Several years ago now, my partner and I took a 1 week course on building with adobe bricks/cob. We are lucky here in New Zealand that some hard working people developed approved building standards for adobe brick construction. Our instructor, Richard Walker was one of the people responsible for this standard.
Our forest property sits on some ideal clay soil and we are interested in sourcing our new construction entirely from materials found on or near the site, so adobe should figure somewhere into our plans.
We did some experiments to test the suitability of our clay and the contents of our soil ( I wasn’t going to cover that here unless someone is interested but suffice it to say that 1 simple test is to fill a jar 3/4 with your soil sample add water and shake – when it settles out measure the layers of sand vs silt vs clay and determine the ratios of each). We determined that our soil was upwards of 50% clay and so was well within the range of suitability.
We made some test bricks about 1 1/2 years ago. A simple quality test we ran after about 8 weeks of drying our bricks – a drop test onto the corner of the brick from about 3 feet up onto a hard surface – indicated that our bricks were weak. It formed cracks along the entire length and the true benchmark was that greater than fist size chunks broke off.
Our conclusion was that we have too much clay in our soil, so this summer we wanted to try another experiment to determine a proper mix.
We got another load of soil from where we would most likely be digging our soil out. The location is ideal for brick making because it is flat ( it’s our logging skid site), open to the sun ( for drying bricks) and near a creek ( for a source of water). We dragged it back however to our current house and prepared 4 different mixes:
- Clay and extra straw – we thought maybe we didn’t have enough binder so we added extra straw this time.
- Clay and straw and some sand. We found a source of sand near the river which runs adjacent to our property, grabbed a couple of buckets of it and added 3 handfuls per brick to the mix from #1.The sand cuts the percentage of clay in the mix. The amount wasn’t very scientifically derived but we figure if we see a difference then we may try another experiment with varying amounts of sand.
- Clay and wood chips. Because this area is not known for much wheat or grain production, any straw we would use would have to be transported from across the country. As this doesn’t quite meet our criteria of sourcing from onsite or nearby materials, we thought we would try woodchips. There is a mill in town and they have mountains of these woodchips. We got a good tubful for our test. We’re also hoping that the woodchip brick might be a tad lighter and have better insulation properties.
- To #3 we again added 3 handfuls of sand per brick and made two more test bricks.
The bricks were formed with our wood form. I have to say the woodchip mixes were the easiest to form, but this may also be because by that time the form had been well soaked. I definitely recommend presoaking the wood mold. On our course we used a metal mold. I might just weld up a metal one and see which one we prefer. I think though the steel one is less sticky and therefore the brick comes out cleaner and easier.
We now have the bricks drying. Since February is our driest, hottest month, it might only take 4 weeks or so for the bricks to harden up and dry out, but we’ll probably leave it for 8 weeks.
We formed the bricks on a concrete pad which is inside a shed in the back of our garage.
Next entry will explore the results.
-- There's no way that machines can build houses, except sterile ones - Lloyd Kahn