Rammed earth

Rammed earth is a technique for constructing foundations, floors, and walls using compacted natural raw materials such as earth, chalk, lime, or gravel. It is an ancient method that has been revived recently as a sustainable building method. Under its French name of pisé it is also a material for sculptures, usually small and made in molds. It has been especially used in Central Asia and Tibetan art, and sometimes in China. Edifices formed of rammed earth are on every continent except Antarctica, in a range of environments including temperate, wet, semiarid desert, montane, and tropical regions. The availability of suitable soil and a building design appropriate for local climatic conditions are the factors that favour its use. The French term "pisé de terre" or "terre pisé" was sometimes used in English for architectural uses, especially in the 19th century. Making rammed earth involves compacting a damp mixture of subsoil that has suitable proportions of sand, gravel, clay, silt and stabilizer, if any, into a formwork (an externally supported frame or mold). Historically, additives such as lime or animal blood were used to stabilize it. Soil mix is poured into the formwork to a depth of and then compacted to approximately 50% of its original volume. The soil is compacted iteratively, in batches or courses, so as to gradually erect the wall up to the top of the formwork. Tamping was historically manual with a long ramming pole by hand, but modern construction systems can employ pneumatically-powered tampers. After a wall is complete, it is sufficiently strong to immediately remove the formwork. This is necessary if a surface texture is to be applied, e.g., by wire brushing, carving, or mold impression, because the walls become too hard to work after approximately one hour. The compressive strength of rammed earth increases as it cures. Cement-stabilised rammed earth is cured for a minimum period of 28 days. In modern rammed earth buildings, the walls are constructed on top of conventional footings or a reinforced concrete slab base.