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Significant efforts have been made in the recent years to use fly ashes in soilstabilization and highway applications (Vishwanathan et al. 1997, Bergeson andBarnes 1998, Consoli et al. 2001, Parsons and Milburn 2003); however, a largepercentage of reuse of fly ash in these mixes has not been studied.

The objective of this study is to investigate the beneficial reuse of Class F fly ashamended mixtures for base layers in highways. To achieve this objective, a battery of tests was conducted on soil-fly ash mixtures prepared with cement and lime asactivators. Unconfined compressive strength (q u), California bearing ratio (CBR), andresilient modulus (M R) tests were conducted to investigate the effect of fines content,curing time, molding water content, activator type, and soil cohesion on engineeringparameters.

MATERIALS AND TEST PROCEDURES

Locally available sandy soil was used in the current study. The soil was classified as

light brown silty sand (SM) according to the Unified Soil Classification System(USCS) and A-2-4 according to the American Association of State Highway andTransportation Officials (AASHTO) Classification System. The soil hasapproximately 18% particles passing the U.S. No. 200 sieve. Specific gravity (G s) of the material is 2.68, and it does not exhibit any plasticity. The fly ash used in thisstudy was low calcium Class F fly ash (0.74% CaO) obtained from Indian RiverPower Plant in Millsboro, Delaware. The fly ash had a dark grayish color, and acarbon content of 6-8%. Approximately 86% of the particles were finer than U.S. No.200 sieve size. The ash had a pH of 7.9 and was insoluble in water. The specificgravity of the ash was 2.24. Type I Portland cement and high calcium (95%)quicklime from Pennsylvania Lime, Inc., were used as activators for the PSM. In

order to investigate the effect of cohesion on engineering parameters, kaolinite wasadded to some mixtures. The kaolinite was obtained from Burgess Pigmet Inc. of Sandersville, Georgia, and had a cation exchange capacity (CEC) of 0.03-0.1 meq/g.Mixes possessing large fractions of ash was used in the testing program. Varyingpercentages of silty (cohesionless) fines and kaolinite were used and specimens werecompacted at optimum moisture content, 4% wet and 4% dry of optimum to examinethe effect of molding water content on the strength parameters. Table 1 provides asummary of the mixes used in the study along with the optimum water contents(OMC) and maximum dry unit weights ( dm) of the mixtures based on compactiontests (ASTM D 698 and D 1557).

The prepared mixes were subjected to unconfined compression and CaliforniaBearing Ratio (CBR) tests. ASTM D 1633 and D 5102 were used to determine theunconfined compressive strength of the cement and lime treated specimens,respectively, After compaction, the specimens were extruded with a hydraulic jack,sealed in plastic wrap, and cured for 1, 7, 28, and 56 days at 100% relative humidityand controlled temperature before testing. The CBR testing was conducted followingthe procedures listed in AASHTO designation T-193 and ASTM D 1883. Thespecimens were compacted at optimum moisture content (OMC) using the standard

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Proctor effort and were cured for 7 days at 100% relative humidity and controlledtemperature before testing. The procedure described in AASHTO T-294 wasfollowed for the resilient modulus test. Specimens of 71.1 mm in diameter and 152.4mm in height were compacted at their OMC in five layers, and were cured for 7 daysat 100% relative humidity following the compaction. Details of the resilient modulus

test apparatus can be found in Aydilek et al. (2003).

RESULTS AND DISCUSSION

Unconfined Compression and CBR Tests

Non-plastic (silty) fines of the soil were varied from 6% to 30% by weight of soilparticles with increments of 12%. That is, soil with 6%, 18%, and 30% fines wasused. Only unconfined compression tests were used to evaluate the effect of finescontent. As seen in Table 2, a consistent trend is not observed between q u andcohesionless fines content. All the specimens invariably show a similar trend of

strength increase with increasing curing period. The postulated mechanism is that therelease of calcium hydroxide (Ca(OH) 2) by Portland cement on crystallization reactswith the fly ash to form calcium aluminium silicates, which in turn hardens thespecimen. The high temperature of the curing chamber and availability of 100%

Table1 . Legend and the composition for the mix designs.

Specimenname

Soil(%)

Siltyfines(% of soil)

Flyash(%)

Cement(%)

Lime(%)

Kaol.(%)

Compac.effort

dm

(kN/m 3) OMC(%)

FA1-C7 60 6 40 7 - - Standard 15.88 16.8FA3-C7 60 18 40 7 - - Standard 15.46 18.1FA5-C7 60 30 40 7 - - Standard 15.34 17.7

FA3K-C7 50 18 40 7 - 10 Standard 15.88 16.7FA3-C1 60 18 40 1 - - Standard 15.45 16.5FA3-C2 60 18 40 2 - - Standard 15.5 17.5FA3-C4 60 18 40 4 - - Standard 15.46 17.2FA3-C5 60 18 40 5 - - Standard 15.39 17.2FA3-L4 60 18 40 - 4 - Standard 15.47 17.7FA3-L7 60 18 40 - 7 - Standard 15.36 17.7

FA3-L10 60 18 40 - 10 - Standard 15.03 18.2FA3K-L7 50 18 40 - 7 10 Standard 15.45 16.8

FA1-C7-M 60 6 40 7 - - Modified 16.92 13.4FA3-C7-M 60 18 40 7 - - Modified 17.15 13.2FA5-C7-M 60 30 40 7 - - Modified 17.16 13.0

Note: dm = Maximum dry unit weight; A total percentage of 100% of soil and fly ash wasconsidered as the base mix, and cement or lime was added at a certain percentage by weight of this base mix. All the specimens were compacted at their optimum moisture contents(OMC). The specimens FA1-C7, FA3-C7, FA5-C7, and FA3K-C7 were also compacted at OMC +4 and OMC -4 molding water contents, but not shown herein.

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relative humidity enhances these cementitious reactions. Similarly, Vishwanathan etal. (1997) reported an increase of 50% in q u from 7 to 28 days for fly ash amendedmixtures.

The effect of water content on unconfined compressive strength of specimens with

6%, 18% and 30% fines is presented in Table 2. The effect is not prominent for thespecimen cured for 28 days; however, for the same mix design, the variation in q uwith compaction water content can be observed more clearly for the specimens curedfor 7 days. For instance, a q u of 5.3 MPa was obtained for the specimen compacted atthe drier side of optimum and 5.0 MPa for the specimen compacted at the wet side at28 days. The values were 4.4 MPa and 0.9 MPa, respectively, for 7-day curedspecimens. The effect of water content on strength can be explained by thecharacteristics of cementitious reactions. The water-to-cement (W/C) ratio isimportant in these reactions, even though it cannot always be optimized insolidification/stabilization work. At W/C > 0.48, cement is over-hydrated, leavingfree water (pore water), and bleed water that appears as standing water on the

surface of the solid mass (Conner 1990). The observed decrease in unconfinedcompressive strength with increasing molding water content was attributed to

Table 2. Summary of unconfined compressive strengths (q u) of all the specimenstested.

Unconfined compressive strength (MPa)Days of curingSpecimen Name

1 7 28 56

CBRat 7 days of

curing (MPa)

FA1-C7 1.2 3.8 5.2 7.5FA1-C7+4 0.4 1.0 5.0 5.5FA1-C7-4 1.8 4.4 5.4 6.5FA3-C7 1.2 3.2 5.0 5.9 140

FA3-C7+4 0.5 1.2 3.1 4.8FA3-C7-4 1.8 3.8 6.0 7.2FA5-C7 1.0 1.6 6.9 7.5

FA5-C7+4 0.5 1.9 3.4 3.4FA5-C7-4 1.5 4.4 6.2 6.9FA3-C1 - 0.6 0.8 - 53FA3-C2 - 1.4 1.8 - 80FA3-C4 - 2.8 3.5 - 93FA3-C5 - 3.2 4.5 - 133FA3-L4 - 0.4 0.7 -FA3-L7 - 0.3 0.4 0.49 36

FA3-L10 - 0.1 0.2 -FA3K-L7 - 0.2 0.4 0.5 26FA3K-C7 2.0 5.5 6.6 8.1

FA3K-C7+4 1.1 4.4 3.1 4.1FA3K-C7-4 2.0 4.1 4.0 4.9FA1-C7-M - 5.3 9.2 -FA3-C7-M - 5.1 11.2 -FA5-C7-M - 5.4 7.6 -

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(Asphalt Institute 1991). The CBR for FA3-C5 and FA3-C7 are greater than 100,which indicates superior bearing capacities. Swell was not observed when thespecimens were kept submerged for five days; hence, mixes appear not to have long-term swell potential (Aydilek et al. 2003).

Figure 1b shows the results of unconfined compression tests performed on the limetreated specimens. The q u decreases with increasing lime content. For instance, the7-day strength of specimens with 4% lime is approximately 448 kPa while thespecimens with 7 and 10% lime exhibit a q u of 276 and 138 kPa, respectively.Similar results were determined after conducting unconfined compression tests on the

0

10002000

3000

4000

5000

6000

0 2 4 6 8

Cement content (%)

U n c o n f i n

e d c o m p r e s s

i v e

s t r e n g

t h ( k P a

)FA3 (7 DAYS)

FA3 (28 DAYS)

0100

200300

400

500600

700800

2 4 6 8 10 12

Lime content (%)

U n c o n

f i n e

d c o m p r e s s

i v e

s t r e n g

t h ( k P a

)FA3 (7 DAYS)

FA3 (28 DAYS)

Figure 1. Effect of cement and lime contents on unconfined compressive strengths

specimens after 28 days of curing. The q u of the specimen with 4% lime is 724 kPawhile that with 10% lime is only 241 kPa . Lime stabilization is usually used for highplastic clays to decrease plasticity, and to increase the strength of the mix. As a result,shear strength parameters of cohesive soils are generally improved by limestabilization. The addition of lime increases shear strength but decreases plasticityindex. For instance, Little (2000) reported an increase in q u from 160 kPato 2,275kPa, and a decrease in PI from 38 to 10 with 6% hydrated lime treatment. Limestabilization can only be beneficial for high plastic clays with a PI greater than 10(Department of the Army, 1983). The fines of natural soil used in the current studywere non-plastic, and this is believed to be the reason for the observed detrimentaleffect of lime on strength.

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0

5 10 4

1 10 5

1.5 10 5

2 10 5

2.5 10 5

3 10 5

3.5 10 5

4 10 5

0 50 100 150 200 250 300 350 400

FA3-C7FA3-C5FA3-C3FA3-C1FA3-C2

R e s

i l i e n

t M o

d u

l u s ,

M R

( k

P a

)

Bulk stress , (kPa)

(a)

0

5 10 4

1 10 5

1.5 10 5

2 10 5

2.5 10 5

3 10 5

0 50 100 150 200 250 300 350 400

FA3-C7FA3K-C7FA3K-L7

R e s

i l i e n

t M o

d u

l u s ,

M R

( k

P a

)

Bulk stress , (kPa)

(b)

Figure 2. Resilient modulus of the specimens with varying bulk stresses.

cement. It can be concluded that the increase in cement content leads to increase inMR, but the rate decreases beyond 4% cement. This supports observations madeconcerning 5% cement being satisfactory for cementitious reactions between fly ashand cement. Attempts were also made to determine the resilient modulus of thespecimen with 7% lime (FA3-L7); however, strength gains were not observed evenafter curing for a period of 7 days. Figure 2b shows the resilient modulus of theselected specimens as a function of bulk stress. A comparison of FA3K-C7 and FA3-

C7 gives a clear indication that cement treatment is more effective for cohesionlesssoils as the M R values for the FA3-C7 are much higher than FA3K-C7. Some loss inMR of the mix that contained kaolinite may be due to the presence of higher finescontent in the mix (Conner 1990).

CONCLUSIONS

Due to the lack of self-cementitious characteristics, Class F fly ash needs anactivator (e.g. cement), and currently only 32% of this ash has been beneficiallyreused. Roadways are the biggest application area and use of this ash could savemillions of dollars annually as most of the ash is landfilled today. A study was

conducted to promote the use of Class F fly ash and to investigate the effect of finescontent, curing period, molding water content, compactive effort, cohesion, andcement or lime addit ion on geomechanical parameters of fly ash amended highwaybases.

The variation of q u with varying amounts of cohesionless fines was not consistent. Onthe other hand, addition of 10% kaolinite generally increased the strength of amixture. It should be noted that for this study, cohesionless fines were varied from

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6% to 30% of sand by weight and hence the observed results should be interpretedonly for this range. An increase in strength can be obtained in the field by compactingthe soil using higher compactive efforts (i.e., modified versus standard effort). Theincrease in strength with curing time was determined for all specimens irrespective of the molding water content. The highest strength was observed at 56 days; however,

strength seemed to increase beyond this curing time. The test results showed that thewater content at compaction could affect the q u of the mix design considerably. Theperformance of the fly ash, soil, and cement mix can be significantly increased bypreventing the intrusion of excess water in the field. It is recommended to compactthe base layer at dry of optimum for higher strength. Alternatively, compaction maybe performed at optimum water content; however, engineers should be carefulconcerning rain or any other addition of unwanted water at the time of compaction.

CBR, q u, and M R increased with increasing cement content; however, the ratedecreased beyond 5% cement. That is, the strength of the mix did not increaseproportionally beyond 5%. This was true for all three test methods. Lime treatment

had a detrimental effect on the mix designs. An increase in lime content decreased thequ of the specimens for both 7-day and 28-day old specimens. On the hand, theincrease in curing period had a positive effect and increased the q u of the lime treatedmixes. The presence of cohesion lowered the q u as well as CBR values during limetreatment, while the M R of the specimen with kaolinite (FA3K-L7) was higher thanthat of its cohesionless companion (FA3-L7).

As part of the study, the thicknesses of highway base layers with different mixdesigns were calculated using the q u, CBR and M R values but not shown herein.Lower thicknesses were required when higher amount of cement is used or highercompactive energies are employed. Presence of lime or cohesive fines generallyrequired higher base thicknesses indicating that use of cohesionless fines, such assandy soils, should be preferred.

REFERENCES

Acosta H. A. (2002). Stabilization of Soft Subgrade Soils Using Fly Ash , M.S.Thesis, University of Wisconsin, Madison, WI, 125 p.

Arora, S. (2003). Suitability of Fly Ash Stabilized Soils as Highway Base Material ,M.S. Thesis, University of Maryland, College Park, MD, 141 p.

Asphalt Institute (1991). Thickness Design Asphalt Pavements for Highways and Streets , Manual Series No. 1, Asphalt Institute, Lexington, Kentucky.

Baykal, G. and Metehan, T. (2002). The Effect of Lime Treatment on the ShearStrength Parameters of The Clay-Concrete Interface, Transportation Research

Board, 81 st Annual Meeting , Washington, D.C.Bergeson, K.L., and Barnes, A.G. (1998). Iowa Thickness Design for Low Volume

Roads Using Reclaimed Hydrated Class C Fly Ash Bases , ISUERI-Ames 98401,Iowa State University, Ames, Iowa.

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Chen, D., Zaman, M., and Laguras, J. (1995). Characterization of Base/SubbaseMaterials under Repetitive Loading, Journal of Testing and Evaluation , ASTM,Vol. 23, No. 3, pp.180-188.

Conner, J.R. (1990). Chemical Fixation and Solidification of Hazardous Wastes ,Van Nostrand Reinfold, New York, 692p.

Consoli, N. C., Prietto P. D. M., Carraro, J. A. H., Heineck, K., S. (2001). Behaviorof Compacted Soil-Fly Ash-Carbide Lime Mixtures." Journal of Geotechnicaland Geoenvironmental Engineering , ASCE, Vol. 127, No. 9, pp. 774-782.

Department of the Army (1983). Soil Stabilization for Pavements , online document(http://www.army.mil/usapa/eng/ )

Little, D.N. (2000). Evaluation of Structural Properties of Lime Stabilized Soils and Aggregates , Mixture Design and Testing Protocol for Lime Stabilized Soils,Prepared for the National Lime Association, Vol. 3. 16 p.

Parsons, R.L. and Milburn, J.P. (2003). Engineering Behavior of Stabilized Soils,Transportation Research Board, 82 nd Annual Meeting , Washington, D.C., CD-ROM, 29 p.

Turner-Fairbanks Highway Research Center (TFHRC) (2002). Coal Fly Ash UserGuideline Stabilized Base ( http://www.tfhrc.gov/hnr20/recycle/waste/cfa55.htm )Vishwanathan, R., Saylak, D., and Estakhri, C. (1997). Stabilization of Subgrade

Soils Using Fly Ash, Ash Utilization Symposium, CAER , Kentucky, pp. 204-211.

http://www.army.mil/usapa/eng/http://www.tfhrc.gov/hnr20/recycle/waste/cfa55.htmhttp://www.army.mil/usapa/eng/http://www.tfhrc.gov/hnr20/recycle/waste/cfa55.htm