2. Rammed earth is a technique for constructing foundations, floors, and walls using
natural raw materials such as earth, chalk, lime, or gravel.
Rammed earth has been used in construction for thousands of years, with
evidence of its use dating as far back as the Neolithic Period.
Commonly used especially in China, the technique was applied to both ancient
monuments and vernacular architecture, with the Great Wall utilizing the
technique.
Though interest in rammed earth declined in the 20th century, some continue to
advocate its use today, citing its sustainability in comparison to more modern
construction methods.
Most notably, rammed earth structures use local materials, meaning they have
low embodied energy and produce little waste. Below, we describe how to build
with this material.
3. To start, working with rammed earth
requires a strong understanding of the
climate and location in which the
structure is to be built.
Typically, the rammed earth technique
works best in climates with high
humidity and relatively moderate
temperatures.
In colder climates, rammed earth
walls may need additional insulators,
while in locations with high rainfall,
they need additional protection
against rain.
4. Making rammed earth involves compacting a damp mixture of subsoil that has
suitable proportions of sand, gravel, clay, and stabilizer, if any, into a formwork
(an externally supported frame or mold).
Soil mix is poured into the formwork to a depth of 10 to 25 cm (4 to 10 in) and then
compacted to approximately 50% of its original volume. The soil is
compacted iteratively, in batches or courses, so as to gradually erect the wall up to
the top of the formwork. Tamping was historically manual with a long ramming
pole, and was very laborious, but modern construction can be made less so by
employing pneumatically-powered tampers.
After a wall is complete, it is sufficiently strong to immediately remove the
formwork. This is necessary if a surface texture is to be applied, e.g., by wire
brushing, carving, or mold impression, because the walls become too hard to work
after approximately one hour. The compressive strength of rammed earth
increases as it cures. Cement-stabilized rammed earth is cured for a minimum
period of 28 days.
5. In modern rammed earth buildings, the walls are constructed on top of
conventional footings or a reinforced concrete slab base.
The construction of an entire wall begins with a temporary frame, the "formwork",
which is usually made of wood or plywood, as a mold for the desired shape and
dimensions of each section of wall.
The form must be durable and well braced, and the two opposing faces must be
clamped together to prevent bulging or deformation caused by the large
compressing forces.
Formwork plays an important role in building rammed earth walls. Historically,
wooden planks tied using rope were used to build walls. Modern builders use
plywood and/or steel to build formwork.
8. Coarse Aggregates and Sands
Soils are classified based on their particle size, with clay having the smallest
particles, increasing through silts, sands and then gravels. Sands and gravels
make up the largest proportion of the soil mix, generally 50-80%, and the
maximum particle size for rammed earth is generally limited to 20mm (medium
gravel).
Binder Material
The type of binder used affects the proportion of clay that is acceptable. In un-
stabilized rammed earth, a higher clay content is used to act as the binding agent.
Clays with lower plasticity are preferred to avoid excessive shrinkage during the
drying of the structure. When exposed to moisture, however, the binding effect of
the clay is lost, as the material loses its strength and becomes susceptible to
erosion. This is only suitable for arid areas, otherwise requiring coatings and
regular maintenance. Where lime or cement is added to form SRE, the clay
content is reduced while the sand and gravel content is increased to ensure the
water in the mix is available for hydration. A cement content of around 7% by
weight is typical, less than half the proportion generally used in concrete.
9. Water
A particular mix will have an optimum water content that is dependent on the
method of compaction and which maximizes the dry density. For SRE, the water
content is typically 7-10% and is judged by “adding water until the mix will hold
together when squeezed in the hand - but not leave a color stain on the hand”. It is
important that the water contains minimal salt and organic matter.
Plasticizer
A plasticizing additive reduces the water absorption of the finished wall, as well as
increasing its strength by improving workability at lower water contents.
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15. The standard thickness of rammed earth walls is generally 300mm.
Compressive strength is a maximum of 1MPa for un stabilized rammed earth and
approximately 10MPa for stabilized rammed earth.
The minimum structural thickness for load-bearing earth wall is 250mm. And, for a
non-load bearing walls is 200mm.
U-value of 300mm rammed earth wall "H 1.5 – 3 W/m2K, therefore insulation needs
adding in external wall applications.
Below is a table detailing the minimum thickness and insulation – R ratings for
rammed earth
16. Electrical conduits and wall boxes are rammed into the wall at locations where
power-points and light switches are required.
Conduits are also placed into the wall to allow plumbing pipes to be inserted
where required. Where an area is to be tiled over or hidden by joinery, plumbing
pipes and electrical conduits can be chased into the wall.
To construct rammed earth over doors or windows, we utilize engineered T-Bar
lintels. These are placed over the opening during ramming to support the earth
above.
T-Bar Lintels: The horizontal of the T-Bar should not exceed 200mm (width) in a
300mm thick wall.
Structural Columns: Placement of structural columns should be avoided in
rammed earth walls where possible. This reduces the risk of walls developing
fractures where columns are embedded.
Rammed earth walls are suitable for construction as an external wall up to four-
hour fire rating or a 300 mm wall can be rated for 90 mins.
17. Distinct appearance
Natural and readily available
Low embodied energy (a level similar to brick veneer construction)
Un stabilized earth is reusable post-demolition
Use of local soils supports sustainability practices
Airtight construction achievable
Traditional form of construction
18. Concerns over durability – requires careful detailing
Poor thermal resistance – external walls require additional insulation
Not all soil types are appropriate
High levels of construction quality control are required
Longer than average construction period
High clay content can cause moisture movement
No proper codes of practice
Adding cement stabilization can compromise environmental credentials
