[Cob] psi testing

[Henry Raduazo]

	This type of testing makes perfect sense for concrete where you have  
five yard batches and every part of a batch is identical to every  
other part of a batch. Now try to apply this to cob where 100 pound  
batches are being mixed up on tarps by crews that may add varying  
amounts of straw, clay, sand and water, and this sort of test is  
nonsense. But engineers love it! I was an engineer for 33 years and I  
think it is comical when code people apply this type of thinking to  
situations where it does not apply.
	I was able to increase the size of my batches and to make them much  
more uniform by mixing on a concrete slab with a rototiller, but the  
batches were wetter and not as strong as foot mixing and I had 3  
inches of shrinkage over a ten foot height wall and proportionally  
less on the tunnel-like structure described below (two 36 foot cinder  
block walls with 4 foot of cob on top). Since my load bearing needs  
are so low the conditions of use are more important. I figure that 24  
hours of a water sprinkler would be sufficient to bring down an 8  
inch thick wall, but this has everything to do with the thickness of  
the wall and the surface treatment and nothing to do with the initial  
psi strength of the material. A strong material that wicks water  
would be more susceptible than a much weaker material that does not  
wick water, but no one measures this. We just assume that no one will  
place water sprinklers to throw water on to an earthen wall. I only  
mention this because I once built a roofless children's playhouse in  
a garden where it got many hours of watering each week and went  
through hard wet winters with no protection. It lasted 12 years till  
they tore it down and I think it might have lasted a lot longer but  
they were nervous about kids climbing on the walls.
	If you give your roof a weight of 50 pounds per square foot (a  
really heavy roof with a snow load) and divide that by the number of  
square inches in your walls you will see that this 350 psi number is  
designed to prevent earth building and has nothing to do with the  
actual weight that the walls will support.

Ed


On Sep 20, 2010, at 12:28 AM, Janet Standeford wrote:

>  Hi Ed,
> Yes. This is code that I had to pass for adobe since they don't  
> have cob codes here.
>
> The important thing is that it passed without adding anything but  
> straw to my sandy clay, which by the way only shrank an eighth of  
> an inch in each direction.
>
> This was 8 of the cylinders full of dirt and 8 small handfuls of  
> loose straw. I'll have to figure out actual measurements on this next.
>
> Janet Standeford OR
> www.buildingnaturally.info (Owned by you)
> A resource for healthy homes.
>
>
>
>>     For my structure 350 psi. would give me an 11200% safety  
>> margin. Does that sound right to you? I don't usually bother with  
>> such calculations because I assume some future owner will do  
>> something really stupid like running a lawn sprinkler against the  
>> wall. Then of course all bets are off.
>>     Is this a code requirement for earth walls? It seems rather  
>> arbitrary to apply something like this to cob unless you are  
>> pushing limits which should not be pushed.
>>
>>
>>
>> Ed
>>
>>