Engineering Properties of Soil-Fly Ash Subgrade Mixtures

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1 Engineering Properties of Soil-Fly Ash Subgrade Mixtures Zachary G. Thomas Graduate Research Assistant Iowa State University Department of Civil and Construction Engineering 394 Town Engineering Building Ames, Iowa Phone: Fax: Word Count: 7302 Midwest Transportation Consortium October 18, 2002

2 Zach Thomas 2 ABSTRACT The effects of adding fly ash to soil were evaluated in some common soil tests. When fly ash and soil are mixed and compacted immediately, the fly ash causes the mixture to have a higher dry unit weight, by filling in voids with ash particles. As a soil-fly ash mixture sits uncompacted, flocculation and agglomeration of the soil particles occurs as the fly ash sets up. This compaction delay time causes the compacted unit weight and strength gain to decrease, especially after the ash sets. Fly ash addition can also increase the freeze/thaw durability of a soil. Strength gain of soil-fly ash mixtures is also affected by curing temperature. Below freezing, 32 F (0 C), the mixtures gain no strength, while the strength gain increases as curing temperature increases. Fly ashes with high sulfur content react with the clay minerals and water in soil to form expansive materials, which break the mixture up, resulting in no long term strength gain, but low sulfur ashes have shown large strength gain in just over two years of curing. Fly ash can also be added to extremely wet soil to dry it out, while, at the same time, increasing the strength of the soil. The engineering properties of fly ash stabilized soil prove fly ash can be useful as a soil stabilizer.

3 Zach Thomas 3 INTRODUCTION Many Iowa soils have high silt and/or clay content, making them unsuitable as building materials. The coal burning power stations in Iowa produce fly ashes that can be used to stabilize these unsuitable soils. Research was conducted to evaluate the engineering properties of fly ash stabilized subgrade soils. Changes in unconfined compressive strength and compacted unit weight were determined as they relate to fly ash set time and compaction delay time. Curing temperature was known to have an effect on strength gain of soil-fly ash mixtures, therefore the unconfined compressive strength was evaluated after samples had been cured at 8 F (-13 C), 70 F (21 C), and 100 F (38 C). Fly ash was added to frost susceptible soils to try to improve their freeze/thaw durability. Since fly ash has initial cementitious and ongoing pozzolanic reactions, the long term strength gain of fly ash stabilized soils was also assessed. In addition, fly ash was tested to determine its potential for use as a drying agent. MATERIALS Soils Tests were conducted using five different Iowa soils, four of which rate as poor subgrade soils and one which is fair for subgrade construction. The soils are: (1) western Iowa loess, (2) central Iowa loess, (3) northeast Iowa glacial till, (4) southeast Iowa paleosol, and (5) western Iowa alluvial clay. The grain size distributions of these soils are shown in Figure 1. Table 1 summarizes the Atterberg limits, specific gravities, percentages of gravel, sand, silt, and clay, and Unified Soil Classification System (USCS) symbol for each soil. The loess soils have low shear strength and, because of the silt content, have freeze/thaw durability problems. The central Iowa loess has higher clay content, suggestive of shrink/swell problems. The paleosol is weathered glacial till and is very common in southeastern Iowa along the Mississippi River. Weathered till soils are usually characterized by expansive clay minerals, which cause numerous shrink/swell related problems. The alluvial clay is also a clayey soil that has expansive characteristics and low shear strength. Glacial till soils are found throughout Iowa to some degree, and the till discussed in this report is typical of most Iowa till. Generally, till has almost equal portions of sand and silt with a small amount of clay, giving the soil medium plasticity. In this case the till is generally considered suitable for subgrade use. Fly Ash Eight different fly ashes from around the state of Iowa were used in a majority of the testing. These ashes are derived from sub-bituminous coal brought in from the Powder River basin in Wyoming. The fly ash source, American Society for Testing and Materials (ASTM) classification, production rates, and set times are shown in Table 2. The ASTM class C indicates the ash is approved for use in Portland cement concrete, but fly ash not designated as class C can be used in soil stabilization as long as it is moderately self cementing and a chemical analysis determines there are not materials, such as sulfur, that will cause expansion. Set time for each ash was determined by mixing fly ash with water to form a paste with a moisture content of 27.5%. The penetration resistance was measured with a pocket penetrometer and the set time occurred at a refusal reading of 4.5 tons/ft 2. Set time is measured to get an idea of the amount of tricalcuim aluminate, which is responsible for initial hardening, in the fly ash. The set time is may be correlated with loss of strength and unit weight of stabilized soil. EFFECTS OF FLY ASH ADDITION ON MOISTURE-DENSITY RELATIONSHIPS Fly ash is composed of very fine, spherical shaped particles which are shown in Figure 14. When ash is added to soil, and the mixture is compacted immediately (<10 minutes), an increase in dry unit weight can be observed, as compared with the soil alone. Because the fly ash particles are much finer than the clay particles in the soil, the ash spheroids act as filler to decrease the volume of voids and drive up the weight of solids, thus increasing the dry unit weight of the soil-fly ash mixture. The increase in dry unit weight due to various ash addition rates can be seen in Figure 2.

4 Zach Thomas 4 EFFECT OF COMPACTION DELAY TIME ON UNIT WEIGHT After fly ash, soil, and water are mixed, the fly ash causes the clay particles in the soil to flocculate and agglomerate. The longer a stabilized material is allowed to sit uncompacted, the more pronounced the flocculation and agglomeration becomes. The flocculated and agglomerated soil particles are less dense and have less of a tendency to be compacted into a tight mass. Figure 3 shows the decrease in dry unit weight at compaction delay times of 0, 4, and 24 hours. The samples in Figure 4 were mixed at one moisture content and then compacted at various delay times up to 4 hours. As can be seen, the unit weight approximately stays the same until the fly ash sets, at which point the unit weight begins to decrease. The Ames and Port Neal #4 samples had rapid set times and large unit weight loss, while the Council Bluffs ash never set up, resulting in a less rapid unit weight loss. The soil used in these tests was the western Iowa loess, which has moderate clay content; a soil with more clay may have had more flocculation and agglomeration occur, leading to a greater loss of unit weight than the soil tested. EFFECT OF COMPACTION DELAY TIME ON STRENGTH GAIN As with unit weight, unconfined strength of fly ash stabilized soil decreases as compaction delay time increases. The flocculation and agglomeration also results in the fly ash being used to cement small groups of soil particles together rather than cementing the soil mass together as a whole. Figure 5 shows the strength results from the same samples that were shown in Figure 3. The strength decreases from 0 to 4 to 24 hour compaction delay times. These samples were soaked for one hour before compression testing and the picture in Figure 6 represents the 24 hour delayed samples after soaking. These samples were poorly cemented because of flocculation and agglomeration, therefore absorbing more water during soaking, which led to lower strength. Figure 7 shows the unconfined compressive strengths of the samples that were discussed in Figure 4. The Ames ash samples exhibit an immediate strength loss, while the strength of the Council Bluffs samples actually increases at longer compaction delay times. The Port Neal #4 sample strengths start to decrease rapidly after the fly ash set time is reached. EFFECT OF CURE TEMPERATURE ON STRENGTH GAIN The temperature at which a fly ash stabilized mixture is cured has a tremendous impact on strength gain. The western Iowa loess and southeast Iowa paleosol were both evaluated for this curing temperature effect. Each soil was mixed with 10%, 15%, and 20% fly ash by dry weight. When fly ash is added to soils there is an initial cementitious reaction and then long-term pozzolanic reactions between the clay minerals and fly ash particles. The results of the paleosol and fly ash are in Figure 7, and the results of the western Iowa loess are shown in Figure 8. The humid cured samples were cured in a 100% humid environment at 70 F (21 C) for seven days. The freezer cured samples were placed in an 8 F (-13 C) freezer immediately after they were compacted, and cured for seven days also. The samples cured in a 100 F (38 C) oven were cured and tested in accordance with ASTM C 593 [Standard Specification for Fly Ash and other Pozzolans for Use with Lime] (1). All sets of samples that were cured in the freezer did not show any strength gain. The temperature was too cold for the initial cementitious reactions to occur. Figure 9 shows the condition of the paleosol freezer samples after one hour of soaking. One can see the large amount of cracking on the surface of the samples, which lowered the strength of the samples. The 10% and 15% loess samples completely disintegrated during soaking and only one 20% sample was able to be tested. The humid cured samples give the type of relationship one would expect, more strength with increasing ash content. It does appear that any ash content greater that 15% has little effect on initial strength gain though. All samples cured in the oven are supposed to simulate the 28 day strength, which is consistent

5 Zach Thomas 5 with the charts, as they show the oven cured samples that were soaked have higher strength than the humid cured samples. The vacuum saturated samples were tested in this manner to determine the freeze/thaw durability of the stabilized material. Theoretically, these samples should have less strength than the oven samples that weren t vacuum saturated, but this is the case in only half of the samples tested. This difference is probably due to the fact that air bubbles were trapped in the soaked samples, therefore causing excess internal damage as the samples were tested in compression. This damage may have then caused premature failure, resulting in lower strengths. Overall the clayey paleosol passes the freeze/thaw durability test, while the silty, low clay western Iowa loess fails the 10% strength difference between plain soaked and vacuum saturated samples standard set by ASTM C 593 (1). The loess would probably still have the same results at longer cure times because there is very little clay material to help in long term pozzolanic reactions, unless an extremely large amount of fly ash was to be used. In Figure 10, the plain soaked and vacuum saturated paleosol samples are shown, and there is virtually no difference in the appearance of these samples, although the strengths did differ up to the allowable limit of 10%. LONG TERM STRENGTH GAIN OF SOIL FLY ASH MIXTURES The real advantage to using fly ash is long-term strength gain for poor subgrade soils under pavements. Figure 11 shows the strength gain of several ashes and ash contents mixed with northeast Iowa glacial till. Most of the ashes showed an overall strength gain when tested at 880 days. These samples had been cured in a 100% humid environment at 72 F (21 C). Soil that was mixed with the University of Northern Iowa (UNI) atmospheric fluidized bed combustion (AFBC) ash and the Prairie Creek stoker ash showed considerable volume increase, as seen in Table 3. The AFBC ash contains large amounts of sulfur, which when mixed with water, calcium, alumina, and silica forms long, needlelike crystals of ettrignite, which then leads to volume expansion. A scanning electron microscope (SEM) image of the till and AFBC is shown in Figure 12. The large amounts of ettrignite crystals are clearly visible. Conversely, an SEM image of till and 20% Prairie Creek fly ash is pictured in Figure 13. The figure shows only a few ettrignite crystals with the fly ash spheres. This leads to the conclusion that the chemical composition of the fly ash is just as important as the ash addition rate on the long-term strength gain of stabilized soil. Some of the samples exhibited shrinkage cracks, due to being tightly sealed and having excess water used up in the pozzolanic reactions, therefore halting further pozzolanic reactions, during the 880-day curing period. This is probably the reason for some strength gains leveling off and not being as high as expected, i.e. the Council Bluffs and Prairie Creek 10% and 15% samples. FLY ASH USE AS A DRYING AGENT Raw fly ash can be added to saturated soils in order to dry them out, which allows construction and paving operations to continue. An added advantage of using ash as a drying agent is that an initial strength gain also occurs, if the added ash content is high enough, which also helps support construction traffic. When saturated, the western Iowa loess has no shear strength and turns to "soup". Table 4 shows the before and after moisture contents of western Iowa loess and fly ash. Fly ash can take up to around 9% moisture out of soil and a reactive ash can take out more as the chemical reactions convert the free water to structural water. Figure 14 shows the extreme cases with the strength gain case; the Sutherland stoker ash took out approximately the same amount of moisture as the Ottumwa ash, but showed greater strength gain. CONCLUSION Self-cementing fly ash has potential for high volume use as a soil stabilizer in the state of Iowa. Soil-fly ash mixtures can add strength and durability to low strength soils, therefore allowing them to be used as subgrade instead of having to waste them. When the reaction characteristics of fly ash are understood and fly ash is used properly, it is beneficial as a stabilizing agent for soils. The following conclusions have been drawn from this research:

6 Zach Thomas 6 1. Fly ash used for stabilization should be self-cementing and contain less than 5% sulfur, in order to reduce the swelling potential of soil-fly ash mixtures. 2. Soil-fly ash mixtures should be compacted as close to completion of mixing as possible, in order to reduce flocculation and agglomeration effects, which result in loss of strength and compacted unit weight. 3. Fly ash set time can be used to predict the maximum compaction delay time. 4. Strength gain of soil-fly ash mixtures is non existent at freezing temperatures, but, above freezing temperatures, is accelerated as curing temperature increases. 5. Long term strength gain of soil-fly ash mixtures has been as high as 300%, thus providing increasing support to overlying structures as time goes on. 6. The freeze/thaw durability of soil can be increased by fly ash addition, on the condition that the clay content is high enough. 7. By adding 30% fly ash, the moisture content of wet soil can be reduced by 9%; while the soil gains strength at the same time. REFERENCES 1. ASTM C 593, Standard Specification for Fly Ash and Other Pozzolans for Use With Lime, Annual Book of ASTM Standards, Vol. 4.01, Philadelphia, PA, ACKNOWLEDGEMENT I would like to thank Dr. David White, Dr. Ken Bergeson, and my parents, Tom and Zona Thomas. The Iowa Fly Ash Affiliates provided the funding for this research.

7 Zach Thomas 7 LIST OF TABLES AND FIGURES 1. Figure 1 Grain Size Distributions of Soils Used in Soil Fly Ash Testing. 2. Table 1 Atterberg Limits, Specific Gravity, Particles Sizes, and USCS Classification of Soils Used in Soil Fly Ash Testing. 3. Table 2 Source Locations, ASTM Classifications, Production Rates, and Set Times of Fly Ashes Used in Soil Fly Ash Testing. 4. Figure 2 Effect of Fly Ash Content on Dry Unit Weight of Soil Fly Ash Mixtures Compacted Immediately After Mixing. 5. Figure 3 Effect of Long Term Compaction Delay on Dry Unit Weight of Soil Fly Ash Mixtures. 6. Figure 4 Correlation Between Compaction Delay and Fly Ash Set Time on Wet Unit Weight of Soil Fly Ash Mixtures. 7. Figure 5 Effect of Long Term Compaction Delay on Strength Gain. 8. Figure 6 24 Hour Compaction Delay Samples After One Hour of Soaking Before Compression Testing. 9. Figure 7 Correlation Between Compaction Delay and Fly Ash Set Time on Strength Gain of Soil Fly Ash Mixtures. 10. Figure 8 Effect of Curing Temperature on Southeast Iowa Paleosol and Fly Ash. 11. Figure 9 Effect of Curing Temperature on Western Iowa Loess and Fly Ash. 12. Figure 10 Picture of Southeast Iowa Paleosol and Fly Ash Samples Cured in Freezer After Soaking Before Compression Testing. 13. Figure 11 Oven Cured Soaked and Vacuum Saturated Southeast Iowa Paleosol and Fly Ash Samples Before Compression Testing. 14. Figure 12 Long Term Strength Gain of Northeast Iowa Glacial Till and Fly Ash Mixtures. 15. Table 3 Volume Change and Strength Gain of Northeast Iowa Glacial Till and Fly Ash Mixtures. 16. Figure 13 SEM Image of UNI AFBC Fly Ash and Northeast Iowa Glacial Till. 17. Figure 14 SEM Image of 20% Prairie Creek Fly Ash and Northeast Iowa Glacial Till. 18. Table 4 Before and After Moisture Contents of Western Iowa Loess Dried With Fly Ash. 19. Figure 15 Short Term Strength Gain of Western Iowa Loess Dried with Fly Ash.

8 Zach Thomas 8 100% 95% 90% 85% 80% 75% 70% 65% Percent Finer, % 60% 55% 50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% Grain Size, mm FIGURE 1 Grain Size Distributions of Soils Used in Soil Fly Ash Testing. W IA Loess C IA Loess NE IA Till SE IA Paleosol Alluvial Clay TABLE 1 Atterberg Limits, Specific Gravity, Particle Sizes, Particle Sizes, and USCS Classification of Soils Used in Soil Fly Ash Testing Western IA Loess Central IA Loess Northeast IA Till Southeast IA Paleosol Western IA Alluvial Clay LL PL PI G s % Gravel 1% 0% 2% 1% 0% % Sand 1% 3% 45% 15% 1% % Silt 87% 71% 44% 68% 73% % Clay 11% 26% 9% 26% 26% USCS ML CL CL CL-CH CL-CH

9 Zach Thomas 9 TABLE 2 Source Locations, ASTM Classifications, Production Rates, and Set Times of Fly Ashes Used in Soil Fly Ash Testing Ash Source ASTM Classification Production, tons/day Set Time, min Port Neal #3 C Port Neal #4 C Ames * Prairie Creek C Sutherland C Council Bluffs C 350 ** Ottumwa C Louisa C * 10% municipal waste burned with coal stream. ** Did not reach refusal in four hours Western Iowa Loess with No Ash, 5% Port Neal #3, 10% Port Neal #4, 15% Ottumwa, and 20% Louisa Ashes. Dry Unit Weight, pcf NO ASH 5% PN #3 10% PN #4 15% OGS 20% LGS % 12% 14% 16% 18% 20% 22% 24% Moisture Content, % FIGURE 2 Effect of Fly Ash Content on Dry Unit Weight of Soil Fly Ash Mixtures Compacted Immediately After Mixing.

10 Zach Thomas Hour R 2 = Hour R 2 = Southeast Iowa Paleosol at Various Moisture Contents Mixed With 20% Prairie Creek 3+4 Fly Ash. Dry Unit Weight, pcf Hour R 2 = Hour Delay 4 Hour Delay 24 Hour Delay Poly. (0 Hour Delay) Poly. (4 Hour Delay) Poly. (24 Hour Delay) % 11% 12% 13% 14% 15% 16% 17% 18% 19% 20% 21% 22% 23% 24% 25% 26% Moisture Content, % FIGURE 3 Effect of Long Term Compaction Delay on Dry Unit Weight of Soil Fly Ash Mixtures Wet Unit Weight, pcf Western Iowa Loess (~18% Moisture) and 20% Fly Ash Penetration Resistance, tons/ft 2 Ames UW Port Neal #4 UW Council Bluffs UW Ames Set Port Neal #4 Set Council Bluffs Set Compaction Delay and Set Time, hours FIGURE 4 Correlation Between Compaction Delay and Fly Ash Set Time on Wet Unit Weight of Soil Fly Ash Mixtures.

Zach Thomas 11 90 80 70 Unconfined Strength, psi 60 50 40 30 0 Hour Delay 4 Hour Delay 24 Hour Delay 20 10 Southeast Iowa Paleosol at Various Moisture Contents Mixed With 20% Prairie Creek 3+4 Fly

11 Zach Thomas Unconfined Strength, psi Hour Delay 4 Hour Delay 24 Hour Delay Southeast Iowa Paleosol at Various Moisture Contents Mixed With 20% Prairie Creek 3+4 Fly Ash. 0 13% 14% 15% 16% 17% 18% 19% 20% 21% 22% 23% 24% 25% Moisture Content, % FIGURE 5 Effect of Long Term Compaction Delay on Strength Gain. FIGURE 6 24 Hour Compaction Delay Samples After One Hour of Soaking Before Compression Testing.

12 Zach Thomas Western Iowa Loess (~18% Moisture) and 20% Fly Ash Unconfined Strength, psi Penetration Resistance, tons/ft 2 Ames Strength Port Neal #4 Strength Council Bluffs Strength Ames Set Port Neal #4 Set Council Bluffs Set Comapction Delay and Set Time, hours FIGURE 7 Correlation Between Compaction Delay and Fly Ash Set Time on Strength Gain of Soil Fly Ash Mixtures Southeast Iowa Paleosol (~ 17% Moisture) Mixed with Various Contents of Louisa Fly Ash. 60 Unconfined Strength, psi % 15% 10% Humid Freezer Oven Soak Oven Vac Sat Cure Location and Temperature, F (72, 8, 100, 100) FIGURE 8 Effect of Curing Temperature on Southeast Iowa Paleosol and Fly Ash.

Zach Thomas 13 50 45 Western Iowa Loess (~ 18% Moisture) Mixed with Various Contents of Port Neal #4 Fly Ash.

13 Zach Thomas Western Iowa Loess (~ 18% Moisture) Mixed with Various Contents of Port Neal #4 Fly Ash. 40 Unconfined Strength, psi % 15% 10% Humid Freezer Oven Soak Oven Vac Sat Cure Location and Temperature, F (72, 8, 100, 100) FIGURE 9 Effect of Curing Temperature on Western Iowa Loess and Fly Ash. FIGURE 10 Picture of Southeast Iowa Paleosol and Fly Ash Samples Cured in Freezer After Soaking Before Compression Testing.

14 Zach Thomas 14 (a) (b) FIGURE 11 Oven Cured (a) Soaked and (b) Vacuum Saturated Southeast Iowa Paleosol and Fly Ash Samples Before Compression Testing.

15 Zach Thomas Northeast Iowa Glacial Till Mixed With Various Fly Ashes and Ash Contents. 225 Unconfined Strength, psi Council Bluffs Fly Ash 15% Prairie Creek Fly Ash 20% Prairie Creek Fly Ash 15% Prairie Creek Fly Ash 10% Prairie Creek Fly Ash 5% UNI AFBC 15% Prairie Creek Stoker Ash 15% Cure Time, Days FIGURE 12 Long Term Strength Gain of Northeast Iowa Glacial Till and Fly Ash Mixtures. TABLE 3 Volume Change and Strength Gain of Northeast Iowa Glacial Till and Fly Ash Mixtures Ash Type Amount New Volume, in 3 Initial Volume, in 3 Volume Change, in 3 Volume Change, % Strength Gain, % AFBC 15% % -75% * PC 20% % 67% PC 5% % 156% CB 15% % 214% PC 15% % 266% PC 10% % -2% * PC Stoker 15% % 308% * Negative Values Indicate Strength Loss

16 Zach Thomas 16 FIGURE 13 SEM Image of UNI AFBC Fly Ash and Northeast Iowa Glacial Till. FIGURE 14 SEM Image of 20% Prairie Creek Fly Ash and Northeast Iowa Glacial Till.

17 Zach Thomas 17 TABLE 4 Before and After Moisture Contents of Western Iowa Loess Dried With Fly Ash Ash Type Addition Rate Initial Moisture, % Final Moisture, % Moisture Change Sutherland Stoker 10% 32.5% 29.9% 2.6% Sutherland Stoker 20% 33.8% 27.1% 6.7% Sutherland Stoker 30% 33.7% 24.8% 8.9% Prairie Creek % 33.3% 28.8% 4.5% Prairie Creek % 32.7% 25.7% 7.0% Prairie Creek % 32.7% 23.7% 9.0% Ottumwa 10% 33.3% 29.1% 4.2% Ottumwa 20% 32.3% 25.7% 6.6% Ottumwa 30% 33.1% 23.7% 9.4% Port Neal #4 10% 32.9% 28.6% 4.3% Port Neal #4 20% 32.8% 26.6% 6.2% Port Neal #4 30% 32.4% 23.7% 8.7% Penetration Resistance, tons/ft Western Iowa Loess (~33% Moisture Before Ash) Dried With Various Contents of Sutherland Stoker Ash and Ottumwa Fly Ash. Sutherland 10% Sutherland 20% Sutherland 30% Ottumwa 10% Ottumwa 20% Ottumwa 30% Set Time, Hours FIGURE 15 Short Term Strength Gain of Western Iowa Loess Dried with Fly Ash.

4. Soil Consistency (Plasticity) (Das, chapter 4)

4. Soil Consistency (Plasticity) (Das, chapter 4) 1 What is Consistency? Consistency is a term used to describe the degree of firmness of fine-grained soils (silt and clay). The consistency of fine grained