Tuesday, June 23, 2009

Soil-Cement Finish Slab

posted by Taj Easton


Outline – Benefits of Approach


While trial and error has shown rammed earth floors to be somewhat impractical, we were determined to find an approach for this project that would allow us to maintain the natural and regionally appropriate aesthetic that would be created by the monolithic earth walls and ecologically sensitive design.


After careful pondering, we determined the best way to achieve this would be to pour a soil-cement slab, which would use the same locally-sourced clays that would constitute the primary component of the rammed earth walls as a coloring agent in the otherwise conventional concrete mix.


In conventional practice, this coloring would be achieved through the addition of chemical dies.  We found our approach to have many apparent benefits over the conventional approach.  These benefits included:


- cost (no chemical dies, slightly lower volumes of concrete required)

- environmental benefits [emissions (from transportation, die production, raw material extraction, etc.), reduce use of chemicals, removal of potential waste stream (if clay materials are extracted from the building site or nearby and will not have other uses), offset cement use]. 

- improved workability, slower set-times

- streamline aesthetics with local ecology and rammed earth walls




Having extracted an abundance of red clay from the building site during excavation and road-building, we had acquired a large stockpile of a beautiful, red kaolin clay that we could use to add to our concrete mix.


We determined through testing that a conventional concrete mix with an additional 6% red clay would produce the aesthetic we were looking for, without significantly changing the character of the cement, reducing compressive strengths, or drastically increasing shrinkage.  We than calculated the amount of water that this clay would demand on top of the water demand for the raw concrete mix. 


In a 1 cu. yd. concrete mixer, we added approximately 18.5 cu. ft. of red clay (to give us the amount we would want to achieve our 6% target for one concrete truck).  We then added water to the clay and begin mixing, carefully measuring the water that was going in to the mix in five gallon buckets.  We continued adding water and mixing (approx. 45 minutes) until the result was a thin liquid slurry that would easily pass through a sump pump and 50 ft., 2” diameter flat hose. 


Having requested the concrete trucks to deliver a very dry mix, we then pumped this slurry into the concrete truck on-site using the aforementioned equipment (sump-pump and flat hoses - which we rented from a local rental yard).  When the clay slurry had all been added to the concrete mix, we than added the rest of the water that would get the final mix to its optimum moisture content.  This water was added directly to the small concrete mixer and pumped through the same system as the clay slurry, assuring that all of the red clay in the mixer itself and the pump lines would make it into the final mix.  We decided to play it safe on the dry side, so we could check the moisture of the mix after a few minutes of mixing and add more water inside the concrete truck if needed. 


The rest of the operation worked like a conventional concrete pour, with the exception of slower-setting times and improved workability.  As the slab dried, we were pleased to see the color blend seamlessly with the surrounding landscape and the stock-piled red clay that would soon be built into the house’s monolithic earth walls… 


  1. Terra! I am waiting for your reports. I even created my own blog in order to post comments on your blog. Please make more pictures. Espesially, those when work in progress. ... and write more often. So, I can get a better idea what and how you do it. )))

    I am possessed by the rammed earth construction after I visited this place --> http://history-gatchina.ru/museum/priorat/eindex.htm