‘Rammed earth’ is a technique for constructing foundations, floors and walls from natural raw materials such as earth, chalk, lime or gravel. With the lowest enviromental impact of all building techniques currently used in the commercial construction industry, rammed earth is receiving growing attention and becoming more and more popular with architects, builders and clients alike. In this short post, we tell you a little more about the technique.
Mixing it up
Rammed earth walls are made by ‘ramming’ a mixture of materials into place between flat, temporary panels called ‘formwork’. The panels are normally made of wood or plywood and they are clamped around the earth to make sure it doesn’t bulge when compressed. The materials rammed into the formwork generally include gravel, sand, silt and a small amount of clay. An additive like lime, cement of asphalt is often added to make it extra strong.
The mixture is compressed inside the formwork by a mechanical ram and compacted to approximately 50% of its original height. The compression takes place in stages, usually in depths of 10cm to 25cm, and this is repeated until the top of the formwork is reached. This is the part of the process that results in the beautiful layers sometimes seen in rammed earth walls.
After the wall has been completed, the formwork is removed and a surface texture can be applied, e.g. by wire brushing or carving. The wall is normally too hard to work on after about one hour. As the walls dry and harden (ideally in warm weather), the compression strength of the rammed earth increases to a maximum of 4.3MPa (620 psi). This is less than concrete but sufficiently strong for domestic buildings.
Cured rammed earth has a high thermal mass. This means that it absorbs heat during the day and releases it at night, which reduces heating and cooling costs. Environmental impacts are also kept low if the amount of processing of rammed earth materials is kept to a minimum (i.e. artificial additives) and material is sourced locally where possible. Most of the energy used in the construction of rammed earth is in quarrying the raw material and transporting it to the site.
The composition of rammed earth allows it to absorb and release humidity from the inside of a building. This improves air quality, particularly for people who suffer from asthma. Rammed earth is also inorganic, which means that it won’t decay and does not support the growth of mould.
Back to our roots
More than 30% of the world’s population uses earth as a building material, hence this technique is not new. Properly constructed rammed earth can also last for thousands of years, demonstrated by the many ancient structures built using this technique which are still standing around the world.
It has received growing attention in recent years as the construction industry and its customers have a growing sense of the need for environmentally friendly and sustainable building practices. Not only does the technique offer opportunities for carbon-neutral building projects, the technique can be employed in a wide range of climatic conditions and for developing housing that would otherwise be constrained due to expensive construction techniques.
It is a technique that we have been exploring at RISE Design Studio, particularly in our drive to use local, sustainably-sourced building materials.