CEB Compressive Strength Test Results

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 14.7.4.23. 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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16 Comments »

  1. Klaus Leiss said,

    June 2, 2010 @ 3:26 pm

    Hello,
    according to the literature i have read the curing time is to short.
    The http://www.sheltercentre.org/library/Compressed+Earth+Blocks+Volume+1+Manual+Of+Production ( page 46 ) talks about at least 7 days curing time in a humid environment. Letting them dry premature stops the curing.

    Klaus Leiss

  2. jim moore said,

    June 2, 2010 @ 4:15 pm

    Cement Stabilized Brick
    The curing time is defiantly short. With concrete typically you take compression readings at 7 and 28 days. As for water to cement ratios go for:
    ~ one third as much water as cement. (for concrete, soil bricks may need more water)
    So a key variable is: How much water is in the earth you are compressing?

  3. LucasG said,

    June 3, 2010 @ 12:57 am

    Press bricks now so they’ll be cured by August 1st?

    Can curing be accelerated with a hot tunnel, and is that desirable?

    It seems the machine is more or less refined, but there’s still some more to learn about the process.

  4. Marcin said,

    June 3, 2010 @ 10:55 am

    The bricks can cure in the wall. They are more than strong enough to be laid immediately, and they harden over time. Laying bricks directly from the machine reduces the wall build time by a factor of 2.

  5. Joel said,

    June 4, 2010 @ 3:47 pm

    Nice! Congratulations on such high numbers.

    What are the confidence intervals on those mesurements? Or if you don’t have them, what are the variance & sample size?

    I think the limit in tower height would depend on your confidence that the lower courses don’t contain any bricks weak enough to give way…95% should be enough for all but the longest walls, but perhaps I’m babeling.

    You might consider testing modulus of rupture and/or impact toughness, as well.

    Also, might I suggest oyster shells over limestone as a source of raw Ca: they come in a convenient, uniform size that maintains ventilation without having a thick cross-section, and they’re renewable to boot.

  6. Klaus Leiss said,

    June 5, 2010 @ 2:21 am

    With cement stabilized blocks you will have to control the drying of the wall. If it dries to fast the curing stops. If you cover the walls in plastic foil you could be OK. You will have to experiment.

    Klaus Leiss

  7. Marcin said,

    June 5, 2010 @ 10:18 pm

    Our sample size was 23 bricks. Tests were performed at least twice on the same type of brick to ensure replicability.

  8. Full Product Release | Open Source Ecology said,

    June 13, 2010 @ 2:18 am

    [...] The Liberator has come a long way since its initial, manual prototype, to our not-so-effective building adventures, to Prototype II, to the soil pulverizer, to the first prototype of the automatic CEB controls, to beta version product release with manual controls, to the second prototype of automatic CEB controls, to initial results for the automatic machine indicating 13 bricks per minute, and to optimization resulting in the present Full Product Release – with fully automatic controls, with a maximum demonstrated pressing rate of 16 bricks per minute. You can also view our compressive strength testing results. [...]

  9. Marcin said,

    June 24, 2010 @ 12:47 pm

    From Ronald Gray:

    I would like you to begin to understand the nature of what you are doing from my standpoint.

    My name is Ron Gray and I live in Reno, Nevada. I am also working on and developing a CEB methodology for building purposes. So — I am quite aware of the amount of effort and time that is required to dedicate yourself to an effort that others see as futile.

    That being said, please, try to understand my criticism.

    The calculation that you are trying to utilize in saying that you can build a CEB wall 795 high is outrageous and very stupid to suppose or suggest. Please do not continue with this type of philosophy or transfer of informatiom to the general public on your website. It actually “undoes” the work that others in the industry are doing in order to bring about rational building codes. To serve the interest of ourselves as well as bring success to the industry.

    I am in a seismic 4 zone in Nevada. I have worked diligently with members of the AIA and various Fields of Engineering in order to move CEB construction forward in the State of Nevada. I have almost lost a marriage and every friendship in regard to my efforts. Still I, like you, press forward to an end goal that will reduce cost and bring about an economical and ecological approach to building. I wish you success in your endeavors, truly.

    Please — do not make unfounded claims in regard to the capacity of the CEB that you are producing unless you have a certified engineering laboratory to support your claims in regard to performance. It is foolish of anyone to do so.

    The particular material that you are working with has a site specific value and cannot be repeated with soil from other regions and under the same mix ratios. Bare this in mind. Others may come to believe that they can achieve the same rate of success as you in regard to their material source and this is incorrect and is not allowable under building departments nationwide.

    I look forward to checking in on your progress from time to time. Good fortune in your endeavors.

    Sincerely,

    Ronald M. Gray
    Independent Inspection Services
    Reno, Nevada

  10. Marcin said,

    June 24, 2010 @ 12:49 pm

    I just left the largest one of the largest structural engineering firms in Reno today and I am submitting a proposal through the firm in order to move forward with seismic studies at UNR on a shake table. We are making every attempt to speak to material design and installation techniques in order to move beyond the present “restrictive” building codes that are in place in the United States.

    I appreciate wholeheartedly your independent efforts that you have put forward to sustainable building, truly! I would like to have the opportunity to engage solid engineering and design practices in all areas of the United States and elsewhere to build earthen based buildings without “specialized” design and engineering.

    I wish you success in your endeavors.

    Sincerely,

    Ronald M. Gray
    Independent Inspection Services
    Reno, Nevada

  11. anon said,

    January 2, 2011 @ 5:12 pm

    CEB strength tests: http://earthbagbuilding.wordpress.com/2010/12/31/bullet-resistance-of-compressed-earth/

  12. Nick said,

    June 21, 2011 @ 10:59 am

    Does anyone know what this machine would weigh when fully assembled? I am interested in testing this machine in a remote area and am considering the logistics of such a device.

    Thanks,

    Nick

  13. Peter said,

    February 27, 2012 @ 11:58 am

    Marcin,

    Do you have any thoughts on the 10″ min thickness of CEB walls found in codes?

  14. JohnBldr said,

    March 23, 2012 @ 6:31 am

    Interesting tests. Unfortunately a little learning may be very dangerous. As an engineer and general contractor (40yrs) building systems have to be engineered for many other forces rather than just uni-dimensional compression. Lateral loading due to wind or earthquake zones create buckling forces in tall walls and columns.
    These can be overcome with proper reinforcing steel etc.

  15. TKN said,

    April 23, 2012 @ 8:26 pm

    I was curious about the mix. Having worked some with cob, I know that coarse, or even round sand adds compressive strength.
    Was there any sand in the mix?

  16. Hojo said,

    December 11, 2012 @ 2:42 pm

    JohnBldr,
    You are correct in that one metric does not a building material make. I would cite that the compressed earth block is over 200 years old and building practices for their use are established world wide. The US is one of the least educated on their properties and use. The two most important reinforcements are sheltering from drop of water and the ring beam. The ring beam is critical in seismic zones.

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