Rammed earth, also known as taipa (Portuguese), tapial (Spanish), and pisé (de terre) (French), is a technique for building walls using the raw materials of earth, chalk, lime and gravel. It is an ancient building method that has seen a revival in recent years as people seek more sustainable building materials and natural building methods. Rammed-earth walls are simple to construct, noncombustible, thermally massive, strong, and durable. They can be labour-intensive to construct without machinery (powered tampers), however, and they are susceptible to water damage if inadequately protected or maintained. Rammed-earth buildings are found on every continent except Antarctica, in a range of environments that includes the temperate and wet regions of northern Europe, semiarid deserts, mountain areas and the tropics. The availability of useful soil and a building design appropriate for local climatic conditions are the factors that favour its use.
Overview of use
Building a rammed-earth wall involves compressing a damp mixture of earth that has suitable proportions of sand, gravel and clay (sometimes with an added stabilizer) into an externally supported frame or mould, creating either a solid wall of earth or individual blocks. Historically, such additives as lime or animal blood were used to stabilize the material, whilst modern construction uses lime, cement or asphalt emulsions. Some modern builders also add coloured oxides or other items, such as bottles, tires, or pieces of timber, to add variety to the structure.
The construction of an entire wall begins with a temporary frame (formwork), usually made of wood or plywood, to act as a mould for the desired shape and dimensions of each wall section. The form must be sturdy and well braced, and the two opposing wall faces clamped together, to prevent bulging or deformation from the large compression forces involved. Damp material is poured in to a depth of 10 to 25 cm (4 to 10 in) and then compacted to around 50% of its original height. The material is compressed iteratively, in batches, gradually building the wall up to the top of the frame. Tamping was historically done by hand with a long ramming pole, and was very labour-intensive; modern construction can be made less labour-intensive by employing pneumatically powered tampers.
Once a wall is complete, it is strong enough for the frames to be immediately removed. This is in fact necessary if a surface texture is to be applied (e.g., by wire-brushing, carving, or mould impression), since the walls become too hard to work after about an hour. Construction is best done in warm weather so that the walls can dry and harden. The compression strength of the rammed earth increases as it cures; it takes some time to dry out, as much as two years for complete curing. Exposed walls should be sealed to prevent water damage.
Where blocks made of rammed earth are used, they are generally stacked like regular blocks but are bonded together with a thin mud slurry instead of cement. Special machines, usually powered by small engines and often portable, are used to compress the earth into blocks.
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.
Environmental aspects and sustainability
Because rammed-earth structures use locally available materials, they usually have low embodied energy and generate very little waste. The soils used are typically subsoils low in clay (between 5% and 15%), the topsoil being retained for agricultural use. Where soil excavated in preparing the building's foundation can be used, the cost and energy consumption for transportation are minimal.
Rammed-earth buildings reduce the need for lumber because the formwork is removable and can be repeatedly reused.
When cement is used in the earth mixture, sustainable benefits such as low embodied energy and humidity control will not be realized. Manufacture of the cement itself adds to the global carbon dioxide burden at a rate of 1.25 tonnes per tonne of cement produced. Partial substitution of cement with alternatives such as ground granulated blast furnace slag has not been shown to be effective, and raises further sustainability questions.
Rammed earth can contribute to the overall energy-efficiency of buildings. The density, thickness and thermal conductivity of rammed earth make it a particularly suitable material for passive solar heating. Warmth takes almost 12 hours to work its way through a wall 35 cm (14 in) thick.
Rammed-earth housing has been shown to resolve problems with homelessness caused by otherwise high building costs and also to help address the ecological impacts of deforestation and the toxicity of building materials associated with conventional construction methods.
Evidence of the early use of rammed earth has been seen in Neolithic archaeological sites of the Yangshao and Longshan cultures along the Yellow River in China, dating back to 5000 BCE. By 2000 BCE, rammed-earth architectural techniques (hangtu 夯土) were commonly used for walls and foundations in China.
In the 1800s, rammed earth was popularized in the United States through the book Rural Economy by S.W. Johnson. The method was used to construct the Borough House Plantation and the Church of the Holy Cross in South Carolina, both US National Historic Landmarks.
Constructed in 1821, the Borough House Plantation complex contains the oldest and largest collection of 'high style' pise de terre (rammed earth) buildings in the United States. Six of the 27 dependencies and portions of the main house were constructed using this ancient technique which was introduced to this country in 1806 through the book Rural Economy, by S.W. Johnson.
The period from the 1920s through the 1940s was active for studies of rammed-earth construction in the US. South Dakota State College carried out extensive research and built almost a hundred weathering walls of rammed earth. Over a period of thirty years the college investigated the use of paints and plasters in relation to colloids in soil. In 1945 Clemson Agricultural College of South Carolina published the results of their rammed-earth research in a pamphlet called "Rammed Earth Building Construction". In 1936, on a homestead near Gardendale, Alabama, the United States Department of Agriculture constructed an experimental community of rammed-earth buildings with architect Thomas Hibben. The houses were built inexpensively, and were sold to the public along with sufficient land for a garden and small livestock plots. The project was a success and provided valuable homes to low-income families.
The U.S. Agency for International Development is working with undeveloped countries to improve the engineering of rammed-earth houses. They also financed the writing of the Handbook of Rammed Earth by Texas A&M University and the Texas Transportation Institute. The handbook was unavailable for purchase by the public until the Rammed Earth Institute International gained permission to reprint it.
Interest in rammed earth fell after World War II when the costs of modern building materials dropped. Rammed earth was considered substandard, and still meets opposition from many contractors, engineers, and tradesmen who are unfamiliar with earth construction techniques. The prevailing perception that such materials and construction do not fare well in earthquake-prone regions has prevented its use in much of the world. In Chile, for example, rammed earth structures normally cannot receive conventional insurance or even government approval in most cases.
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