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Stone construction practices are fairly standard. Attention needs to be paid to the load capacity of foundations and footings because of the weight of the material. Veneers need non-combustible support such as concrete grade beams or footings. Pay particular attention to grade beams when designing interior stone wall applications. Anchoring of veneers must follow Uniform Building Code (UBC) guidelines.
Indigenous stone
Limestone: A rock that is formed chiefly by the accumulation of organic remains (shells or coral) that consist mainly of calcium carbonate.
Marble: Crystallized limestone, ranging from granular to compact in texture.
Granite: A very hard, indigenous rock formation of visibly crystalline texture formed essentially of quartz and orthoclase or microcline.
Sandstone: A sedimentary rock consisting usually of quartz sand combined with some binding elements such as silica or calcium carbonate.
Flagstone: A hard, evenly stratified stone that splits into flat pieces suitable for paving.
Fieldstone: Stone in its unaltered form.
The same guidelines apply to brick masonry as those for stone. Brick has value as a recyclable material. Used brick, available through local salvage companies, is often desired for its weathered, antique appearance. In addition, brick seconds or brick that is damaged can be crushed and recycled and either returned to the manufacturing process to make more brick, or used as a landscaping material in its crushed form.
Some American brick manufacturers are making brick with sewage sludge. Sludge material is mixed with clay normally used in the manufacturing process. The resulting brick is equally attractive and strong. Another alternative material for brick production is petroleum contaminated soils. Such soils, when combined with clay and fired at very high temperatures, yield brick which is free from hydrocarbon contamination. These techniques are not currently being used in the Austin area.
Soil for rammed earth, compressed earth blocks, cob, and superadobe construction is abundant in the Austin area. However, soils that are bentonitic or highly expansive are unsuitable for earth construction because of their capacity to shrink and swell. Soil that cracks after rainfall may indicate expansiveness. Soil must be tested to determine its suitability as a building material. For instance, soil intended for use as flooring will not need the same strength as that used to construct a load-bearing wall.
Desirable qualities for soil construction materials include strength, low moisture absorption, limited shrink/swell reaction, and high resistance to erosion and chemical attack.
Soil testing
Soil testing is done in three phases: laboratory testing, construction mix testing, and quality control testing. Laboratory testing should always be done early in the design process, using representative samples of soil intended for use. (See Resources section for laboratories.) Engineering properties for which soils are tested include permeability, stability, plasticity and cohesion, compactibility, durability, and abrasiveness. Shrinkage, swelling, and compressive strength are important aspects of soil suitability.
It is possible to alter soils to make them suitable for construction by stabilizing them. Stabilizing soil helps to inhibit the shrink and swell potential and aids in the binding of soil components. Soil can be stabilized through chemical or mechanical means or both. For information on mechanical methods, see the section below on rammed earth.
Chemical soil stabilization
Lime, cement, and pozzolan mineral admixtures (high silica volcanic ash) can be used as chemical additives. Lime is most effective on clay soils, and can be used in combination with cement and pozzolan. Hydrated lime, as opposed to quick lime, should be used. Lime is inexpensive, but care must be taken to protect workers from breathing in lime dust.
Cement is relatively inexpensive, but requires large energy inputs in its production process. However, cement produces the stronger block than lime. Pozzolan exists in plentiful supply in Texas, but is not readily available commercially. The Center for Maximum Potential Building Systems (CMPBS) in Austin is experimenting pozzolan additives and offers considerable expertise in earth materials use (see the Resources section).
What are pozzolans?
"Finely ground mineral substances that, when water is added react with calcium hydroxide (the primary ingredient in cement) to form compounds with cement-like properties. Pozzolans include industrial byproducts, such as flyash, ground granulated blast furnace slag, and silica fume. Other types come from natural materials, such as volcanic glass and tuff, diatomaceous earth, and calcinated clay."
Strength of tested earth and caliche block
Unfired caliche block with 5-10 percent cement added can easily pass the Uniform Building Code standards for compression with an average of 960 psi.
Rammed earth walls have been tested with a compressive strength of 30 to 90 psi immediately after forming. Ultimate compressive strength should reach 450-800 psi. If cement is added, compressive strength will increase significantly.
The Uniform Building Code for single and two story buildings requires block-bearing capacity of 300 psi bearing strength. Blocks manufactured with a hydraulic press have been tested with a bearing capacity immediately after production of 700 psi. Such soil block continues to cure, until blocks reach a typical bearing capacity of 1000 psi, far exceeding requirements of the Uniform Building Code and HUD standards. Cement can be added to the soil block mixture to reach a bearing capacity of 2500-3900 psi.
Soil handling
The use of soil and caliche as building materials is inexpensive for materials costs. However, the right equipment and coordinated labor are important in the soil material construction process. Even a small structure may require at least 15 tons of earth. This material must be moved and handled several times. A bulldozer, front-end loader, or tractor equipped with a shovel or backhoe will be necessary for on-site extraction of soil materials. A large, flat area with good drainage is necessary for hand molding of blocks, making the clay lumps for cob, or mixing superadobe. The building footprint should be accessible by truck for rammed earth construction.
Materials
Caliche is used in our area as a road base material, and in the production of cement and lime. Although not commonly used as a building material, there are historical as well as current examples of caliche for construction. For an in-depth treatment of the subject, see The Caliche Report (see Resources ).
Caliche occurs in abundance in the Austin area, where it is often found at the construction site. It can also be purchased from area suppliers. Be sure to test the source for the correct clay-to-sand ratio. The use of soil as the basic block material is also possible, but will have slightly different stabilization demands. The same methods described here can be used with soil block.
Block production methods
A bulldozer or front-end loader is needed to extract caliche or soil on-site. Between 5 and 10 percent cement must be added to caliche, depending on the quality of the caliche. For mixing, caliche must be dried and screened. Some soil may not require this step. The soil components are mixed in a mortar or concrete mixer.
Molding techniques may be in the form of monolithic walls (see the following section on Rammed Earth ) or molded into blocks or bricks. For the latter, the mix is poured into molds, or pressure molded using special machinery. These methods provide for a variety of standard and custom sizes and shapes of block. With the hand mold technique, the prepared mix is poured into damp or oiled molds, spread evenly, and the molds are shaken slightly to ensure even filling of the forms. The blocks are then removed and allowed to cure before stacking.
Air curing must occur for 10-14 days before the block can be used in construction. Protect the blocks from direct sunlight for 5 days and from rain throughout the curing process. Drying bricks may be temporarily covered with tarps or plastic sheeting, but these must be removed for curing to continue. Once bricks are sufficiently cured, they can be set on end to continue drying.
With a wheelbarrow and gang forms, a crew of two can produce 300 to 400 bricks per day. With the addition of a plaster mixer and gang forms for 500 bricks, this production can be doubled. The addition of a front-end loader with a driver will additionally increase production.
Compressed caliche or soil block can be manufactured on site with a variety of block-making machines, including hydraulic presses, mechanical presses, and various combinations. Some mechanical presses are small enough to be operated by hand (Cinva-Ram, for instance). With a mobile industrial block machine powered by a diesel engine, 800 blocks can be produced per hour. Compressed soil blocks can be used immediately. They continue to cure and gain strength after they are installed. When green (before they are cured), they can be readily shaped or nailed with hand tools.
Mortaring
Mortar for blocks must be applied to the entire surface of the block, as opposed to ribbon mortar beds often used with conventional brick. Full surface mortaring allows for maximum compressive strength. The same soil used in block making, mixed with water to form a slurry, is usually used as a mortar for binding blocks together into floors and walls. Cement can be added to the mortar mix, but this increases the cost. The main advantage of cement mortar is its quick drying speed.
Design methods
Block size can be varied easily to accommodate a variety of designs. Walls can be sculptured, rounded, or formed into keystone arches to create custom effects. Relatively unskilled labor can be used in construction with compressed earth and caliche block.
Design of structural walls using caliche or soil material block must take into account wall height and thickness, size of block, insulation value, and the desired style and finish. Wall height-to-thickness ratio must be adequate for stability to meet energy standards. For more information on structural design, see Buildings of Earth and Straw, listed in Resources .
The relatively low insulation value of soil or caliche block may make additional insulation necessary. In Central Texas, a 12 inch wide block provides an appropriate mass / insulation value.
Soil or caliche block structures need not have the "pueblo" style if this is not desired. Many architectural styles are possible.
A bond or collar beam is necessary if the roof is supported by the walls. This will serve to spread the loads over the entire wall, and stabilize the tops of the walls from horizontal movement.
Vertical reinforcement is difficult with solid block walls, but can be accommodated with the use of reinforced concrete columns at corners, wall openings, and at intervals in the wall. In this case, the soil block becomes an infill panel. Alternatively, walls made more than one block thick may have internal reinforcing between blocks, and have additional insulation between panels. With this method, care must be taken to ensure that the lower block courses are completely dry before additional courses are added.
Soil blocks are typically stuccoed to prevent them from getting wet. Clear finishes or a variety of plasters may be applied on the interior.
Rammed earth , an ancient building technique, may have originally been developed in climates where higher humidity and rainfall did not permit the production of soil block. For soil block to cure uncovered, there must be at least 10 straight days that are rain-free. 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 when being cured as blocks.
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 task. 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 or a front-end loader can also be used effectively for this task.
Formwork
Formwork 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 fiberglass.
Systems for keeping formwork in position vary. Small clamps adapted from concrete formwork techniques work well, although small holes are left when the clamps are removed. Other methods include locking hydraulic jacks, or formwork built on steel posts. Steel I-beams and plywood are sometimes used. For more discussion of form work design, organization, and moving, see the Earth Construction Primer and Adobe and Rammed Earth Buildings listed in Resources .
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. Curing can take from several months to several years, depending on weather and humidity.
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. A helpful hint is to put windows and doors immediately below the bond beams because it is difficult to shore up the concrete lintel and then ram on top of it. Water flow and moisture control is critical to protect structural walls. Special detailing for 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 because of the corrosive action of moisture from the soil material. Lintels can be concrete, stone, or wood. Careful attention to roof and any opening details is necessary to protect the structure from water damage. The addition of a small percentage of cement can increase the strength and the longevity of the rammed earth walls.
Foundations required by most codes are concrete reinforced with steel. Soil block 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 good 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.
Cob or cobb is a very old method of building thick walls out of hand-formed lumps of earth and straw. It has been most widely used in the United Kingdom and other parts of Western Europe. The lumps or "cobs" are stacked or packed together to create walls that take on any shape or form. This method lends itself to using creative sculptural efforts to achieve unique walls, curves, doorways, built-in furniture, arches, window forms, etc. This method is very time and labor consuming, yet offers the opportunity for unique wall forms. Cob walls are usually covered with a natural plaster and can be naturally pigmented.
This relatively new method combines elements of rammed earth technique and ancient building forms, such as domes and vaults. Cal Earth Institute has done engineering testing and successfully obtained permits for this type of construction in California where seismic concerns have resulted in the strictest structural codes.
Superadobe buildings can be designed and built with sand bags ranging from standard sizes to very long (continuous) bags that are coiled up from the foundation base to form domes. Vaulted forms and other variations are also possible.
Earth (usually from site), sand, cement (relatively small quantities), straw, and water are combined and stuffed by hand or pumped with standard concrete equipment into sandbags. The bags are layered to achieve the designed form. The layers are held together by compression and 4 point barbed wire, which is available from most building suppliers. No mortar is used between bags. Proper engineering is critical to insure structural stability.
The use of the sandbag allows a wider variety of soil types to be used than in many other earth methods. Because the bags are laid up wet, it is a faster method than adobe or formed blocks, which must dry before use. The forms used for rammed earth construction are not necessary.
Exterior walls can receive traditional stucco or a variety of finishes. Interior walls can receive simple earth plasters or even sheetrock. Standard doors and windows can be incorporated. Concerns regarding their details are similar to rammed earth. Some structures incorporate chimneylike "windcatchers" for natural cooling.
Thermal characteristics are similar to rammed earth and straw bale structures. Embodied energy is relatively low. Fly ash can be used in the mud mixture to reduce the amount of cement and to increase strength. See Resources section for more information on publications, workshops and training, house plans, and supplies.
Earth floors are most often used in outbuildings and sheds, but if properly installed they 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 watertight 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 (see Finishes ), 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 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 or sealing. Color can be integral or topical (just on the surface).
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 six to nine cm thick, and can be set on a two cm layer of mortar.
The outer or finished soil materials may be vulnerable to weather unless they are stabilized with cement and have adequate roof overhangs. Normally, the clay content of the finished material is naturally somewhat moisture resistant.
Structures made of soil materials are durable and can last from 50 to hundreds of years. The U.S. 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 1600s.
Two basic approaches for finishing soil-based construction materials exist: stabilized or natural finishes. Stabilized finishes such as cement stucco are more permanent and more expensive initially. Natural finishes such as mud plaster are less expensive for materials and less durable and will require ongoing maintenance to ensure a high quality finish.
Breathability is a quality with which to be concerned. Generally, natural plasters are much more breathable than cement-based stuccos. Masonry stucco or lime plasters are often good compromises between breathability and durability. Rammed earth walls may be sealed with a breathable water-based sealer alone, leaving the attractive soil walls visible with no need for plaster or stucco.
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 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, floated, or tinted to achieve a variety of textures and color variations, and reapplied as many times as necessary to achieve the desired affect or to make repairs. When dry, the mud plaster surface will take on a firm finished surface 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. However, cement stucco has a different expansion coefficient than the wall material. This may eventually lead to separation from the wall, and may conceal structural erosion problems that 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, because they may 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 in the 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 mixture.
In addition to stucco or plaster, interior walls may also be treated with a variety of non-traditional and traditional interior veneers including gypsum wallboard (drywall).
Thermal Characteristics
The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE ) laboratory tests give a 10-inch thick adobe wall with æ inch of stucco on the exterior and * inch of gypsum plaster on the interior an R-value of 3.8. A 14-inch thick earth 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.
Some dynamic testing of high mass walls has indicated that their "performance" is actually much better than these low R-values might suggest because they "keep the weather out".
K-value is much more important than R-values for earth walls. K-value refers to the heat capacity of the wall's mass. Earth walls do not actually "resist" the movement of temperature changes through them like an insulated stud wall does. Instead, their wall thickness, combined with its density and the low conductivity characteristics of earth materials, greatly slows down the heat exchange process between the inside and outside.
A wall thickness from 12 to 14 inches is generally considered optimum for thermal mass performance in Central Texas. In colder regions, insulation may need to be added. Keeping a thermal mass wall shaded during the heat of the summer is an important design consideration in Central Texas.
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 usually offset initial costs.
Embodied energy
Figure 1, adapted from Adobe and Rammed Earth Buildings , reflects 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 commonly used materials.
Figure 1
portland cement |
94 lb sk |
381,624 BTU |
hydrated lime |
100 lb sk |
440,619 BTU |
common brick |
1 block |
13,570 BTU |
concrete block |
1 block |
29,018 BTU |
earth/adobe block |
1 block (10x4x14") |
2,500 BTU |
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