Characteristics
The compressive strength of rammed earth can be up to 4.3 MPa (620 psi). This is less than that of concrete, but more than strong enough for use in domestic buildings. Indeed, properly built rammed earth can withstand loads for thousands of years, as many still-standing ancient structures around the world attest. Rammed earth using rebar, wood or bamboo reinforcement can prevent failure caused by earthquakes or heavy storms. Adding cement to clay-poor soil mixtures can also increase a structure's load-bearing capacity. The USDA has observed that rammed-earth structures last indefinitely and could be built for no more than two-thirds the cost of standard frame houses.
Soil is a widely available, low-cost and sustainable resource, and utilizing it in construction has minimal environmental impact. This makes rammed-earth construction highly affordable and viable for low-income builders. Unskilled labour can do most of the necessary work, and today more than 30 percent of the world's population uses earth as a building material. Rammed earth has been used around the world in a wide range of climatic conditions, from wet northern Europe to dry regions in Africa.
While the cost of material is low, rammed-earth construction without mechanical tools can be very time-consuming; however, with a mechanical tamper and prefabricated formwork, it can take as little as two to three days to construct the walls for a 200 to 220 m2 (2,200 to 2,400 sq ft) house.
One of the significant benefits of rammed earth is its high thermal mass; like brick or concrete construction, it can absorb heat during the day and release it at night. This moderates daily temperature variations and reduces the need for air conditioning and heating. For colder climates, rammed earth walls can also be insulated with a Styrofoam or similar insert. It must also be protected from heavy rain and insulated with vapour barriers.
Rammed earth can effectively control humidity where unclad walls containing clay are exposed to an internal space. Humidity is held between 40% and 60%, the ideal range for asthma sufferers and for the storage of such susceptible items as books. The material mass and clay content of rammed earth allows the building to "breathe" more than concrete structures do, avoiding condensation issues without significant heat loss.
Untouched, rammed-earth walls have the colour and texture of natural earth. Moisture-impermeable finishes, such as cement render, are avoided because they impair the wall's ability to desorb moisture, necessary to preserve its strength. Well-cured walls accept nails and screws easily, and can be effectively patched with the same material used to build them. Blemishes can be repaired using the soil mixture as a plaster and sanded smooth.
The thickness, typically 30 to 35 centimetres (12 to 14 in), and density of rammed-earth walls make them suitable for soundproofing. They are also termite-resistant, non-toxic, inherently fireproof and ultimately biodegradable.
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