Rammed Earth - Overview of Use

Overview of Use

Building a rammed-earth wall involves compressing a damp mixture of earth that has suitable proportions of sand, gravel and clay (sometimes with an added stabilizer) into an externally supported frame or mould, creating either a solid wall of earth or individual blocks. Historically, such additives as lime or animal blood were used to stabilize the material, whilst modern construction uses lime, cement or asphalt emulsions. Some modern builders also add coloured oxides or other items, such as bottles, tires, or pieces of timber, to add variety to the structure.

The construction of an entire wall begins with a temporary frame (formwork), usually made of wood or plywood, to act as a mould for the desired shape and dimensions of each wall section. The form must be sturdy and well braced, and the two opposing wall faces clamped together, to prevent bulging or deformation from the large compression forces involved. Damp material is poured in to a depth of 10 to 25 cm (4 to 10 in) and then compacted to around 50% of its original height. The material is compressed iteratively, in batches, gradually building the wall up to the top of the frame. Tamping was historically done by hand with a long ramming pole, and was very labour-intensive; modern construction can be made less labour-intensive by employing pneumatically powered tampers.

Once a wall is complete, it is strong enough that the frames can be removed immediately. This is necessary if a surface texture will be applied (e.g. by wire-brushing), since the walls become too hard to work after about an hour. Construction is best done in warm weather so that the walls can dry and harden. The compression strength of the rammed earth increases as it cures; it takes some time to dry out, as much as two years for complete curing. Exposed walls should be sealed to prevent water damage.

In modern variations of the method, rammed-earth walls are constructed on top of conventional footings or a reinforced concrete slab base.

Where blocks made of rammed earth are used, they are generally stacked like regular blocks but are bonded together with a thin mud slurry instead of cement. Special machines, usually powered by small engines and often portable, are used to compress the earth into blocks.