Rammed earth construction, is also known as “pisé de terre” or “pisé”, is an ancient building technique that has been recently revived by people looking for low-cost, environmentally friendly construction materials and more natural building techniques.
Rammed earth buildings essentially compress a dampened mix of soil, gravel, cement, sand and clay, into a supported shutter frame structure this in turn forms a sturdy earthen wall when the shutter frames are removed.
The required building materials are often very cheap, free and are recyclable. This makes it a very viable building option within low-income groups with an large availability of skilled and unskilled labor.
Creative use of exterior and interior wall finishings that look like pillars and the passage is curved & flowing into the rest of the home complimented by natural light and wood. You can download some nice desktops of rammed earth here. Images also stolen from Sirewall.
Properties of rammed earth construction:
- Rammed earth has around 300-psi compressive strength rating.
- Walls can be plastered, painted, sealed or left exposed for beautiful and distinct appearance.
- Rammed earth could be used in virtually all styles of domestic building design with any kind of roofing.
- Rammed earth buildings are amazingly well insulated keeping the home cool in summer and warm in winter (saving on energy costs).
- Natural and readily available resources local soils can be used in many cases.
- Modern methods are tried, tested and proven in many European, North American and Australasian countries.
Stabilised rammed earth conforms to the Building Regulations for UK, Europe, Australia as an accepted form of low-rise building in residential and public construction.
Rammed Earth Component Mix
As soil varies from site to site, so it is always worthwhile making some test blocks of various mix concentrations to see exactly how well they hold up.
The denser the soil to clay ratio the greater the amount of cement is required. So, lesser amounts of cement are required on sites with less clay content in the soil.
Warning : Silty soil is not good as it does not have the required cohesive properties used in rammed construction.
The rule of thumb
- 3 to 6 % Portland cement.
- 10% Moisture content.
- The rest is your soil.
In 15 parts soil
- 6% cement = 1 part.
- 10% water = 1.5 parts.
Load bearing rammed earth walls are 45cm to 60cm thick. Non load bearing rammed earth walls are about 25cm to 35cm thick.
It is possible to run PVC, steel or any other conduit in the walls for facilities such as water and electricity (simply position the pipes as you go). Although chasing with an angle grinder is possible (after the fact) you may not want to beak your exposed earth walls aesthetics.
The Ramming Process
Once thoroughly mixed (mix it again) then add about 20cm of the mix into the shutter forms. Ram down the 20cm about 13cm (as far as is possible). To be ideally compacted you are looking at compacting roundabout 35% of it’s original soil depth going into the shutter frame.
The ramming process can be performed manually with a weighted rammer / tamper / steel plate attached to a long pole (Any hand tamper / rammer should weigh at least 5kg). The compressing process is a very time consuming and labor intensive, so it is recommended that rammed earth builders hire a pneumatic tampers for the job (this will also shortens the construction time significantly).
The forms are usually built with plywood or sheet metal and can be re-used. The forms need to be additionally reinforced from the outside with beams to prohibit bending of the wall faces during the ramming process.
The shutter forms in theory can be removed immediately. The walls begin to cure and get harder as time goes by (duration for curing depends on the climate).
Consumerenergycenter.org reckons that with “David Easton’s” “Pneumatically Impacted Stabilized Earth” techniques you can look at building 1000 square feet of 18″ thick wall per day, that’s 304.8 square meters a day of 45.7cm thick walling. Rammed earth can take anywhere from a few months to years to entirely cure depending on the climate, mix and application. Generally a wall section should be usable within 24h.
Rammed Earth History
In the old days rammed earth buildings were quite common in arid and semi-desert regions where wood and other building materials like straw are in limited or no or short supply. Rammed earth structures like the Great Wall of China, Potala Palace in Lhasa and the Alhambra building in Granada are constructed using rammed earth. Quite a few European castles were constructed with rammed earth techniques and the center core of pyramids in Egypt are made of rammed earth.
Problems with Rammed Earth
- Not all soil types can be used in construction.
- There are usually some concerns over durability
- Increased quality control is a vital element in both the mixing process and the ramming process.
- Higher clay in the soil can cause structural “moisture movement” which may challenge the integrity of the structure.