Rammed earth specifications and history

Rammed Earth (pisé)

rammed earth walls are constructed by ramming a mixture of selected aggregates, including gravel, sand, silt and a small amount of clay, into place between flat panels called formwork.

Traditional technology involved repeatedly ramming the end of a wooden pole into

the earth mixture to compress it. Modern technology replaces the pole with a mechanical ram. Stabilised rammed earth is a variant of traditional rammed earth that adds a small amount of cement (typically between 5 and

10 per cent) to add strength and durability. Stabilised rammed earth walls need little added protection but are usually coated with a permeable sealer to increase the life of the material – this varies with circumstance and there are thousands of unstabilised rammed earth buildings around the world that have given good service over 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 provides some insulation and excellent thermal mass.

The term pisé is of Latin origin from pisé de terre. First used in Lyons, France in 1562, the term applied to the principle of constructing walls at least 50cm thick by ramming earth between two parallel frames that are then removed, revealing a completed section of hard earth wall. While 50cm thick walls can still be constructed if desired, with or without cement, most modern rammed earth walls in Australia are built using cement at 30cm thick for external walls and 30cm or 20cm

for internal walls.

Performance and appearance 

The colour of rammed earth walls is determined by the earth and aggregate used. The ramming process proceeds layer by layer and this does introduce horizontal stratification to the appearance of the walls. The stratification due to ramming can enhance the overall appearance and can be controlled as a feature or eliminated. Aggregates can be exposed and special effects can be created by the addition

of different coloured material in some layers and elements such as feature stones, alcoves or relief mouldings can be incorporated into rammed earth walls, at a price. Brushed finishes help reduce formwork marks that can create

a concrete-like appearance, but this is only necessary with fine grain size ingredients.

Unusual finishes can be achieved by including shapes in the formwork that can be released after the wall has been rammed. Other possibilities include embedding rocks and other objects in walls for aesthetic effect.

Layers of ramming are visible as are the chamfered corners that are required to allow the walls to be easily released from the formwork.

It is possible to form vertical curves, made by carefully ramming along a drawn guideline on the interior of the formwork. Horizontal curves are also possible but require specialized formwork.

Structural capability

Rammed earth is very strong in compression and can be used for multi-storey load-bearing construction. Research in New Zealand indicates that monolithic earth walls perform better under earthquake conditions than walls made of separate bricks or blocks. There is a five storey hotel in Queensland built of stabilised rammed earth. Rammed earth can be engineered to achieve reasonably high strengths and can be reinforced in a similar manner to concrete, although horizontal reinforcement is not recommended and excessive vertical reinforcement can cause cracking problems. Construction Systems

Interesting structural features, including leaning walls, have been constructed in rammed earth. Any difficulties associated with placing and ramming around reinforcement can be eased by careful management of the construction process and need not add significantly to cost.