COAL FLY ASHUser Guideline

Asphalt Concrete

INTRODUCTION

Asphalt concrete is a composite material consisting of an asphalt binder and mineral aggregate that is laid down in lifts and compacted to a sufficient density to allow dynamic loading by traffic. For highway and airfield applications, hot mix asphalt concrete (HMA) is most commonly used and is generated by heating the aggregate to around 300° F and the asphalt cement to 200° F before mixing. Flexible pavement comprises more than 93 percent of all roadways in the U.S. and increased use of fly ash in this application would dramatically decrease the amount of landfilled fly ash while potentially decreasing costs.⁽¹⁾

Fly ash can be used as a mineral filler to fill the voids and provide contact points between larger aggregate particles in asphalt concrete mixes.⁽²⁾ Use of fly ash as a mineral filler is governed by ASTM D242 and AASHTO M-17, which outline specifications for mineral filler. Beneficial mineral fillers are sought out to increase asphalt mortar mix stiffness, improve rutting resistance, and increase mix durability.⁽³⁾ Asphalt mixtures containing low levels of fly ash, approximately 5 percent by dry weight of aggregate, exhibit mix design properties that are usually comparable to asphalt mixtures containing natural fillers such as hydrated lime or stone dust. Gradation, organic impurities, and plasticity characteristics ordinarily associated with mineral filler specification requirements can normally be met without difficulty. The characteristics of mineral fillers that are most related to asphalt paving mixture performance are particle sizes in microns corresponding to D60 and D10 of the P200 material and the methylene blue test.⁽⁴⁾

Fly ash can also be used as an asphalt cement extender. Testing has shown 10 and 30 percent replacement of asphalt cement with fly ash does not significantly negatively affect mix properties like strength and durability.⁽⁵⁾ Because of the difference in unit weight between asphalt cement and fly ash, the partial replacement of asphalt cement with fly ash increases the amount of binder, which counterbalances the decrease in the percentage of the asphalt cement.⁽²⁾ Replacement of a portion of asphalt cement with fly ash in paving mixtures could provide an economical alternative to using costly asphalt as well as provide longer pavement service life due to improved pavement properties.⁽⁵⁾ Although testing has shown the possible benefits of this application, further testing, especially in the field, is required to prove the legitimacy using fly ash as a partial asphalt cement replacement.

PERFORMANCE RECORD

Research conducted over many years has determined that fly ash is a suitable mineral filler material. The earliest study of this application dates back to 1931, when the Detroit Edison Company compared the physical properties of fly ash with those of limestone dust. Fly ash was shown to have comparable physical properties to limestone dust, to possess good void filling characteristics, and be hydrophobic, meaning it sheds water easily, thus reducing the potential for asphalt stripping.^(6;3)

The Federal Highway Administration (FHWA) compared the strength of asphalt mixes containing various mineral fillers by means of the immersion-compression test.⁽⁷⁾ This test is used as an indicator to evaluate resistance to stripping. Four sources of fly ash were evaluated, along with silica dust, limestone dust, mica dust, and traprock dust. Similarly, North Dakota State University compared lignite fly ash as a mineral filler with hydrated lime and crusher dust.⁽⁸⁾ In both investigations, mixes containing the fly ash fillers had higher strengths than the other filler sources tested, indicating that fly ash fillers can be expected to provide excellent resistance to stripping.⁽⁹⁾

Further confirmation of the beneficial anti-stripping characteristics of fly ash mineral fillers was provided from an investigation of two western coal fly ashes (one Class C and one Class F) in combination with, or as a replacement for, Portland cement or hydrated lime. All mixes which contained fly ash showed comparable or improved retained strengths in the immersion-compression test using two different sources of aggregate.⁽¹⁰⁾ A study of lignite fly ash indicated fly ash as a mineral filler retards the rate of age hardening of asphalt cement. The higher lime content also appears to be particularly beneficial as an anti-stripping agent for polish-susceptible aggregates.⁽¹¹⁾

The American Coal Ash Association has reported that approximately 26,720 metric tons (29,450 tons) of ash were used as asphalt mineral filler in 2006.⁽¹²⁾ Past surveys of state transportation agencies indicated that the majority of states use fly ash as a mineral filler. In general, states reported fair to good performance on the survey.⁽¹³⁾

MATERIAL PROCESSING REQUIREMENTS

Drying

Fly ash must be in a dry form when used as a mineral filler. This means that moisture-conditioned fly ash and reclaimed ponded fly ash are unsuitable for use in asphalt concrete.

Storage

Fly ash is collected at the power plant and stored in watertight silos in a dry form. As a result, fly ash can readily be loaded into pneumatic trucks and delivered to a hot mix asphalt plant.

ENGINEERING PROPERTIES

The physical requirements for mineral filler in bituminous paving mixtures are defined in AASHTO M-17 and are shown in Table 4.^(3;14) These requirements include gradation, organic impurities, and plasticity characteristics. Other properties of interest include fineness and specific gravity.

Table 4. AASHTO M-17 specification requirements for mineral filler use in asphalt paving mixtures.

Particle Sizing		Organic Impurities	Plasticity Index
Sieve Size	Percent Passing
0.006 mm (No. 30)	100	Mineral filler must be free from any organic impurities	Mineral filler must have plasticity index not greater than 4
0.003 mm (No. 50)	95-100
0.075 mm (No. 200)	70-100

Gradation: The AASHTO specification limits for mineral filler range from 70 to 100 percent passing the 0.075 mm (No. 200) sieve. Most fly ashes typically fall within a size range of from 60 to 90 percent passing the 0.075 mm (No. 200) sieve.⁽¹⁵⁾

Fineness: Although most sources of fly ash are capable of meeting the AASHTO gradation requirements for mineral filler, consistency of gradation is important, especially the size and shape of the particles finer than a 0.075 mm (No. 200) sieve. Theoretically, higher fineness may indicate a more effective mineral filler, although the higher fineness also means a greater surface area of particles that must be coated, resulting in an increase in asphalt content of the mix. Fly ash fineness is often specified by the percentage by weight retained on the 0.045 mm (No. 325) sieve, especially when used in Portland cement concrete;⁽¹⁶⁾ however, this is not a standard for fly ash used as a mineral filler.

Specific Gravity: The specific gravity of fly ash varies from source to source. Specific gravity may be as low as 1.7 to as high as 3.0, but is more often within a range of 2.0 to 2.8. Most conventional mineral fillers have a specific gravity in the 2.6 to 2.8 range. The bulk unit weight of fly ash is typically less than conventional mineral filler; therefore, a given weight percentage of fly ash will usually occupy a greater volume than that of a conventional filler material.

Rigden Voids: As determined by the modified Rigden's void test, asphalt binder tends to become over stiffened when composed of mineral fillers with more than 50 percent voids. This is generally not a concern when using fly ash as a mineral filler as most fly ashes have a Rigden measured percentage of voids of less than 50 percent.⁽³⁾

Organic Impurities: Some fly ash from boilers that burn oil during start-up periods may contain residual oil in the fly ash. Although no standard for carbon content or loss on ignition (LOI) is specified for fly ash used as a mineral filler, it is more practical to use a fly ash with a relatively low LOI (less than 5 or 6 percent) to minimize the potential absorption of asphalt by carbonaceous particles. The LOI of fly ash may not be a significant factor affecting performance as a mineral filler, especially for fly ash with a low calcium content. Laboratory tests performed to evaluate the effectiveness of LOI have shown that asphalt mixes incorporating fly ash with LOI up to 10 percent perform satisfactorily.⁽³⁾

Plasticity: Fly ash is a nonplastic material. Thus, plasticity is not an issue when using fly ash as a mineral filler.

DESIGN CONSIDERATIONS

Mix Design

The same mix design methods that are commonly used for hot mix asphalt paving mixtures are also applicable to mixes in which coal fly ash is used as a mineral filler. The percentage of fly ash filler to be incorporated into the design mix is the lowest percentage that will enable the mix to satisfy all the required design criteria.

A small amount of hydrated lime (usually ½ to 2 percent by weight) improves the anti-stripping characteristics of an asphalt paving mix. Because Class C fly ash contains 30 percent or more calcium, the use of Class C fly ash may improve asphalt stripping. In addition, fly ash is hydrophobic in nature and is believed to reduce asphalt stripping,⁽³⁾ although further testing is needed to confirm that fly ash improves asphalt stripping characteristics.

Structural Design

Conventional AASHTO pavement structural design methods are applicable to hot mix asphalt incorporating fly ash as mineral filler in the mix.

CONSTRUCTION PROCEDURES

Material Handling and Storage

Fly ash can be a dusty material and may result in more dust generation than normally experienced with conventional mineral fillers.

At a hot mix plant, the fly ash can be discharged directly into a storage silo, like conventional mineral fillers, prior to input into the mixing plant.

Placing and Compacting

Typical placement and compacting equipment can be used with asphalt mixes containing fly ash.

ENVIRONMENTAL CONSIDERATIONS

A field study of pavements where fly ash and bottom ash were used as a partial substitute of fine aggregate showed no increase of metal concentration in surrounding soils. Soil was collected three months after placement from underneath the pavement section that contained 6 percent fly ash-bottom ash mix by weight, and also beneath a control pavement. Concentrations of arsenic, barium, cadmium, chromium, mercury, lead, selenium, and silver showed minimal differences between the two sections.⁽¹⁾

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of coal fly ash literature is available here.

Churchill EV, Amirkhanian SN. Coal ash utilization in asphalt concrete mixtures. J Mater Civ Eng 1999;11:295-301.
Butul B. Performance characteristics of coal fly ash and wood ash-modified asphalt binder. United States -- Florida: Florida Atlantic University; 2000:ID: 192.
Federal Highway Administration (FHWA), American Coal Ash Association (ACAA). Fly ash facts for highway engineers. Federal Highway Administration (FHWA); 2003 06-13-2003. Report nr FHWA-IF-03-019.
Kandhal PS, Lynn CY, Parker F. Characterization tests for mineral fillers related to performance of asphalt paving mixtures. Transportation Research Record 1998;1638:101-10.
Simms SA. Use of coal fly ash in asphalt concrete mixes. Canada: DalTech - Dalhousie University (Canada); 1998:ID: 190.
Zimmer FV. Fly ash as a bituminous filler. In: Proceedings of the second ash utilization symposium. Pittsburgh, Pennsylvania: U.S. Bureau of Mines; 1970.
ASTM D1075-07 standard test method for effect of water on compressive strength of compacted bituminous mixtures. In: Annual Book of ASTM Standards, West Conshohocken, Pennsylvania: ASTM; 2007.
Brahma SP. Use of lignite fly ash as a mineral filler in bituminous concrete. Fargo, North Dakota: North Dakota State University, Engineering Experiment Station; 1968. Report nr Series No. 3.
Carpenter CA. Cooperative study of fillers in asphaltic concrete. Public Roads 1952;27:101-10.
Rosner JC, Chehovits JG, Morris GR. Fly Ash as a Mineral Filler and Anti-Strip Agent For Asphalt Concrete. Challenge of change - 6th international ash utilization symposium proceedings. Reno, NV, United States Department of Energy, Morgantown Energy Technology Center.
Galloway BM. A review of the use of mineral filler in asphalt-aggregate mixtures. Fly ash applications in 1980 conference; 1980.
American Coal Ash Association (ACAA). 2006 coal combustion product (CCP) production and use. Aurora, CO: American Coal Ash Association; August 24, 2007.
AASHTO. Use of waste materials in highway construction. subcommittee on construction. Washington, DC: American Association of State Highway and Transportation Officials; August, 1994.
AASHTO. Standard specification for mineral filler for bituminous paving mixtures. Washington, DC 20001: American Association of State Highway and Transportation Officials; 2007. Report nr M 17-07.
Di Gioia AM, Nuzzo WL. Fly ash as structural fill. Journal of the Power Division 1972 June;98(1):77-92.
ASTM C618-05 standard specification for fly ash and raw or calcined natural pozzolan for use as mineral admixture in Portland cement concrete. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2005.

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Last Update 7/28/08