Wed 2 Jun 2010
Posted by Marcin
Our compressive strength tests have revealed that we could, in principle, build a 795 foot high wall with our unstabilized compressed earth blocks. Here’s how we arrived at this.
Prototype I of The Liberator lay gathering dust for over a year after it served its initial purpose. We have converted it into a 20 ton shop press, and used it for testing the compressive strength of the blocks produced by the The Liberator Beta v2.0.
We obtained 795 psi as the maximum compressive strength for unstabilized blocks made from on-site soil, and over 1200 PSI for blocks made from reject lime stabilized with 10% Portland cement by volume. The latter cured for 2 days only. For comparison, building code requirements are 300 psi compressive strength for earthen construction. Here are some of the results:
The first block you saw in the video went up to 3000 psi on the gauge, and the last one was the unstabilized brick that reached 1800 psi. This is converted to compressive strength as reported above – by considering the 4.5″ hydraulic cylinder, and 36 square inch pressing area upon the brick.
The experimental method is based on the 2003 New Mexico Earthen Building Materials Code paragraph J of Title 18.104.22.168. Paragraph J defines the compressive strength as load per area, where the load is applied with a ‘swivel head to accommodate nonparallel bearing surfaces’. We used a cylinder with a swivel head, and you can see in the video how that enabled the pressure to be applied evenly over the brick surface. The plate was pressing on a surface of 36 square inches. We increased the pressure roughly at 2000 psi per minute to save time, as there were no immediately visible differences in test results when the pressure increase rate was raised from 500 to 2000 psi per minute.
The closeup of the pressure gauge in the video shows the pressure going up to 1800 psi on the gauge, for a hydraulic cylinder of 4.5″ diameter bore. This translates to 795 psi for the 36 square inches of brick surface area that was receiving the cylinder force.
What does 795 psi mean? It means that we can build a wall at least 795 feet high before the bottom course of bricks would start to collapse under the weight of the wall. Each 4″ tall brick weighs about 20 pounds, and has a surface area of 72 square inches. This implies 0.3 psi created by the weight of each brick. It takes 3 bricks to go up a foot, so every foot of height creates under 1 psi of force from the weight of the bricks. Hence, we can build a wall at least 795 feet high with unstabilized bricks, and over 1200 feet high with the stabilized lime bricks that we demonstrated. If we settle for a single story house in the meantime, we should not have any structural problems.
An example of the stabilized reject lime bricks is the front brick in this picture:
We have heard from Floyd, the local CEB builder, that the reject lime bricks are strong and waterproof when stabilized. They are easy to produce, as reject lime flows easily, and it costs about $5 per ton at the quarry. We have verified the strength, though we must comment that prior to setting, these bricks are extremely fragile, and one must take utmost caution lifting these from the production line. We expect significant breakage from handling these, as they will break in half from their own weight when held by the far corners. That is what we experienced, but there may be tricks for handling these more effectively. There must be a good method for handling these by placing them or rolling them onto trays for carrying.
The cost of cement in the 10%-stabilized reject lime brick was about 25 cents. 5% stabilization, or about 12 cents per brick in stabilizer cost, would probably yield good results as well. In the future, it would be useful to burn local limestone to generate lime for stabilization – as part of a natural economy.
We did not have much luck with stabilized soil bricks. After 2 days of curing, the cement did little to increase their strength. We went from 2% to 14% stabilization by volume, and the best result (about 600 psi) was lower than our best unstabilized, but well-cured, result. We suspect these possible causes: (1), our soil composition was poor, including presence of some organic matter; (2), we did not have enough curing time; and, (3), the brick moisture level was improper. Unless we figure out why the stabilized bricks are not showing significant improvement in compressive strength, we may use the stabilized reject lime bricks for the bottom course in our building, or wherever moisture stabilization is required. We welcome suggestions from those with experience as to the details involved with successful brick stabilization with Portland cement.
For fun, we also did a compression test on a fired clay brick that we had lying around. It cracked under a pressure of about 2000 psi.
With further experience, we may be able to increase the compressive strengths significantly – where soil composition appears to be the major issue. It is not that our soil is inadequate, but it produces suboptimal results, if we believe Wikipedia. According to Wikipedia, compressive strenghts of 1200-1400 psi are common. As we stand, the pressed bricks that we obtained acquire a strength higher than the strength used to compress them – about 1.5 times as large.
To sum up, we’ve got a powerful machine on our hands. We’ve gotten outstanding results on the pressing rates, and decent results in compression tests. The prospects for effective construction are great, and now we only need an improved soil pulverizer and LifeTrac II for absolute robustness in our forthcoming building adventures. With the increased brick production rates achieved, the Soil Pulverizer Prototype I may be insufficient to keep up with The Liberator.
The great part about all of this is that our work is open source, and The Liberator is the first authentically replicable device coming from our skunkworks. We finally have product, as opposed to unfinished work in need of improvements. We’re preparing detailed fabrication and operation documentation at present, and we welcome replication of this open enterprise by those with the merit to move the work of creating resilient communities forward. In our view, open enterprise is the only substantial route to changing the world, and we are not being materialistic when we say that. We are just observing historical progress and regress of humanity. We await the positive economic impact that may happen from replication and adoption of this work in a distributed fashion. Openness of the project is one key to this.
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