Carrying on with our series on the fascinating subject of bricks, today we’re going to take a look at adobe, a material that’s not just used for making bricks, but for many other parts of the construction process. The word “adobe” stems from the Arabic word for mud brick and adobe is an incredibly durable building material in dry climates, accounting for some of the oldest existing buildings in the world as it is one of the earliest building materials to be discovered by man.
Adobe is basically sun-dried earth and the earth is often mixed with straw, dung or other organic materials which bind the adobe together and help to ensure an even drying process to avoid cracking due to shrinkage. Adobe bricks are made by pressing the mud mix into an open timber frame which is removed after the initial setting. After drying out for a few hours, the bricks are turned on edge to finish drying. Drying slowly in the shade helps to reduce cracking.
The same mud mix without the straw is used to make both mortar and plaster for use on both interior and exterior walls and some ancient cultures would use lime based cement for the plaster to protect against rain damage.
Depending on the form into which the mix is pressed, adobe bricks can come in just about any shape or size as long as the drying process is even and reinforcements are included in the production of larger bricks. Reinforcement can be provided by adding manure, straw, cement, rebar or wood. The inclusion of reinforcements has been demonstrated to produce the strongest, most crack-resistant bricks.
To test whether soil is suitable for making adobe, a sample is mixed with water in a clear container to produce a saturated liquid. The container is shaken vigorously and then the contents are allowed to settle for a day until the soil settles into layers. Heavier particles will settle first, followed by sand and silt while very fine clay and organic matter will stay in suspension for days. Once the water has cleared, percentages of the various particles can be determined. 50 – 60% sand and 35 – 40% clay will result in the strongest of bricks.
Adobe can also be poured or puddled – building up layers of soft adobe, rather than making it into bricks. This is today known as cob and cob houses are becoming increasingly popular in many parts of the world as more people look for eco-friendly ways of building their own homes.
When building with adobe, the ground supporting the structure will need to be compressed in order to prevent the foundation settling and causing cracks in the wall. Adobe is heavy and modern construction codes require the use of reinforcing steel in the footing and stem wall. Adobe walls are load bearing and rarely rise above two stories due to the low structure strength of the material. Lintels are necessary when creating door and window openings to support the subsequent lines of brickwork. When the last courses of brick are laid, bond beams made from heavy wood or reinforced concrete are laid to provide a horizontal bearing plate for the roof beams. Finishes from mud plaster, whitewash or stucco can be applied to protect the adobe wall from water damage and these will need regular reapplication.