Rammed earth, is a technique for building walls using natural raw materials such as earth, chalk, lime or 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. Rammed-earth walls are simple to construct, noncombustible, thermally massive, strong, and durable.
They can be labor-intensive to construct without machinery (powered tampers). Most of the energy used in the construction of rammed earth is in quarrying the raw material and transporting it to the site. Use of on-site materials drastically lessens energy consumed in construction. Rammed earth gives limited insulation but excellent thermal mass.
Rammed-earth buildings are found on every continent except Antarctica, in a range of environments that includes the temperate and wet regions of northern Europe, semiarid deserts, mountain areas and the tropics. The availability of useful soil and a building design-appropriate for local climatic conditions are the factors that favor its use. This method of building has a long history. Parts of the Great Wall of China were constructed using the technique and are still standing more than 2,000 years later.
As the name implies, the primary material used in rammed earth construction is the earth itself. There are five basic types of soil (gravel, sand, silt, clay, and organic), and the dirt in a given location is generally some combination of all or most of these types. Historically, the longest lasting rammed earth walls were made of soil that was 70% sand and 30% clay.
Conventional concrete slab or strip footings are generally used, subject to soil conditions. The walls are anchored with re-bar, the same as a concrete wall sitting on top of a footing. More elaborately engineered structures may require reinforcement or frames that work with the load bearing capacity of rammed earth. Simple and commonly built rammed earth buildings do usually not require such measures.
Rammed earth is made by compacting a mixture of gravel, sand, silt, clay (and cement if required by local building regulations) between formwork in a series of layers approximately 10 cm thick.
Wood or steel sheets are both used in making formwork, which is superficially similar to the formwork used for pouring concrete, but with its own specific requirements. Walls are built in sections and the rise of each level of formwork is often visible in the final finish. As the wall rises it is possible to take out the lower portions of formwork provided the wall has set strongly enough.
Door and window lintels must be carefully designed if they are required to bear the weight of rammed earth above their openings. Many types of lintels are used in rammed earth, as well as rammed earth arches.
A bond beam at the top of the rammed earth walls provides a point of attachment for the roof structure and maintains structural integrity at the top of the walls and can be mandatory by local building regulations.
The off-form finish of stabilised rammed earth generally requires no additional finish. A clear water-repellent coating may be needed in some instances and non-stabilised rammed earth walls should be protected by eaves, overhangs or render, as they are more prone to erosion. Walls can be wire brushed shortly after being released from the formwork to eliminate the visual impact of the joins between the formwork and achieve an appearance closer to monolithic sandstone. Selection of the ingredients for rammed earth also affects final appearance.
Pros of rammed earth:
- Natural and plentiful resource
- Low carbon emissions
- Material is completely reusable
- Due to high moisture mass, the humidity of the building is well regulated
- Good at regulating internal temperature of buildings
- Airtight construction is possible
Cons of rammed earth:
- Issues may arise concerning durability in extreme climate conditions
- Insulation requires additional materials (i.e polystyrene insulation)
- Only certain types of soil can be used in construction of this type
- Construction is very labor intensive
- The addition of other construction materials to stabilize (i.e cement or concrete) increases carbon emissions therefore diminishing environmental benefits.
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