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COAL BOTTOM ASH/
BOILER SLAGUser Guideline


Asphalt Concrete

INTRODUCTION

In the United States, about 3.7 million km (2.3 million miles) or 55 percent of all roadways are surfaced with asphalt concrete, making asphalt concrete a critical component of the nations infrastructure.(1;2) Asphalt pavements in new roadway construction typically requires a large amount of virgin materials. Bottom ash and boiler slag can be used as an aggregate replacement, providing a substantial savings to both highway agencies and utility companies. Bottom ash has been used as aggregates in hot mix asphalt (HMA) base courses, in emulsified asphalt cold mixes, and in shoulder construction. Boiler slag has been used in the same applications as bottom ash and also as wearing surfaces, emulsified asphalt cold mix bases or surfaces, and asphalt surface treatments or seal coats.

Bottom ash produced in dry bottom boilers is usually sufficiently well-graded to meet gradation requirements for asphalt concrete. However, bottom ash particles are less durable than conventional aggregates. Consequently, bottom ash is better suited for use in base course and shoulder mixtures or in cold mix applications, as opposed to wearing surface mixtures. Most bottom ash paving applications have been cold mix projects on low-volume secondary roadways.

Boiler slag produced in wet bottom boilers is uniformly sized, and consists of hard, durable, glassy particles. Boiler slag is typically blended with other fine aggregates to meet gradation requirements of asphalt concrete, but because of the hard, durable particles and resistance to surface wear boiler slag is used more frequently in asphalt paving than bottom ash. Boiler slag enhances HMA wearing surfaces, because boiler slag has an affinity for asphalt and has a dust-free surface, which increases adhesion and anti-stripping characteristics. Boiler slag has also been used successfully as a seal-coat aggregate for bituminous surface treatments to enhance skid resistance.(3)

PERFORMANCE RECORD

Bottom ash and boiler slag have been used as fine aggregates in asphalt paving mixtures since the early 1970's. The American Coal Ash Association reported that, 40,800 metric tons (45,000 tons) of boiler slag and over 17,200 metric tons (19,000 tons) of bottom ash were used in asphalt paving during 2006.(4)

Bottom Ash

Bottom ash may contain pyrites or lightweight, porous "popcorn" particles. As a result, bottom ash is not an ideal aggregate source in HMA paving mixes, especially wearing surfaces. Bottom ash has been used more frequently in cold mix emulsified asphalt mixtures, hot mix base course mixtures, and in shoulder construction, where durability considerations are not as critical as in hot mix wearing surface mixtures.(5)

The most extensive use of bottom ash in bituminous paving has been in West Virginia, where, during the 1970's and 1980's, bottom ash was cold mixed with 6 to 7 percent by weight of emulsified asphalt and used in the paving of secondary roads where durability concerns are reduced. Similar paving has also been done in eastern Ohio.(6) To improve the characteristics of a cold mix containing bottom ash, boiler slag can be included.

Because of the potential market for bottom ash as a partial replacement for aggregate in HMA, bottom ash continues to be researched and fielded tested in HMA. In West Virginia, there have been indications of problems with paving mixtures containing bottom ash in which pyrite contamination in the bottom ash had not been considered. Pyrite particles will weather in service, despite being coated with asphalt cement, causing popouts and deep red stains in the pavement surface.(7) Iron pyrites should be removed before bottom ash is used as an aggregate replacement material.(2)

Recent studies have indicated that bottom ash may possess desirable engineering properties and will not degrade HMA performance properties when used to replace a portion of the fine aggregate in an asphalt mix.(2) No more than 30 percent of bottom ash as aggregate replacement is recommended, mixes with 50 percent or more of bottom ash in asphalt pavements were found to have unacceptable stabilities(8). In a study where 15 percent bottom ash replaced aggregate, HMA mixes prepared with bottom ash did not show any significant degradation in performance properties when compared to control mixes. The use of bottom ash as 15 percent replacement of aggregate in HMA mixes maintained desirable strength properties, low temperature properties, and rutting properties. However, the addition of bottom ash required an increase in asphalt content.(1)

Boiler Slag

Boiler slag is used more frequently in HMA than bottom ash because of the hard, durable particles and resistance to surface wear. In addition, boiler slag has an affinity for asphalt and a dust-free surface that aids in asphalt adhesion and resistance to stripping.(3) Boiler slag has a black color that does not fade, which aids in the melting of snow from the road surface.(9) Since boiler slag is poorly-graded, blending with other aggregates is needed to meet specifications for use in asphalt mixtures. The recommended percentage of boiler slag should be less than 50 percent to maintain paving mixture stability.(7)

Boiler slag was first used in asphalt paving in Hammond, Indiana, where, on an experimental basis, bottom slag was blended with conventional aggregate to solve a problem of aggregate polishing. The success of that project and several other demonstration projects in Indiana led to the acceptance and use of boiler slag in Indiana and several other states, including Ohio, Michigan, Missouri, and West Virginia. Boiler slag has also been used as an aggregate in HMA paving in a number of cities such as Cincinnati and Columbus, Ohio, as well as in Tampa, Florida.(10)

Boiler slag has been used successfully as a seal coat aggregate for bituminous surface treatments. Boiler slag provides better coverage per mile than limestone chips and retains a rich black color that is an excellent contrast to road strip colors. The attributes of boiler slag often lead to specifying boiler slag for rehabilitation of airport runways and taxiways.(11)

MATERIAL PROCESSING REQUIREMENTS

Cleaning

Pyrites, which are more prevalent is high sulfur coal, are typically removed from coal during pulverizing and/or cleaning operations prior to burning. Commingling the rejected pyrite with bottom ash is a practice at some power plants. Material handling operations should be modified to keep pyrite and bottom ash-boiler slag separate.(12) Pyrites (iron sulfide) are volumetrically unstable, expansive, and produce a reddish stain when exposed to water over an extended time period. If pyrites are present in the bottom ash or boiler slag, they should be removed by electromagnets, media separation, or other means. Technologies typically used for bottom ash processing can provide a cost-effective method to remove impurities (i.e. unburnt coal and pyrite) so that bottom ash meets product quality targets.(13)

Screening

Oversize or agglomerated popcorn particles may be present in some bottom ash sources and should be removed by screening the material with a 19 mm (¾ in) or 12.7 mm (½2 in) screen.

Blending

Boiler slag will almost always require blending with other aggregate sources to meet gradation specifications. Although more well-graded than boiler slag, bottom ash may require blending with aggregates.

Drying

Aggregates used to produce HMA are dried before blending with asphalt cement; therefore, moisture that may be present in bottom ash or boiler slag should be removed. Excessive moisture in the aggregates will reduce the production rate of paving material due to the additional drying time required. Both bottom ash and boiler slag are relatively easy to dewater, particularly boiler slag, which consists of glassy particles. Ponded ash, which is usually a mixture of fly ash and bottom ash or boiler slag, should be stockpiled and allowed to drain to a surface dry condition.

When used in a cold mix application, bottom ash should be at least surface dry so that moisture does not interfere with the coating of the ash particles by the emulsified asphalt. Boiler slag should also be in a surface dry condition when used as a seal coat aggregate.

ENGINEERING PROPERTIES

Properties of bottom ash and boiler slag that are of particular interest when used in asphalt concrete include gradation, specific gravity, absorption, and durability.

Gradation: ASTM D1073 defines a fine aggregate in asphalt paving mixtures as an aggregate that passes the 9.5 mm (3/8 inch) sieve.(14) Boiler slag typically meets maximum size requirement for fine aggregate, while bottom ash sources may require minimal screening of oversize material to satisfy the definition of a fine aggregate. Because boiler slag is typically poorly-graded, blending with other aggregates may be needed to meet gradation requirements. Bottom ash is typically a well-graded sand-sized material.

Specific Gravity:Specific gravity is a good indicator of the quality of a material. The specific gravity of bottom ash and boiler slag depend on the mineralogical composition of the material as well as the porosity of the particles. A dry bottom ash with a high iron content may have a specific gravity as high as 3.4,(12) while a bottom ash with a large percentage of both porous and popcorn particles may be as low as 1.6.(2) In general, the lower the percentage of porous particles, the higher the specific gravity.

Absorption:Bottom ash generally has lower specific gravity and higher absorption values than limestone sand, while boiler slag is comparable in specific gravity with lower absorption than limestone sand.(15) The water absorption for bottom ash reported in the literature ranges from 0.3(15) to 6.10(16) percent. Due to the porous nature of bottom ash particles, the absorption of asphalt binder is higher than conventional fine aggregate. Hence, from a purely economic standpoint, bottom ash may not be a cost effective choice for asphalt aggregate.(17)

To reduces the asphalt consumption and improve friability of bottom ash particles, sulfur modification to coat bottom ash particles is being researched. Sulfur-modified bottom ash mixtures in which bottom ash represents 50 to 100 percent of the aggregate fraction compare favorably with typical surface course mixtures in terms of strength, durability, and asphalt demand.(18)

Durability: Bottom ash and boiler slag exhibit marginal durability as measured by the Los Angeles abrasion test, with bottom ash percent loss values between 30 and 50 and boiler slag somewhat lower. Most bottom ash samples have some friable particles, while boiler slag normally does not. Los Angeles Abrasion test results have shown that bottom ash samples are not as sound or durable as natural aggregate. However, the test results fall within the specifications of a maximum 50 percent loss by abrasion.

Soundness: Soundness values are generally found to be within ASTM C88(19) weight loss specifications of not more than 15 percent after five cycles when sodium sulfate is used, or not more than 20 percent after five cycles when magnesium sulfate is used.(6) Values for both boiler slag and bottom ash are generally found to be less than 10 percent.(15) Boiler slag has lower sulfate soundness loss because of the glassy surface texture and low porosity while bottom ash with porous popcorn particles has the highest loss.(12)

DESIGN CONSIDERATIONS

Mix Design

Bottom Ash

Bottom ash is used less than boiler slag in asphalt paving because bottom ash has a higher asphalt absorption, some particles may be friability, and there is a potential for pyrites to be mixed into bottom ash from some sources. Most of the paving experience with bottom ash has been in cold mixes.

Because of the presence of friable popcorn particles in dry bottom ash that can break down under compaction, bottom ash is more appropriate as a base course rather than surface mixtures.(20) In addition, the porous particles in bottom ash absorb asphalt; therefore, asphalt contents of mixes containing bottom ash will be higher than mixes with conventional aggregates. Although the asphalt contents of mixes containing bottom ash will be greater than the asphalt contents of conventional asphalt paving mixes, the total weight of asphalt cement used should not be significantly greater because of the low unit weight of the bottom ash. Bottom ash mixes are also likely to have relatively high air void contents. The high air voids are attributable to the rough surface texture of bottom ash particles, which also produces a high angle of internal friction.(21)

Bottom ash may contain iron pyrites that reduce pavement strength. For this reason, no more than 30 percent of the aggregate in a asphalt pavement mix should be replaced with bottom ash.(2)

The comparatively high optimum asphalt content of mixtures using bottom ash as the only aggregate can be reduced by combining bottom ash with conventional aggregates. Research has shown that sulfur modified bottom ash mixes containing 50 to 100 percent bottom ash aggregate replacement can be achieved with 7.5 percent or less asphalt.(18)

Moisture-induced damage of asphalt pavement, referred to as stripping, reduces pavement life. Stripping occurs when moisture causes a loss of bond between aggregate and asphalt binder. Lime additives minimize the moisture susceptibility of pavement mixes.(2) The addition of lime in bottom ash mixtures also improves the indirect tensile strength and the tensile strength ratio.(20) Generally, 1 to 1.5 percent lime by dry weight of aggregate is used. Mixes containing large amounts of fine aggregate will require additional lime because of the increased surface area of the aggregate. Usually three forms of lime are used: hydrated lime (CaOH2), quick lime (CaO), and Dolomitic limes (both types S and N). Coating dry aggregate surfaces with dry hydrated lime may be difficult. Therefore, dry hydrated lime can be added to damp aggregate that contains 3 to 5 percent water content by weight of lime.(2)

Boiler Slag

The uniform gradation and smooth surface texture commonly associated with most boiler slags require that these materials be blended with other aggregates for use in asphalt paving mixtures. The blend proportions of boiler slag and conventional aggregates are designed to meet gradation specifications. Percentages of aggregate replaced by boiler slag range from 40 to 50 percent by weight of the total mix.(7) The type of aggregate used along with the relative proportions of boiler slag and aggregate influence the engineering properties of a pavement.(7)

Marshall stability and flow values decrease as the percentage of boiler slag increases for a given compactive effort. Mixes blended with rounded siliceous aggregates, such as uncrushed river sand, result in lower quality mixtures than blends containing crushed stone, which possess more desirable angularity and surface texture. Blending crushed stone aggregates with boiler slag is recommended because boiler slags lack microtexture that increases the aggregate-asphalt bond and to provide skid resistance.(15)

Optimum skid resistance using boiler slag is achieved in open graded sand mixes where boiler slag is the top-sized aggregate. However, such mixes should limit the percentage of boiler slag in the mix and avoid low filler content. Rounded river sands should also be avoided. Boiler slag does not appear to be as helpful in terms of skid resistance in coarse graded mixtures, especially if the coarse aggregate is polish susceptible.(7)

The laboratory effort and method of compaction effects pavement properties of mixes containing boiler slag. Kneading compaction improves the stability and flow characteristics compared to Marshall drop hammer compaction. Obtaining adequate compaction is essential with boiler slag mixtures. Optimum compaction is produced by blending boiler slag with well-graded, angular, rough-textured aggregate and limiting the percentage of boiler slag to 50 percent. Porous boiler slag can be used in greater percentages, but excessively porous slag are weak and can crush.(21) In addition, porous boiler slag may absorb more asphalt than typical boiler slag, thereby requiring a higher percentage of asphalt cement.

Boiler slag asphalt mixtures, evaluated using the Marshall immersion-compression test,(22) retain stability in the presence of water.(7)

Structural Design

Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating bottom ash or boiler slag.(23)

Similarly, pavement thickness design procedures for cold mix overlays containing bottom ash or boiler slag are the same procedures used for cold mix overlays using conventional aggregates. Modified structural numbers (SN) for cold mix overlays containing bottom ash and/or boiler slag are the same as conventional cold mix overlays.

CONSTRUCTION PROCEDURES

Bottom Ash and Boiler Slag

Material Handling and Storage

Bottom ash and boiler slag can be stored, stockpiled, and handled using the same methods and equipment as conventional aggregates. However, pyrites should be removed prior to using bottom ash in asphalt paving. Boiler slag mixtures with acceptable skid resistance that employ boiler slag as the top size aggregate can be designed by limiting the percentage of boiler slag in the mix. Boiler slag asphalt pavement mixtures should avoid open-graded mixtures with low filler content.

Mixing, Placing, and Compacting

The same methods and equipment used for mixing, placing, and compacting conventional pavements are applicable for asphalt pavements containing bottom ash or boiler slag. In hot mix applications, bottom ash or boiler slag are typically blended with other aggregates using conventional equipment. Dry bottom ash used in cold mix applications may not require blending and can be prepared by mixing with emulsified asphalt at a central pugmill mixing plant. Cold mix asphalt containing bottom ash or boiler slag can be prepared in advanced and stockpiled for 10 or more days.(8)

Cold mixes containing bottom ash can be placed with a paver, spreader box, or can be end dumped and leveled with a grader. Laydown characteristics of dry bottom ash cold mixes placed with either a spreader box or a conventional paving machine are the same as conventional mixes. Spreader box lifts of up to 200 mm (8 in) uncompacted mix can be placed. Lifts greater than 200 mm (8 in) in loose thickness may be difficult to compact.(8) Adequate compaction is usually achieved with several passes of a pneumatic roller followed by a steel-wheeled roller.(8)

ENVIRONMENTAL CONSIDERATIONS

Leaching of metals and air quality during construction are two environmental issues associated with using bottom ash or boiler slag in asphalt pavement. Bottom ash and boiler slag consist of the same chemical components as fly ash; therefore there exists the potential to leach trace elements. Because bottom ash and boiler slag have larger particles and less surface area per unit volume, the potential to leach trace elements is reduced. In addition, coal combustion products mixed in asphalt pavement is considered an encapsulated application that further reduces the potential to leach elements. A recent leachate study was conducted on test strips of asphalt concrete with bottom ash. Although trace elements were observed in the leachate, there was no evidence that the use of coal ash in asphalt pavements was the source.(24)

Air quality during construction is and additional environmental concern involving the use of bottom ash as a highway material.(2) Material handling precautions should be taken to protect workers and the public from dusting during delivery and construction.(25)

UNRESOLVED ISSUES

Bottom ash that contains friable particles should be avoided in asphalt surface mixes unless precrushed before being mixed with asphalt. The performance of wearing surface mixes with precrushed bottom ash aggregate should be evaluated in comparison with more conventional asphalt paving mixes. Additional research into modifying bottom ash, i.e. sulfur modification,(18) to reduce friability and asphalt absorption is encouraged.

Bottom ash and boiler slag possess unique physical and engineering properties that are different from conventional pavement materials; therefore, standard test methods may reject bottom ash or boiler slags that would provide acceptable performance. New or modified test methods are needed to characterize bottom ash and boiler slag properties that influence pavement performance. Improved characterization is needed for both abrasion loss and particle size degradation that may occur during compaction.

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. Ksaibati K, Conner GL. Laboratory evaluation of bottom ash asphalt mixes. Department of Civil & Architectural Engineering, University of Wyoming; Laramie, WY: 2004 May.
  2. Ksaibati K, Sayiri, S. R. K. Utilization of Wyoming bottom ash in asphalt mixes. Department of Civil & Architectural Engineering, University of Wyoming; Laramie, WY: 2006 March.
  3. NETL National Energy Technology Laboratory. Clean coal technology: Coal utilization by-products. Department of Energy Office of Fossil Energy; Washington, DC: 2006 August. Topical report no. 24.
  4. American Coal Ash Association (ACAA). 2006 coal combustion product (CCP) production and use. American Coal Ash Association; Aurora, CO: 2007.
  5. Robnett QL. Use of boiler bottom ash as a paving material A technical database. Georgia Institute of Technology, School of Civil Engineering; Atlanta, GA: 1983.
  6. Root RE, Williams EG. West Virginia turns waste material into useful aggregate. Asphalt 1976;29(2).
  7. Usmen M, Anderson DA. Use of power plant aggregate in asphaltic concrete. In: Proceedings of the fourth international ash utilization symposium, report no. MERC/SP-76/4. Washington, DC: U.S. Energy Research and Development Administration; 1976.
  8. Moulton LK, Seals RK, Anderson DA. Utilization of ash from coal burning power plants in highway construction. Transportation Research Record 1973(430):26-39.
  9. Kerkhoff GO. Bottom ash and wet bottom slag. In: Presented at the annual soils engineers meeting of the Michigan department of transportation. Lansing, Michigan: Michigan Department of Transportation; 1968.
  10. Cockrell CF, Shafer HE, Leonard JW. New or undeveloped methods for producing and utilizing coal ash. In: Proceedings of the second international ash utilization symposium, information circular no. 8488. Washington, DC: U. S. Bureau of Mines; 1970.
  11. American Coal Ash Association (ACAA). Boiler slag. American Coal Ash Association; Aurora, CO: 2004 August. Resource Bulletin 23.
  12. Huang WH. The use of bottom ash in highway embankment and pavement construction. West Lafayette, IN: Purdue University; 1990. p. 317.
  13. Groppo J, Robl T. Construction fill sand production from bottom ash at Mill Creek Station. EPA; 2003 December. Case study No. 7.
  14. ASTM D1073-06 standard specification for fine aggregate for bituminous paving mixtures. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2006.
  15. Moulton LK. Bottom ash and boiler slag. In: Proceedings of the third international ash utilization symposium. U.S. Bureau of Mines; Washington, DC: 1973.
  16. 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.
  17. Ramme BW, Tharaniyil M. Coal combustion products utilization handbook. We Energies; Milwaukee, WI: 2004.
  18. Estakhri CK, Saylak D. Sulfur-modified bottom ash as aggregate in hot-mix asphalt concrete: Field demonstration project. Transportation Research Record 2000(1723):57-65.
  19. ASTM C88-05 standard test method for soundness of aggregates by use of sodium sulfate or magnesium sulfate. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2005.
  20. Asokan P, Saxena M, Asolekar S. Coal combustion residues – environmental implications and recycling potentials. Resources, Conservation and Recycling 2005;43:239-62.
  21. Anderson DA, Usmen M, Moulton LK. Use of power plant aggregate in bituminous construction. In: Presented at the 55th annual meeting of the transportation research board. Washington, DC: Transportation Research Board; 1976.
  22. ASTM D1075-07 standard test method for effect of water on compressive strength of compacted bituminous mixtures. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2007.
  23. AASHTO. Guide for design of pavement structures. American Association of State Highway and Transportation Officials; Washington, DC,:1993.
  24. Churchill EV, Amirkhanian SN. Coal ash utilization in asphalt concrete mixtures. Journal of Materials in Civil Engineering 1999;11:295-301.
  25. 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