Resurrection: Rammed Earth Construction Michael Padavic. Professor Mark Mulligan GSD 6204 Building Technology January 11, 2002

Transcription

1 Resurrection: Rammed Earth Construction Michael Padavic Professor Mark Mulligan GSD 6204 Building Technology January 11, 2002

2 Padavic 1 Construction materials are at a peculiar stage in history. Decades ago, traditional building methods were dropped in favor of the economic, disregarding the embodied energy of these newer materials. Technology and environmental awareness have advanced enough, however, to allow purely economic building methods to be questioned. One of the most interesting materials affected by this heightened awareness is rammed earth a process by which a soil mixture is lifted into formwork and compacted to make a strong, monolithic wall (figure 1). Building with earth is well known to be an ancient practice, occurring more frequently once the human race began evolving from a nomadic lifestyle to an agricultural one. Thousands of years later, technology is allowing the reexamination of earth construction. Developments prove Figure 1 that earth construction is still a viable economic and environmentally conscious technique. Further, the subcategory of rammed earth construction is worthy of financial support and advancement. To see this, first it is important to have a deeper look at the history of earth construction. Then it is informative to see recent examples of building with rammed earth. Next, it is necessary to have a detailed explanation of the rammed earth construction method, thus outlining its benefits and weaknesses. Putting these all together, including innovations that target weaknesses, it will be evident that rammed earth is a viable construction technique, safer for the environment, yet comparable in price. Rammed earth is a logical building material that has a common place in today s economy and environment. There is a good amount of information known about the early building techniques of the human race. The nomadic lifestyle of the hunter didn t allow for a fixed dwelling. However, knowledge and development of agriculture rendered the need for permanent shelter. Globally, a chronology is difficult to understand, because civilizations developed independently and at different speeds. In all places, though, dependant upon locale was the use of wood, stone, or

3 Padavic 2 neither, but in all cases was the use of earth in the form of mud 1. An early advancement was brick building to solve the need for an effective, rapid technique. Adobe was used most often, but soil blocks were developed where adobe wouldn t work 2. In the arid Middle East, buildings of brick with no internal structure at all were developed 3. During the expansion of the western United States, bricks and lumber would be provided locally, with settlers completing construction in the local tradition or style. It is difficult to specifically date rammed earth in particular, but it is known to have been in use on the east coast through mid-19 th century. A treatise was published in 1839 extolling the virtues of rammed earth construction. It is a viable medium in virtually any climate 4. A gradual decline of building with earth came with the advent of railroads providing an abundance of materials that could emulate the styles of the east. The big change, however, came when the switch was made to economical, modern materials after WWII. 5 Today, rammed earth is reasserting itself in a few ways. Two of these ways are environmentally and pragmatically. Environmentally, the Autonomous Environmental Centre (Ateic), designed by Pat Borer with David Lea in Wales, showcases a number of alternative building materials and has raised some discussion in the U.K. 6 In addition to non-reinforced rammed earth walls there is locally sourced timber, woodwool sheathing, and lime-crete as opposed to cement due to its high production of carbon dioxide 7. Whereas the Ateic building Figure 2 expresses the environmental nature, Rick Joy s architecture expresses its pragmatic desirability. In 1998, Joy designed the 2800-square-foot Palmer/Rose residence outside Tucson, Arizona (figure 2). It could almost go unsaid how beautifully the residence blends in with the 1 McHenry, Paul Graham. Adobe and Rammed Earth Buildings: Design and Construction. New York: John Wiley & Sons, Page 2. 2 McHenry, 4. 3 McHenry, 6. 4 McHenry, 7. 5 McHenry, 8. 6 Hannay, Patrick. Ground Force. RIBA Journal 107 (Nov 2000): Page Hannay, 36.

4 Padavic 3 Figure 3 Sonoran and the Santa Catalina Mountains (Figure 3). Described as a modernist who designs with an elemental palette of light, space, and soil, Joy uses rammed earth because of its low level of skill involved. As his own contractor, skilled labor can be concentrated on other parts of the design, creating a greater contrast with the shell. 8 The specific construction of rammed earth consists of lifts or layers of earth poured into formwork at a depth of eight inches and then compacted to five inches. 9 This creates a striated earthen wall like a geological cut through time 10. Similar to concrete, it is stronger than other forms of earth construction because it is compacted in place and contains no mortar joints 11. Rammed earth construction begins with site selection and the acquisition of very specific materials: soil mix, water, and an optional stabilizer. Since topsoil is organic and biodegrades, a mix of subsoil is used 12. The approximate proportion of subsoil is thirty percent clay/silt to seventy percent sand/gravel 13. Water has a direct impact on the strength of finished walls, and depending on the soil mix, is eight to sixteen percent of the mix 14. An optional stabilizer may be added four to twelve percent depending on conditions such as bonding strength of the clay, seismic activity, desired construction process, or desired wall proportions 15. Stabilizers include cement, lime, or pozzolan added to the mix 16. There are numerous field and laboratory tests to be run at all stages of the material gathering process, each to determine the specific mix peculiar to the site. These include density, compressive strength, bond strength, and erosion and wear resistance tests Giovanni, Joseph. Earthwork: The Palmer/Rose Residence, Tucson, Arizona. Architecture 87 (Dec 1998: Page Keable, Julian. Rammed Earth Structures: A Code of Practice. London: Intermediate Publications, LTD, Giovanni, King, Bruce. Buildings of Earth and Straw. Sausalito, California: Ecological Design Press, Page King, Keable, Keable, King, Keable, King, 59.

5 Padavic 4 If a good sand/clay mix is not attainable at the site, the lacking part may be brought to the site and blended in. A variety in particle size is necessary so that the soil bonds to itself and other materials (such as a concrete foundation or slab). Particle sizes range from gravel (one inch 3/16 inch) to sand (3/16 inch a few thousands of an inch) to fines (less that three thousands of an inch). Fines are a combination of clay and silt, with concentration on clay levels too little won t bond the soil while too much will cause excess water absorption and cracking. 18 The soil must be mixed, usually manually by shovel, to allow the variety of particles to be spread throughout 19. At this time, a stabilizer is blended if desired. Reasons for adding a stabilizer far outweigh not, as it speeds the construction process, improves durability, allows thinner walls, and requires less of a surface treatment. However, with a stabilizer, the process is a bit more complicated and expensive 20. Finally, water is added to the mix, sprinkled on as to not saturate any part of the mix. If a stabilizer has been added (especially cement), lifting the wall must begin immediately thereafter. Formwork (figure 4) is similar to that for concrete, and should be in place before water is added to the soil mix. However, different from concrete, the formwork cannot have ties running through it. This is because all areas of the form must be accessible to tamping 21. This fact also renders the minimum thickness of a wall to about sixteen to eighteen inches, so that a laborer can move within the form. Typical thickness for walls is sixteen, eighteen, and twenty-four inches with a height to weight Figure 4 18 King, Keable, Keable, Keable, 44.

6 Padavic 5 ratio of approximately ten 22. Concentration for formwork is on strength and stiffness, lightness, and ease of alignment. Possible materials include plywood, wood planks and framing, mild steel, or a combination for a desired finished appearance. Like concrete, sheet materials give good offform finished surfaces. Dependant on the quality of soil and presence of stabilizer is the height of the formwork. Unstabilized soil is concentrated horizontally, and alignment is from layer to layer of wall. Stabilized soil is concentrated vertically, and alignment is between forms. Openings for doors and windows within the wall are created with blockouts. Corners are made stronger if created as one piece as opposed to solely having a joint between two walls 23. Figure 5 Once formwork for a wall section is complete, the soil is mixed, blended and hydrated. Soon after, as mentioned before, earth is laid in and compacted (or tamped see figure 5). Both manual and pneumatic tampers can be used. Care is taken so that larger stones are moved away from the form. The edges are rammed first, and then the center until no further impressions result from blows from the tamper 24. After the wall completely goes up and is cured (twentyeight to fifty-six day period) any fixtures may be added 25. The roof is tied into the wall, and window and doorframes are added. Fixings are buried deep within the wall to retain structural integrity 26. In addition, utilities and systems, 22 King, Keable, Keable, King, Keable, 92.

7 Padavic 6 determined before construction, may pass within the wall to a certain degree 27. The final part of the construction process is to apply a wall finish, if desired or required 28. Through the construction process, it is possible to outline the strengths and weaknesses of rammed earth construction. It is environmentally conscious, especially since the main material is abundant and usually taken from the site itself 29. Also, although a poor insulator, rammed earth has good thermal mass properties, storing heat from the sun and releasing it during the cooler night 30. Finally, it s a low cost, rapid construction technique of high compressive strength ( psi) and desirable aesthetics 31. On the other hand, the main known weakness includes its low-tensile strength (susceptibility to seismic activity) 32, while perceived weaknesses are its absorption of and erosion by water 33. Additionally, there is one standpoint that is generally unclear about rammed earth construction. Some claim that it is possible to achieve with unskilled labor. Yet, one would assume that it takes a high degree of skilled labor to construct. The conclusion that can be made from the given information, listed here and in other readings, would suggest that a contractor skilled in rammed earth would have to be available for acquisition of materials, testing, and designing formwork. However, after that, the process of mixing, hydrating, lifting, and tamping is possible by unskilled labor 34. Since earth construction had been seldom used in the past sixty years, expertise on the matter had all but disappeared. However, continuous research and experimentation in recent years has improved knowledge about rammed earth. With knowledge of concrete construction, many of the tensile problems of rammed earth are solved in the same way. Seismic activity, or any overbearing force, causes cracking within the wall that in turn reduces its overall strength. Exact calculations are not yet known, but problem areas can be targeted. Steel rebar can be placed into the form and bound to the earth during the ramming process. This can occur at 27 Keable, Keable, Keable, section King, Keable, section Keable, 2.

8 Padavic 7 corners and over wall openings to provide stability and tensile strength 35. Instead of rebar, lintels of various materials or even arches can also be used over openings 36 (figure 6). Much of the general concern with rammed earth, though, is its susceptibility to water. If the wall absorbs water, the bonds between particles lessen and crushing strength is reduced. Driving rain loosens smaller particles, which create larger holes and cracks within the wall. Water can be combated easily though. Common design techniques, such as deep over hangs (usually one-third the height of Figure 6 the wall), can begin to protect the wall. The addition of a stabilizer will help with the water absorption (another advantage to adding a stabilizer) 37. But as far as water damage goes, surface treatment can provide the best protection for a rammed earth wall. Breathable finishes should be used to allow for water evaporation. Traditionally, stucco or plaster has been used and then painted over. Also, a lime wash (whitewash), bitumen emulsion with paint, emulsion paint, or oil-based paint can protect the surface. Still, with the desire to express the earthen quality of rammed earth, polymer emulsion (PVA) has more recently been used to seal the wall and protect it from wind and rain, left transparent for aesthetics 38. Rammed earth is a logical building material in its economy, environmental awareness, and construction technique. It has been accepted as a safe construction material that has an aesthetic advantage. Once accepted on a larger scale for more frequent use and is supported financially, further experimentation may be done to further this building material, and allow the resurrection of earth construction. 35 Keable, Keable, Keable, Keable, 94.

9 Padavic 8 Bibliography 1. Giovanni, Joseph. Earthwork: The Palmer/Rose House, Tucson, Arizona. Architecture 87 (Dec 1998): Hannay, Patrick. Ground Force. RIBA Journal 107 (Nov 2000): Keable, Julian. Rammed Earth Construction: A Code of Practice. London: Intermediate Publications, LTD, King, Bruce. Buildings of Earth and Straw. Sausalito, California: Ecological Design Press, McHenry, Paul Graham. Adobe and Rammed Earth Buildings: Design and Construction. New York: John Wiley & Sons, Illustration Sources cover: Giovanni, 94. figure 1: figure 2: Giovanni, 94. figure 3: Giovanni, 94. figure 4: King, 50. figure 5: King, 56. figure 6: King, 70.