Rammed Earth Attributes

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Rammed earth, an ancient building technique, may have originally been developed in climates where humidity and rainfall did not permit the production of soil block. For soil block to cure uncovered, there must be at least 10 rain-free days. Soil mixtures for rammed earth are similar to those for soil block. Soils with high clay content may be more suitable for ramming, as they tend to crack in blocks when curing.

Preparation and Transport of Soil

Rammed earth soil mixes must be carefully prepared by screening, pulverizing, and mixing. Pulverizing is important to ensure a uniform mix and to break up any clumps.

Transporting the soil mix to the forms is a demanding tasks. Large quantities of soil must be moved and transported vertically for placement in the forms. This process is not the same as pouring concrete, because the material is not liquid. Traditionally, workers passed baskets or buckets of earth up to where it was needed. Hoists can also be used effectively for this task.

Form work

Form work for rammed earth must be stable and well-built in order to resist pressure and vibration resulting from ramming. Small, simply designed forms that are easy to manage are most effective. Ease of assembly and dismantling should be considered when designing forms. A variety of materials can be used for forms, including wood, aluminum, steel, or glass fiber.

Systems for keeping form work in position vary. Small clamps adapted from concrete form work techniques work well, although small holes are left when the clamps are removed. Other methods include locking hydraulic jacks, or form work built on posts. For more discussion of form work design, organization and moving, see the Earth Construction Primer, and Adobe and Rammed Earth Buildings listed inResources .

The Ramming Process

Once a soil “lift” of 6 to 8 inches in thickness is in place, the soil is rammed. Ramming can be accomplished manually or mechanically. Manual ramming is an ancient technique using a large, specially shaped tool with a long handle called a rammer. Rammers weigh around 18 pounds, and have heads of wood or metal. Differently shaped heads are designed to perform ramming for various form shapes, especially for corners.

Mechanical impact ramming uses pneumatic ramming machines. Only rammers specifically designed for soil are effective (rammers which are too powerful or too heavy will not work). Such equipment is quite expensive, but impact ramming is highly effective, and if the soil mixture is good, creates high quality rammed earth. Rammed earth will begin to cure immediately, and can take from several months to several years, depending on weather and humidity to complete the process.

Design Methods

Rammed earth walls have low tensile strength, and should be reinforced by providing a bond or collar beam. Beams can be constructed of concrete, wood or steel. Vertical reinforcing may also be done, and may be required by some building officials.

All openings in rammed earth walls, such as windows and doors, must have lintels to span the opening width. Water flow and moisture control is critical to protect structural walls. Special detailing to accommodate manufactured windows may be necessary to accommodate wall thickness. All openings for doors and windows will require a frame. Wood, as opposed to metal, is recommended due to the corrosive action of moisture from the soil material. Lintels can be concrete, stone or wood. Careful attention to both roof and opening details is necessary to protect the structure from water damage.

Foundations required by most codes are concrete reinforced with steel. Soil block material may be used as a filler material between piers of a reinforced concrete pier and beam foundation. Historically, many structures built with earth materials had no foundation, or used sand and gravel foundations. The latter are excavated trenches filled with two parts sand to three parts gravel. Trench bottoms should be graded to provide positive drainage. Soil material block should not be used in below grade walls unless supported on both sides. Natural moisture from the ground may infiltrate the block, resulting in reduced compressive strength.


Earth Flooring

Earth floors are most often used in outbuildings and sheds, but if properly installed, can also be used in interior spaces. For interior use, earth floors must be properly insulated and moisture sealed. Earth floors must be protected from capillary action of water by sealing with a water tight membrane underlayment.

Construction preparation includes removal of any vegetation under the floor area followed by ramming of the area. The ground must be dry before installation of the floor. After the surface is moisture-proofed, a foundation of stone, gravel or sand is installed, 20 to 25 cm. deep. Then, an insulating layer is installed, such as a straw clay mixture.

An appropriate soil stabilized mixture for the load-bearing layer of the floor is then installed. The load bearing layer should be 4 cm. thick. The floor can be finished with a thin layer of cement grout mixed with sand. Sawdust can also be .i.concrete: rammed earth and, added as a filler, in proportion of one part sawdust, one part sand, and one part cement. Sawdust should be treated first with lime and dried. The final stage of floor finishing is waxing and/or coloring.

Other construction options include monolithic earth floors which are poured in layers within guide forms. Each layer must have curing cracks filled, be treated with a mixture of linseed oil and turpentine, and allowed to dry for a week before the next layer is applied. The final floor surface can be waxed and polished.

Soil material flooring can also be installed using stabilized bricks or tiles. Such materials should be from 6 to 9 cm thick, and can be set on a 2 cm layer of mortar. If soil is not used for flooring, concrete or masonry are other options. Tile and wood floors are possible.


Earth Material Durability and Finishes

Soil materials in construction are often believed to be vulnerable to weather. This is true only of the outer, or finished surfaces. If proper roof and structural design is done, rainfall or severe weather will not affect the structural properties of the wall or the interior wall. Only the cosmetic surface of the earth material will be affected. Normally, the clay content of the material resists extensive wetting.

Structures constructed of soil materials are durable, and are said to last more than fifty years. The US. government has documented over 350,000 currently existing houses and commercial structures of earthen construction in the US. Many of these have been in existence with minimal maintenance for the past 100 years. Some were built as long ago as the 1600’s.

Several options are available for finishing soil based construction materials. Two basic approaches exist: waterproof or breathable finishes. Waterproof finishes such as cement stucco are more permanent and more expensive initially. Such finishes will contain and trap moisture, which may be problematic; permeable finishes such as mud plaster are less expensive, less durable and will allow the wall to absorb and give off airborne moisture.

Investigate qualities and claims of products before purchasing. If possible, test wall finishes before purchasing large quantities of materials.

Plaster

Mud plaster is usually applied in two coats for both exterior and interior surfaces. The addition of straw is recommended in the mud plaster mix. This will help to reinforce the plaster, allowing for thicker coats and surface leveling. In addition, this will decrease the tendency for cracking of the plaster as it dries. High clay content soils in mud plaster may tend to result in a poor bond of the plaster to the wall.

The finish coat is made of screened, fine materials. This layer is applied as thinly as possible while achieving full coverage. Plaster can be troweled or floated to achieve a variety of textures, and reapplied as many times as necessary to achieve the desired affect or to make repairs. When dry, the mud plaster surface will take a hard, firm set similar in hardness and texture to conventional plaster.

The same stabilizers used in the preparation of the structural soil mix may be used to stabilize the plaster. Thorough mixing of the plaster mix is necessary to avoid an uneven finish.

Stucco

Traditional cement stucco may be used on walls for a low-maintenance finish. While this may seem desirable, cement stucco also has disadvantages in that it has a different expansion coefficient than the wall material. This may eventually lead to separation from the wall, and may conceal structural erosion problems which may result from leaky pipes or roofs. Stucco netting is recommended to accommodate any settling and cracking of the stucco. Exterior stucco walls should not be painted with traditional exterior paints, which will increase moisture impermeability. A final colored coat of stucco or texture finishes may be used decoratively. For more information on both interior and exterior cement stucco preparation and application, see Adobe and Rammed Earth Buildings (Resources section).

Interior Walls

Interior earth walls may be painted more successfully, and may also be treated with sealing compounds to reduce the tendency for dust to develop and rub off on furniture and clothing. Oil-based varnishes and resinous liquids can be diluted for such use. If paint is to be used, a sealing or sizing coat should be applied first. Whitewash can be prepared with equal parts of lime and white cement mixed with water. Natural earth pigments may be added to this.

In addition to stucco or plaster, interior walls may also be treated with a variety of veneers including gypsum wall board or other interior veneers.


Earth as a Material and its Energy

Thermal Characteristics

Earth material walls are not especially good insulators. ASHRAE laboratory tests give a 10 inch thick adobe wall with 3/4 inch of stucco on the exterior and 1/2 inch of gypsum plaster on the interior an R-value of 3.8. A 14 inch wall with similar construction is assigned an R value of 4.9. In spite of these fairly low values in laboratory conditions, earth materials do have good thermal mass characteristics. Wall thickness of from 12 to 14 inches are generally considered optimum for thermal mass performance.

Double wall construction can greatly enhance insulation value. Applied insulation can be in the form of rigid material or spray on insulation. Spray on insulation must be covered with stucco to protect it. Although the addition of insulation will increase construction costs, the resulting energy savings will offset initial costs. Some dynamic testing of high mass walls have indicated better thermal performances than the calculated thermal values would indicate.

Embodied Energy

The following figures, adapted from Adobe and Rammed Earth Buildings , reflect the embodied energy in BTU’s required for the production and use of various materials. Soil block has a much lower embodied energy than many traditional materials.

Portland Cement94 lb sack381,624 BTU
Lime, hydrated100 lb sack440,619 BTU
Common brick1 block13,570 BTU
Concrete block1 block29,018 BTU
Earth (Adobe) block (mechanized production)1 block (10X4X14)2,500 BTU

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