Earth - soil - is the oldest and most widely used building material in the world today. For at least 9,000 years, people have mixed dirt with water to make adobe to build their homes.
Today adobe and several variations on this earth-based building technique are experiencing a new popularity. These "green" building methods are becoming economically competitive with standard construction methods. Other features like long-term energy savings, ease and speed of construction, and the knowledge that by building "green" you're doing less harm to the environment make these construction alternatives even more attractive.
Made from available soil, adobe is abundant, inexpensive and energy-efficient. A sustainable material, it has little impact on the environment and uses little or none of the planet's finite resources like fossil fuels.
Adobe has many environmental benefits, including low energy costs. Because of its high thermal mass, it works well in hot climates with cool nighttime temperatures.
Adobe bricks are made by blending together soil and water into a goopy mix. Traditionally straw is added to the mixture to provide strength and to prevent cracking. The adobe is then shaped in a mold or by hand and dried by the sun. The resulting bricks can be almost any shape or size and are laid with a mortar made of the same material - soil and water. In the summer, it takes about a week of hot, dry weather for the bricks to cure adequately before they can be handled and used.
Until recently, the techniques of building with adobe changed little from the mud brick homes built during the Neolithic period, around 7100 BCE. Since the mixture of mud is created by human power and the bricks are fired by the sun, adobe requires a very minimal manufacturing process. Because skilled labor is not necessary, it offers a viable solution to low-cost housing.
Adobe buildings can last hundreds of years, providing their walls are protected from moisture. Adobe can be effectively sealed and waterproofed by applying a cement stucco to the exterior.
Recent variations to the adobe mix - adding small amounts of liquid asphalt emulsion stabilizers or Portland cement - make it less crumbly and more resistant to moisture. In another recent innovation, some manufacturers now squeeze their oversized mud bricks in a hydraulic press. The resulting brick - called a pressed adobe block - has twice the strength of ordinary adobe, even though it weighs only one-third more.
These changes help to overcome one of the drawbacks to earthen construction: adobe walls don't span open spaces such as windows or doorways well. Without adequate door and window lintels, walls tend to crack near openings.
Then, too, adobe by itself it does not support weight well enough to build high walls of more than two stories. That's why California's high-ceiling early Spanish missions built from adobe have tapering walls that are much thicker at the bottom - thicker than it is typically practical to build. Labor costs for a two-story adobe home would be very high.
In the family of building materials, rammed earth is a first cousin to adobe. It was used to create parts of the Great Wall of China.
To create a masonry wall using rammed earth techniques, a builder pounds layers of moist, sifted soil - mixed with a small amount of Portland cement - into removable forms. This soil is tamped down until its volume has been compacted by approximately 25 percent. When the earth is stable, the wood or metal form is removed, placed on top of the first layer of the wall, and the process is repeated.
The best mixture for rammed earth contains approximately 30 percent clay and 70 percent sand, with a small amount of cement added. These components are nontoxic, inexpensive and readily available.
Rammed earth buildings are extremely durable and can last for centuries. Experts claim that rammed-earth walls continue to harden - or cure, in their parlance - during the first year after construction. Although finished walls are somewhat water resistant, they can be stuccoed, plastered painted or left natural and sealed to better waterproofing them.
Like adobe, rammed earth buildings have the advantage of having very thick walls that retain heat. Their thickness, or thermal mass, helps to even out temperature fluctuations between day and night, making them easy to heat and cool.
Unfortunately, rammed earth construction is very labor intensive. It can take many hours to properly tamp dirt into the forms, then take the forms down and set them up again for the next section of a wall. This laborious process makes rammed earth construction more expensive than traditional frame and stucco building, and most of the homes currently built in this way are custom homes.
Builder David Easton recently developed a new, less labor-intensive way of creating a rammed-earth-type home. The process is called Pneumatically Impacted Stabilized Earth - named PISÉ by Easton in honor of the French tradition of rammed earth construction.
PISÉ is another earth-based building material that looks and performs much like rammed earth or adobe. PISÉ has all the environmental benefits of rammed earth, but with one important advantage - its quick-lock forming systems, which speeds up the building process. Trained crews can complete up to 1000 square feet of 18" thick wall per day.
The process, however, is more complex than traditional rammed earth. A mixture of earth, cement and water is sprayed through the oversized hoses of a gunite machine - equipment normally used in the construction of swimming pools - and builds up against the one-sided forms. The carefully controlled mixture is just moist enough to stack up against the form without slumping. The nozzle operator moves along the wall, spraying the PISÉ mix 18-to-24 inches thick and in layer two-to-three feet high. As the wall begins to firm up, any irregularities are shaved off to create a plumb, smooth wall. In 30 minutes to an hour, another two-to-three feet high layer can be added to the wall. Once the PISÉ dries, the forms are removed.
Because the forms against which the PISÉ mixture is sprayed are one-sided, they can be set up and removed quickly. Their open design allows plumbers and electricians to easily put pipes and conduits in place before the wall is cast. And one-sided forms make it easy to incorporate reinforcing steel - rebar - in the wall being sprayed, an important addition for earthquake safety. The addition of steel makes it possible to build taller walls that bear more weight.
PISÉ technology is relatively new, so it is not widely available. It requires sophisticated equipment and meticulous control over mix designs. PISÉ continues to be researched and developed to improve its quick-lock forming systems, versatile soil mixes, soil amendments, and improved delivery equipment.