Formwork in a Rammed Earth wall construction is the supporting frame or mould in which the earth mixture is compacted. Formwork is required to have sufficient strength, support and stability to withstand pressures afflicted when in setup, when being compacted and dismantled.
With rammed earth wall construction the formwork can be removed and reused almost immediately after being rammed, so it is important to plan for efficient use of the formwork throughout the construction process. It can take up to three times as long to organise – strip, clean and setup formwork sections compared to actual time ramming the earth walls.
Stabilised Rammed Earth walls are constructed by compacting a mixture of aggregates; usually sand, gravel and soil with clay content combined with 5% - 20% cement to generate stability. However the mix is made (straw and horse hair etc is also used traditionally) the next stage is to ram the mix into place between flat panels or shutters – known as formwork.
Rammed Earth forms are a carefully considered type of boxing, created to reflect the desired shape and finish of a specified wall. Varieties to the wall finish can be achieved by brushing immediately after the formwork is removed or by including different shapes and patterns in the formwork.
When planning the formwork for a Rammed Earth construction consider:
· Strength – Will it withstand Assembly, Ramming and Dismantling.
· Stiffness – Formwork material needs to have minimal or no flexibility.
· Durability – Can the formwork material be reused easily, repeatedly.
· Ease of handling – can each component be handled easily by 1 – 2 labourers.
· Ease of alignment – can each wall section be aligned easily with other sections.
· Ease of compaction – Practicality of carrying out the ramming process on top of the formwork.
Formwork principles
There is a number of different formwork processes utilised in modern rammed earth builds. Formwork can be created using plywood or with steel sheets along with sufficient propping and temporary stays. Ties or bolts can be used to fix the formwork together with spacers to keep the wall thickness consistent.
This type of earth wall construction has the ability to reach a higher structural specification, largely due to the fact there is no grout or mortar required. This makes a Rammed Earth Wall a homogeneous material, which blends well with timber and other naturally finished building materials.
Rammed Earth walls are built in formwork panels approximately 3.5m in length. For longer wall sections a recess is built into the end of one wall section and locks with a recess on the second wall for lateral stability.
It is often possible to reuse formwork panels for other sections of the building; the key to success is attention to detail. It is critical to meticulously carryout each step involved in setting up every section of formwork. Any mistakes can be difficult to identify until after a wall has been rammed making rectification very costly.
There is no substitute for experience in these matters and Terra Firma Ltd have rammed over 40,000 tonnes of RE walls. We have long since identified paths to wall construction failure and have instilled risk minimizing, checking systems that ensure a high quality and consistency to RE detail, density and finish.
