- Soils for Rammed Earth, Caliche Block, and Soil Material Construction
- Caliche and Soil Block Construction
- Rammed Earth Construction
- Soil Materials Flooring
- Soil Material Durability and Finishes
- Soil Material and Energy
025 128 to 025 166
042 050 to 042 100
042 500 to 042 550
044 050 to 044 700
The type of materials available locally will of course vary depending upon the conditions in the area of the building site.
In many areas, indigenous stone is available from the local region, such as limestone, marble, granite, and sandstone. It mat be cut in quarries or removed from the surface of the ground (flag and fieldstone). Ideally, stone from the building site can be utilized. Depending on the stone type, it can be used for structural block, facing block, pavers, and crushed stone.
Most brick plants are located near the clay source they use to make brick. Bricks are molded and baked blocks of clay. Brick products come in many forms, including structural brick, face brick, roof tile, structural tile, paving brick, and floor tile.
Caliche is a soft limestone material which is mined from areas with calcium-carbonate soils and limestone bedrock. It is best known as a road bed material, but it can be processed into an unfired building block, stabilized with an additive such as cement. Other earth materials include soil blocks typically stabilized with a cement additive and produced with forms or compression.
Rammed Earth consists of walls made from moist, sandy soil, or stabilized soil, which is tamped into form work. Walls are a minimum of 12″ thick. Soils should contain about 30% clay and 70% sand.
The use of locally available and indigenous earth materials has several advantages in terms of sustainability. They are:
- Reduction of energy costs related to transportation.
- Reduction of material costs due to reduced transportation costs, especially for well-established industries.
- Support of local businesses and resource bases.
Care must be taken to ensure that non-renewable earth materials are not over-extracted. Ecological balance within the region needs to be maintained while efficiently utilizing its resources. Many local suppliers carry materials that have been shipped in from out of the area, so it is important to ask for locally produced/quarried materials.
Both brick and stone materials are aesthetically pleasing, durable, and low maintenance. Exterior walls weather well, eliminating the need for constant refinishing and sealing. Interior use of brick and stone can also provide excellent thermal mass, or be used to provide radiant heat. Some stone and brick makes an ideal flooring or exterior paving material, cool in summer and possessing good thermal properties for passive solar heating. Caliche block has been produced for applications similar to stone and brick mentioned above. Caliche or earth material block has special structural and finishing characteristics.
Rammed earth is more often considered for use in walls, although it can also be used for floors. Rammed earth and caliche block can be used for structural walls, and offer great potential as low-cost material alternatives with low embodied energy. In addition, such materials are fireproof.
Caliche block and rammed earth can be produced on-site. It is very important to have soils tested for construction material use. Some soils, such as highly expansive or bentonite soils, are not suitable for structural use. Testing labs are available in most areas to determine material suitability for structural use and meeting codes.
Soils for traditional adobe construction are not found in some areas, but other soils for earth building options are available. Many areas have a high percentage of soils suitable for ramming (approximately 19,610 acres in the Austin, TX area, according to the US. Department of Agriculture). Caliche is also abundant in many areas (covering 14 % of the Austin geographic area, for instance) and is readily available locally.
|Satisfactory in most conditions|
|Satisfactory in Limited Conditions|
|Unsatisfactory or Difficult|
Stone cutting, brick production and masonry techniques are mature technologies. Rammed earth and caliche block construction are not well known by most builders and architects today, although there are some architects and builders who are experienced with these materials.
There are numerous suppliers of indigenous stone and local brick in many regions. Caliche block and rammed earth are not available commercially, but can be created on site. There are contractors who can provide machinery for manufacturing compressed soil block, and in some places such block is commercially available.
Brick: approximately $2.00 per square foot (4 inch material) and up depending on thickness. Stone: $4.00 to $15.00 per square foot (material) depending on type. Compressed soil block: approximately $1.80 per square foot (9 inches thick). Earth block made from labor intensive methods cost significantly less.
Stone and brick materials do not pose a problem for lending institutions, and are often valued positively for increased property value and fire rating. Rammed earth, compressed soil block, and caliche block may pose problems for traditional financing. Proper testing and building code compliance will assist lenders in accepting their products.
Stone and brick construction are considered desirable, although their use for interior thermal mass is not common in many areas. Rammed earth and caliche block are little known, and may not currently receive wide public acceptance.
In structural applications, materials must be rated for appropriate load requirements. Unfired caliche blocks can easily pass Unified Building Code standards for compression with an average of 960 p.s.i. Rammed earth and caliche block construction will require a building code review if used structurally. Regulatory acceptance will be based on precedents for the material as accepted in other jurisdictions and/or upon independent tests that demonstrate methods and performance required by code for masonry materials are satisfied.
Stone construction practices are fairly standard. We do not recommend any stone applications that would require non-traditional methods. Attention needs to be paid to the load capacity of foundations and footings due to the excessive 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.
1.2 Indigenous Stone Description
- Limestone: A rock that is formed chiefly by the accumulation of organic remains (shells or coral), consist mainly of calcium carbonate.
- Marble: Crystallized limestone, ranges from granular to compact in texture.
- Granite: A very hard igneous 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 unaltered form as taken from the field.
The same guidelines in Section 1.0 above also apply to brick masonry.
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.
3.0 Soils for Rammed Earth, Caliche Block, and Soil Material Construction
Soils that qualify for both Compressed Earth Block and Rammed Earth are common in many areas. Consider that most of the continents are granitic and decomposed granite is normally perfect having the ratios of feldspars to quartz that are appropriate for compaction. Basaltic soils are a little more difficult and many times require additional clay added. The basic formula is 30% clay and the balance loam and small aggregate. Caliche (which is usually a misnomer for decomposed limestone soils) is the common subsoil of the alluvial plain which dominates the south Texas landscape, much of the Midwest and most of the deep south as well as most of the Caribbean . In The Dominican Republic it is named for the coral reefs that underly the island and is somewhat compactable depending on the area. The use of decomposed limestone can be problematic unless modified with either the addition of clay, portland cement or lime if necessary.
Soils that are bentonitic or highly expansive are normally unsuitable for earth construction without modification. The shrink and swell capacity of these soils, related to their clay content can cause the block to be highly susceptible to moisture, even high humidity, however the acid test is how the clays actually perform under compaction and even poor performance can be offset by stabilization. Soil cracking after rainfall may indicate expansive soil. Soil must be tested to determine its suitability. The ideal is a block or wall that looks pretty and has a lot of strength but even ugly block and marginal soils can be used to build a structure that will last for centuries.
Desirable qualities for soil construction materials include:
- Low Moisture Absorption
- Limited Shrink/Swell Reaction
- High Resistance to Erosion and Chemical Attack
3.1 Soil Testing
Soil testing techniques vary, and include laboratory as well as field testing. 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.
Again, 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 Section 5.0 on rammed earth.
3.2 Chemical Soil Stabilization
Lime, portland cement, and other pozzolans (high silica volcanic ash, rice hull ash, etc) can be used as chemical additives. Lime is most effective on clay soils, and can be used in combination with portland 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 and puts approximately an equal weight of carbon dioxide into the atmosphere. However, cement produces the strongest block and will substitute for clay poor soils where lime will not and the normal usage of between 5 and 10% minimizes the embodied energy especially when compared to concrete and lumber products * . Pozzolan exists in plentiful supply in many areas, and is sometimes readily available commercially in the form of coal fly ash .
The Center for Maximum Potential Building Systems (CMPBS) in Austin, Tx is experimenting with the use of pozzolan as an additive and offers considerable expertise in earth materials use. See the Resources section.
3.3 Strength of tested earth and caliche block
Unfired Compressed Earth Block with addition of 5-10% cement 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.
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.
3.4 Soil Handling
The use of earth as building materials is inexpensive for materials costs, but emphasizes labor in construction methods. 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 at least twice. A front end loader, skidsteer or tractor equipped with a shovel or back hoe will be necessary for on-site extraction of soil materials as well as processing the soil and loading the machinery. A large flat area with good drainage is necessary for handling and processing the materials as well as making the blocks. The building footprint should also be accessible by truck for rammed earth construction.
4.0 Caliche and Soil Block Construction
Caliche is used in many areas 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 and may be possible to get from the construction site. However, if this is not possible, caliche may be purchased from area suppliers. Be sure to test the source. The use of soil as the basic block material is also possible, but will have slightly different stabilization demands.
Subsoils are the basics of Earth Block Construction. With a clay content of plus or minus 30% and a water content of 6% (equivalent to soil that has received an inch of rain a week previous. No straw, roots, twigs, leaves, etc.
4.2 Block Production Methods
A backhoe and/or a front end loader will be needed to dig the soil on-site or handle soils imported. Soils obtained from the site may need to be dried and screened prior to mixing. Soils should be tested to prove their compactability and to determine any needed additions such as sand or clay. The next step of hydrating and mixing has traditionally been the largest labor and time investment being done either by hand or with a front end loader. The use of concrete and stucco mixers have proven ineffectual for large projects such as a home, however there are earth mixing or blending machinery available that are especially cost effective for adding portland cement or lime and for adding water in dry areas.
Sun Dried Adobe
- Molding techniques may be in the form of monolithic walls (See 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 size 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. Protection from direct sunlight for 5 days and protection from rain throughout the curing process are important. 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 Earth Block
- Compressed earth 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 as many as 800 blocks can be produced per hour. Compressed soil blocks can be used immmediately. They continue to cure and gain strength after they are installed. When green (before they are cured), they can be readily shaped or nailed into with hand tools.
- Compressed Earth Block come in two basic types, The vertical press where the block are normally 10″ x 14″ (there are many variations) that are fixed with the height of the block nominally 3″ which is variable due to the variability of the soil. These block are treated like Adobe in that they need to be mortared and cut to fit. The Horizontal Press are of a fixed dimension normally 4″ x 14″(again there are variations) with a length of the block variable from 2″ to 12″ depending on the machine. These blocks do not require mortar and can be dry stacked with ease by basic skilled workers, the block can also be custom sized to minimize cutting for electrical, plumbing and wall changes.
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 stabilization.
4.4 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 utilized in construction with compressed earth block.
Design of structural walls using any soil material block must take into account wall height and thickness, size of block, mass value * , and the desired style and finish. Wall height-to-thickness ratio must be adequate for stability * .
Because thermal mass equates to insulation in soil block a minimum of 12 inches is needed for a comfortable abode.
Earth block structures need not have the “pueblo” style if this is not desired. In fact a gable or hip roof can protect the home better while offering solar protection from western exposures. 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. (See code)
- Soil blocks are typically stuccoed or plastered to prevent them from getting wet, however, any veneer or siding can be used on Pressed Earth Block as they can hold a nail or staple. Interior finishes are normally plaster (structolite) or earth plasters that are simple to apply and maintain. Petroleum based finishes do not work well with unstabilized earth block and cement plasters do not stick to asphalt stabilized adobe. A common mix for a stabilized interior mud plaster is 5% portland cement to 30% minimum clay fine screened with window screen. Exterior mud plaster will need 6 to 10% portland cement with 30% minimum clay and 1/8″ screen.
- Fully stabilized structures do not require any exterior finish unless desired for aesthetics.
5.0 Rammed Earth Construction
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.
5.1 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.
5.2 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 in Resources .
5.3 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.
5.4 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.
6.0 Soil Materials 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.
7.0 Soil 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.
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.
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).
7.3 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.
8.0 Soil Material and Energy
8.1 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.
8.2 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 Cement||94 lb sack||381,624 BTU|
|Lime, hydrated||100 lb sack||440,619 BTU|
|Common brick||1 block||13,570 BTU|
|Concrete block||1 block||29,018 BTU|
|Earth (Adobe) block (mechanized production)||1 block (10X4X14)||2,500 BTU|