“Fieldwork” by Doug Fogelson [education]
Rammed earth, also known as taipa[1] (Portuguese), tapia (Spanish), pisé de terre or simply pisé (French), is a technique used in the building of 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, incombustible, thermally massive, very strong and hardwearing. Conversely they can be labour-intensive to construct without machinery (powered rammers), and if improperly protected or maintained they are susceptible to water damage. Traditionally, rammed earth buildings are found on every continent except Antarctica. From temperate and wet regions of north Europe [2]to semi dry deserts, mountain areas and the tropics. The availability of useful soil and building design for the local climatic conditions are the factors which favour its use.
Features and benefits
The compression strength of rammed earth can be up to 4.3 MPa (620 psi). This is less than the value of a similar thickness of concrete, but more than strong enough for use in domestic buildings.[3] Indeed, properly built rammed earth can withstand loads for thousands of years, as many still-standing ancient rammed earth structures around the world attest.[4] Rammed earth using re-bar, wood or bamboo reinforcement can prevent failure caused by earthquakes or heavy storms. Mixing cement with the soil mixture can also increase the structure’s load bearing capacity but can only be used in clay-poor mixtures. The USDA observed that rammed earth structures last indefinitely and could be built for no more than two-thirds the cost of standard frame houses.[5]
Rammed earth has been used around the world in a wide range of climatic conditions, from wet northern Europe to dry regions in Africa.
Soil is a widely available, low cost and sustainable resource, and harvesting it for use in construction has minimal environmental impact.[6] This makes rammed earth construction highly affordable and viable for low-income builders. Unskilled labor (often friends and family) are able to provide most of the necessary labor, and today more than 30 percent of the world’s population uses earth as a building material.[3]
While the cost of material is low, constructing rammed earth without mechanical tools can be a very time consuming project; 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.[3]
One of the significant benefits of rammed earth is its excellent thermal mass; like brick or concrete construction, it can absorb heat during the day and release it at night. This can even out daily temperature variations and reduce the need for air conditioning and heating. However rammed earth, also like brick and concrete, often requires insulation in colder climates. It must also be protected from heavy rain and insulated with vapor barriers.[4]
Rammed earth wall surface detail. Apart from the patches of damage, the surface shows regular horizontal lines from the wooden form work used in constructing the wall and subtler horizontal strata from the successive compacted layers of earth used to build the wall.
Untouched, the walls have the color and texture of natural earth. Blemishes can also be patched up using the soil mixture as a plaster and sanded smooth. Care needs to be taken to avoid moisture-impermeable finishes such as cement render, as these will impair the ability of the wall to desorb moisture, leading in turn to a loss of compressive strength.
The thickness and density of rammed earth walls, typically 300 to 350 millimetres (12 to 14 in) thick, lends itself naturally to soundproofing. Rammed earth walls are also termite-resistant, non-toxic, inherently fireproof and ultimately biodegradable.
Nails or screws can be driven easily into well-cured walls, and they can be effectively patched with the same material used to build them.
[edit] Environmental aspects and sustainability
Rammed earth trombe wall built by the University of Utah’s Design Build Bluff project
Because rammed earth structures use locally available materials, they typically have low embodied energy and generate very little waste. The soils used are typically subsoils low in clay, between 5% and 15% typically with the topsoil retained for agricultural use. Ideally, the soil removed in order to prepare the building foundation can be used, further reducing cost and energy used for transportation.[6]
Rammed earth buildings reduce the need for lumber because the formwork used is removable and can be continually reused.[7]
A Taipa section of the Great Wall of China
Rammed earth can effectively control humidity where unclad walls containing clay are exposed to an internal space. Humidity is held between 40% and 60% which is the ideal humidity range for asthma sufferers and the storage of susceptible items, such as books.
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[8]. Partial substitution of cement with alternatives such as ground granulated blast furnace slag has not been shown to be effective and brings other sustainability questions with it.[9]
Rammed earth can contribute to the overall energy-efficiency of buildings. The density, thickness and thermal conductivity of rammed earth makes it a particularly suitable material for passive solar heating. Warmth takes almost 12 hours to work its way through a 350-millimetre (14 in) thick wall.[3]
The material mass and clay content of rammed earth allows the building to “breathe” more than concrete structures, avoiding condensation issues without significant heat loss.[3]
Rammed earth housing has been shown to resolve problems with homelessness caused by otherwise high building costs, as well as to help address the ecological dilemma of deforestation and toxic building materials associated with conventional construction methods.[10]
