U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
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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 bottom ash and combined ash that has been processed to remove ferrous and nonferrous metals and to achieve the appropriate particle size gradation can be blended with other aggregates for use in an asphalt paving mix. Due to the larger fraction of finer, grain-sized material in processed MSW combustor ash than coarse, grain-sized material, the ash is primarily used as a substitute for fine aggregate in a paving mix. Although up to 50 percent, by weight of aggregate, has been used in some test pavements, it is recommended that the substitution rated be limited to 25 percent ash in binder or base course and 15 percent or less in surface mixes to ensure satisfactory paving material production and field performance.
There is no large-scale commercial use of municipal waste combustor ash in asphalt paving mixes in the United States at the present time.
During the 1970’s and early 1980’s at least six test pavements containing municipal solid waste (MSW) combustor ash were installed under Federal Highway Administration (FHWA) sponsored programs. Base course test sections were placed in Baltimore,(1) Houston,(2) and Washington, DC.(3) Wearing or surface course test sections were placed in Delaware County, PA, Philadelphia, PA, and Harrisburg, PA.(4) In 1980 a binder and surface course section was placed in Lynn, MA.(5) More recently (during the past 10 years) test pavements have been placed in Shelton, CT,(6) Tampa, FL,(7) Rochester, MA,(8) Laconia, NH,(9) and Elizabeth, NJ.(10)
A listing of specific design details associated with many of the referenced test pavement demonstrations is presented in Table 10-4.
All of the 1970 pavement demonstrations were considered successful by FHWA, with the exception of the Harrisburg pavement where considerable stripping was reported during the first year. In general, the results of the early FHWA test demonstrations suggested that municipal waste combustor ash could be mixed, placed, and compacted using conventional bituminous construction equipment.
Difficulties were noted with residues that contained high organic content (measured as loss on ignition), compared with residues with low organic content. Residues having LOI’s greater than 10 percent exhibited high and uneven absorption of asphalt during the asphalt production process. High asphalt demand and dusting during drying was attributed to the high fines content in the ash. The need for close temperature control at the asphalt plant to control the drying process, particularly with high moisture content ash, was also noted.(5)
Base course applications were generally considered to be better suited for use as ash pavements than wearing coarse applications.
Table 10-4. MSW combustor ash paving demonstrations.
The results of more recent demonstrations, conducted during the past 10 years, found ash sections in general to be comparable in performance to conventional mixes; however, some difficulty in the asphalt production process because of baghouse clogging(9) and high ash moisture content(10) were reported. A breakdown in ash particles to a finer graded product during the production process was also reported.(9,10)
During the asphalt production process, the introduction of aggregates blended with a large fraction of ash (that contains a high moisture and fines content) could result in potential operational problems. This includes lower plant throughput rates to provide greater time for drying and potential clogging of the asphalt plant baghouse with excessive fines from the ash.
Asphalt pavements incorporating municipal waste combustor ash can be expected to benefit from the low unit weight of the ash compared with conventional aggregate and the resultant higher yield expressed in terms of volume per ton. Pavements that contain higher percentages of ash require higher percentages of asphalt binder (cement) compared with conventional aggregate mixes. This results from the highly absorptive characteristics of ash particles. Due to the relatively low durability of ash particles compared with that of natural aggregates, and the high percentage (approximately 20 to 30 percent) of glass that is present in ash, the introduction of high percentages of ash into wearing course mixes could result in ravelling or stripping problems.
MATERIAL PROCESSING REQUIREMENTS
Segregation of Ash Streams
The separation of the coarse ash particles (grate ash) from the fine ash particles (fly ash and the boiler ash) at the combustion facility, and the use of the coarse ash, is a preferable ash collection strategy. From a paving perspective the reduction in fines would help reduce the highly absorptive fine fraction and also help to alleviate potential baghouse clogging problems during the asphalt production process.
Screening to a minus 19 mm (3/4 in) top size is necessary to produce an aggregate substitute material that can meet most gradation specifications. Screening to a smaller top size, such as minus 12.7 mm (1/2 in), is more desirable to produce a material with a lower plus 12.7 mm (1/2 in) fraction, since this fraction can consist of weak particles (clinkers) that readily break down during handling. Screening of ash with a relatively high moisture content to a minus 12.7 mm (1/2 in) size, however, could clog screening equipment or slow screening throughput rates.
Ferrous and Nonferrous Metal Removal
Ferrous removal using magnetic separators is mandatory, and nonferrous metal removal using eddy current separators is preferable to produce a metal-free nonreactive ash product. The presence and oxidation of ferrous metal in surface pavements could result in popouts.
To satisfy the gradation requirements of AASHTO T27 (11) municipal waste combustor ash must be blended with conventional aggregate.
High temperature melting processes that vitrify the ash have been commercially used in Japan and have been demonstrated in the United States (12,13) The process of vitrification can produce a glass product that can alleviate many of the aforementioned problems associated with ash fines and moisture. Such processing, however, is energy intensive and costly and has not been adopted in the United States.
Some of the engineering properties of MSW combustor ash that are of particular interest when MSW combustor ash is used as aggregate in asphalt paving applications include gradation, unit weight, durability, moisture content, and absorption.
Gradation: Minus 19 mm (3/4 in) municipal waste combustor ash is a well-graded material. Approximately 60 percent of both bottom and combined ash falls into the category of a fine aggregate material. The measured silt content (minus 0.075 mm (No. 200 sieve) fraction) of MSW combustor ash can be expected to range from 5 to 15 percent. The well-graded nature of minus 19 mm (3/4 in) ash makes it a relatively easy product to blend in high percentages into most paving mixes, where 50 percent of the natural aggregate can be readily replaced with ash in most base course mixes (and still comply with gradation specifications). For use in hot mix asphalt, MSW combustor ash must meet the same gradation requirements as conventional aggregate as per AASHTO T27.(11)
Unit Weight: The lower unit weight of MSW combustor ash, which is approximately 965 to 1290 kg/m3 (60 to 80 lb/ft3), can be expected to result in higher asphalt paving yields (paved area per mass of asphalt), when compared with natural aggregate paving yields.
Durability: MSW combustor ash exhibits marginal durability, as measured by the Los Angeles Abrasion test (40 to 50 percent). Breakdown of coarse particles can be expected in handling and asphalt production, which could potentially reduce the quality of the mix.
Moisture Content: MSW combustor ash moisture content can vary a great deal, ranging from approximately 30 to 60 percent on a dry weight basis. This moisture content, as previously noted, can be expected to impact asphalt production operations.
Absorption: MSW combustor ash is a highly absorptive material, with absorption values ranging from 5 to 17 percent. This manifests itself in asphalt cement requirements that could be significantly higher (10 to 20 percent) than that required when conventional aggregate materials are used.
Some of the asphalt mix properties that can be affected by the use of MSW combustor ash include mix stability and stripping (moisture susceptibility).
Stability: Stability of mixes has been reported to be comparable to natural aggregate mixes.(9) These results are expected if municipal waste combustor ash is used to replace primarily the fine aggregate fraction of the mixes.
Stripping: During demonstration programs in the 1970's, hydrated lime was added to MSW combustor ash as an antistripping agent in surface courses. During demonstrations in the late 1980's and 1990's, no hydrated lime was added. When ash is used as a fine aggregate substitute and mix percentages are kept low (approximately 15 percent), stripping problems should be minimal. Higher percentages may warrant the addition of an antistripping agent.
Asphalt mixes containing MSW combustor ash can be designed using standard laboratory procedures. Care must be taken, however, to ensure that the asphalt cement content is adequate to account for the high absorption of ash particles during the mix design process. Recent work has suggested that Marshall mix design methods could lead to overestimating the required asphalt content of the mix and could result in subsequent pavement failure due to rutting. The use of a Gyratory Test Machine as specified in ASTM D3387 has been proposed as an alternative for preparation of the mix, using gyratory compactibility and stability indices to determine the optimum asphalt content.(9)
Lower ash contents may be more suitable in paving mixes to reduce the asphalt cement requirements resulting from the introduction of a large fraction of highly absorptive ash particles into the mix.(14) As long as percentages of ash introduced into a mix remain low (less than 20 percent), additional asphalt cement requirements should be reasonably low. Lower ash contents in surface mixes can also eliminate the need for antistripping agents such as hydrated lime.
No special accommodation is required for aggregate gradations and conventional hot mix gradations can be used; however, it is advisable to anticipate some measurable breakdown of ash particles during handling. This could increase the fine aggregate and silt fraction of the mix during the production process and open up new, uncoated absorptive particles resulting in unanticipated asphalt demand.
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating MSW combustor ash in the mix.
Material Handling and Storage
In some European countries, where MSW combustor bottom ash is used as a granular base material, the ash is stored from 30 days to 6 months prior to use.(15) This storage period provides time for potentially hydratable and expansive salts to react prior to the use of the material in a construction application. The use of combined ash that may contain free lime could further aggravate this problem. Storage of ash for at least 30 days in asphalt paving applications is probably adequate when using 10 to 20 percent ash in the mix. Longer periods of storage may be needed if higher percentages of ash are introduced into a mix.
Mixing, Placing, and Compacting
The same methods and equipment used for conventional asphalt production are applicable to paving mixes containing MSW combustor ash. During asphalt production, special care must be provided to control plant temperature, which could be impacted by the moisture content of the ash. This will probably necessitate a reduction in plant throughput rates. Excessive fines in the ash and high ash contents (greater than 20 percent) could result in excessive carry-over of fines into the baghouse. This will be more problematic in batch plants where the ash will most likely be dried by itself prior to storage in hot bins.
The same methods and equipment used for placing and compacting conventional pavement are applicable to asphalt pavements containing MSW combustor ash.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing MSW combustor ash. Mixes should be sampled in accordance with AASHTO T168,(16) and tested for specific gravity in accordance with ASTM D2726, (17) and in-place density in accordance with ASTM D2950.(18)
The use of municipal waste combustor ash in hot mix asphalt has yet to be commercialized on a large scale in the United States There are no specifications regarding minimum processing requirements, ash properties, or mix characteristics. In addition, ash contains levels of trace metals (particularly lead) and high soluble salt levels. Potential impacts need to be more fully addressed to ensure that the use of MSW combustor ash does not result in any adverse environmental impacts. Additional data regarding asphalt plant operations and air emission quality, when MSW combustor ash are introduced into a plant, is needed.