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COAL BOTTOM ASH/
BOILER SLAGUser Guideline


Granular Base

INTRODUCTION

Coal bottom ash and boiler slag have been used as a granular base material in road and parking lot construction. Bottom ash or boiler slag need to satisfy material specifications for gradation, soundness, and abrasion loss. Bottom ash and boiler slag can be blended with natural aggregate to produce a mixture that meets specifications. Due to higher value uses of boiler slag, bottom ash is used more prevalently as a stabilized or unstabilized base or subbase material.

PERFORMANCE RECORD

Bottom ash has been successfully used as granular base since the early 1970's. The American Coal Ash Association reported that 740,000 metric tons (815,000 tons) of bottom ash were used as road base or subbase materials in 2006.(1) The road base or subbase category used by the American Coal Ash Association includes the use of coal bottom ash as an unbound base or in stabilized subbase or stabilized base material. Bottom ash is being studied and used as a granular base in both public(2;3;4;5) and private projects, although private use is not well documented in the literature.

MATERIAL PROCESSING REQUIREMENTS

Dewatering (Moisture Control)

Bottom ash is typically free-draining, therefore, unless saturated, the moisture content has little influence on compaction characteristics.(6) Short-term stockpiling of bottom ash, typically less than two days, may be required to reduce moisture content. Reclaimed ponded ash may require longer-term stockpiling, up to two weeks, to reduce moisture content to an appropriate level. The moisture content of bottom ash should be high enough to prevent dusting during material handling.

Screening

Bottom ash may meet granular base specifications without processing,(2;7) although the ash may require screening, washing, or blending with conventional aggregate to meet specifications. Oversize or agglomerated popcorn particles may be present in some bottom ash sources and should be removed by screening. Because boiler slag is a poorly-graded material, screening is typically not needed, but blending with conventional aggregate may be required.

Deleterious Materials

Deleterious materials, such as soluble sulfates or coal pyrites, should be removed from bottom ash, boiler slag, or pond ash before use as a granular base. Pyrites should be removed from the coal prior to burning and handled separately from the ash. The pyrites should not be commingled with the ash stream. Although an added cost, processing techniques do exist to remove pyrites from bottom ash.8

ENGINEERING PROPERTIES

Engineering properties of bottom ash and/or boiler slag that are important in granular base applications are gradation, specific gravity, unit weight, compaction characteristics, degradation under compaction, shear strength, bearing strength (CBR), resilient modulus, corrosivity, and hydraulic conductivity.

Gradation: Bottom ash and boiler slag are considered fine aggregates in a granular base.(9;10) To improve grain size distribution characteristics of bottom ash or boiler slag, a conventional aggregate or a slag aggregate may be blended with the ash.

Specific Gravity: The specific gravity of bottom ash is in the range of 2.1 to 2.7,(11) but values as low as 1.9 and as high as 3.4 have been recorded.(12) Bottom ash with relatively low apparent specific gravity is often indicative of the presence of porous particles trapping gases that effect the test results. Bottom ash with relatively high specific gravity may indicate a high iron content. The specific gravity of boiler slag is in the range of 2.3 to 2.9.

Dry Unit weight: The dry unit weight of bottom ash is in the range of 7.07 to 15.72 kN/m3 (45 to 100 lb/ft3) while the dry unit weight of boiler slag is in the range of 9.43 to 14.15 kN/m3 (60 to 90 lb/ft3).(11) The dry unit weight of bottom ash can reach as high as 18.08 kN/m3 (115 lb/ft3) while the dry unit weight of boiler slag may be as high as 17.29 kN/m3 (110 lb/ft3).(12)

Compaction Characteristics: The laboratory compaction curves for dry bottom ash are similar in shape to those of typical cohesionless materials. These curves are characterized by a relatively high dry density for the air-dried condition, a lower dry density at intermediate moisture, and a high dry density at or near saturation. Generally, field compaction curves also exhibit maximum dry density at either an air-dried condition or a flushed or wet condition.(12) Although difficult to maintain an air-dried condition in the field, a "flushed" condition can be maintained, which should produce a maximum dry density.(13) When compared to conventional granular materials, bottom ashes have lower maximum dry densities.(14) Bottom ash studied by WE Energies exhibited a maximum dry density of 13.91 kN/m3 (88.5 lb/ft3) and an optimum moisture content of 28 percent.(15) Conventional aggregates have maximum densities in the range of 16.51 to 18.87 kN/m3 (105 to 120 lb/ft3) at optimum moisture content typically in the range of 8 to 16 percent.(15)

Degradation Under Compaction: To quantify the extent of degradation under compaction, an index of crushing has been developed by calculating the mean size of a material before and after compaction and expressing the index of crushing as the percent reduction between the two mean sizes. The higher the index, the easier a material crushes. The index of crushing for coal bottom ash from a pulverized coal boiler was found to be roughly twice that of conventional aggregates, whereas the index of crushing for boiler slag is essentially the same as that of conventional aggregates. The index of crushing for bottom ash from a stoker-fired boiler was found to be about three times greater than the index of crushing for bottom ash from a pulverized coal boiler.(12)

Shear Strength: Bottom ash, having a rough surface texture and angular particles, has a slightly higher friction angle than conventional granular soils when compacted to a high relative density. Direct shear tests conducted on dry bottom ash samples under loose and dense relative density conditions indicated that the angle of internal friction for most bottom ashes ranged from 35 to 50 degrees, depending on the extent of densification, with some dense bottom ash samples exhibiting friction angles as high as 55 degrees. The angle of internal friction for boiler slag was found to fall within the same range as that of most natural granular soils (between 36 and 46 degrees).(11)

Bearing Strength: California Bearing Ratio (CBR) tests on bottom ash indicate that soaking does not negatively affect CBR.(12) Although the maximum CBR is obtained for bottom ash compacted slightly dry of optimum, where optimum is approximately a flushed conditioned, the general trend for CBR values is to increase with increasing moisture contents.(13) The CBR values of bottom ash compacted a high moisture contents were found to be higher (CBR 40 to 70) than the CBR values of bottom ash compacted at low moisture contents (CBR 35 to 60) indicating that compacting at high moisture contents is advantageous.

In general, more coarsely graded and more angular materials tend to exhibit greater stiffness and tend to distribute load more evenly. When the same thickness is used, bottom ash exhibits less load distribution characteristics and would be more flexible than conventional aggregates, (15) even though bottom ash falls in the categories of "good subbases" and "good gravel bases" on the basis of CBR values.(13).

Resilient Modulus: Resilient modulus is a measure of the modulus of elasticity during rapidly applied loadings. Resilient modulus is related to the long-term performance of materials under service loads. Unbound bottom ash tested following AASHTO T 292 protocol produced a resilient modulus of 200 to 400 MN/m2.(2) Resilient modulus is effected by the state of stress, but even when corrections are applied, the laboratory modulus for bottom ash tends to be lower (by a factor of 4) than the modulus calculated from field tests.(3)

Corrosivity: Bottom ash or boiler slag used as a granular base may potentially corrode metal structures.(16) Parameters of interest that are related to corrosivity are pH, electrical resistivity, soluble chlorides, and soluble sulfates. A study of 11 bottom ash or boiler slag samples from Indiana indicated that seven of the samples were considered corrosive, principally because of low electrical resistivity,(16) although pH measurements may exhibit high alkalinity indicating low corrosion potential.(17) Therefore, bottom ash, boiler slag, and ponded ash should be investigated for corrosivity with multiple methods if there is a potential that the ash will come in contact with metal.

Hydraulic Conductivity: The hydraulic conductivity of bottom ash varies between 1 and 10-3 cm/s which is comparable to natural materials with similar grain size distribution.(18) Bottom ash is well draining material and is non-susceptible to frost heave.

The physical properties of coal bottom ash, boiler slag, and ponded ash will vary depending on the type, source, and fineness of the parent fuel, as well as the operating conditions of the power plant;(19) therefore, material specific testing is recommended.

DESIGN CONSIDERATIONS

Pavement design that includes bottom ash, boiler slag, or ponded ash as an unbound or granular base or subbase material can follow AASHTO methods provided in Guide for Design of Pavement Structures.(16) The AASHTO method accounts for the predicted loading (the predicted number of 80 kN equivalent single axle loads), required reliability (degree of certainty that a design will function properly during the design life), serviceable life (ability to maintain quality during the pavement life), the pavement structure (characterized by the structural number), and subgrade support (related to the resilient modulus of the subgrade).(16)

The structural number of a pavement design accounts for the relative strength of the constructed materials. The total structural support from the surface course, base course, and any subbase course needs to equal the required structural number. Layer thicknesses are calculated using layer coefficients that define the structural support. The layer coefficients can be obtained from the relationship provided by AASHTO based on CBR or MR.(16)

A layer coefficient value of 0.10 can be used for the design of flexible pavement systems in which bottom ash, boiler slag, or reclaimed ponded ash are used to construct an unbound or granular base or subbase. A coefficient of 0.10 for bottom ash and/or boiler slag recognizes that bottom ash and/or boiler slag are not structurally equivalent to crushed stone, which is typically given a larger coefficient of 0.15.

Bottom ash used at approximately 1.5 times the thickness of conventional aggregates achieves a comparable stress level in the underlying subgrade. For equivalent deformation, the thickness of bottom ash should be two times the thickness of conventional aggregates to maintain similar deflection at the surface of the base course layer.(15)

Compacted unbound bottom ash used as a working platform and subsequently as a contributing subbase member in flexible pavement design has been studied.(2;20;4) Laboratory and case study results show that with proper design and construction, compacted bottom ash provides adequate support as a working platform or subbase material.(2;20) Design charts for selecting the equivalent thickness of compacted bottom ash for working platforms are provided in reference 20, where a methodology for including the structural contribution of working platforms made from bottom ash or other alternative material is presented in 4.

CONSTRUCTION PROCEDURES

Material Handling and Storage

Both bottom ash and boiler slag can be handled and stored using the same methods and equipment that are used for conventional aggregates.

Placing and Compacting

Bottom ash and boiler slag can be dumped and spread with a motor grader or bulldozer or for more accurate grade control, these materials can be placed with a spreader box or paving machine. Bottom ash and boiler slag should be compacted at, or slightly above, optimum moisture content as determined by standard Proctor compaction procedures.(1) Bottom ash loses stability at low moisture contents; therefore, high moisture contents should be maintained to allow construction equipment to operate. The addition of up to 30 percent fines in the form of fly ash may remedy the loss of stability upon drying.(11) Compaction of bottom ash and boiler slag bases and subbases can be accomplished by static steel-wheeled rollers, pneumatic rollers, or vibratory compaction equipment.

After compaction, a bottom ash granular base layer should be protected. A prime coat of asphalt emulsion can be applied to the base material to prevent moisture evaporation, stabilize the surface, and provide a bond between the base layer and an asphalt or Portland cement concrete wearing surface. An asphalt binder, wearing surface, or concrete pavement should be constructed within a reasonable time after sealing a granular base layer to minimize traffic loads on the base layer.

ENVIRONMENTAL CONSIDERATIONS

As described in the Coal Bottom Ash\Boiler Slag Material Description, the use of bottom ash or boiler slag as an unbound granular base is an unencapsulated use and therefore has the potential to leach trace elements. Use of bottom ash or boiler slag as an unbound granular base requires good management and care to ensure that there is no negative impact on the environment. In particular, areas with sandy soils possessing high hydraulic conductivities and areas near shallow groundwater or drinking aquifers should be given careful consideration. An evaluation of groundwater conditions, applicable state test procedures, water quality standards, and proper construction are all necessary considerations in ensuring a safe product that does not adversely affect the environment.(21)

UNRESOLVED ISSUES

Bottom ash and boiler slag aggregates possess somewhat unique engineering properties when compared to conventional aggregates. For example, bottom ash may contain some particles that can crush or degrade easily, while boiler slag is very uniformly graded. Some sources of bottom ash or boiler slag, which provide satisfactory performance as an unbound granular base, may not satisfy all specification requirements. This is particularly the case for particle size distribution specifications and abrasion loss requirements for graded base courses. Performance based specifications rather than material characteristic specifications would allow for an increased use of unconventional materials as unbound granular bases.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of bottom ash and boiler slag literature is available here.

  1. American Coal Ash Association (ACAA). 2006 coal combustion product (CCP) production and use. American Coal Ash Association; Aurora, CO: 2007.
  2. Edil TB, Benson CH, Bin-Shafique MS, Tanyu BF, Kim W, Senol A. Field evaluation of construction alternatives for roadways over soft subgrade. Transportation Research Record 2002;(1786):36-48.
  3. Tanyu BF, Kim W, Edil TB, Benson CH. Comparison of laboratory resilient modulus with back-calculated elastic moduli from large-scale model experiments and FWD tests on granular materials, In: G. Durham, A. Marr, W. De Groff, editors. Resilient modulus testing for pavement components. West Conshohocken, PA: ASTM; 2003.
  4. Tanyu BF, Kim W, Edil TB, Benson CH. Development of methodology to include structural contribution of alternative working platforms in pavement structure. Transportation Research Record 2005(1936):70-7.
  5. Seals RK, Moulton LK, Ruth BE. Bottom ash: An engineering material. Journal of the Soil Mechanics and Foundations Division 1972 April; SM 4:311-25.
  6. ASTM E2277-03 standard guide for design and construction of coal ash structural fills. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: American Society for Testing and Materials; 2003.
  7. Moulton LK, Seals RK, Anderson DA. Utilization of ash from coal burning power plants in highway construction. Transportation Research Record 1973(430):26-39.
  8. Groppo J, Robl T. Construction fill sand production from bottom ash at mill creek station. EPA; 2003 December. Case study No. 7.
  9. ASTM. ASTM D1241-07 standard specification for materials for soil-aggregate subbase, base, and surface courses. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: ASTM; 2007.
  10. ASTM D2940-03 standard specification for graded aggregate material for bases or subbases for highways or airports. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: American Society for Testing and Materials; 2003.
  11. Moulton LK. Bottom ash and boiler slag. In: Proceedings of the third international ash utilization symposium. Washington, DC: U.S. Bureau of Mines; 1973.
  12. Lovell CW, Ke TC, Huang WH, Lovell JE. Bottom ash as highway material. In: 70th annual meeting of the transportation research board. Transportation Research Board; Washington, DC:1991.
  13. Huang WH. The use of bottom ash in highway embankment and pavement construction. Purdue University; West Lafayette, IN: 1990. p. 317
  14. Usmen MA. A critical review of the applicability of conventional test methods and materials specifications to the use of coal-associated wastes in pavement construction.: West Virginia University; Morgantown, West Virginia: 1977.
  15. Ramme BW, Tharaniyil M. Coal combustion products utilization handbook. We Energies; Milwaukee, WI: 2004.
  16. AASHTO. Guide for design of pavement structures. American Association of State Highway and Transportation Officials; Washington, DC,: 1993.
  17. Kim B, Prezzi M. Compaction characteristics and corrosivity of indiana class-F fly and bottom ash mixtures. Construction and Building Materials; In Press, 2007.
  18. Rogbeck J, Knutz A. Coal bottom ash as light fill material in construction. Waste Management 1996;16(1-3):125-8.
  19. Özkan Ö, Yüksel I, Muratoglu Ö. Strength properties of concrete incorporating coal bottom ash and granulated blast furnace slag. Waste Management 2007;27(2):161-7.
  20. Tanyu BF, Benson CH, Edil TB, Kim W. Equivalency of crushed rock and three industrial by-products used for working platforms during pavement construction. Transportation Research Record 2004(1874):59-69.
  21. Environmental Protection Agency (EPA), Federal Highway Administration (FHWA). Using coal ash in highway construction - A guide to benefits and impacts. ; 2005. Report nr EPA-530-K-002:ID: 151.

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Last Update 7/28/08