Compressed earth block

Building a CEB project in Midland, Texas in August 2006

A compressed earth block (CEB), also known as a pressed earth block or a compressed soil block, is a building material made primarily from damp soil compressed at high pressure to form blocks. Compressed earth blocks use a mechanical press to form blocks out of an appropriate mix of fairly dry inorganic subsoil, non-expansive clay and aggregate. If the blocks are stabilized with a chemical binder such as Portland cement they are called compressed stabilized earth block (CSEB) or stabilized earth block (SEB). Typically, around 3,000 psi (21 MPa) is applied in compression, and the original soil volume is reduced by about half.

Creating CEBs differs from rammed earth in that the latter uses a larger formwork into which earth is poured and manually tamped down, creating larger forms such as a whole wall or more at one time rather than building blocks. CEBs differ from mud bricks in that the latter are not compressed and solidify through chemical changes that take place as they air dry. The compression strength of properly made CEB can meet or exceed that of typical cement or mud brick. Building standards have been developed for CEB.

CEBs are assembled onto walls using standard bricklaying and masonry techniques. The mortar may be a simple slurry made of the same soil/clay mix without aggregate, spread or brushed very thinly between the blocks for bonding, or cement mortar may also be used for high strength, or when construction during freeze-thaw cycles causes stability issues. Hydraform blocks are shaped to be interlocking.

Development

CEB technology has been developed for low-cost construction, as an alternative to adobe, and with some advantages. A commercial industry has been advanced by eco-friendly contractors, manufacturers of the mechanical presses, and by cultural acceptance of the method. In the United States, most general contractors building with CEB are in the Southwestern states: New Mexico, Colorado, Arizona, California, and to a lesser extent in Texas. The methods and presses have been used for many years in Mexico, and in developing countries.

The South African Department of Water Affairs and Forestry considers that CEB, locally called "Dutch brick" is an appropriate technology for a developing country, as are adobe, rammed earth and cob. All use natural building materials.^[1] In 2002 the International Institute for Energy Conservation was one of the winners of a World Bank Development Marketplace Award for a project to make an energy-efficient Dutch brick-making machine for home construction in South Africa. By making cheaper bricks that use earth, the project would reduce housing costs while stimulating the building industry.^[2] The machine would be mobile, allowing bricks to be made locally from earth.^[3]

Various types of CEB production machines exist, from manual to semi-automated and fully automated, with increasing capital-investment and production rates, and decreased labor. Automated machines are more common in the developed world, and manual machines in the developing world.

Advantages

There are many advantages of the CEB system. On-site materials can be used, which reduces cost, minimizes shipping costs for materials, and increases efficiency and sustainability. The wait-time required to obtain materials is minimal, because after the blocks are pressed, materials are available very soon after a short drying period. The uniformity of the blocks simplifies construction, and minimizes or eliminates the need for mortar, thus reducing both the labor and materials costs. The blocks are strong, stable, water-resistant and long-lasting.

• CEB can be pressed from damp earth. Because it is not wet, the drying time is much shorter. Some soil conditions permit the blocks to go straight from the press onto the wall. A single mechanical press can produce from 800 to over 5,000 blocks per day, enough to build a 1,200 square feet (110 m²) house in one day. A high performance CEB press, of open source design, named "The Liberator", can produce from 8,000 to 17,000 or more blocks per day. The production rate is limited more by the ability to get material into the machine, than the machine itself.
• Shipping cost: Suitable soils are often available at or near the construction site. Adobe and CEB are of similar weight, but distance from a source supply gives CEB an advantage. Also, CEB can be made available in places where adobe manufacturing operations are non-existent.
• Uniformity: CEB can be manufactured to a predictable size and has true flat sides and 90-degree angle edges. This makes design and costing easier. This also provides the contractor the option of making the exteriors look like conventional stucco houses.
• Presses allow blocks to be consistently made of uniform size, while also obtaining strengths that exceed the ASTM standard for concrete blocks (1900 psi).
• Non-toxic: materials are completely natural, non-toxic, and do not out-gas
• Sound resistant: an important feature in high-density neighborhoods, residential areas adjacent to industrial zones
• Fire resistant: earthen walls do not burn
• Insect resistant: Insects are discouraged because the walls are solid and very dense, and have no food value
• Mold resistant: there is no cellulose material - such as in wood, Oriented Strand Board or drywall - that can host mold or rot
• In India, CSEB's with cement stabilization have shown to be very beneficial. The observed compressive strength, flexural strength at 28 days of aging with 9% cement stabilization has been observed to be 3.2 MPa and 1 MPa respectively.^[4]

Presses

CEB had very limited use prior to the 1980s. It was known in the 1950s in South America, where one of the most well-known presses, the Cinva Ram, was developed by Raul Ramirez in the Inter-American Housing Center (CINVA) in Bogota, Colombia. The Cinva Ram is a single-block, manual-press that uses a long, hand-operated lever to drive a cam, generating high pressure.

Industrial manufacturers produce much larger machines that run with diesel or gasoline engines and hydraulic presses that receive the soil/aggregate mixture through a hopper. This is fed into a chamber to create a block that is then ejected onto a conveyor.

During the 1980s, soil-pressing technology became widespread. France, England, Germany, South Africa and Switzerland began to write standards. The Peace Corps, USAID, Habitat for Humanity and other programs began to implement it into housing projects.

Finishing

Completed walls require either a reinforced bond beam or a ring beam on top or between floors and if the blocks are not stabilized, a plaster finish, usually stucco wire/stucco cement and/or lime plaster. Stabilized blocks can be left exposed with no outer plaster finish. In tropical environments, polycarbonate varnish is often used to provide an additional layer of wet-weather protection.

Foundations

Standards for foundations are similar to those for brick walls. A CEB wall is heavy. Footings must be at least 10 inches thick, with a minimum width that is 33 percent greater than the wall width. If a stem wall is used, it shall extend to an elevation not less than eight inches (203 mm) above the exterior finish grade. Rubble-filled foundation trench designs with a reinforced concrete grade beam above are allowed to support CEB construction.

Strength

Using the ASTM D1633-00 stabilization standard, a pressed and cured block must be submerged in water for four hours. It is then pulled from the water and immediately subjected to a compression test. The blocks must score at least a 300 pound-force per square inch (p.s.i) (2 MPa) minimum. This is a higher standard than for adobe, which must score an average of at least 300 p.s.i. (2 MPa)

References