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[Cob] straw in cob or concrete?

john fordice otherfish at
Mon Aug 18 11:02:25 CDT 2008

Damon & all,
Cob and concrete are both compressive strength materials & relatively  
low in tensile strength.
so when considering adding a tensile component to either  it is  
cogent, the difference in compressive strength between cob & concrete.
Cob is around 200 psi ( pounds per square inch ) & concrete is  
normally at least 2000 psi minimum.
The tensile strength provided to both cob & concrete by adding a  
tensile component ( such as fiber in the case of straw & rebar in the  
case of concrete) will depend on three things:
1. the tensile capacity of the added component
	a. concrete is very high & measured in K (1000's of pounds)
	b. straw is unknown, but apparently quite low when compared to steel
2. the ability of the compressive material to grip the tensile  
	a.  rebar has an irregular surface to transfer the tensile strength   
into the compressive capacity of concrete
	b. straw is essentially smooth so the transfer will be low
3. the development length of the tensile component , or how long the  
tensile component needs to be to take advantage of the ability of the  
compressive material to grip it.

So, with out more data, it's difficult to say how straw will perform  
as a tensile component in concrete.  My guess is not to good.
But, as you point out, the only way to know for sure is try it.  Lots  
of variables to consider & the result is quite apt to vary too!
Cob on.

john fordice  -  Cob Research Institute

On Aug 18, 2008, at 8:08 AM, Damon Howell wrote:

> Hi all,
> 	To Ocean: I originally posted the question about straw in concrete
> and your reply was "Instead of asking the coblist, why not float the
> idea to the concrete people - they'd probably laugh their asses
> off..."  - Thanks Ocean, but since those guys don't experiment and
> cobbers do, I figured this list would be more appreciative and
> supportive of the idea! I think I'll try it anyway as an experiment
> because I KNOW FOR A FACT the straw in cob doesn't deteriorate even
> though moisture penetrates the wall. And I also know when I add the
> straw into my clay/sand mix it gets stiff which probably means it
> works well as reinforcement (at least to keep it together in an
> earthquake). Straw doesn't give the cob compressive strength, sand
> does that. But, as I said before, straw in concrete may not work
> because concrete works chemically where cob works physically. But,
> also as I said before, soaking the straw in water/sodium chloride
> neutralizes the carbohydrates which keeps the concrete from drying.
> I'm no cob "expert" so I do like to hear from intelligent people like
> yourself Ocean. It's just that it seems to me most cobbers are
> willing to try a new idea instead of saying "oh no that won't work
> and you shouldn't even try it." We severely limit our chance of
> survival when we say there is only one way to do something.
> Damon in GA
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