Stabilized Earth FAQ

Definitions (2)

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“Stabilized Earth” (SE) or “Stabilized Rammed Earth” (SRE) is compressed or rammed earth, in block or monolithic form, that has been strengthened with rebar and/or additives, such as Portland cement, lime, asphalt emulsion, waterproofing agents, sodium silicate (liquid glass), manure, earth fibers (e.g., straw or hemp), and other inorganic or organic compounds. The purpose of adding stabilizers is to approach or surpass the strength of poured concrete while at the same time adopting more “green” processes and ingredients. The raw “earth” materials of stabilized earth include gravel (usually 3/4 inch or 3/8 inch), sand, chalk, clay, silt, local soil, coloring oxides such as iron, and other natural ingredients, such as crushed limestone. The development of stabilized earth block and walls is both half science and half art.

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For many earth builders, there is no distinction between the two. However, purists would argue that earth walls and earth bricks that use stabilizers, such as Portland cement, are really concrete structures and go against the movement to adopt more “green” alternatives and technologies in home and commercial building structures. For the idealists, however, the practice of using non-green additives to earth is an essential and intermediary stage of the development of a modern Stabilized Earth Science. Building inspectors in many developed countries are reluctant to approve earth-only structures. The adoption of cement, rebar, and waterproofing agents, allows home builders an easier path to meet current building codes.

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Secrets of the Mix (2)

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There is no rule-of-thumb for proper mix ratios in SE walls. It is a process of testing various mixes and curing processes. Mix ratios depend upon what kind of soil you have locally, as well as what kind of access you may have to inexpensive local materials. Typically, an SE wall will have between 5 to 15% cement with 10% being the most common ratio. Higher concentrations of cement are used for higher-load bearing walls, as well as retaining walls or basements. Road base and sand can be added to the cement along with a waterproofing agent to create a wall with PSI ranges from 1500 to 3000 PSI. Some rammed earth builders have achieved PSI ratings of 3800, stronger than commercial concrete. The science of stabilized earth is still evolving and hopefully one day will reach a stage where earth builders will be able to create near-stone like materials such as sandstone or granite using minimal amounts of energy.

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For an SRE mix, water is added  until the mix can compact into a ball without breaking apart. When the ball is dropped, or broken up by the fingers, it breaks apart into small pieces of clumped parts. A mix that is too dry will be difficult to compact while a mix that is too wet will be impossible to compact. Many rammed earth builders develop a feel for the mix knowing by color and texture when the mix is ready. The matter is further complicated by the way slightly wet and slightly dry mixes compact. A slightly wet mix compacted hard will ten to create a smooth marble like texture at its top, while a slightly dry mix compacted more lightly will create a rougher sandstone like texture.

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