[Cob] cob or concrete: hydrophobic vs hydrophillic considerations

[Ocean Liff-Anderson]

OK, I sure caused a bit of a storm by suggesting we talk to the  
concrete professionals, who we all know would laugh their a**es off  
at the suggestion to add a degradable material to concrete.  My off  
the cuff response is due to my frustration with the coblist - there  
have been many speculative suggestions of relatively serious or silly  
nature, which may or may not have severe consequences to the  
inhabitants of the buildings proposed.  Let's not forget the poor  
fellow killed when his cob dome collapsed on him due to failure of  
the straw by water damage.

I really suggested the concrete professionals because the original  
poster seemed to want to use the material they would know so much  
about - Portland cement.  Just because this is an "alternative" list  
doesn't mean we should shirk the expertise of the conventional  
building industry, especially when suggesting to merge an industrial  
material with a degradable material - straw.

Another consideration about the concrete-straw hybrid which I didn't  
mention:  Hydrophilic vs. Hydrophobic characteristics of cement vs.  
clay:

As mentioned by Damon (and countless cob professionals including  
Ianto Evans), in a dry cob wall straw can last hundreds or thousands  
of years.  This is due to the chemical nature of clay, which is  
hydrophobic under proper conditions - namely no surface source of  
water, plenty of air circulation - clay dries completely, protecting  
the straw that cob depends upon for shear strength.  Ianto also  
speculates that rain blown onto the surface of an un-plastered wall  
will not penetrate further than a few fractions of an inch because  
the surface clay hydrates and expands, in a sense creating a barrier  
to prevent rain from penetrating deeper into the wall, where it might  
soak the straw and cause it to fail.

Portland cement, on the other hand, is highly hydrophilic, meaning it  
holds onto water, and actuallys wick water from a below ground source  
- why most cobbers will not use concrete as a footing for a cob  
building.  The straw in the cob immediately adjacent to the concrete  
footing can rot due to water wicked through the concrete.  I would  
speculate that rain blown onto a concrete-straw wall would be wicked  
into the wall, ultimately causing the straw to degrade and the wall  
to fail.  Additionally there is the damage done chemically by the  
caustic cement during the initial wet concrete phase (which doesn't  
happen in cob - clay isn't caustic, wet or dry).  Given these  
concerns, I wouldn't trust straw mixed into concrete for shear strength.

One last point:  I don't think I ever said that "all cement is bad".   
I have in fact used a lot of cement, learned a great deal about it  
from the professionals who have helped by projects, including Kiko  
Denzer.  I've also seen cob get soaked at the concrete footing / cob  
interface, due to the wicking nature of the concrete.  I speculate  
that the wall may eventually fail in this case.


On Aug 17, 2008, at 11:12 PM, Lance Collins wrote:

> In my response to Ocean's comments I was reacting to his 'all cement
> is bad' tone and saying things about concrete which conflict with my
> experiences.
>
> I believe in using appropriate technology.  Concrete is a very good
> technology and the fact that some people get the design and building
> of their structures wrong does not support arguments against concrete.
>
> The principal problem with concrete is it's environmental impact
> (which is one of the reasons this list exists).
>
> Straw might work as a concrete re-inforcement but to me it doesn't
> seem worth the bother.  Once you have paid for your concrete for only
> a little extra you can get some steel and also great confidence that
> the steel and concrete will work together to give you a  
> satisfactory result.
>
> For people building their own dwelling probably the only place where
> concrete might be considered is for foundations or retaining walls
> where ground water is a problem.
>
> Lance
> (in Aus)
>
>
>
>
>
>
>
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