MECHANICAL PROPERTIES OF CLASS F FLY AND BOTTOM ASH MIXTURES FOR EMBANKMENT APPLICATION

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MECHANICAL PROPERTIES OF CLASS F FLY AND BOTTOM ASHMIXTURES FOR EMBANKMENT APPLICATION

IGC-2005, 17-19 December 2005, Ahmedabad, INDIA

Kim B.Senior Researcher, Dam Safety Research Center, Korea Institute of Water and Environment,462-1, Jeonmin-Dong, Yusung-Gu, Daejon, Korea

[email protected]

Prezzi M.Asst Professor, School of Civil Engineering, Purdue University, West Lafayette, IN 47907, U.S.

Salgado R.Professor, School of Civil Engineering, Purdue University, West Lafayette, IN 47907, U.S.

Lee J.Assoc Professor, School of Civil and Environmental Engineering, Yonsei University134 Shinchon-Dong, Seodaemun-Gu, Seoul, Korea

[email protected]

ABSTRACT: Fly ash and bottom ash are the solid residues from coal combustion process for power generation. InIndiana, U.S., most coal-fired utility plants produce class F fly ash and bottom ash with a typical production ratio of80% fly ash and 20% bottom ash, and most of them are usually disposed of together as a waste in utility disposal sites.Direct use of these materials in construction projects consuming large volumes of materials, such as highwayembankment construction, not only provides a promising solution to the disposal problem, but also an economicalternative to the use of traditional materials. This paper presents the investigation of mechanical properties of class Ffly/bottom ash mixtures. Representative samples of class F fly and bottom ash were collected from a utility power plantin Indiana, U.S. and tested for evaluation of strength, stiffness, and compressibility. Test results showed that overall, ashmixtures compare favorably with conventional granular materials.

1. INTRODUCTIONIn the Unites States (US), the coal ash producedannually by coal-burning power plants amounts to morethan 100 million tons (ACAA 2001). A large fraction ofthe coal ash (about 70% of the total production) istypically disposed of as a waste in utility disposal sites.Use of coal ash in construction projects requiring largematerial volumes, such as highway embankmentconstruction, is a highly promising solution to thedisposal problem.

In common disposal practice, fly ash and bottom ash,which account for most coal ash production, are either

ponded or landfilled together in mixtures to minimizedisposal costs. According to the American Coal AshAssociation (ACAA, 2001), in the US, both the

production and disposal ratios of fly ash and bottom ashare approximately 80:20 by weight, which representseither ponded or landfilled ash consisting of high

proportions of fly ash. In Indiana, the majority of thecoal power plants produce class F fly ash and bottomash with a typical production ratio of 80% fly ash and20% bottom ash.

The objective of this paper is to evaluate the suitabilityof class F fly/bottom ash mixtures with high fly ashcontents as construction materials for highway

embankments. Although there have been investigationsinto the properties of separated single types of ash,studies of the fly/bottom ash mixtures, especially withhigh fly ash contents, are very limited. Evaluations ofthe environmental, physical and chemical characteristicsof Indiana fly and bottom ash have been done

previously (Diamond 1985; Huang 1990; Ke 1990).This study focuses mainly on fly/bottom ash mixtureswith high fly ash contents. Since the intended use ofthese materials is as embankment constructionmaterials, emphasis is given to the determination oftheir mechanical characteristics, including strength,stiffness, and compressibility.

2. TESTING MATERIAL

2.1 Ash Sources and Sampling

Ash samples used in this study were extracted from theWabash River power plant in Indiana, U.S. The plant

produces non-pozzolanic class F fly ash and bottom ashwith a typical production ratio of 80% fly ash and 20%

bottom ash.

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which uses a pneumatic pressure loading system.

Each ash mixture sample was compacted in six layers ina split mold with a collar using a standard manual sleevecompaction rammer. The split mold had a diameter of72 mm (2.8 in.) and a height of 163 mm (6.4 in.), which

was equal to the specimen size. Prior to back-pressuresaturation, the specimens were percolated by CO gas2and de-aired water for about one hour each, to helpobtain high B-values. Upon completion of the back-

pressure saturation, the specimens were isotropicallyconsolidated by applying the desired effective confining

stresses ( = 50, 100, and 200 kPa). Following the3consolidation, triaxial compression (shearing) was

performed on the saturated specimens previously

consolidated to a given under strain-controlled3conditions.

4. RESULTS AND DISCUSSION

4.1 Compressbility

Figure 4 shows the one-dimensional compression curvesof the compacted ash mixture samples with F = 50%,75% and 100%. A general observed trend is that as Fdecreases from 100% to 50%, the ash samples becomeslightly more compressible. Two possible mechanismscan explain the increasing compressibility withdecreasing F (i.e. with increasing B): 1) angularity and

porous surface texture of bottom ash particles and 2)particle crushing. Bottom ash particles are mostlyangular and irregular in shape. Granular materials with

angular particles are typically more compressible thanthose with well-rounded particles because the sharpedges in the angular particles tend to be overstressedduring movement and reorientation of the particles, andthus break to allow compression. Particularly weak

bottom ash particles, which in most cases have internalpores, can break at relatively low stress levels asdiscussed by Huang (1990). Furthermore, it is possiblethat fine fly ash particles adjacent to or filling theexternal pores of the bottom ash particles may besqueezed out with increasing stress, resulting in anincrease in deformation.

4.2 Shear strength

< Stress-strain and volumetric behavior >

Figure 5 shows stress-strain and volume changebehaviors of ash mixture samples with F = 100% and75%, compacted at R = 95% and 90% and sheared in a

triaxial test at a = 100 kPa. For the specimens at R =395%, the stress-strain and volumetric behaviorstypically resembled those of a sandy soil in a dense

state. The deviatoric stress ( = - ) increased upd 1 3to a peak (i.e. peak shear strength) with axial strain ( ).aAfter the peak, dropped and gradually approached ad

plateau as increased further. The specimens at R = 90a% exhibited behavior that is similar to that of a granular

soil in a loose state. The increased gradually up to adpeak level and then stayed practically unchanged with

increasing . The volumetric strains ( ) werea vcontractive throughout shearing.

When comparing F = 100% and F = 75%, for R = 95%,

as F decreased from 100% to 75%, the peak deviatoricstresses increased slightly while both thed(peak)maximum rate of dilation and the maximum dilationdecreased slightly. For R = 90%, the behavior of ashmixtures became slightly less stiff and contractive, that

is, increased with decreasing F (i.e. as B increases).v

Figure 6 represents the effect of confining stress on3the volumetric behavior of the ash mixtures (R = 95%, F

= 75%). Dilation decreased gradually as increased. It3is noted that at = 200 kPa, the volumetric behavior3

became contractive. Slight dilation observed at

relatively small turned to contraction as increaseda a

further.

< Peak friction angle >peak


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Both non-pozzolanic class F fly ash and bottom ash arebasically frictional materials. For frictional materials,

the peak friction angle () is a measure of thepmaximum shear strength that the material can develop.

As shown in Figure 7, for ash mixtures is a functionpof the relative compaction (R), the confining pressure

( ) and the fly ash content (F), in order of decreasing3significance. The reduction of R from 95% to 90%

decreased significantly. Also, decreased as p p 3increased from 50 kPa to 200 kPa. The samples at R =

95% displayed notable reductions in with increasingp due to reduced dilatancy.3

As compared to the effects of R and , however,3varying F does not appear to change significantly.pThe overall trend was that decreased slightly as Fpincreased from 50% to 75% and 100%. In other words,

addition of bottom ash to fly ash increased . This mayp

be primarily due to the angularity of bottom ashparticles, which provides higher resistance to particlerearrangement for sustained shearing.

It is known that the of compacted sandy soilsptypically ranges from 31 to 45 (U.S. Navy 1986). The

test results suggest that, although the of the ashpmixtures varies in a wide range, depending on the ash

sources and conditions (i.e. R, , F), ash mixtures3generally exhibit fairly comparable shear strength tothat of typical compacted sandy soils.

5. CONCLUSIONS

From a mechanical point of view, non-pozzolanic classF fly and bottom ash mixtures generally comparefavorably to conventional granular materials. It appearsthat high volume fly ash mixtures are suitable for use in

highway embankments, if proper design andconstruction procedures are followed. Prior to use, thematerials must pass the appropriate environmentalrequirements set by state regulatory agencies. As theenvironmental requirements are satisfied, the fly/bottomash mixtures can provide fill materials of comparablestrength and compressibility to most soils typically usedas fill materials, while having the advantage of smallerdry unit weights.

REFERENCES

American Coal Ash Association, ACAA, (2001), CCPSurvey, Retrieved from http://www.acaa-usa.org/PDF/ACAA2001CCPSurvey.pdf.

Diamond, S. (1985), "Selection and Use of Fly Ash forHighway Concrete," Report No. FHWA/IN/JHRP-85/8,Final Report, Purdue University, W. Lafayette, IN.,USA.

Huang, H. W. (1990), "The Use of Bottom Ash inHighway Embankments, Subgrade, and Subbases," JHRP Final Report, FHWA/IN/JHRP-90/4, PurdueUniversity, W. Lafayette, IN., USA.

Ke, Te-Chih (1990), "The Physical Durability andElectrical Resistivity of Indiana Bottom Ash," JHRPFinal Report, FHWA/IN/JHRP-90/6, Purdue University,W. Lafayette, IN., USA.

U.S. Navy (1986), "Design Manual - Soil Mechanics,Foundations, and Earth Structures," NAVFAC DM-7,Department of the Navy, Washington, D.C.


MECHANICAL PROPERTIES OF CLASS F FLY AND BOTTOM ASH MIXTURES FOR EMBANKMENT APPLICATION

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