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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Although there is little documented use of nonferrous slags as aggregate in embankments or fill, both air-cooled and granulated nonferrous slags are potentially useful for these applications. Nonferrous slag that is suitable for use as a granular base (copper, nickel, and phosphorus slags) will generally exceed specifications for embankment and fill construction. The high stability of nonferrous slag aggregates can be used advantageously to provide good load transfer to weaker subgrades.
Some of the desirable features of nonferrous slags for embankment construction include their high stability, good drainage characteristics, and negligible plasticity. Slags from the production of copper, nickel, lead and zinc have higher unit weight, resulting in lower yield and potentially increased long-term settlement if placed over compressible soils.
While limited toxicity testing data indicate that the leachate from specific copper and phosphorus slags are not hazardous(1,2) (as measured by EPA hazardous waste testing procedures), nonferrous slags produced from sulfide ores may contain leachable sulfur. If placed in poor drainage conditions and in extended contact with stagnant or slow-moving water, sulfur odor and water discoloration may result.
Copper slag (air-cooled and granulated) suitable for use as granular road base material has suitable engineering properties for use in embankments or fill applications.(3) Reverberatory copper slag (copper slag derived from reverberatory furnaces used for the smelting of copper concentrates)(4,5) is covered by conventional specifications for granular aggregate in Michigan.
Nickel slag (air-cooled and granulated) suitable for use as granular road base material has suitable engineering properties for use in embankment or fill applications.(6) Nickel slag is considered as a conventional granular aggregate and railway ballast in Ontario, Canada.
Due to galvanic corrosion concerns, steel pipelines, services and piles should be separated from nickel slag (for instance, using a 150 mm (6 in) thick layer of natural aggregate material).
Phosphorus slag has suitable engineering properties for use in embankment or fill applications, and large quantities of phosphorus slag have been used in Montana for aggregate in base courses.(6) The low unit weight of phosphorus slag aggregates results in higher yield (greater volume for the same weight), reduced dead load (leading to reduced long-term settlement of compressible soils), and reduced lateral pressures compared with conventional fill materials.
Lead, Lead-Zinc, and Zinc Slag
Although no documentation was found regarding the use of lead or zinc slags in embankments or fill in North America, these slags are considered to have suitable engineering properties for such use.(5) Lead-zinc slag from production of lead and zinc using the proprietary Imperial Smelting Process (ISP) has been used in Japan as fill in land reclamations and in the United Kingdom as granular base below grade for large buildings.(7)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
Other than crushing of large, air-cooled pieces, nonferrous slags require minimal processing to satisfy the physical requirements for use in embankments.
If necessary, the nonferrous slag aggregates can be blended with conventional embankment or fill materials (rock, soil, aggregates) to meet required gradation specifications.
Some of the engineering properties of nonferrous slags that are of particular interest when nonferrous slags are used in embankment or fill applications include gradation, unit weight, absorption, soundness, stability, drainage, and corrosivity.
Gradation: Reverberatory copper slag can be processed into coarse or fine aggregate material for use in embankment applications. Copper slag can readily satisfy the gradation and physical requirements of AASHTO M145.(11)
Unit Weight: Copper slag has a unit weight of 2800 to 3800 kg/m3 (175 to 237 lb/ft3).(8) The unit weight is somewhat higher than for conventional aggregates, resulting in increased density asphalt concrete (lower yield).
Absorption: Air-cooled copper slag absorption is typically very low (0.13 percent).(9) Granulated copper slag has a higher absorption than air-cooled slag.
Soundness: The excellent soundness exhibited by copper slag aggregate reflects good resistance to freeze-thaw exposure.(15)
Stability: The high angularity and friction angle (up to 53 )(3) of copper slag aggregates contribute to excellent stability and load bearing capacity.
Drainage Characteristics: Copper slag aggregates are also free draining and are not frost susceptible.(3)
Corrosivity: No data were identified to permit an evaluation of the potential corrosivity of copper slag.
Gradation: No specific data were identified for this application; however, no problems are anticipated in meeting the appropriate gradation requirements.
Unit Weight: The unit weight of crushed air-cooled nickel slag tends to be as high as 3500 kg/m3 (219 lb/ft3).(5) Granulated nickel slag is more vesicular, and has lower unit weight than air-cooled nickel slag.
Absorption: Air-cooled nickel slag has quite low absorption (0.37 percent).(5) Granulated nickel slag is more vesicular and has higher absorption than air-cooled nickel slag.
Soundness: Nickel slag aggregates display very good soundness (resisting freeze-thaw deterioration), are harder than conventional granular aggregates, and have good resistance to wear.(10)
Stability: The high angularity and friction angle (approximately 40 )(3) of nickel slag aggregates contribute to excellent stability and load bearing capacity.
Drainage Characteristics: Nickel slag aggregates are free draining and are not frost susceptible.(3)
Corrosivity: There is some evidence that nickel slag can contribute to the corrosion of ferrous metals in the presence of moisture, probably due to galvanic effects (differences in electrochemical potential between the nickel slag and iron or steel).
Gradation: Air-cooled phosphorus slag can readily satisfy the AASHTO M145(11) gradation and physical requirements for embankment aggregates.
Unit Weight: The unit weight of crushed air-cooled phosphorus slag ranges from 1360 to 1440 kg/m3 (85 to 90 lb/ft3). The unit weight of expanded phosphorus slag is about 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.
Soundness: Phosphorus slags exhibit excellent soundness, which corresponds to good resistance to freeze-thaw exposure.(13)
Stability: No data were available regarding the stability characteristics of phosphorus slag; however, a properly graded material should be capable of yielding a stable fill.
Drainage Characteristics: Phosphorus slag aggregates are generally nonplastic, free draining, and not frost susceptible.(3)
Corrosivity: No data were identified to permit an assessment of the potential corrosivity of phosphorus slag
Structural design procedures employed for embankments containing nonferrous slag are the same as design procedures for conventional embankment materials.
There are no standard specifications covering nonferrous slag use as embankment or fill. The designer may be required to satisfy moisture content criteria according to AASHTO T99(14) and implement appropriate material handling practices to avoid segregation and breakage.
Material Handling and Storage
The same methods and equipment used to store or stockpile conventional aggregates are applicable for nonferrous slag.
Precautions may be required to ensure that stockpiles containing nonferrous slag materials are sufficiently separated from watercourses to prevent leachate contamination. The material should be placed in a manner that allows free drainage and prevents ponding within or against the material.
Placing and Compacting
Due to their high angularity, additional effort (for instance, using vibratory rollers) may be required to compact copper, nickel, and phosphorus slags to their maximum densities.
The same test procedures used for conventional aggregate are appropriate for nonferrous slag. Standard laboratory and field tests for compacted density and field measurement of compaction are given by AASHTO T191,(T 191-86, Part II Tests, 14th Edition, 1986.) T205,(16) T238,(17) and T239.(18)
The main unresolved issue pertaining to the use of nonferrous slags in embankment construction is that of environmental suitability. The material for each application must be assessed for leachate toxicity. Phosphorus slag must also be assessed for potential radioactivity.
Further, standard methods and clear guidelines to assess the suitability of nonferrous slags that may be in contact with groundwater or watercourses should be established. The potential corrosion risk to buried utilities within nickel as well as other nonferrous slag fills should be evaluated.