Tuesday, 25 October 2011

Testing, Testing 1 2 3 4 5 6 7 8 9 10 11 12

The first thing I had to do was test the soil on and around our plot to see if it was suitable for building with cob. A local carpenter made brick moulds to my specifications from wood from forestry land. We employed a young man from our village to collect different sands and clay in his scotch cart from areas close by. Cob needs to have the right proportions of clay and sand, we found there was a very good clay deposit just below our plot, the sand on the plot is very fine so river sand was also collected as a good particle range is needed. Straw is essential for tensile strength in the mix, wheat is supposed to be the best. There is no farming of wheat in our area so for the tests I used wild grass that had been baled once it had dispersed it's seeds.
I made 12 mixes, using different proportions of sand and clay, the most fun method of mixing cob is on a  tarp with your feet. All the sand water and clay are mixed with your feet and then by rolling the tarp in different directions to make sure everything is well mixed, the straw is added last.
"You know your job, you dance like an elephant" 'Yabani' on seeing me mixing cob with my feet!

The next step was to make bricks using the brick moulds, this is only for testing the strength of the mix as cob is applied wet to the foundation/stem wall. I made the bricks numbering each mix and small balls to test the compressive strength.

After a week of drying(and turning every couple of days) the bricks were ready to test. To do this I dropped the bricks on a hard surface to see how and if they broke! Most of them crack at least a bit, but I found the perfect mix!! 2 1/2 buckets of pit sand(dug from the plot) 1 1/2 buckets of river sand and 1 bucket of clay, plus the straw makes a very strong mix, so I am very pleased!

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