Rammed earth

Rammed earth, also known as taipa, pisé de terre or simply pisé, is a technique used in the building of walls using the raw materials of mud, 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. Because of the nature of the materials used it is incombustible, thermally insulating and very strong and hardwearing. It also has the added advantage of being a simple way to construct walls. Traditionally, rammed earth buildings are common in arid regions where wood is in scarce supply.

Using it involves a process of 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 that molds the shape of a wall section creating a solid wall of earth. Traditional stabilizers such as lime or animal blood were used to stabilize the material, but cement or asphalt emulsions have been the stabilizers of choice for modern times. (Lime, while hazardous to handle, is also making a comeback.) However the use of cement is contentious as its manufacture creates 10% of man made carbon emissions. After compressing the earth the wall frames can be immediately removed and require an extent of warm dry days after construction to dry and harden. The structure can take up to two years to completely cure, and the more it cures the stronger the structure becomes. When the process is complete it is much like constructing a hand made wall of solid rock.

Formwork is set up creating the desired shape of the section of wall; damp material is poured in to a depth of between 100 to 250 mm (4 to 10 in). A pneumatically powered backfill tamper — something like a hand-held pogo stick with a flat plate on the bottom or even a manual tamper — is then used to compact the material to around 50% of its original height. Further layers of material are added and the process is repeated until the wall has reached the desired height. The wall is so solid that, if desired, the forms can be removed immediately. This is necessary if wire brushing to add texture is desired; otherwise walls become too hard to brush after around 60 minutes. Walls take some time to dry out completely, but this does not prevent further work on the project. Any exposed walls may be sealed to prevent water damage — there are several proprietary products specifically designed to seal earth walls.

In modern variations of the method, rammed earth walls are constructed on top of conventional footings or a reinforced concrete base, sometimes with extra ground insulation from a horizontal layer of styrofoam. Some builders also add coloured oxides or other items such as bottles or pieces of timber to add variety to the structure.

Once completely cured the walls are very workable. It is easy to drive a nail or screw into them and they can be patched if necessary with the result being undetectable if the same material was used.

One of the significant benefits of rammed earth constructions is its excellent thermal mass; it heats up slowly during the day and releases its heat during the evening. This can even out daily temperature variations and reduce the need for air conditioning and heating. On the other hand, rammed earth is not a good insulator. Like brick and concrete (which also have excellent thermal mass), rammed earth is often insulated in colder climates. The thickness and density of the walls lends itself naturally to soundproofing and the materials used in the walls make them virtually fireproof.

Rammed earth has been used around the globe for millennia in a wide range of climatic conditions, from wet northern Europe to dry regions in Africa. Rammed earth walls may be placed within the weatherproof fabric of the building. Depending on conditions walls may also have external insulation, soft plaster, timber cladding or a number of locally specific finishes which are applied to masonry buildings.

Because rammed earth structures utilize locally available materials, they typically have a low embodied energy rating and generate very little waste. Earth used for building is a widely-available resource and harvesting it for use in construction has minimal environmental impact. The soils used are typically subsoils, retaining organic topsoil for agricultural use. Ideally, soil from the site where the construction takes place can be used, further reducing cost and energy used for transportation. The materials are often inexpensive or free, making it highly affordable and a viable building material for low-income builders, as unskilled labor, friends and family are able to contribute a great deal of the necessary work. Today more than 30 percent of the world's population uses earth as a building material.

Compressing the earth can be done manually using a tamper made of a heavy, flat-bottomed plate connected to a long vertical handle. Using a pneumatically-powered tamper, the material can be compressed with much less manual labor. Although 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 forms it can take as little as two to three days to construct the walls for a 2000–2200 sq ft house.

Rammed earth buildings reduce the need for lumber because the forms used are removable and can then be reused for different rammed earth wall construction. The forms are usually made of reinforced plywood, but sheet metal and even glass fiber can be used. The form wall faces are externally reinforced with laterally running beams to prevent outward bending of the wall faces during the compression process. The two opposing wall faces are clamped together and the wall edges securely compressed between the form faces, to withstand the high amounts of pressure created during compression.

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. Rammed earth can carry a heavy load and 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 problems arise in the mixing of cement with material containing clay. As a result common practice in the areas using cement on a regular basis (Australia and California) require sand and gravel mixes without a clay content. This is known as concrete by any other name except for the fact that the walls are built with a lower (too low?) water content which leaves the walls weaker than if they had been mixed as conventional concrete. The compression strength of rammed earth can be up to 625 pounds per square inch. This is only two-thirds the value of a similar thickness of concrete, but a rammed earth building is still a useful durable material. Termites will not infest rammed earth walls and the material is reusable, biodegradable and highly fire resistant. Properly-built rammed earth can withstand loads for thousands of years as the history of rammed earth structures around the world has proven. 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.

Rammed earth is a green material for a number of reasons. It provides good thermal mass, which implies good heat storage and absorption. It also controls humidity where walls contain clay which is exposed to an internal space. Humidity is held between 40% and 60% (coincidentally the lower and upper trigger points for asthma sufferers) as well as the ideal humidity for the storage of susceptible items such as books. When cement is used these effects are not present, and ecological benefits may not be realised. Cement adds to the global carbon dioxide burden at a rate of 1.25 tonnes per tonne of cement produced. Given cement blocks and walls rammed with a cement content vary in cement content in a range between 5% and 13%, a 300 mm thick 'earth' wall with cement in will typically have a higher emissions burden than a 115 mm concrete block wall, and therefore cannot seriously be considered green. However this may be substantially offset by the partial substitution of cement with alternatives such as ground granulated blast furnace slag.

Rammed earth is not only an economically viable construction technique, it results in pleasant, energy-efficient buildings. The density and thickness of rammed earth means cold temperature penetration has a slow rate of thermal conductivity. Warmth takes almost 12 hours to work its way through a 14-inch (360 mm) thick wall. The walls provide good thermal mass, which helps keep indoor temperatures stable, particularly in regions with dramatic daily temperature changes. The half-day rate of heat transfer and thermal mass of the material makes rammed earth a particularly suitable material for passive solar buildings. Rammed earth has been a popular choice for buildings where temperature fluctuations need to be kept to a minimum. It can be used in cooler climates but must be protected from heavy rain and insulated with vapor barriers.

Typically rammed earth walls are about 12 to 14 inches (360 mm) thick making them ideal for humidity control and noise barriers from traffic, furnaces, compressors, fans or ducts. Rammed earth also allows more air exchange than concrete structures, as the material mass allows the building to "breathe", avoiding condensation issues without significant heat loss.

By its very nature, earth is one of the best sustainable building materials as it is historically the longest used material by man. It is universally a naturally available product, with a heavy thermal mass and a natural barrier to cold winds and forces of nature including insects and rodents. The material is not rationed or monopolized, is fire proof, and sound proof. 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.

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