Earth Building Association of New Zealand Inc
Promoting the Art and Science of Earth and Natural Building
Adobe or mud-brick building is an ancient technique dating back at least to Jericho (8300 BC). The oldest continually inhabited structures in the world are adobe. Some built in North America around 900 years ago are still in use.
Mud bricks are probably the simplest and easiest form of earth building. Most of the shrinkage takes place in the brick itself before the walls are built, so shrinkage cracking is much less of a problem. The bricks can be cast from a greater variety of soils, and can cope with more clay content than is suitable for the in-situ techniques.
You cast mud bricks in open moulds on the ground, using a blend of earth and water with the consistency of cake mix. You can withdraw the moulds either straightaway or the next day.
Once they are firm enough you lift the bricks onto their sides, trim them, and then stack them to air-dry and cure. You can cast different shapes of bricks to create wedges for corners or arches, 45-degree corners, or rebates for jambs. You can cut holes in newly cast bricks for reinforcing or services.
You lay the bricks up into the wall with a mortar similar to the brick consistency, although modified sometimes for shrinkage and workability (often by adding sand).
The finished walls are bagged as they are laid, a practice whereby the mortar mix is rubbed thinly into and over the walls to fill in gaps and cracks.
Often a mud render is applied as a surface coating. This coating will sometimes have a dust retardant in it - a traditional and successful mixture uses clay and sand mixed with fresh cow manure. You can also use wallpaper size or boiled linseed oil as an anti-dust protector. The wall surface tends to be a bit uneven unless plastered. It has a "soft" look, although the material is quite hard when dry.
Sometimes adobe bricks are stabilised with a small percentage of hydrated lime or cement to improve durability. Mortar shrinkage is more of a problem with this type of construction than shrinkage of the bricks themselves. The necessary tests for adobe bricks and mortars are detailed in NZS 4298.