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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Both waste rock and mill tailings have physical properties that are suitable, in most cases, for use in asphalt paving applications. Coarse coal refuse is generally unsuitable for such use.
Waste rocks derived from most ore processing sources can be considered for use in asphalt paving applications provided they satisfy conventional asphalt paving aggregate requirements. Waste rock should not contain deleterious components and must not be commingled with unsuitable materials. Waste rock from iron ore processing is usually either trap rock or granite, which makes an excellent source of aggregate.
Mill tailings have successfully been used as aggregate in asphalt paving applications. Generally, the coarser, sand-size fractions of mill tailings can also be considered for use as coarse aggregates provided there are no harmful or reactive chemical components concentrated from the host rock and the tailings can satisfy the conventional paving aggregate requirements. Despite the fine size of most mill tailings, these materials can be blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range. It is also possible to separate the coarser fraction of tailings by classifying prior to disposal. Depending on the source of the mill tailings, there may be some concern with trace metals remaining after ore processing that could potentially leach from fine-grained tailings, which have a high surface area. (1)
Over the years, there have been numerous examples of the use of mineral processing wastes, notably waste rock and mill tailings, in asphalt paving applications. The current use of mineral processing wastes as aggregate in hot mix asphalt is not a common practice, due in great part to the relatively remote location of many sources of these wastes. Although examples of mineral processing waste utilization are not well documented, it is known that waste rock has been used as coarse aggregate in asphalt paving in at least 8 states and that mill tailings have been used in asphalt paving in at least 13 states.(1) There are also two states (Kentucky and Pennsylvania) where coal refuse has been used to a limited extent in asphalt paving. Table 9-5 is a summary of the known U.S. mining and mineral processing wastes in asphalt paving in at least 19 different states. A few state agencies have been involved in recent research or field trials, but only five states (Kansas, Missouri, Nevada, New York, and Oklahoma) report any ongoing or upcoming research on the use of mineral processing wastes as aggregate or mineral filler in hot mix asphalt.(2)
Table 9-5. Summary of the use of mining and mineral processing wastes in asphalt paving mixtures in the United States.
Colorado and South Dakota have used crushed rock waste from gold mining operations in road construction, including asphalt paving. Lead waste rock has been used in bituminous mixtures in Missouri. Lead-zinc waste rock has been used for resurfacing by local and county agencies in Washington. Trap rock from iron ore processing has been crushed to meet standard specification requirements for hot mix aggregate in New Jersey and Pennsylvania.
In Missouri and Illinois, iron waste rock has been used as a skid-resistant aggregate for asphalt paving. In New Mexico, waste rock from molybdenum mining operations has been used as aggregate in asphalt paving with satisfactory performance.(3) Most waste rock is generated in the western United States, particularly in copper mining areas such as Arizona and Utah.
In Minnesota, taconite tailings improved the frictional resistance of asphalt overlays, and, on this basis, this material is still used in the northern part of the state for hot mix resurfacing. In New Mexico, the coarse tailings from molybdenum mining operations have been used as aggregate in asphalt paving with satisfactory performance.(3) At least 12 states, including Alabama, California, Illinois, Kansas, Minnesota, Missouri, Nevada, New Jersey, New Mexico, New York, Oklahoma, and Wisconsin, have used or continue to use mill tailings for asphalt paving applications. Utah reportedly has used the fines from copper mining operations as a mineral filler in asphalt.(2)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
Many sources of waste rock are geologically similar to natural sources of construction aggregate, and, therefore, can be crushed and/or screened using conventional aggregate processing equipment. Waste rock from iron ore or taconite processing may be heavier than conventional aggregate.
Mill tailings may have to be dried to reduce the moisture content, or may require selective screening and dewatering prior to being introduced into a hot mix asphalt plant. When reclaimed from a tailings pond, stockpiling and air drying for a period of time may be sufficient to reduce the moisture content of some tailings by evaporation, especially in arid areas.
Crushing and Screening
Screening and/or crushing may be required in some cases to produce a suitable aggregate like product from mill tailings or to meet gradation specifications. Crushing is not normally required, with the possible exception of some coarse tailings that may require size reduction of oversize particles. Some fine-sized tailings, such as copper mill tailings, can be classified prior to disposal in order to separate the coarser fraction of the tailings for subsequent reuse.
Some of the properties of waste rock that are of interest when used in asphalt paving applications include gradation, shape and texture, specific gravity, shear strength, and abrasion resistance.
Gradation: Waste rock is often homogeneous but can vary widely in size from boulders down to gravel, due to variations in ore formation and different mining techniques. In general, most sources of waste rock can be reduced to a desired gradation by normal crushing and sizing methods.
Shape and Texture: Waste rock is coarse, hard, and angular in shape and can vary in size from large boulders or blocks down to gravel.
Specific Gravity: The average specific gravity of waste rock is about 2.65, with a range from 2.4 to 3.6 depending on the nature of the mineral constituents. Specific gravity may be used to determine other important properties such as void ratio, porosity, and degree of saturation.(4)
Shear Strength: Typical values for the angle of internal friction for waste rock materials often exceed 35 degrees and contribute to high bearing capacity and stability.
Abrasion Resistance: Most sources of waste rock are able to satisfy abrasion loss requirements. Waste rocks from the processing of iron ore or taconite are usually quite dense and often have relatively low abrasion loss values.
Some of the properties of mill tailings that are of interest when mill tailings are used in asphalt paving applications include gradation, shape and texture, specific gravity, absorption, unit weight, and stripping resistance.
Gradation: Mill tailings are usually very fine-graded, cohesionless materials. They consist of hard, angular siliceous particles with a high percentage of fines. Typically, mill tailings range from sand to silt-clay particle size with 40 to 90 percent passing a 0.075 µm (No. 200) sieve. They are disposed of in slurry form by pumping into large ponds.(5)
Shape and Texture: Mill tailings are uniform in particle shape and texture. Mill tailings typically consist of hard, angular, siliceous particles with a high percentage of fines.
Specific Gravity: The specific gravity of tailings ranges from about 2.0 to 3.5, depending on the mineralogical composition.
Absorption: Water absorption values for lead, zinc, copper and iron ore tailings are typically higher than the standard maximum limit of 1.0 percent for fine aggregate in asphalt paving mixes.(6,7)
Unit Weight: Iron ore tailings and taconite tailings from northern Minnesota have high unit weight values, up to as high as 2750 kg/m3 (170 lb/ft3). The dry rodded weight of most other tailings sources is expected to range from 1450 kg/m3 (90 lb/ft3) to 2200 kg/m3 (135 lb/ft3), which is comparable to that of most natural aggregates, which are approximately 2000 kg/m3 (125 lb/ft3) to 2300 kg/m3 (140 lb/ft3).(5) Stripping Resistance: Iron ore and taconite tailings do not appear to be susceptible to stripping. Mill tailings from other sources should be evaluated for stripping potential as part of the normal asphalt paving mix design procedures.
Waste rock for use in hot mix asphalt must comply with the requirements for coarse aggregate in bituminous mixtures.(8) Asphalt mixes containing waste rock can be designed using standard laboratory procedures.
The potential for stripping of asphalt mixes containing waste rock should be assessed in the laboratory as part of the overall hot mix asphalt mix design. Stripping resistance can be enhanced by adding hydrated lime or a proprietary antistripping additive.
There are no standard specifications for the use of mill tailings in hot mix asphalt paving. There are, however, a few states that have historically used different types of mill tailings as a fine aggregate, a mineral filler, or in some cases, as a coarse aggregate in asphalt paving mixes. Tailings should meet the appropriate specification requirements for their intended use, either as a source of fine aggregate(9) or mineral filler.(10) Asphalt mixes containing mill tailings can be designed using standard laboratory procedures. The potential for stripping of asphalt mixes containing mill tailings should also be assessed in the laboratory as part of the overall design.
Conventional AASHTO pavement structural design methods(11) are appropriate for the thickness design of asphalt paving mixtures incorporating waste rock as the coarse aggregate.
Conventional AASHTO pavement structural design methods(11) are also appropriate for the thickness design of asphalt paving mixtures incorporating mill tailings as the fine aggregate, mineral filler, or coarse aggregate.
Material Handling and Storage
The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste rock and mill tailings. However, users of those materials should be aware that such materials usually have an acid potential and that leaching may occur during stockpiling or heating in the asphalt plants.
Placing and Compacting
The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing waste rock or mill tailings. Compaction operations should be visually inspected on a continuous basis to ensure that the specified degree of compaction can be achieved and there is no movement under the action of compaction equipment.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing mineral processing wastes. Mixes should be sampled in accordance with AASHTO T168,(12) and tested for specific gravity in accordance with ASTM D2726,(13) and in-place density in accordance with ASTM D2950.(14)
There is a need to establish general environmental criteria for the selection of mining or mineral processing by-products to be used in paving applications. More knowledge is needed concerning the variation in mineral processing operations that can alter the quality of such by-products.
More specifically, there is a need to investigate and analyze the environmental impact of some waste rock and mill tailings sources that may contain inorganic metal and sulfide-based metallic ore constituents to assess the level of leachability, if any, when used in hot mix asphalt. Mill tailings may contain concentrations of certain inorganic metal constituents that may be leachable. Some waste rock and tailings have been leached with cyanide as a means of further ore extraction. Certain sources of taconite are known to contain asbestiform fibers. Uranium mill tailings can be a source of residual radiation, and phosphate rock can be a source of low-level radiation resulting from radon gas.