Rammed earth is one of the oldest construction methods, having been in use for thousands of years throughout human history and in a variety of climates. Recently, rammed earth construction has seen new interest, as people look for more sustainable methods of construction.
Rammed earth involves the process of compressing a mixture of earth, sand, gravel, clay, and occassionally cement into formwork to create a structural wall, generally 12-14 inches thick (18-24 if the wall is load bearing). The wall is layered with 4-10 inches of material at a time, which is then compressed using a peumatic tamper before the next layer is added. This process creates what can be a very beautiful layering effect present in rammed earth walls. After the formwork has been filled to the desired height, the formwork can be removed, allowing the wall to dry and harden. Exposed walls can be sealed to prevent water damage.
Generally, with newer construction, rammed earth will be constructed on top of conventional concrete footings or slabs.
Rammed earth is often used in dry, warm climates, but can be used in colder, wetter climates if protetected from heavy rain and insulated with vapor barriers.
Advantages
There are numerous advantages to using rammed earth construction:
Thermal Mass Because of the density and shear thickness required, rammed earth walls absorb heat, and slowly release it through the day and night. This results in a building that sees less thermal swings throughout the day, and reduces the need for less sustainable heating and cooling methods.
Noise Reduction
Because of the shear mass of a rammed earth wall, noise transfer is kept to a minimum, making it an ideal solution for reducing unwanted noise.
Maintenance
Rammed earth walls are pretty much maintenance free, and can be repaired quite easily. Nail holes can be filled seamlessly if the same mixture is used as the original wall.
Non-Combustible
Rammed earth doesn't burn, making it a great material for use in areas where there is a heightened fire danger. Fire ratings of four hours for a rammed earth wall is common.
Cost
Most of the costs associated with rammed earth is with labor, not material, which can be advantageous for projects where labor is inexpensive or donated. According to the USDA, rammed earth structures las indefinitely, and can potentially be built for two-thrids the cost of a standard frame house.
Less Construction Waste
Formwork can be reused for numerous projects, reducing the amount of waste as a result of construction.
Aesthetics The look of rammed earth offers an opportunity to create a rustic feel by using modern practices and methods. The colors, textures, and patterns created by using the rammed earth can create beautiful spaces, and using local soils help to tie a structure to it's location while reducing energy spent transporting materials to the construction site.
Disadvantages
Insulation
Rammed earth is not a good insulator, so heat or cooling is easily lost through a rammed earth wall. If being constructed in a colder climate, rammed earth walls can contain an insulative cavity to alleviate heating/cooling loss.
Construction Considerations
For rammed earth to cure, there must be at least 10 rain-free days (or the walls need to be covered to prevent water infiltration
The soil mix needs to be balanced between clay, sand, and aggregate (what is the right mix?). Granatic soils work well for rammed earth, while basaltic soil is a little more difficult and may require additional clay added to the mix. Typically, 30% of the mix needs to be clay.
If cement is used, a wider range of soils can be used
Addititives such as Kaolin and Bentonite clays are occassionally used to improve the strength of rammed earth walls.
