Everything about Rammed Earth totally explained
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. 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's 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 250mm (4 to 10 inches). 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 doesn't 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 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 isn't 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.
Prior to the use of
cement as a stabilizer, rammed earth buildings were most successful in dry climates with 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 by the application of external insulation and weatherproofing.
History
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."
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. 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.
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 foot house. 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. 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. 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 won't necessarily be much thinner than an equivalent in rammed earth.
Rammed earth isn't 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 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 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's 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 isn't 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.
Further Information
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