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.
Church of the Holy Cross (Episcopal) Stateburg or Holy Cross Episcopal Church in Stateburg, South Carolina, built of rammed earth in 1850–1852
Rammed earth walls form part of the entrance building for the Eden Project in Cornwall, England.
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 trombe wall built by the University of Utah's DesignbuildBluff project
Overview of Use
Using the rammed earth technique 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. Traditionally, substances such as lime and animal blood were used to stabilize the material, but cement has been the stabilizer of choice for modern times. After the earth is compressed, the wall frames can be immediately removed; they 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, the result is much like a wall of solid rock.
Formwork is set up to create the desired shape of the section of wall; damp material is poured in to a depth of between 100 and 250 mm (4–10 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. Immediate removal of the forms is necessary if wire brushing is gogin to be done to reveal texture; otherwise, walls become too hard to brush after around 60 minutes. Walls take some time to dry out completely, but drying time does not prevent further work on the project. Any exposed walls should 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 colored 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 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 are good for soundproofing, and the materials used in the walls make them virtually fireproof.
Prior to the use of cement as a stabilizer, rammed-earth buildings were most successful in dry climates where there was a limited availability of building materials other than earth. Rammed earth has become a viable material in wetter climates, either through the use of cement stabilization, through placing the earth walls within the weatherproof fabric of the building, or through the application of external insulation and weatherproofing.
History
Partially rammed-earth wall (with the upper 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 the 1820s
In the 1800s in the United States, rammed earth was popularized by the 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' pisé 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."
From the 1920s through the 1940s, millions of dollars were spent by the US government and several western universities to research rammed earth construction. South Dakota State College carried out extensive research and built almost 100 weathering walls of rammed earth. 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 (USDA) 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.
The US Agency for International Development has spent millions of dollars teaching undeveloped countries building science about rammed-earth houses. The agency 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.
Interest in rammed earth fell after World War II when the costs for conventional building materials dropped. Rammed earth became viewed as substandard, and it suffered from the prejudice that it seemed too basic in the face of new technology and was 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 customary method of construction seems easier, and using rammed earth for a profitable investment seems too uncertain, so rammed earth construction is often neglected in conventional building cultures.
Rammed Earth in Green Building
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 soil can be used right from the construction site, so that the costs and energy use associated with transportation are reduced. Rammed earth 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% 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 of the cost of standard frame houses. Rammed earth can carry a heavy load, and using rebar, 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. The compression strength of rammed earth can be up to 625 lb/in.2. 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 no toxic treatments and have no risk of off-gassing toxic fumes, making the dwellings ideal for chemically sensitive people. 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 lb/in.2) 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 lb/in.2) from a cube strength fcu = 30N/mm2–40N/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 not only an economically viable construction technique, but it also 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 in. 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 the thermal mass of the material make 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 in. thick. At this thickness, the walls are ideal for controlling humidity and also for reducing the noise from traffic, furnaces, compressors, fans, or ducts. Rammed earth also allows more air exchange than concrete structures. Increased air exchange means the building can "breathe"; it does not become clammy, but it also does not have significant heat loss, since the material mass absorbs the temperature.
By its very nature, earth is one of the best sustainable building materials, and it is historically the longest-used material. 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, and it is fireproof and soundproof. Greater use of rammed earth could help solve much of the world's homelessness problem and could also help alleviate the ecological damage caused by deforestation and toxic building materials.
External Links
- Earth Architecture — A website whose focus is contemporary issues in earth architecture.
- Historic Rammed Earth — A good site about the history of rammed earth throughout the world, and research into preservation techniques.
- Rammed Earth Home Building — Learn about how rammed-earth homes are built and find firms that build them across the US.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "
Rammed earth."
