Rammed earth

Rammed earth walls form part of the entrance building for the Eden Project in Cornwall, England.

Rammed earth, also known as pisé de terre or simply pisé, is a type of construction material. It is an age-old building method that has seen a revival in recent years as people seek low-impact building materials and natural building methods. Traditionally, rammed earth buildings are common in arid regions where wood is in scarce supply.

Overview of use

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 has been the stabilizer of choice for modern times. 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 reveal 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 the 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.

History

Partially rammed earth wall (with the upper level portion of mud brick) located at Jiayuguan, China, built during the Ming Dynasty (1368–1644).

One of many pictures available of buildings of the Borough House Plantation, built in 1820's, in Stateburg, South Carolina.

The age of this proven earth building technique isn't exactly known. What is known is that in the following 2000 year chronology Portland cement has not been a feature up until the late 20th century. Pliny's report about Hannibal's rammed earth watch towers shows it in evidence a couple of hundred years before Christ. The idea of stuffing or tamping mud into a form had to have occurred long before that, as adobe making began thousands of years B.C. and certainly the two methods suggest each other.

Evidence of the early use of rammed earth as been seen in Neolithic archaeological sites of the Yangshao culture and the Longshan culture in China along the Yellow River dating back to 5000 BCE. By 2000 BCE, the use of rammed earth architectural techniques was commonly used for walls and foundations in China.

In the 1800s in the United States, rammed earth was popularized by a book Rural Economy by S. W. Johnson. For example, it was used to construct Borough House Plantation and Church of the Holy Cross in South Carolina, which are two National Historic Landmarks of the United States. The National Historic Landmark description for one states:

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.

Church of the Holy Cross (Episcopal) Stateburg or Holy Cross Episcopal Church in Stateburg, South Carolina, built of rammed earth in 1850–1852

An outstanding example of rammed earth construction in Canada is St. Thomas Anglican Church (Shanty Bay, Ontario) built between 1838 and 1841.

During the 1920s through the 1940s millions of dollars were spent by the US Government and several western universities researching rammed earth construction. South Dakota State College carried out extensive research and built almost 100 weathering walls of rammed earth. Over a period of thirty years of exploration the college researched the use of paints and plasters relation of colloids in soil. In 1945 Clemson Agricultural College of South Carolina published their results on 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 at a very reasonable cost and sold to the public, along with tracts of land sufficient enough for a garden and small livestock plots. The project was a success and provided valuable homes to low-income families.

Rammed earth trombe wall built by the University of Utah's Design Build Bluff project

The U.S. Agency for International Development has spent millions of dollars teaching undeveloped countries building science about 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 never available for purchase by the public until the Rammed Earth Institute International gained permission to reprint it.

Interests in rammed earth fell after World War II when the costs of modern building materials dropped. Rammed earth became viewed as substandard and it suffered from the prejudice that using earth technique seemed too basic in the face of new technology and too dependent on labor intensive methods. Soil as a building material meets opposition with many contractors, engineers, and tradesmen who are unfamiliar with earth construction techniques. Often the modern method of construction seems easier. Profitable investment seems too uncertain so rammed earth construction is often neglected in modern building cultures.

Rammed earth in green building

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.

Rammed earth structures are beneficial for natural building because they can utilize locally available materials with little embodied energy and harmful waste. Earth is a widely available building material with virtually no side effects associated with harvesting for use in construction. The earth used is typically subsoil, leaving topsoil readily available for agricultural uses. Often the soil can be used on the site where the construction takes place reducing cost and energy used for transportation. It is also affordable to build with, as the materials are inexpensive or free. It is a viable building material for low- income builders with help from unskilled workers, friends, or family. 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 bottom 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 is a time consuming project. With a mechanical tamper and the forms ready it can take about 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 or even glass fiber can be used. The form wall faces must be externally reinforced with laterally running beams to prevent outward bending of the wall faces during the compression process. The two opposing wall faces must be clamped together and the wall edges need to be 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 won’t infest rammed earth walls and the material is reusable, biodegradable and highly fire resistant. The walls require non toxic treatments and have no risk of off-gassing toxic fumes, making it ideal for chemically sensitive dwellers. Properly built rammed earth can withstand loads for thousands of years as the history of rammed earth structures around the world has proven. Stucco can finish the walls in almost any color or style; 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.

In the UK it has been suggested that a compression strength of 2N/mm² (290 pounds per square inch) should be assumed in the absence of data derived from testing of the earth that will be used. Concrete typically used in UK construction is mixed off site and has a compression strength of 12-16N/mm² (1700–2300 pounds per square inch, from a cube strength fcu = 30N/mm² to 40N/mm²), around seven times stronger than rammed earth. However, there are many factors that affect the width of a wall, so a plain concrete wall will not necessarily be much thinner than an equivalent in rammed earth.

Rammed earth is a green material for a number of reasons. It does provide good thermal mass, widely recognised as the only way of storing heat and coolth. It also controls humidity where walls contain clay which is exposed to an internal space. Humidity is held between 40% and 60%, the lower and upper trigger points for asthma sufferers as it happens, as well as the ideal humidity for the storage of items such as books. These effects come as part of the package when cement is not used. When cement is used these effects are not present. Cement also adds to the global carbon dioxide burden at a rate of 1.25tonnes 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 300mm thick 'earth' wall with cement in will typically have a higher emissions burden than a 115mm concrete block wall, and therefore cannot seriously be considered green.

Rammed earth is not only an economically viable construction technique, it results in pleasant, and energy-efficient buildings. The density and thickness of rammed earth makes it so that hot or 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 practical 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 allowing the building to breathe and not become clammy without significant heat loss as the material mass absorbs the temperature as the wall breathes.

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 can contribute to a solution for much of the world of homelessness caused by high costs as well as today ecological dilemma caused by deforestation and toxic building materials.

See also

• Adobe
• Building construction
• Cob (material)
• Compressed earth block
• Earthbag construction
• Dutch brick
• Mudbrick
• Super Adobe

References

• Western Living "earth to saltspring", October 2003.