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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Municipal solid waste (MSW) combustor ash (bottom ash or combined ash) can be used as a granular base material in road construction applications. The high stability of MSW combustor ash provides for good load transfer to the pavement subgrade. Its somewhat marginal durability, however, could result in breakdown during compaction and the relatively high percentage of fines in combustor ash could result in frost susceptibility. The ash must also be stored prior to use to minimize the possibility of any volumetrically expansive hydration reactions from occurring after placement.
The use of MSW combustor ash as a granular fill material in the United States has been largely limited to its use in a few test demonstrations.(2) In Europe, however, the same material has been in commercial use as a granular base or fill for use in road base and embankment applications for almost two decades. In Denmark, Germany, and The Netherlands, more than 50 percent of the bottom ash generated is used in granular base or fill applications.(1)
In the United States test demonstrations have been undertaken in Texas, Massachusetts, and California.(2) In addition to its use as a granular base, municipal waste combustor ash has also been blended with lime and used or proposed for use as a stabilized base material.(3) A listing of some locations where MSW combustor has been used in road base applications is presented in Table 10-5.
Although there has been little documented postconstruction monitoring on any of the test demonstrations, all of the demonstrations are believed to have performed in a satisfactory manner.
Some of the more desirable features of MSW combustor ash in granular base applications include its extremely high stability and low unit weight. It is not a highly durable material, however, and is subject to particle size breakdown under heavy equipment load. MSW combustor ash also contains concentrations of trace metals and soluble salts that may warrant some environmental concern.
Table 10-5. MSW combustor ash road base/subbase demonstrations.
MATERIAL PROCESSING REQUIREMENTS
Segregation of Ash Streams
In most countries in Europe where ash has been used as a granular base material, the grate ash has been separated from the boiler ash and fly ash. This helps to reduce the fines (minus 0.075 mm (No. 200 sieve)) fraction from the ash, producing a more suitable granular base material. It also eliminates the inclusion of free lime associated with the fly ash fraction, which could produce expansive reactions, and provides for a material with lower trace metal and trace organic content than the combined ash stream.
Ferrous and Nonferrous Metal Removal
Ferrous and nonferrous metal should be removed prior to use. Metal removal is necessary to produce a suitable granular material, because ferrous and some nonferrous metals (such as aluminum) are known to produce adverse reactions. For example, free aluminum that is present in the ash could potentially react with water to form hydrogen gas.
The organic content in the ash should be minimized. In some countries bottom ash samples found to contain greater than 5 percent LOI are unacceptable for use. The organic limitations are primarily the result of concern over gas evolution and subsidence problems.(1) These organic content limitations can normally be met if the combustor is operating efficiently. If the organic content is higher, then additional processing, such as air classification, may be required.
In some European countries it is recommended that the ash be screened to a maximum grain size of 50 mm (2 in), with less than 10 percent of the total weight less than 0.06 mm (No. 300 sieve) in size.(1)
It is recommended that bottom ash should be stockpiled for a 1- to 3-month period to allow swelling, hydration, carbonation, and oxidation aging reactions to occur.
Some of the engineering properties that are of particular interest when MSW combustor ash is used in granular base applications include gradation, density, stability, durability, and drainage characteristics.
Gradation: MSW combustor ash must be screened to produce, at a minimum, a minus 19 mm (3/4 in) material and preferably a minus 12.7 mm (1/2 in) material. Larger-sized particles tend to be less granular and typically comprise pieces of ceramic or metal. Processed MSW combustor ash should meet the gradation requirements of AASHTO M147.(4)
Density: The compacted unit weight of processed municipal waste combustor ash is in the range of 1280 to 1760 kg/m3 (80 to 110 pcf), which is somewhat lower than that of conventional aggregates.(4,5) Maximum density has been reported at moisture contents in the range of 12 to 16 percent, by total weight.(4)
Stability: Minus 19 mm (3/4 in) municipal waste combustor ash is a well-graded material.(5,6) The apparent angularity of the particles and high friction angle (40 to 45o)(7,8) contribute to its high bearing capacity. California Bearing Ratio (CBR) values have been shown to range from approximately 75 to 150 percent, which is similar to crushed stone.(4,9)
Durability: MSW combustor ash is not a highly durable material as measured by the Los Angeles Abrasion test method (values approximately 40 to 60 percent), but does exhibit resistance to freezing and thawing as measured by sodium soundness tests (values generally less than 10 percent).
Drainage: Although compacted bottom ash is a free-draining material (with permeabilities ranging from 10-2 to 10-4 cm/sec), compacted combined ash, particularly compacted combined ash containing hydrated lime (introduced into the ash through the air pollution control system) is a relatively impermeable material with compacted permeabilities ranging from 10-5 to 10-7 cm/sec.(6,7,8)
Structural design procedures for granular base containing processed MSW combustor ash are the same as design procedures for bases containing conventional aggregate. Combined ash with excess lime can be expected to exhibit properties similar to that of a lime-stabilized base material. In such cases AASHTO T99(10) or T180(11) procedures should be used to prepare a mixture with optimum moisture and maximum compacted density. ASTM C593(12) compressive strength and durability testing should be undertaken to assess the properties of the compacted mix.
Material Handling and Storage
The same equipment and procedures used to stockpile, handle, place and compact conventional aggregates can be used for processed ash in granular base applications.
MSW combustor ash should probably be stored for at least 3 months prior to use. The relative low optimum moisture content of ash coupled with the high moisture content of fresh ash suggests that ash be stored, not only to permit aging reactions to occur, but to reduce the moisture content prior to use.
Placing and Compacting
It has been recommended that a minimum 10-ton vibratory roller be used to achieve maximum compaction, and that aged, well-drained ash be used to enable maximum density to be achieved in the field.(13) In addition, due to the marginal durability of the ash, a breakdown in particles toward the fine side can be expected.
The same field test procedures used for conventional aggregate are recommended for granular base applications when using MSW combustor ash. Standard laboratory and field test methods for compacted density are given by AASHTO T191,(14) T205,(15) T238,(16) and T239.(17)
The most pressing issue to be resolved relative to the use of incinerator ash in granular base applications is the environmental suitability of using these materials. Although no adverse impacts associated with using these materials in Europe have been reported, the high trace metal and salt contents associated with these residues have raised concerns regarding the environmental suitability of these materials.
Additional work is also needed to address aging requirements and the potential for hydrogen gas formation due to free metal and water reactions.