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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Coal fly ash can be used as a mineral filler in hot mix asphalt paving applications. Asphalt mixtures containing low addition levels (approximately 5 percent by dry weight of aggregate) of fly ash as a mineral filler 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.
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 to be hydrophobic, meaning it sheds water easily, thus reducing the potential for asphalt stripping.(1)
The U.S. Bureau of Public Roads (now the Federal Highway Administration (FHWA)), compared the retained strength of asphalt mixes containing various mineral fillers by means of the immersion-compression test.(2) 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.(3) In both investigations, mixes containing the fly ash fillers had higher retained strengths than the other filler sources tested, indicating that fly ash fillers can be expected to provide excellent resistance to stripping.(4)
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.(5)
A study of Texas lignite fly ash indicated that the use of these fly ashes as mineral filler retards the rate of age hardening of asphalt cement. The high lime content of these fly ashes also appears to be particularly beneficial as an anti-stripping agent for polish-susceptible aggregates.(6)
A 1994 survey of all 50 state transportation agencies indicated that eight states have made some recent use of fly ash as a mineral filler in asphalt paving. These states included Connecticut, Louisiana, Michigan, Nebraska, New York, Ohio, Oregon, and Pennsylvania. Most of these states reported that the performance of fly ash as a filler material was fair to good. However, in two states (Michigan and Nebraska), fly ash reportedly performed poorly as a filler material and was either discontinued or eliminated from further use.(7)
An earlier survey of all state transportation agencies in 1992 also indicated that eight states reported using fly ash as a filler. In that survey, Arkansas and Kansas were noted in place of Connecticut and Oregon. There were also two states that reported poor performance, these being Nebraska and New York.(8) It has also been indicated by at least one asphalt producer that using fly ash as a filler material during hot weather has resulted in tender mixes that tend to push or shove under the action of a steel-wheeled roller.(9)
Previous surveys of state transportation agencies by the American Coal Ash Association have indicated that at least 14 other states, besides those noted above, reported having used fly ash at one time or another as mineral filler. These states included Alabama, Arizona, Colorado, Illinois, Kentucky, Maryland, Minnesota, Montana, Nevada, New Mexico, North Carolina, North Dakota, West Virginia, and Wyoming. There is no indication from these surveys regarding fly ash performance. The American Coal Ash Association has reported that approximately 96,000 metric tons (107,000 tons) of fly ash were used in 1995 as a mineral filler in asphalt.(10)
MATERIAL PROCESSING REQUIREMENTS
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 this application.
Fly ash is collected at the power plant and stored in silos in a dry form. As a result, it can readily be loaded into pneumatic hauling vehicles and delivered to a hot mix asphalt plant.
The physical requirements for mineral filler in bituminous paving mixtures are defined in AASHTO M17 and are shown in Table 5-3.(11) These requirements include gradation, organic impurities, and plasticity characteristics. Other properties of interest include fineness and specific gravity.
Table 5-3. AASHTO M17 specification requirements for mineral filler use in asphalt paving mixtures.
Gradation: The AASHTO specification limits for mineral filler call for a range of 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.(12)
Fineness: Although most sources of fly ash are capable of meeting the AASHTO gradation requirements for mineral filler, consistency of gradation is also 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;(13) 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. It 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; therefore, a given weight percentage of fly ash will usually occupy a greater volume than that of a conventional filler material.
Organic Impurities: Some fly ash from boilers that burn oil during start-up periods may have some 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 probably 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.
Plasticity: Fly ash is a nonplastic material with no plasticity index.
DESIGN CONSIDERATIONSMix 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.
One of the most recognized ways to improve the anti-stripping characteristics of an asphalt paving mix is the addition of a modest amount (usually 1/2 to 2 percent by weight) of commercial hydrated lime into the mix. Lignite or subbituminous fly ash contains up to 30 percent or more calcium, compared with anthracite or bituminous fly ash, which may only contain 4 to 6 percent of calcium. The use of high calcium fly ash may improve asphalt stripping with many aggregates, although there is not a great amount of field performance data to corroborate this assumption.
Conventional AASHTO pavement structural design methods are applicable to asphalt pavements incorporating fly ash as a mineral filler in the mix.
Material Handling and Storage
Fly ash is a dusty material and its use may result in more dust generation than that normally experienced from using conventional filler sources.
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
In isolated instances, asphalt paving mixes with fly ash as the mineral filler have been observed to be tender and difficult to compact during hot weather. This does not appear to be a widespread problem and also does not appear to be universally true for all sources of fly ash during hot weather, or even at other times of the year.
Although most fly ash sources are capable of satisfying specification requirements for asphalt mineral filler, which relate mostly to particle sizing, not all fly ashes have performed satisfactorily in asphalt paving mixtures. The reasons for this are not altogether clear, but are probably related to the fineness of the fly ash, its chemical composition, and its affinity for the asphalt cement used in a paving mix. A better means of classifying fly ash for use as a mineral filler is needed.
A method for assessing the potential suitability of a given source of fly ash as a mineral filler in asphalt paving is needed. The loss on ignition (LOI) of fly ash, especially fly ash with a low calcium content, may not be a significant factor affecting its performance as a mineral filler. The calcium content, and in particular the free or available lime (CaO) content, of fly ash with a high calcium content is believed to be instrumental in its performance as a filler, especially as an aid in the prevention of asphalt stripping. Additional field performance data are needed to draw valid conclusions regarding these factors.