[Cob] length of straw etc

[Dean Sherwin]

At 03:00 PM 3/12/2009, you wrote:
>I would say the poor folks who built most
>of those cob houses in England didn't go to natural building school
>or do a lot of research into it. They simply dug the dirt up, mixed
>straw into it, and slapped it into a wall. What more is there to
>know?

I applaud the sentiment and it would be ridiculous to be held to 
ransom by codes & requirements dreamt up in distant 
laboratories.   ON THE OTHER HAND, those cob builders did have 
expertise and handed-down knowledge, and they had a living tradition 
and lots of examples of what worked - and no doubt what didnt.  I 
come from Devon in England, cob country, and only very specific soils 
in that particular region made for viable cob.  Elsewhere in UK there 
may be earth building traditions but they are not cob.
Of course other soils do work, we know that, and I've seen monolithic 
earth buildings - as opposed to adobe - in India, N. Africa and 
elsewhere, and they can be found in China and so on.  And most of 
those places dont seem to use much straw, by the way.  So there are 
solid - haha - reasons for soil testing and as necessary earth remediation.
On the subject of straw, there is probably no minimum length unless 
you are getting down to below one inch.    Longer (than say 6") is OK 
especially if you are going to plaster.  Other wise it can make for a 
furry appearance.  Most of the binding benefit of straw is during the 
building process.  As the cob dries it, according to some 
authorities, helps the clay to have slippage zones and prevents 
cracking.  Hassan Fathy had a theory that as the straw starts to rot 
it produces lactic acid which improves waterproofness.  There is 
probably little useful tensile strength once the cob is set  (which 
as we all know can be quite a while...)

Dean Sherwin CPE
Certified Professional Estimator
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CONSTRUCTION COST MANAGEMENT
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