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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
A variety of nonferrous slags (air-cooled or granulated) including phosphorus, copper, nickel, and zinc slags, can be used as coarse and/or fine aggregate in hot mix asphalt pavements. Processed air-cooled and granulated copper, nickel, and phosphorus slags have a number of favorable mechanical properties for use as hot mix aggregate, including good soundness characteristics, abrasion resistance, and stability (high friction angle due to sharp, angular shape). However, some nonferrous slags are vitreous or "glassy," which can adversely affect their frictional resistance properties. Some glassy nonferrous slags may also be susceptible to moisture-related damage (stripping)
Nonferrous slags tend to be produced in few, generally remote geographic locations situated some distance from potential urban asphalt paving markets. As a result, nonferrous slags are not well utilized.
Phosphorus slag aggregates have been used in dense graded hot mix surface course mixes for asphalt concrete pavements in Tennessee, particularly where high wet skid resistance was desired. Some use of air-cooled phosphorus slag aggregate in hot mix asphalt has also occurred in Montana (as fine aggregate) and in Tennessee and Florida. The use of phosphorus slag is addressed within conventional materials specifications in Tennessee and Florida for both coarse and fine hot mix asphalt aggregates.
Pavements incorporating phosphorus slag aggregate that have been properly selected, processed, and tested for specification compliance are reported to have demonstrated very satisfactory performance.(1) However, the use of unsuitable (glassy) phosphorus slag in open graded asphalt concrete pavements can result in inadequate frictional resistance properties.(1,2,3)
There has been limited use of copper slag aggregates in hot mix asphalt pavements. Copper oxide blasting grit (fine copper slag) has reportedly been used in hot mix asphalt pavements in California, and granulated copper slag has reportedly been incorporated into asphalt mixes in Georgia to improve stability. Although it is rarely used, Michigan Department of Transportation specifications consider reverberatory copper slag to be a conventional coarse and fine aggregate for hot mix asphalt pavements.(4)
Very little documentation is available regarding the use of nickel slag in asphalt pavements in North America. Road trials in Ontario, Canada, incorporating air-cooled nickel slag in asphalt mixes are reported to have exhibited poor frictional resistance,(5) which was attributed to the glassy, smooth texture of the nickel slag aggregate. However, the more vesicular and porous granulated nickel slag produced in Japan has been successfully used as a skid resistant aggregate in surface course hot mix asphalt pavements.(6) Nickel slag aggregates are currently being used in hot mix paving asphalt mixes for the construction of highway pavements in the Dominican Republic.(7)
Although studies performed in Oklahoma for four types of zinc smelter wastes (fine slags) indicate that they are suitable for use as fine aggregate in hot mix asphalt concrete,(8) no North American documentation was identified regarding the use of zinc or lead slags in asphalt paving. A road trial completed in the United Kingdom using lead-zinc slag as the fine aggregate in asphalt concrete, in conjunction with limestone coarse aggregate, indicated good wearing properties but only moderate frictional resistance.(9)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
Air-cooled nonferrous slags to be used as aggregate in asphalt must be crushed and screened to the desired gradation (coarse or fine aggregate). This can be undertaken using conventional aggregate processing plant equipment.
Because granulated nonferrous slags tend to be uniform in size, they may require blending with other suitable materials to satisfy aggregate gradation requirements for asphalt mixtures.
Some of the properties of nonferrous slags that are of particular interest when nonferrous slags are used as an aggregate in asphalt paving applications include particle shape and texture, gradation, unit weight, absorption, stability characteristics, wear resistance, frictional properties, adhesion, and resistance to freezing and thawing. Specific physical, chemical, and mineralogical properties of nonferrous slags depend in great part on the type of slag, method of production, type of furnace, and cooling procedures associated with their respective production processes. Consequently, each nonferrous slag aggregate must be considered by mineralogical type on a source-specific and cooling (air-cooled or granulated) basis.
Shape and Texture: Air-cooled phosphorus slag aggregates are black to dark gray in color and are generally flat and elongated. Individual particles tend to be vitreous (glassy) with sharp fracture faces similar to broken glass (irregular shape). Granulated phosphorus slag is made up of regularly shaped, angular particles.
Gradation: Air-cooled phosphorus slag can be processed into coarse or fine aggregate for hot mix asphalt to satisfy ASTM D692(10) and AASHTO M29(11) requirements. Granulated phosphorus slag can be used (after blending with other suitable material to satisfy AASHTO M29 gradation requirements) as a fine aggregate material.
Unit Weight: Crushed air-cooled phosphorus slag has a unit weight ranging from 1360 to 1440 kg/m3 (85 to 90 lb/ft3). Expanded phosphorus slag has unit weight of 880 to 1000 kg/m3 (55 to 62 lb/ft3).(12) Granulated phosphorus slag is more vesicular than air-cooled slag and consequently has lower unit weight.
Absorption: The absorption of air-cooled phosphorus slag is about 1.0 to 1.5 percent.(13) Both expanded and granulated phosphorus slags have higher absorption than air-cooled slag due to their more vesicular nature.
Stability Characteristics: No data on stability characteristics of phosphorus slag were identified, but processing and blending with other suitable material should be capable of yielding a stable material.
Wear Resistance: The high abrasion resistance of phosphorus slag can be expected to correspond to good wear resistance.
Frictional Properties: The favorable frictional resistance properties and abrasion resistance of crystalline, air-cooled phosphorus slag can be expected to contribute to good wet skid resistance in asphalt pavements.(1)
Adhesion: Good adhesion to asphalt cement is facilitated by the moderate absorption (1.0 to 1.5 percent) of air-cooled phosphorus slag aggregates.
Soundness: Phosphorus slags exhibit excellent soundness, which corresponds to good resistance to freeze-thaw exposure.(13)
Shape and Texture: Air-cooled copper slag aggregates are black in color, and typically have a glassy appearance. Granulated copper slag aggregates are similar to air-cooled copper slag aggregates but more vesicular.
Gradation: Reverberatory copper slag can be processed into coarse or fine aggregate material for use in hot mix asphalt. It should be crushed and screened to produce aggregate that satisfies the gradation requirements for hot mix asphalt including ASTM D692(10) and AASHTO M29.(11) Granulated copper slag can be blended with other suitable material (to satisfy gradation requirements for AASHTO M29) as a fine aggregate for asphalt mixtures.
Unit Weight: Crushed air-cooled copper slag has a unit weight of 2800 to 3800 kg/m(3) (175 to 237 lb/ft(3)).(14) The unit weight is somewhat higher than for conventional aggregates, resulting in increased density asphalt concrete (lower yield). Granulated copper slag is more vesicular and therefore has a lower unit weight than air-cooled slag.
Absorption: Air-cooled copper slag absorption is typically very low (0.13 percent).(15) Granulated copper slag has a higher absorption than air-cooled slag.
Stability Characteristics: The high angularity and friction angle (up to 53°)(16) of copper slag aggregates contribute to excellent stability and load bearing capacity. In Georgia, granulated slag is reportedly added to hot mix asphalt mixes in conjunction with limestone aggregate to increase stability and reduce Marshall flow.(17)
Wear Resistance: The superior hardness and abrasion resistance of copper slag aggregates compared with most conventional aggregates contribute to good wear resistance.(8,15)
Frictional Properties: No specific data were identified.
Adhesion: No specific data were identified, but low absorption values and the glassy nature of copper slag suggest that stripping might be a concern. Soundness: The excellent soundness exhibited by copper slag aggregate reflects good resistance to freeze-thaw exposure.(15)
Shape and Texture: Air-cooled nickel slag aggregates are reddish brown to brown-black in color. It can be crushed to angular particles but has a massive, angular, smooth, amorphous texture. Granulated nickel slag is essentially an angular, black, glassy slag "sand."
Gradation: No specific data were identified, but no problems are anticipated in producing the appropriate gradation.
Unit Weight: The unit weight of crushed air-cooled nickel slag tends to be as high as 3500 kg/m(3) (219 lb/ft(3) ).(18) Granulated nickel slag is more vesicular and has a lower unit weight than air-cooled nickel slag.
Absorption: Air-cooled nickel slag has quite low absorption (0.37 percent).(18) Granulated nickel slag is more vesicular, and has higher absorption than air-cooled nickel slag.
Stability Characteristics: The high angularity and friction angle (approximately 40°)(16) of nickel slag aggregates contribute to excellent stability and load bearing capacity.
Wear Resistance: No data were identified, but the high hardness and good soundness properties suggest favorable wear resistance.
Frictional Properties: No data were identified, but the high angularity and potential wear resistance could be expected to result in favorable frictional properties.
Adhesion: No specific data were identified.
Soundness: Nickel slag aggregates display very good soundness (resisting freeze-thaw deterioration), are harder than conventional granular aggregates and have good resistance to wear.(8)
Lead, Lead-Zinc and Zinc Slags
Shape and Texture: Lead, lead-zinc, and zinc slags are black to red in color and have glassy, sharp, angular (cubical) particles.
Gradation: No specific data were identified.
Unit Weight: The unit weight of granulated lead, lead-zinc, and zinc slags can vary from less than 2500 kg/m(3) to as high as 3600 kg/m(3) (156 to 225 lb/ft3).(8, 14)
Absorption: Granulated lead, lead-zinc, and zinc slags tend to be porous, with absorptions up to about 5 percent.(12)
Stability Characteristics: Although no specific data were identified, it is anticipated that these slags would produce acceptable stability characteristics.
Wear Resistance: Although no specific data were identified, it is anticipated that these slags would produce acceptable wear resistance characteristics.
Frictional Properties: Although no specific data were identified, it is anticipated that these slags would produce acceptable frictional properties.
Adhesion: Although no specific data were identified, it is anticipated that these slags would produce acceptable adhesion characteristics.
Soundness: Although no specific data were identified, it is anticipated that these slags would exhibit adequate soundness properties.
Conventional asphalt mix design methods (e.g., Marshall, Hveem, SHRP) are applicable for the design of hot mix asphalt containing nonferrous slag (particularly air-cooled phosphorus and reverberatory copper slag) aggregates. No special procedures are required for aggregate gradations. Both coarse and fine nonferrous slag aggregates can be incorporated in hot mix asphalt, provided that the physical requirements of ASTM D692(10) and/or AASHTO M29(11) are satisfied. No special provisions are required for nonferrous slag, and conventional hot mix gradations specifications may be used. Blending with other suitable hot mix asphalt aggregates may be necessary to achieve gradation specifications compliance. Due to the difference in unit weights, mix designs are usually calculated on a volumetric basis.
Some glassy nonferrous slags may be susceptible to moisture-related damage (stripping) and therefore, the mix design should include a stripping resistance or retained stability test (AASHTO T283(19) and MTO LS-283(20) ) and the addition of hydrated lime or other anti-stripping agents may be warranted. Due to their glassy nature, some air-cooled nonferrous slags may exhibit poor frictional properties.
Conventional AASHTO pavement structure design methods are appropriate for asphalt paving incorporating nonferrous slag aggregates.
The same production methods and equipment used for conventional hot mix asphalt can be used for production of hot mix asphalt containing nonferrous slag.
Placing and Compaction
The same equipment and construction procedures used for conventional hot mix asphalt aggregate can be used for hot mix asphalt paving mixtures incorporating nonferrous slag aggregates.
Standard and field and laboratory tests for compacted bituminous mixes are given by AASHTO T168,(21) T166,(22) and ASTM D2950.(23) ASTM D4792(24) should be considered for nonferrous slags where significant quantities of hydratable oxides may be present.
There is a need to investigate the poor frictional resistance properties associated with the use of air-cooled nickel slag and lead-zinc, slags which limit their use as aggregates in asphalt pavements. Particular attention is required to avoid using glassy aggregates as they impart poor frictional resistance properties to asphalt concrete pavements.
Also, there is a need to investigate the potential use of nonferrous slag in surface treatments and cold mixes. Very little documentation data are available regarding their use in this application.
Finally, there is little documentation regarding the engineering properties and performance/ serviceability of lead, lead-zinc, and zinc slags.
Environmental research, including chemical and leaching analyses and environmental assessments, is required to evaluate potential environmental issues associated with the use of these slag materials in asphalt paving applications.