What is Rammed Earth?
Rammed earth walls are created from a mixture of selected aggregates, including gravel, sand, silt and a small amount of clay. The aggregates are placed between flat panels called formwork and they are then compacted down. Traditionally this was done with a wooden pole, however the modern method is to use a mechanised ramming device.
Adding a small amount of cement (typically 5–10%) to create a stabilised rammed earth which increases the strength and durability of the walls. This type of rammed earth walls may be coated with an air-permeable sealer to increase the life of the material – though this is not necessarily required. It’s also worth noting that non-stabilised rammed earth buildings around the world have survived for many centuries.
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 can lessen energy consumed in construction. Rammed earth gives limited insulation but excellent thermal mass.
First used in Lyons, France, in 1562, the term pisé walls — from the Latin origin pisé de terre was applied to the principle of constructing walls at least half a meter thick by ramming earth between two parallel frames that were then removed creating a compressed earth wall.
The Strength of Rammed Earth
Rammed earth is very strong in compression and is therefore suited to use as load-bearing element in multi-storey constructions. Research in New Zealand indicates that monolithic earth walls perform better under earthquake conditions than walls made of separate bricks or blocks.
The Appearance of Rammed Earth
Rammed earth walls can be created in a wide variety of colours and styles – not just the brown that is often seen. They can include graphical and text elements, alcoves, relief mouldings and many other features. Their natural appearance works well when combined with timber and natural materials but can create a beautiful contrast when combined with clean modern materials such as steel.
The Silence of Rammed Earth
The monolithic mass of rammed earth is an excellent form of sound insulation. In addition, rammed earth has excellent sound reverberation characteristics and does not generate the harsh echoes characteristic found in concrete.
The Fire Resistance of Rammed Earth
Without any flammable components, a rammed earth wall has very good fire resistance. Testing by the CSIRO shows that a 150mm thick Cinva-rammed earth block wall (similar to rammed earth) achieved a fire-resistance rating of nearly four hours.
The Thermal mass of Rammed Earth
Like heavyweight masonry, rammed earth walls have a high thermal mass which helps level out the temperature of the building throughout the day. It’s cooler on hot days and warmer on cold days – releasing the heat that was stored up during the day through the night.
Used correctly, and in the right climate, the thermal mass of rammed earth can delay heat flow through the building envelope by as much as 10 to 12 hours and can even out daily temperature variations. Rammed earth walls are most effective when the difference between day and night outdoor temperatures is at least 6°C. Where the diurnal range is greater than 10°C, appropriate design can exploit the high thermal mass of rammed earth to very good effect. This makes them perfectly suited to much of Western Australia.
The Vermin Resistance of Rammed Earth
There is no cavity to harbour vermin and nothing in the material to attract or support them so its resistance to vermin attack is very high.
Durability and moisture resistance
The basic technology has been around for thousands of years and there are many rammed earth buildings still standing that are centuries old. Rammed earth does possess a generally high durability but all types of rammed earth walls are porous by nature and need protection from driving rain and long term exposure to moisture.
In general, rammed earth does have moderate to good moisture resistance and most modern Australian rammed earth walls do not require additional waterproofing. New water repellent additives that waterproof the walls right through may make rammed earth suitable for very exposed conditions, including retaining walls, but may inhibit the breathability of the material.
Breathability and toxicity
Provided it is not sealed with material that is impermeable to air molecules, rammed earth maintains its breathability. Finished walls are inert but take care in the choice of waterproofing or anti-dust finishes to avoid adding toxicity to the surfaces.
The Environmental Impacts of Rammed Earth
Rammed earth uses natural materials and has low manufacturing impacts and the embodied energy of a rammed earth building is low, especially when compared to brick and cement.
Further Information
More general information about rammed earth can be found at: http://www.yourhome.gov.au/materials/rammed-earth
Rammed Earth Construction
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