compression and moisture resistance of sustainably stabilized
RAMMED EARTH
brittany hodges - university of kansas abstract
theory
This research is a materiality study of a vernacular type of construction used for centuries called rammed earth, where soil is compressed to form a structurally strong, insulating and fundamentally beautiful building elements. This specific research will study how linseed oil and cow’s blood stabilize and strengthen a Kansas silty loam soil. Rammed earth is a solution for sustainable construction for it’s low embodied energy, low toxicity, recyclability, renewability, and compressive strength. This specific research expands developing knowledge on sustainable stabilization to improve the compressive strength and moisture resistance of earth. A series of tests to find the optimum moisture content and to classify the soil preluded the process of ramming. The four test groups include the control, an addition of linseed oil, an addition of blood, and the combination of linseed oil and blood. Cylinders are rammed in standard sizing used in concrete tests, along with the same testing days as concrete tests. The cylinders are crushed with a machine used to test the compressive strength of concrete. In addition, the cylinders are also submerged in water, again using the same standards for other materials, in order to test the moisture resistance. Results showed an increase in strength with the added stabilizers, and an impressive increase to moisture resistance. What this proves is that rammed earth can be stabilized without the use of unsustainable cement. Further research is needed on how different types of soil react and how different combinations of stabilizers can create a structural and sustainable building material.
This research will expand current knowledge by studying how a Kansas silty loam soil can be strengthened by using stabilizers of cow’s blood and slacked lime. Stabilizers typically cement in modern use, improve compressive strength and erosive resistance (Maniatidis 29). However, the use of cement limits recyclability, releases Carbon Dioxide gas, and encompasses a high embodied energy. This research is to study the effects of less detrimental and more natural and renewable stabilizers. Both blood and linseed oil have been studied for rammed earth in past studies. Linseed oil has been proven to strengthen weak earthen walls when applied as a paint (Browne 7). Blood has been proven to improve strength. This research will study their effects on strength and water erosive resistance on a more clay based soil that is typical of eastern Kansas and other Midwestern soils. This will encourage an alternative and more sustainable stabilizer use in rammed earth and will establish knowledge of how this soil type interacts with said stabilizers whereas past research has been done with quarried and nonlocal soil.
methodology Research begins with choosing a specific soil type. Soil was chosen from a property in Anderson County in Kansas with property owner’s permission. Soil type and profile was chosen based on with the website for the USDA Web Soil Survey and typical soil found in the area in order to study the typical Eastern Kansas soil applicability for rammed earth. The depth of soil excavated was between the vegetation layer and the clay layer, found deeper in the soil.
introduction The field of architecture and the built environment is increasingly focusing itself on sustainable practices because of the large amounts of embodied energy and waste in buildings and construction. Earthen architecture, specifically rammed earth, is a solution because of its low embodied energy, low toxicity, recyclability (with the exception of soil stabilized with concrete), and compressive strength. Rammed earth is a vernacular style of construction that has been used for centuries, some examples being Hakk Tulou in Fujian, China and the Castle of Banos de la Encina, Andalucia (Jaquin, “A Chronological Description of the Spatial Development of Rammed Earth Techniques”). It’s capability to be used over the centuries proves is usability and reliability, as well as a surge of interest in the subject in current research and practice. However, pure rammed earth has a compressive strength too low to be used structurally in most cases, and is highly threatened by erosion for exterior functions. Much of the research revolving around rammed earth, including this research, studies different stabilizers added to the mix of dirt and water prior to ramming in order to increase strength and resist water damage. This research studies linseed oil, cow’s blood, and the two combined. To complete the research, dirt was excavated in Anderson County, Kansas. It was then tested to find the particle size distribution, the plasticity index, and the optimum moisture content. Then, 10 cylinders were rammed for each test group, nine to be tested for compressive strength, and one to study moisture resistance. The nine were tested at days 7, 14, and 28 to find the optimum curing length. The moisture resistance required the cylinders be submerged in water and weighed at different time intervals to measure the amount of soil eroded.
The soil was then tested and characterized using the University of Kansas School of Engineering facilities. Testing included particle size analysis (ASTM D 422-63), liquid limits (ASTM D 4318), plastic limits (ASTM D 4318), moisture content (ASTM D 2216-05) and optimum moisture content (ASTM D 698-07 Method A). Soil is sieved and organic material removed because of its effects on the soil’s tendency to shrink, biodegrade, and be posed to insect attacks (Maniatidis 16). The liquid limit is found in order to determine the point of soil saturation. Soil needs to be completely saturated with liquid (the stabilizers in the test cylinders and artificial ground water in the control cylinders) without being over saturated. This will maximize their workability when ramming without sacrificing compressive strength, which can be ill affected with over saturation. Plasticity index indicates “the ability of a soil to undergo irreversible deformation while still resisting an increase in loading” (Maniatidis 20). It is the percentage of dry weight (or water content increase) needed for the soil to pass from a plastic to liquid state (Maniatidis 20). After soil is characterized, sieved and an ideal liquid content is found, a project compaction test with water will test the saturation level of the soil. The amount of soil typically needed for one cylinder is 4 kg. Once the soil is weighed, the water was added with the weight found from the ideal liquid content percentage applied to the 4 kg of soil. Water and soil was thoroughly mixed with gloved hands on sanitized metal surface until all water is absorbed and all particles are saturated. Metal cylinder form of 4” diameter and 8” tall was clamped to table. Lifts of 2 inches are preferable, but for the test cylinder it was estimated to be 90 g for each lift. For each lift, an air compressor powered rammer was used to compact soil in cylinder until completely compacted. Between each lift, soil was scored to allow lifts to adhere to each other. There were 10 cylinders for each test group to test compressive strength. Test groups include: Control (water and soil), Linseed Oil (boiled linseed oil and soil), Blood (Cow’s blood and soil), and Linseed Oil+Blood (boiled linseed oil, cow’s blood, and soil).
Yuchanglou , a tulou in China, circa 1308. Photo source: Gisling, Wikimedia Commons
Castillo Baños de la Encina, Spain, circa 10th century Photo source: Paul Jacquin, Auroville Earth Institute
Nk’Mip Desert Cultural Centre Photo source: Dialog Design
Testing of the moisture resistance will take place on day 28 from date of ramming following ASTM C67 Immersion Testing standards. First, the cylinder was weighed. Then the cylinder was submerged in water with at least two inches above the top of the cylinder. Then the weight was recorded after 1 hour, 4 hours, and 24 hours from initial submersion, with the cylinder always returning to be submerged after weighing.
results Prior to ramming the cylinders, soil was tested in order to characterize it. A particle size test was completed (figure 1), along with the plasticity index, liquid limits, and optimum moisture content (figure 2). Soil was also characterized according to the USDA Web Soil survey which categorized it as a silty loam soil based on its geographical location.
Figure 1. The soil particle size distribution showing the percentage of the sample that is each particle size.
Compressive Strength (psi)
300 250 200 150 100 50 0 7 Days Control
Testing/crushing of cylinders occurred at days 7 and 14 from date of ramming. These dates are used because they are typical test dates used for concrete compressive testing. This way the results can be easily compared. The three data points of this test will show the improvement (or lack of) in the material’s strength over time. Cylinders were weighed before testing. Testing of the compressive strength was facilitated by the KU School of Engineering whose concrete cylinder compression testing machines was used. Testing adhered to ASTM 2007 standard D1633-00, modified to use 4 x 8 inch steel molds and neoprene pad caps.
Cow's Blood
14 Days Linseed Oil
Oil and Blood
Figure 3. Comparison of compressive strengths of unstabilized earth (control), cow’s blood, linseed oil and the combination of oil and blood. Error bars represent the range of strength values between three trials.
Figure 2. Maximum dry unit weight and optimum moisture content is found at the standard effort of compaction
The addition of linseed oil and the addition of cow’s blood proved to increase the strength of cylinders with average compressive strengths of 186 psi and 192 psi respectively. Compared to the control’s strength of 153 psi, the oil increased by 22% and the blood increased it by 25%. However, the combination of the oil and the blood decreased the strength of the cylinders with the average strength of 146 psi—a decrease of 5% (figure 3). While not much research has been done in the realm of oil based stabilizations, animal blood has been studied to suggest that the proteins and hemoglobin the blood bond the soil while drying (Winkler, “The Effects of Blood on Clay”). The water resistance proved to be parallel with the compressive strength of the variables. The percentage of weight lost (the amount of soil eroded) was drastically high compared for the control compared to the oil or blood. While the combination of the oil and blood proved to be more resistant than the control, it was not as resistant as the other two variables (figure 4).
Figure 4. The total weight lost during the water immersion study to test water resistance of soil. Negative values indicate water absorbed was greater than soil lost.
The testing of the cylinders finds the compressive strength of the different stabilizers. The difference in weights finds a correlation with amount of water evaporated and strength of cylinder. The immersion tests determine the moisture and erosion resistance of the stabilized earth to find which stabilizer is most resistant to water. This helps with determining which stabilizer is used for relatively wet climates like eastern Kansas.
Control
Linseed Oil
Blood
Oil + Blood
Photos of cylinders after 24 hours fully submerged in water
ASTM D422-63(2007)e2, Standard Test Method for Particle-Size Analysis of Soils, ASTM International, West Conshohocken, PA, 2007, www.astm.org ASTM D4318-10e1, Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils, ASTM International, West Conshohocken, PA, 2010, www.astm.org ASTM D2216-05, Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass, ASTM International, West Conshohocken, PA, 2005, www.astm.org ASTM D698-07, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12 400 ft-lbf/ft3 (600 kN-m/m3)), ASTM International, West Conshohocken, PA, 2007, www.astm.org ASTM C67-14, Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile, ASTM International, West Conshohocken, PA, 2014, www.astm.org ASTM D1633-00(2007), Standard Test Methods for Compressive Strength of Molded Soil-Cement Cylinders, ASTM International, West Conshohocken, PA, 2007, www.astm.org Browne, Gordon. “Stabilised Interlocking Rammed Earth Blocks: Alternatives to Cement Stabilisation.” International Conference on Non-conventional Materials and Technologies (2009): n. page. Web. 26 Mar. 2014. Jaquin, P.A., C. Augarde, and C.M. Gerrard. “A Chronological Description of the Spatial Development of Rammed Earth Techniques.” International Journal of Architectural Heritage : Conservation, Analysis, and Restoration. 2.4 (2008): 377-400. Web. 26 Mar. 2014. Jaquin, P. A., D. G. Toll, D. Gallipoli, and C. E. Augarde. “The Strength of Unstabilised Rammed Earth Materials.” Geotechnique 59.5 (2009): 487-90. Print. Kraus, Chad. “Compressive Strength of Blood Stabilized Earthen Architecture.” Litt’s Drug Eruption and Reaction Manual, 21st Edition 21 (2014): 216-20. Print. Maniatidis, Vasilios, and Peter Walker. “A Review of Rammed Earth Construction.” ‘Developing Rammed Earth for UK Housing’. (2003): n. page. Web. 26 Mar. 2014. Winkler, Erhard. “The Effect of Blood on Clays.” Soil Science 21 Dec. 1956: 157-72. Print. Winkler, Erhard. “Method of Stabilizing Soil with Soluble Dried Blood.” Patent 723,607. 24 March 1958.
1. Testing soil plasticity and other soil classifications 2. Measuring combinations of linseed oil and cow's blood 3. Mixing of soil and blood 4. Pre crushing 5. Post crushing 6. Water wash
discussion The potential of stabilized rammed earth is encouraged with these results, but not as promising as other research. Past research has shown a greater strength of blood stabilized soil. In a similar experiment, blood increased the strength 200% in comparison to water stabilized soil (Kraus, 219). The process undergone was very similar, suggesting that the particular soil in this experiment did not have a similar enough make-up to bond well with the blood. Other research has determined that the amount of clay in the soil determines the effectiveness of blood stabilization, with non-clay soils unlikely to be very affected (Winkler, “Method of Stabilizing Soil with Soluble Dried Blood”). Perhaps this silty loam soil does not respond well to either of these particular stabilizations, or it does not bond well when rammed. However, the ultimate conclusion of the matter is that blood stabilization and linseed oil stabilization do increase compressive strength of rammed earth. Another notable aspect of this experiment is the failure of the 4th variable, the combination of blood and oil stabilization, to increase the strength of the soil. While both liquids independently affect the soil, together they may block each other from bonding the soil. However, this is simply an assumption and further research would be needed to accurately determine why together they do not strengthen it. As seen from the graphs, compressive strength decreased over time. This is unique to most other experiments of rammed earth, which may show plateauing, but not decreasing. Research has shown that soil that hasn’t dried properly loses strength potential, which may also be a factor here (Jaquin, “The Strength of Unstabilised Rammed Earth”). The moisture resistance tests proved to be successful with the blood stabilized and oil stabilized cylinders losing the least amount of material. Even the combination of blood and oil didn’t lose as much as the control, which may signify that although this combination of the two doesn’t increase strength, it does increase water resistance. Why this is would need more research.
Special thanks to Chad Kraus, Jared Pechauer, Brad Prewitt, and Ragan Allen