CAST EARTH: TECHNOLOGY FOR COST EFFECTIVE, ENERGY EFFICIENT CONSTRUCTION
Restoring a Traditional Building Material
A structural material made with earth and calcined gypsum can replace wood or steel framing in residential and light commercial buildings, yielding energy and environmental benefits. Cast Earth has been demonstrated successfully in residences and commercial scale structures. It is a true "breakthrough technology", producing a product with all the properties of traditional earth construction, augmented by superior esthetics, rapid construction, and affordable cost.
Cast Earth does not involve laying bricks or blocks or slowly compacting earth by mechanical or pneumatic action. Instead, it consists of rapidly pouring an entire building in place, removing forms shortly after the pour. What makes this possible is calcined gypsum's fast set rate to a wet strength sufficient to support a wall, at an unexpectedly low concentration. Fifteen percent calcined gypsum provides surprising strength immediately after setting. Steel reinforcing is not used.
In a new development, not yet completely reduced to practice but put into limited field use in late 2003, the mix is treated after placing in forms to cause its normal one to two hour set time to take place in a few minutes. This set acceleration can speed up the rate of building significantly and opens new opportunities to create enhanced esthetics. During 2004, this exciting feature will be fine tuned and become a common part of the process.
For a short description of the Cast Earth Process, use the The Cast Earth Process Description link on the Home Page. It is not directly linked here in the hope that readers will stay with this description.
For centuries, residences and light commercial buildings were built from adobe, particularly in arid climates but not limited to them. Rammed earth structures were built in a wider climactic range, including some very rainy places. Many of these structures may still be seen, along with a few new earth buildings erected in recent years.
Architectural styles of earth construction are not limited to the pueblo, hacienda or territorial facades often associated with the medium. Many earth structures have been built which do not differ in external appearance from frame buildings incorporating contemporary, classical, or other designs.
Following the Second World War, labor costs escalated to the point at which manufacturing and laying of adobe bricks or of ramming earth walls became prohibitive. Today, traditional earth housing is only built for the very wealthy, or by the very poor.
In contrast, when calcined gypsum is added to an earth-water slurry, some startling possibilities emerge. The resulting earth walls are built in a few hours by machinery with minimal manual labor, instead of by heavy labor use over many days. Innovative designs are feasible since the "framing" material is assembled in a plastic state.
Discovering that it takes a relatively small portion of calined gypsum to yeild the desired results was one facet of developing the technology of Cast Earth. The other key feature was overcoming the drastic set acceleration of calcined gypsum when it is mixed with soil and water. Calcined gypsum that by itself sets in about 30 minutes will set in two to ten minutes when in the presence of soil, usually tending towards the shorter end of that interval. Obviously, this would not be a practical building material.
The set retarders that are known to work with gypsum would not function with the Cast Earth mix, because it takes so much of them to give a suitable working time that their presence destroys the strength of the resulting product. Finding new materials that give an "open" time of one to three hours without undue strength degredation was the other key to solving the rapid set obstacle.
Cast Earth Building Cost Considerations:
Cast Earth building costs are job and site specific. The cost generalities below assume that equipment common to the concrete industry is available and that materials, labor, water and site access costs are not inflated by unusual local conditions. The size of the building project has considerable influence, since equipment mobilization is a smaller cost component per cubic yard for a large job. The only precise way to estimate costs is with the aid of a builder or architect who is trained in Cast Earth construction.
Costs of building with Cast Earth are usually significantly lower than adobe or rammed earth, and under various conditions may be competitive with frame walls. These conditions include size of project, lumber costs, choice of exterior and interior finishing (unfinished cast earth is richly textured and aesthetically beautiful). Lifetime costs are substantially less than frame construction because of the high energy efficiency and low maintenance expenses of earth walls.
If the job and site conditions are favorable and local infrastructure for building with Cast Earth is available at reasonable cost, it can be expected that a smaller, one-off Cast Earth project will be about one half to three quarters the cost of rammed earth or adobe. For large jobs, or multiple buildings (as in a subdivision), the economies of scale of mechanized building can be considerable. A project construction crew develops an operating "rhythm" after several dozen yards have been poured, accelerating the job so that Cast Earth costs may become competitive with frame construction initial cost.
The economic competitiveness extends across the construction quality spectrum, including high end custom homes where earth is regarded as a prestigious material, to mass housing.
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Earth -- A Premium Building Material:
People have been living in houses built of earth for thousands of years. Even so, a properly built earthen structure is not primitive. Because of its mass, it provides substantial energy efficiency, and the thick walls deaden sound and lend an atmosphere of subtle power to the structure. As already mentioned, the architecture can be traditional, contemporary, or nearly anything compatible with one or two story construction. It is fire and termite proof, and is already accepted as an "environmentally correct" building technique.
Earth construction has been successfully utilized in all earthquake zones. It is a matter of structural engineering to design an earth wall to withstand shocks anticipated in a given zone. Reinforcing steel is not necessary, and neither is it desirable, since it has a much different frequency of vibration and coefficient of expansion than the earth walls. The massive earth walls are also natural protection against wind damage in areas which are subject to hurricanes or other powerful atmospheric disturbances.
In the old Southwest, an earth home is regarded as a premium item. Traditional earth structures are still built, but only in special cases because of their high labor intensiveness. Cast Earth will provide affordable earthbuilt structures for the many people who know the advantages of earth, but have not been able to afford it. A larger market will develop as interest in Cast Earth expands.
In addition to the traditional benefits, Cast Earth is differentiated from conventional construction in several respects:
Insulation is sometimes used with earth construction in areas of climatic extremes. When this is necessary, it is very easily incorporated within a Cast Earth wall by placing a rigid insulation panel inside the forms, and pouring Cast Earth on each side. Integral insulation is not directly possible with adobe or rammed earth, in which two separate walls must be built to enclose the insulation. North and east walls might be insulated in cold climates since they will not act as solar collectors, whereas south and west walls might warrant insulation in extremely sunny and hot areas to stifle the collector effect during summer.
A link to another page on this site describing this insulative process appears on the Hope Page but is not directly linked here to avoid a distraction to those who are reading this section.
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Cast Earth is a technology which requires quite a bit of training in order to do it without problems. It's easy when you know how, but probably impossible if you don't.
This requires that you learn the system, and that we teach it to you, which consumes considerable time and expense on both our parts. For a builder, the effort and expense can be spread over multiple structures and years, but for a single event, it may be questionable whether it's worth it. Also, it takes substantial equipment and operating knowledge, which requires both you and us to spend time locating the concrete industry resources in your area, and identifying suitable soils.
Because of this initial effort, we require a signup or licensing fee, training costs, and a royalty on a per yard basis. This would authorize a licensee to do unlimited building.
Your first job will be down on the learning curve, although a trainer will be present during the wall building phase to help smooth the assimilation of the technology. Your initial project should be a bit less costly than rammed earth or adobe, and compared to them, will seem a relaxing exercise. By the second job, you'll be pretty knowledgeable, and with good planning, the cost will be perhaps half that of the old technologies. The real economies in Cast Earth flow when repetitive building can be done, as opposed to a single, "one off" job. Equipment and personnel efficiencies are maximized when a rhythm is established in sequential building. We believe that at this level, the cost can be competitive with frame construction, but haven't reached that point ourselves.
The initial sign on fee is presently $15,000 per licensee, which entitles you to training at rates set by Cast Earth. The current training hourly rate is $60, and the estimated training time is one to two weeks. Actual travel and subsistence expenses will also be charged for training at your jobsite; if you choose to train at a location where a job is being undertaken by the trainer, you will be responsible for your own costs.
At present, the royalty is set at $5.00/yard, but will be waived for your first two hundred yards to help you offset startup inefficiencies. Cast Earth will develop a distribution and sales division for calcined gypsum and other materials used in the process. Calcined gypsum sold by Cast Earth will be lower in cost than you can usually find as an individual. If you choose to purchase calcined gypsum from Cast Earth (which you are not required to do), the royalty will not apply for yardage poured from that calcined gypsum. In the future, the trend will probably be for the license fee to rise, and for the royalty to drop. You would not be assessed additional fees should the rate increase, but you will be eligible for royalty reductions if they are implemented.
Beyond the training and materials supply considerations, as a Cast Earth Licensee, you will be able to attend our meetings, exchange ideas and technology with other licensees and receive updated training as new developments occur.
These considerations are important if you are only intending to build a single job, but represent a small investment if you foresee greater volume. After you have considered these thoughts, and have decided you'd like to go further, we should get to know each other. Send us an eMail if you have other questions, and we'll answer you in writing or by phone, depending on which seems appropriate.
Furthermore, the real economies in Cast Earth flow when repetitive building can be done, as opposed to a single, "one off" job. Equipment and personnel efficiencies are maximized when a rhythm is established in sequential building.
If an existing builder or builders are already in your locale, your search is simplified. If a builder needs to be recruited, with your assistance, you can be assured that your project will receive our intense attention as the pilot job for your area.
If you contact a Cast Earth Licensee, or contact Cast Earth directly through the following eMail address, you will need to supply some information in order to process your inquiry. The data that you should send includes the following because we can’t give you definitive information until we have a better idea of what you plan to do. Please send a general description of your project and be sure to describe each issue below, point by point:
1. size
2. timing
3. budget
4. exact location
5. existence of drawings
6. permitting status
7. site access
8. water supply
9. any other such information.
An earth home is a vastly different and moving experience, which you will begin to appreciate at first contact, and which will grow as the years pass until it evolves to the point at which you and the earth structure acquire a mutual personality, and if it's possible with an inanimate object, a bonding occurs. Start now on a journey you will never regret, and which will take you into a realm which languished in obscurity for fifty years but has now been reinvented to move ahead into the new century.
Contact Cast Earth: eMail Cast Earth
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Natural gypsum is a crystal of calcium sulfate in combination with two molecules of water. When it is carefully heated in the process of converting it to calcined gypsum, exactly three fourths of the water is driven off:
CaSO4x2H2O = CaSO4x1/2H2O + 1 1/2 H2O
The chemical name of the dry product containing one half molecule of water is calcium sulfate hemihydrate. In these Web pages, we refer to it as calcined gypsum. In a very pure and milled form, it is the well known Plaster of Paris used in casting, normally at a high concentration and with little if any aggregate.
This seemingly simple and mundane product has an unusual property. When it is mixed with water at ambient temperatures, it recombines with the same amount of water lost during calcination, and sets to form a strong gypsum crystal lattice:
CaSO4x1/2H2O + 1 1/2 H2O = CaSO4x2H2O
The setting property of calcined gypsum is uncommon and does not occur to such an extent with most other crystals when they rehydrate after loss of water. It imparts a very useful functionality to an otherwise common and uninteresting material.
Traditional adobe mud can be poured into a wall-sized form and left in place, but takes weeks to dry sufficiently to remove the forms. An immense inventory of forms would be necessary for construction of multiple buildings. Because soil composition is not ideal, massive shrinkage cracking almost always results during the drying process.
High labor cost problems occur with rammed earth, in which forms must be extremely strong and are left in place for days while the wall is tamped layer upon layer.
When walls are poured with the Cast Earth mixture, the forms can be removed on the same day as the pour, since the wet material is strong enough to support a complete wall. Lower levels set before the wall height grows significantly, allowing use of lightweight forms and fewer ties. Obtaining an early set with the newly developing set acceleration technology will enhance this feature.
Because of the cementitious nature of the calcined gypsum and its slight expansion on setting, shrinkage and cracking are not a problem. Consequently, it is possible to use a much wider range of soils than have historically been employed for earthbuilding. Steel reinforcing is not used in Cast Earth.
Addition of retarding agents will increase the open time and dispersing agents will increase fluidity or allow lower water usage. Water and additive requirements vary from soil to soil and also with changing water sources. Benefits of these reagents are limited, as excessive use impairs both wet and dry strength. Application of correct retarders can expand the "open" time of the slurry to three hours or more, without reducing strength to nonviable levels. Without retardation, the mix sets in four to ten minutes.
Retarder selection is extremely important, since most common retarding agents will not yield sufficiently long open times without drastic strength loss. In developing the technology, powerful retarders which do not seriously reduce strength were discovered. They are not materials which are currently known to the gypsum industry as retarders. Retarder studies are ongoing, since new possibilities come to light from time to time.
The need for well defined water addition and additive chemistry for various soils is a key to the system. Cast Earth and some of the Licensees maintain laboratories to determine additive mix and water levels for various soil types.
Suitable Soils For Cast Earth Construction
Conventional earthbuilding requires soils containing sand fractions and a smaller fines fraction, which is fairly low in clay content and has a cohesive nature. It is commonly believed that adobe is made from clay. This is not the case, although some clay (roughly 15 - 40%) must be present to provide cohesion. The balance should be a range of sand fractions with a maximum size of about 1/4 inch. If these soils are not cohesive, they are referred to as "dead" and are not suitable.
Because of the cementitious character of calcined gypsum, soils for building with Cast Earth are not restricted by a criterion of natural cohesiveness; that property is provided when the gypsum lattice sets. Thus a much wider range of soils is available for use. The principal determinant for soil suitability is fines content, which determines water demand. A certain clay content is acceptable and can benefit Cast Earth. The presence of gypsum counteracts the tendency of many clays to expand and shrink with changing moisture content.
The principles in identifying suitable soils for Cast Earth are that, (1) the largest particles should not exceed about 3/8" (10mm) and preferably, most of the larger particles should be 1/4" (6mm) and smaller; and (2) the material should contain a range of sizes, varying from the top size down to a portion of fines, small particles grading downward from about 100 mesh (0.006" or 150 microns). The fines should constitute between roughly 10% to 20% of the total solids. If material of this nature is not available, it is often possible to combine two different aggregates to synthesize a soil within the desired range -- if one aggregate is too course, it can be mixed with another that is quite fine, or if too fine, mixed with some coarse material.
At first glance, it would appear desirable to simply build with soil from the construction site. If the soil at specific building sites is suitable, it may be utilized, but in many cases it is preferable to haul in a known material. A typical low cost soil is the reject fraction which is obtained at sand and gravel quarries when landfill soil is screened to recover its gravel content. This is nearly a waste fraction and is often available in abundance at less than $5 per yard.
In most locations, it should not be difficult to find suitable building soils using these parameters. As with any masonry application, soils with a high salt content should be avoided.
Historical Uniqueness
Until about sixty years ago, laying adobe bricks was low cost and there was no impetus to develop a poured wall system of earth construction. Lack of understanding of retardation and operating techniques of working with this material may have discouraged any earlier experimenters. Inexpensive wood framing has dominated the light construction business since that time, but recent developments are changing the cost of lumber.
Today, the custom earthbuilding field is dominated by small contractors and owner-builders obsessed with the mystique and environmental advantages of traditional adobe and rammed earth construction. These are people who have decided to build with earth regardless of the cost. Their efforts result in a few houses which are beautiful and functional representatives of an art which was widely practiced in the US until mid-century and is still widespread in many areas, including Australia. Many low income families make adobes and build their own crude adobe homes. Since they do not place a value on their labor in this context, this is for them a low cost housing choice.
Very little has changed from ancient to modern earth construction. A limited amount of obvious mechanization has been introduced in making adobes or in filling and tamping rammed earth walls, but these changes are peripheral and only mildly reduce the great labor intensiveness of the basic unit operations of wall building.
About the only structural differences between historic and modern earth walls are the introduction of a six inch high concrete stem at the wall sole to keep the lowest earth portion above minor floods, and incorporation of a concrete or wood "bond beam" around the upper perimeter. Bond beams are also common in brick and concrete block masonry, to tie the top of the wall together and provide a plate to secure the roof. The stem and bond beam have been incorporated in ICBO earthbuilding codes and are part of the Cast Earth technique.
Cast Earth is the first breakthrough technology in earthbuilding. A fundamental change in wall chemistry is introduced, which permits substitution of machinery for manpower in the major labor consuming crafts. While other attempts to mechanize earth construction have only resulted in marginal increases in production rates (up to about 12 yards per hour, but commonly at 20 yards per day), Cast Earth placing rates of 30 to 45 yards per hour are easily achieved.
Literature searches and a patent search revealed no prior disclosures of similar technology. It seems that no one historically worked with the Cast Earth system, or that any activity which may have taken place was never published.
Following the patent and literature searches, a large volume of compressive strength data was accumulated. Prototype walls were then built, followed by six earth homes. Various equipment configurations have been tested, including the familiar Batch Plant/ReadyMix truck configuration, and Concrete Mobile Mixers. Both pumps, crane-and-bucket systems and front loaders have been used to place the mix.
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