THE MAKING OF MUD BRICK

For this project, the Clean Living group will be constructing a mud brick wall at the residence of a team member at Warragul, using this amazing clay rich red soil from the site itself to make the bricks. On 22nd March all 5 members of Clean Living met on site to commence process by making the mud bricks.

PREPARATIONS

Soil testing to determine the right mixture
As discussed in the introduction, a typical mud brick recipe includes clay for adhesion and mass, sand to improve compressive strength and reduce shrinkage, and fibre such as straw for binding and tensile strength. The group conducted a couple of soil tests to determine the ratio of clay to sand present in the soil that we intended to use for the bricks. This test helps to determine if and how much additional sand is required.

This test is conducted by first roughly sifting a sample of soil to remove any stones and compressed clay. Next, we filled a straight sided jar part way with soil and topped it up with water. Shaking the jar vigorously disperses the clay content into the water. In theory, once placed to rest, the heavy sand content of the soil sample settles quickly at the base of the jar, indicating the quantity of sand present in the sample. The clay content takes longer to fully settle, several hours. By measuring the two layers formed in the jar once settled, you can calculate the ratio of sand to clay. The test of our soil sample was inconclusive. While seemingly some heavier elements settled at the bottom of the jar, this was not completely separated from the clay, making it difficult to determine the ratio of sand to clay content.

Another simple (but very unattractive) test of clay/sand content is to slightly wet a handful of soil and knead it into a cylinder or sausage shape. If you are able to then hold the sample at one end without it breaking off by it’s own weight, this indicates high clay content. If the sample breaks off quickly, the sand and silt content is high. This test informed us the clay content in our soil was high, which suggested we would need additional sand.



In addition to sand and clay, a typical mud brick mix includes a fibrous organic material such as hay, pine needles or pea straw to help hold the materials together, improve tensile strength and prevent cracking. Our mud brick recipe included straw.

Waterproofing
To improve waterproof capacity of the mud brick, lime, asphalt and/or cement are commonly added to the mix. As stated, this project is being carried out on a team member’s family property, and after the project is completed, the wall will be dismantled and crushed or repurposed for garden works. In order to have the least impact to the land and vegetation we are building on, the group decided not to use any of these additives as lime, asphalt or cement runoff could be harmful to future productivity of the soil. For the purpose of this project the dense clay bricks should provide adequate waterproofing, without the need for admixtures. 

So, after testing and a bit of research, the group determined a simple recipe of 2 parts clay rich soil, one part sand, and one part straw. 

Brick size and moulds

A crucial element in the production of the bricks is the brick mould. According to the government website yourhome.gov.au a typical mud brick is 300–375mm long, 250–300mm wide and 1100-125mm high, and can weigh up to 18kg. Single skin mud brick walls should be a minimum of 250mm in depth in order to achieve adequate structural thickness, and 300mm to provide effective thermal mass. Based on this information we designed out bricks to be 300x250x125mm and, using discarded/off-cut timber and film-face ply wood, constructed 2 moulds of 5 bricks, fixed together with self-taping screws. The moulds after use can be dismantled an repurposed, or perhaps hosed off and used as shelving in the shed or garden.

MAKING THE BRICKS
Preparing the brick mix

On site, the group dug a small trench in the clay rich soil to act as a big mixing bowl for our mud brick solution, adding ingredients incrementally at a ratio of:

• 2 parts soil

• 1 part sand

• 1 part straw

• Approx 1 part water, or as needed to achieve the desired consistency. 

This approximate ratio of water to solid materials produces what is known as a dry mix.

Several methods were employed to thoroughly combine the ingredients, including stomping barefoot, shovelling and raking, mixing with paint mixer attached to a cordless drill, punching and kneading by hand to a consistency resembling cottage cheese.

The ideal dry mud brick mixture should compact when squeezed in the hand but should not easily breach through the fingers. Benefits of a dry mix include faster drying time and most importantly, only one mould is required as moulds can be removed directly after being filled. A wet mix, as the name suggests is a much more fluid consistency. And while a wet mix is more workable and slower drying time improves strength, it requires several days in a mould to solidify its form and is therefore far less efficient in terms of time and materials.

Forming the bricks

When the desired consistency and quantity had been reached, the mud was transferred via wheel barrow from the trench to the moulds, which had been set up in a location that receives adequate sun to dry the bricks, and is relatively level and out of the way of the daily activities of the residence. The moulds were lightly housed down to allow the brick to slide out of the mould more easily. Together we filled the moulds, making sure to pack the mixture into the corners and massaging any air pockets out of the brick.

We then levelled off the top of the brick by raking the straight edge of a timber off-cut against the top edge of the moulds, moistening the surface as we raked to ensure a smooth, level finish. The underside of the brick will moulds to the ground beneath it. This is why a level ground on which to set the bricks is ideal.

After each of the moulds was filled, we removed the moulds by levering them up gently with a spade, then lifting them vertically, taking care to remain level so as not to warp the wet brick. The moulds came off relatively smoothly, leaving uniform rows of 5 bricks with a 2cm gap between each for air circulation.

This initial mix created in the ground trough yielded approx 20 bricks. We went on to mix a further 3 smaller batches of mix in a bucket and wheel barrow. The impervious nature of these vessels resulted in significantly less water use as the water did not leach into the surrounding soil. These vessels, however, produced much smaller yields due to the limitation of their size. 

At days end, we had produced 34 full size 300x250x125 mud bricks. After drying, the bricks can be cut to support the proposed design, ie half bricks for the end of every second course, and cut-outs to allow for a lintel above the window. Alternatively, moulds can be prepared to the specific shapes required. For flexibility, however, we chose to make uniform bricks, and cut to size after curing.

Resource consumption and environmental impacts

So far, this project was largely human powered with manual labour using hand tools. A few electrical tools were used at this stage of the process- namely a small cordless drill used to drive the screws into the moulds and (with mixer attachment) blend the mud brick mix, and a bench saw to cut the off-cut and recycled timber to size for the moulds.

As discussed previously, we chose to leave out any admixtures from the brick mix and instead using the materials in the rawest form available to us. Omitting additives from the mix mitigates the greenhouse emissions and embodied energy resulting from the production and refinement of material such as cement, asphalt or Bondcrete.

The greatest consumption of fuel and power was in the transportation of people, tools and materials to the building site. A more central site and car pooling where possible would have alleviated some of this fuel consumption. 

A considerable amount of water went into both the brick mixture and the continuous clean-up required.

Text by Mary Walbrook

Photos by Alex Tomas, Melina Vlastou and Mary Walbrook

References: www.Yourhome.gov.au