But I think the walls circling a small bathroom and closet, if made thick 
> enough, would be able to support your water tank.  But best to get some 
> engineering...

Well, the numbers I have are nowhere near a concrete wall.  A study done 
by an engineering student a few years back using samples provided by 
experienced cobbers in this area gave a range from 65 to 129 psi.  His 
search of the literature found that most past testing by others had ranged 
up to 175 psi, but as far as I can tell, most of it seems to center around 
100 psi except for one outlier which cited a range of 400 to 610.  In 
otherwords, there is a lot of variation.

It is important to note that cob is built with local materials and 
optimized for a variety of tradeoffs which may not include maximizing 
tensile strength.  The soil and sand available where I live give a mixture 
that is more ductile and with much lower compressive strength than mixes I 
have worked with at other locations (some of which are probably easily 
over twice the compressive strength of my local mix which is around 100 
psi).  Of course if I had need of higher compressive strength, careful 
selection of materials for the mix and thicker walls would allow for the 
handling of quite substantial loads.

Shannon C. Dealy      |               DeaTech Research Inc.
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