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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
The use of copper, nickel and phosphorus slag aggregates as granular base has occurred primarily in rural areas close to the remote locations where these slags are produced. No North American use of lead or zinc slags as granular base has been confirmed. The unit weights of copper and nickel slags tend to be greater than those of conventional aggregates, with a corresponding lower yield and increased transportation and placement costs. The lower unit weight of phosphorus slag aggregates compared to conventional aggregates results in somewhat higher yield (greater volume for the same weight).
Copper and nickel slags have been used for many years as granular base in mining roads,(1) where they have demonstrated satisfactory performance in what are generally considered to be very severe traffic and operating conditions. In Michigan, reverberatory copper slag is considered to be a conventional aggregate and is covered by state specifications for granular base. Similarly, nickel slag is considered a conventional granular aggregate in Ontario, Canada, and is used extensively as road base in areas near where it is produced. Phosphorus slag has also been used in large quantities in Montana for aggregate in base courses.(2)
Some of the desirable features of copper, nickel and phosphorus slags in granular base applications include high stability and good drainage characteristics (3,4) as well as good resistance to freeze-thaw exposure and mechanical degradation.(4,5,6)
While limited toxicity testing data indicate that the leachate from specific copper and phosphorus slags are not hazardous(3,7) (as measured by USEPA hazardous waste testing methods), 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. Due to concerns regarding the leachability of heavy metals, most lead, lead-zinc and zinc slags are generally considered to be unsuitable for use in granular base. However, there has reportedly been use of specially processed lead and zinc slag aggregates as bulk fill in land reclamations in Japan and granular base for floor slabs in buildings in the United Kingdom.(8)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
Copper, nickel and phosphorus slags must be crushed and screened to produce a granular base aggregate. This can readily be accomplished using conventional crushing and screening plant and equipment.
Crushing of air-cooled nickel slag produces a low quantity of finer particles, and consequently blending with additional crushed fine material may be necessary to satisfy gradation requirements.(1,5) Other nonferrous slag aggregates may also require blending with conventional granular aggregates to optimize aggregate properties. Crushed fines, having high angularity, should be used (rather than natural sand) to "lock-up" the smooth, hard nickel slag aggregates.
Some of the engineering properties of nonferrous slag aggregates that are of particular interest when nonferrous slags are used in granular base applications include gradation, specific gravity, durability, stability, and drainage characteristics.
Copper Slag Aggregates
Gradation: Copper slags can be crushed and screened to satisfy the AASHTO M147(9) gradation requirements for granular aggregates.
Specific Gravity: With specific gravities ranging from 2.8 to 3.8, copper slag aggregates are decidedly heavier than conventional granular material.(1)
Durability: Copper slag aggregates display very good soundness (resisting freeze-thaw deterioration), are harder than conventional granular aggregates and have good resistance to wear.
Stability: The high angularity and friction angle (up to 53°) of copper slag aggregates contribute to excellent stability and load bearing capacity.(3)
Drainage Characteristics: Copper slag aggregates tend to be free draining and are not frost susceptible.(3)
Nickel Slag Aggregates
Gradation: Nickel slags can be crushed and screened to satisfy the AASHTO M147(10) gradation requirements for granular aggregates.
Specific Gravity: Like copper slag, nickel slag aggregates are substantially heavier than conventional granular aggregates (specific gravity to 3.5).(1)
Durability: Nickel slag aggregates exhibit higher soundness, hardness and abrasion resistance properties than conventional aggregates.(4)
Stability: Nickel slag granular base aggregates exhibit good stability and high bearing capacity due to their angular shape and high angle of internal friction.(5)
Drainage Characteristics: Nickel slag aggregates are free draining and non-frost susceptible.
Phosphorus Slag Aggregates
Gradation: Phosphorus slags can be crushed and screened to satisfy the AASHTO M147(11)gradation requirements for granular aggregates.
Specific Gravity: Crushed air-cooled phosphorus slag aggregates are somewhat lighter than conventional granular aggregates.(12)
Durability: Phosphorus slag aggregates exhibit very good soundness (high resistance to freeze-thaw deterioration) and good resistance to mechanical degradation.(6)
Stability: Due to their sharp, angular shape, phosphorus slag aggregates demonstrate good stability.
Drainage Characteristics: Phosphorus slag aggregates have good drainage characteristics.
Properly processed copper, nickel and phosphorus slag aggregates can readily satisfy the gradation and physical requirements of AASHTO M147(9) and ASTM D2940(13)
The high stability of properly graded, crushed, nonferrous slag aggregates provides good load transfer to a weaker subgrade. Due to the low fines generated by crushing nickel slags, it is often necessary to supplement nickel slag aggregates with suitable fine aggregate material. Standard AASHTO pavement structural design procedures can be employed for granular base containing nonferrous slag aggregates. The appropriate structural number for nonferrous slag aggregates should be established by resilient modulus testing.
Material Handling and Storage
The same equipment and procedures used to stockpile and handle conventional aggregates can be used for nonferrous slag aggregates.
Due to their high angularity, greater care should be taken when stockpiling and handling nonferrous aggregates to avoid segregation. Precautions may be required to ensure that nonferrous slag aggregate stockpiles are sufficiently separated from watercourses to prevent leachate contamination.
Mixing, Placing and Compacting
Nonferrous slag aggregates can be difficult to compact and may require additional effort (for instance vibratory rollers) to achieve adequate compaction.(4,5)
The same test procedures used for conventional aggregate are appropriate for granular base applications when using nonferrous slag. Standard laboratory and field tests for compacted density and field measurement of compaction are given by AASHTO test methods T191,(14) T205,(15) T238(16) and T239.(17)
The most pressing issue that needs to be resolved is the environmental suitability of nonferrous slags for granular base applications. Materials from each source must be assessed for heavy metals content and leachability. Phosphorus slag radioactivity concerns should also be investigated.
Further, there is a need to establish standard methods and clear guidelines to assess the suitability of nonferrous slags that may be in contact with groundwater or watercourses.