Rammed Earth is a form of building technology that involves compacting a damp mixture of local soils that have suitable properties of sand, gravel, clay, silt, and a soil stabilizer (lime/cement) into a formwork to produce a high-density, stone-like wall.
Mixture:
The subsoil composition is as follows: clay (15-25%), sand (50-60%), and gravel (15-20%), with or without a small percentage of stabilizers such as lime powder (7-10%) and minimum water (8-12%)
Mixing the soil
Formwork and Molding:
The soil gets tamped between shutters with manual or pneumatic rammers. The shuttering is best designed so that a whole wall can be built in one go, thus eliminating joints that would cause cracks within panels as much as possible.
The formwork/shuttering system comprises end panels which can be quickly assembled and disassembled to provide end stops to support side panels without the need for through-ties. The end panels comprise integral handholds for access to the full height of the formwork. The end panels are stackable, rotatable and interchangeable, such that one set of formwork may be used to create several different wall configurations. The finished wall contains an internal support structure and insulating core, without through-holes.
wooden formwork
Rammed earth walls are limited to the shapes that can be built with removable shuttering and left as they are, with the natural-looking strata pattern from the ramming process.
Consistent workmanship is critical for both the appearance and strength of the rammed earth walls; therefore, site works have to be of high quality. An area of wall that is not mixed or rammed correctly can ruin a whole panel.
ramming the walls
Compressive Strength:
Too much clay in the soil increases its compressibility, while too little prevents it from binding. That is why, in some cases, stabilization materials are added, like a small percentage of cement or lime powder.
Maximum compressive strength is achieved by ensuring that there is maximum dry density in the mix, and proper formwork, along with accurate ramming procedures, are also key.
The average compressive strength should not be less than 1.5 N/mm2 generally, or 2.0 N/mm2 for walls of height between 3.0 m and 6.0 m at a minimum age of 7 days of curing.
Durability:
Basically, the ability of the materials/structure to maintain its functionality over time, and in this case it’s about resistance against water-induced erosion that weakens it over time.
There are different decay mechanisms such as shrinkage, surface erosion, sacrificial erosion, freeze/thaw cycles, organic growth, delamination, and dampness.
Shrinkage is the most common, as it leads to cracking and is sorted by adding sandy/gravelly materials.
Common Quality Tests:
- Drop test – Checks the optimum moisture content in the sample mixture.
In this case, you take a sample of damp mix from the larger mixture and squeeze it into a ball of 40 mm diameter in between your hands.
Stretch out your arm to shoulder level at a height of 1.5m, with a piece of plywood(or any other soft wood) right below; drop it.
If the mix remains intact, it shows it’s too wet, so let it dry and try again.
If the mix breaks into many pieces, it shows it’s too dry; hence, add water and try again.
If the mix breaks into fewer pieces but still holds together, it shows that it’s near the optimum moisture content.
- Roll test – Checks the clay content in the mixture as it’s key to cohesion.
In this case, you take a sample of damp mix from the larger mixture and squeeze it into a rod size 25 mm in diameter and 200 mm long, between your hands.
Place the roll on a table and push it gently over the edge to check how long it gets before it breaks off and, consequently, the length of the piece that drops off.
If the roll breaks off less than 80 mm, there is not enough clay. If the roll breaks off longer than 120 mm, there is too much clay.
Benefits:
- Thermal comfort – They can absorb heat during the day and release it slowly at night. This helps regulate indoor temperatures, providing a naturally comfortable living environment with less reliance on heating and cooling systems.
- Sustainable – Construction generates minimal waste, as most materials come from the site itself. It also has low embodied energy compared to conventional materials like concrete, making it environmentally friendly.
- Natural aesthetics – The natural colors and textures of rammed earth formed from the layering process create a unique, earthy look that blends harmoniously with the environment. The unique walls create a warm, organic feel.
- Affordable – Earth is abundant, thus making rammed earth buildings very economical. The construction method is seamless, with low unskilled labor intensity.
Rammed earth is a frontier in the generation of low-cost sustainable housing in developing countries for both in our rural and urban areas..
Very popular in West Africa and Ghana in particular. In East Africa, Uganda is leading the way, followed closely by Tanzania (Zanzibar) and Rwanda. We are in the process of popularizing it in Kenya.
