[Cob] More on Cob vs Cement

[Ian Marcuse]

Hi Ocean,

Yes, I agree, many more tests would be great to account for all the 
variables. We did not build a foundation for one thing. Keep in mind 
the power of a 2 g force quake which equals greater then 9 richter at 
the epicenter. I am convinced that even the best built cob will 
experience severe damage, but perhaps not to the extent that other 
building would. Is a scale model representative? Perhaps not fully, 
though the engineers took the scale into account. Because we had this 
great opportunity to do this test, we made sure we built to the best 
level of quality we could. All of us have worked and trained with 
Ianto and were originally trained in his methods.

We used alot of straw. Test bricks were made prior to building and 
the strongest was used. We also did a number of materials testing to 
examine different ratios of everything. The clay we used on the model 
was local and contained a certain low percentage of silt. This was a 
concern, but we went with what is commonly representative of many 
clay samples in our region. The building went up quickly so as to 
avoid drying between layers, but it does look like some of the 
failures occured between layers. I also suspect that alot of cob 
buildings that take years or months to build have these less 
integrated layers so perhaps a degree of less integrated layers is 
representative. We used cobbers thumbs and I would say that we 
worked the walls way more than I ever saw anyone else doing it. We 
really wanted to make sure that the straw was well connected.

At any rate, this test was of course not definitive. I would say that 
for a model, it was very well built and engineered. To some extent it 
was overbuilt, particularly the roof connection. It would be great to 
see other tests done, but  It was an incredible amount of work done 
to in the end have to see fall apart

Ian Marcuse



>Most of us in the cob building movement love the curvilinear mode of 
>construction, both for strength and aesthetics.  Have you ever been 
>in a "round" building, or a cob house?  There is a sense of flowing 
>form, "coziness", comfort - all qualities which can't be engineered 
>or quanitified.  Still, many cob cottages in Wales are rectilinear, 
>gaining their strength from being massively overbuilt - 3 foot thick 
>walls!
>
>I wonder why we are so concerned about the potential failure of cob 
>(under a massive 9.0 quake) after just one seismic test.  Was the BC 
>test conducted in a manner representative of quality cob 
>construction?  Any test of a cob structure must account for numerous 
>variables.  I have several questions regarding the BC seismic test:
>
>1. Why was there failure laterally, between cob "layers"?
>2. Was the straw sewn through layers with "cobbers thumbs" as Cob 
>Cottage Company teaches?
>3. How much straw was added?  How was the ratio determined (test 
>bricks, etc.)?
>4. What was the clay/sand ration?  How was the ratio determined 
>(test bricks, etc.)?
>5. Did the layers dry between application?  If so, were the surfaces 
>built with "ribs" to allow keying between layers?
>
>Ianto and Linda emphasize the aforementioned techniques when they 
>teach "Oregon cob", which they have developed to response to the 
>seismic concerns of our region.  Was Ianto consulted on the method 
>of construction for the BC test?  BTW: Have seismic tests been done 
>on rammed cement-soil houses, or strawbale houses?  I'd be 
>interested in these results for comparison.
>
>One correction for Layth:  Concrete construction commonly contains 
>15-20% portland cement, 80% aggregate sand/gravel, not 100% portland 
>cement.  So some cement-soil construction actually approaches the 
>same amount of portland cement as concrete!  Ianto Evans oft quotes 
>that cement kilns produce 10% of the worlds greenhouse gasses. 
>Hospital waste (including radioactive material) is sometimes used to 
>fire the kilns, resulting with cement that contains radioactive 
>material!  I think we natural builders should use as little as 
>possible.  In some cases it is necessary, such as outdoor patios, 
>foundations, etc.
>
>Lastly, I wonder if the need to conform to conventional rectilinear 
>construction so as to be more acceptable to the "general public" 
>should be a goal for natural builders.  I happen to believe most of 
>the public would love to live in a curvilinear house, and once 
>exposed to a cozy cob cottage wouldn't think twice about abandoning 
>their stick framed box.  One anecdote: we have hosted the Seattle 
>"Tribes Project" tour with an overnight stay in the cob Kiva at 
>Ahimsa Sanctuary for the was four years.  This group is a 
>multicultural theatre troupe of inner city high school kids who 
>present a performance on race and empowerment.  Without exception, 
>each year the kids are amazed, ecstatic about the beauty Kiva.  One 
>young man even burst out saying, "I'm going to come back and get 
>married here!" ( http://www.tribesproject.org )
>
>I encourage all natural builders to continue thinking "outside the 
>box".  Sometimes the coblist seems to have a lot of conjecture, 
>speculation, intellectual ponderings, too much for my taste.  At 
>those times I just want to tell us all to get out there and get our 
>hands and feet in the mud, to get building and post some 
>pictures...I'll be getting some up of the Kiva soon!  Let's all keep 
>cobbing!
>
>Blessings,
>Ocean
>
>Steward, Ahimsa Sanctuary http://www.peacemaking.org
>Proprietor, Intaba's Restaurant
>http://www.intabas.com
>
>On Dec 11, 2004, at 12:37 AM, laythss at yahoo.com wrote:
>
>>Ian,
>>   WHat I am trying to do in my thesis, is to see the eligibility of 
>>cob (among other earth buildings) to be used in the mainstream 
>>residential housing.  My only problem (for the general public) is 
>>the test that was made in BC was for a round structure.  A round 
>>structure is known to be stronger than a rectangular structure. 
>>This is all good for the exception that a round (curved wall) 
>>structure is not so much a main-stream structure.  and yes using 
>>rebar might not be the smartest thing, yet with my limited 
>>engineering background I think it is still better than building 
>>with wood or concrete, since they impact the environment far more 
>>than cob or cements/earth rammed earth structures even if they are 
>>rebarrred.
>>Ocean in regards to rammed earth, the cement/rammed earth uses 
>>between 5%-15% cement in it, isn't this better than a 100% cement 
>>structure environmentally. I know it is not the best, but wouldn't 
>>be a good start?
>>
>>Layth
>
>
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