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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Sewage sludge ash can be used as a mineral filler substitute or as a portion of the fine aggregate in hot mix asphalt paving. The introduction of sewage sludge ash at levels of approximately 2 to 5 percent by weight of aggregate has been shown to produce mix design properties that are comparable to those of mixes containing conventional fillers such as hydrated lime and stone dust.
During the past 15 years a number of laboratory investigations have been undertaken, and field demonstration pavements have been constructed, using sludge ash that has been incorporated into hot mix asphalt as either a mineral filler or as fine aggregate substitute. Although data from these bench scale tests and field installations suggest that asphalt mixes containing sludge ash can readily be prepared and should perform in an acceptable manner, there is no widespread commercial use of this material in the United States at this time.
The former Metropolitan Waste Control Commission (MWCC) of Minneapolis, Minnesota, (presently the Metropolitan Council of Environmental Services) undertook the most extensive investigations on the use of sludge ash in hot mix asphalt production. Table 17-3 lists a number of field demonstrations undertaken in Minnesota. The results of these demonstrations reportedly revealed no visible difference between the pavement sections containing sludge ash and adjacent sections containing conventional materials.(1)
Table 17-3. Listing of Minnesota test pavements.
In 1993, the Suffolk County, New York, Department of Public Works constructed test pads containing sludge ash that was introduced into a New York State Department of Transportation surface mix as a sand substitute.(2) The prepared mixes contained a total of 5.5 percent sludge ash by weight, which accounted for a replacement of 15 percent of the sand or fine aggregate fraction of the mix. Testing of the test pad focused on the surficial texture (skid resistance and surface runoff properties of the ash mixes). Control mix test patches (without ash) were also constructed, and test results of the control mix test patches were compared to the sludge amended mixes. The results provided data that indicated that the presence of ash had no significant effect on the skid resistance of the pavement and was comparable to the control mix.
Previous testing and field demonstration data suggest that sludge ash can be introduced as either a mineral filler or as both a fine aggregate substitute and a mineral filler into an asphalt concrete mix.
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
The specification requirements for mineral filler covered in AASHTO M17-83,(3) and listed in Table 17-4, require that 100 percent of the mineral filler must pass the 0.6 mm (No. 30) sieve. To comply with this specification, some sludge ash sources may require crushing and/or screening to remove oversize particles. If used as a combination mineral filler and fine aggregate substitute, no processing will generally be required.
Table 17-4. AASHTO M17-83 specification requirements for Mineral filler for use in bituminous paving mixtures
There are several plants located in Japan that vitrify sewage sludge ash. A recently closed facility operated for several years in Monticello, New York, producing a frit-like aggregate that was suitable for use as a fine aggregate substitute material,(4) and was approved for such use by the New York State Department of Transportation. Vitrification produces a fine aggregate material (not a mineral filler), which permits the introduction of higher percentages of sludge ash into a mix.
Some of the properties of sludge ash that are of particular interest when sludge ash is used in asphalt concrete include particle size, plasticity, and organic content.
Particle Size: Depending on the source of ash, some sludge ash may have a significant fraction of particles greater than 0.6 mm (No. 30 sieve) in size. If this is the case, then sludge ash will not meet the requirements of AASHTO M17 for mineral filler and may have to be processed (crushed and screened) or introduced into the mix as a combination mineral filler and fine aggregate. Sludge ash particles greater than 0.6 mm (No. 30 sieve) in size are expected to comply with gradation and soundness requirements for fine aggregate material outlined in AASHTO M29.(5)
Plasticity: Sludge ash is a nonplastic material and meets the plasticity requirements for mineral filler or fine aggregate outlined in AASHTO M17(3) and AASHTO M29,(5) respectively.
Organic Impurities: Sludge ash can be expected to contain some small percentage (generally less than 2 percent) of organic material, depending on the efficiency of combustion.(6) This small organic fraction does not appear to affect the performance of sludge ash as a mineral filler.
The properties of an asphalt paving mix containing sludge ash that are of particular interest include stability, mix density, air voids, asphalt demand, durability, and asphalt cement viscosity.
Stability: The addition of sludge ash in paving mixes up to approximately 5 to 6 percent by weight of aggregate reportedly increases the stability of the mix.(2,3)
Mix Density: The addition of sludge ash can be expected to decrease the density of the mix.(2,3)
Air Voids and Asphalt Demand: An increase in sludge ash concentration can be expected to result in an increase in air voids and a corresponding increase in the asphalt cement demand of the mix.(2,3)
Durability: Mix durability (measured in the laboratory) may be slightly improved by the addition of sludge ash.(2,3)
Viscosity: The addition of sludge ash to an asphalt cement reduces the ductility of the binder and its penetration values and increases the corresponding viscosity of the binder, producing a high consistency binder.(2)
Although sludge ash can essentially meet the requirements of a mineral filler, as outlined in AASHTO M17, it is likely that when introduced into a paving mix, some of the larger sludge ash particles act as a mineral aggregate material (filling the interstices between coarser aggregate materials). At the same time the silt-sized particles act as an asphalt modifier, forming a high-consistency asphalt grade binder. A high-consistency asphalt grade binder exhibits relatively high viscosity and hardness and low ductility, which could increase the susceptibility of the pavement to thermal cracking.
Asphalt mixes containing sludge ash can be designed using standard laboratory procedures.
It is recommended that sludge ash in paving mixes be limited to less than 5 percent by weight of aggregate (3 percent for wearing courses). Higher percentages of sludge ash appear to result in excessively high stabilities.(2)
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating sludge ash as a mineral filler or fine aggregate.
Material Handling and Storage
Sludge ash is a porous, lightweight material that must be introduced into a mineral filler storage silo pneumatically. Due to its lightweight nature, additional silo storage capacity may be required. Wet sludge ash that has been quenched as part of the sludge incineration process cannot be managed in this manner.
Mixing, Placing and Compacting
Sludge ash may be introduced into paving mixes by means of its own hopper or through a mineral filler silo. The same methods and equipment used for mixing, placing, and compacting conventional pavements are applicable to asphalt pavements containing sewage sludge ash.
A quality control program should be adopted when sludge ash is being used in order to monitor the quality (particularly the size distribution) of the source material. The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing sewage sludge ash. Mixes should be sampled in accordance with AASHTO T168,(7) and tested for specific gravity in accordance with ASTM D2726(8) and in-place density in accordance with ASTM D2950.(9)
There has been some concern expressed regarding the presence of trace metals in sludge ash. Recent studies in Minnesota(10,11) indicate that leaching does not appear to be a problem, which would seem apparent due to the small percentages of sludge ash actually incorporated into a mix (less than 5 percent). Nonetheless, environmental criteria are needed to establish acceptable sludge ash trace metal content criteria. Additional data are also needed to define specifications regarding acceptable loss on ignition or organic impurity data.