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Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction


STEEL SLAG User Guideline

Granular Base


Steel slag can be used as aggregate in granular base applications. It is considered by many specifying agencies to be a conventional aggregate and can normally exceed the aggregate requirements for granular aggregate base. The high bearing capacity of steel slag aggregates can be used advantageously on weak subgrades and in heavy traffic applications. Good interlock between steel slag aggregate particles provides good load transfer to weaker subgrades. Because of their similar particle shape and angle of internal friction, blast furnace slag aggregates have at times been blended with steel slag aggregates to improve yield, without substantial reduction in stability.



Experience in the United States, Belgium, Japan, The Netherlands, and Germany has shown that steel slag, properly selected, processed, aged, and tested, can be used as granular base for roads in above-grade applications. Steel slag aggregates exhibit a number of very favorable mechanical properties for use in granular base, including very high stability and good soundness. It is not widely used for granular base applications where lower quality (and less expansive) aggregates will often suffice. Only four state agencies (California, Indiana, Louisiana. and Michigan) are monitoring the use of steel slag aggregate in base course.(1)

Since volumetric instability of steel slag granular base (due to lime and dolime hydration reactions) has resulted in expansive reactions, steel slag aggregate granular base should not be utilized in confined applications, such as backfill behind structures, granular base, subbase confined by curb and gutter, and trenches.

In addition, the formation of tufalike precipitates (white, powdery precipitates formed by the chemical reaction of atmospheric carbon dioxide and free lime (CaO) in the steel slag) has resulted in deposits that have clogged subdrains and drain outlets.(2,3) The clogging of drainage paths creates water retention and soft pavement conditions. Frost action on the retained water can result in severe pavement distress.



Quality Control

Special quality-control procedures are needed during steel slag production (at the steel-making plant) and during aggregate processing to ensure that steelworks "rubbish" (furnace brick, wood, incompletely fused fragments, lime, rock, etc.) is not included as part of the steel slag aggregate.

In addition to control problems associated with volume instability and tufa precipitate formation, only suitable high-quality furnace slags that do not contain significant quantities of unreacted lime and dolime should be used. Belgium and The Netherlands limit the free lime content of steel slags used for granular base applications to 4.5 percent and require that the processed material be weathered at least 1 year to limit volume instability.(4)

Studies indicate tufa formation is likely to occur in highway subdrain applications if the original total lime content (CaO) of steel slags exceeds 1 percent.(5,6)

Although weathering is useful to control the volumetric instability of steel slags, it does not appear to prevent the formation of tufa precipitates.


Recent recommendations suggest that steel slag aggregates should be washed and should contain less than 3 percent by mass of nonslag constituents, less than 0.1 percent wood content, and have no detectable soft lime particles or lime-oxide agglomerations present.(7)

Crushing and Screening

Prior to use as a granular base material, ferrous components of the steel slag are magnetically separated. Steel slag must be crushed and screened to produce a suitable granular aggregate gradation using processing equipment similar to that for conventional aggregates.



Some of the important properties of steel slag that are of particular interest when steel slag is used as an aggregate in granular base include gradation, specific gravity, stability, durability, corrosivity, volumetric instability, drainage, and tufa formation.

Gradation: Steel slag can readily be processed to satisfy the AASHTO M 147(8) gradation requirements for granular aggregates.

Specific Gravity: Due to the relatively high specific gravity (3.2-3.6) of steel slag, steel slag aggregate can be expected to yield a higher density product compared with conventional mixes (2.5-2.7).

Stability: Steel slag aggregates have high angle of internal friction (40° to 45° ) that contribute to high stability and California Bearing Ratio (CBR) values up to 300 percent.

Durability: Steel slag aggregates display good durability with resistance to weathering and erosion.

Corrosivity: The pH value of the steel slag aggregate generally ranges from approximately 8 to 10; however, leachate from steel slag can exceed a pH value of 11. This can be corrosive to galvanized or aluminum pipes placed in direct contact with the slag.

Drainage Characteristics: Steel slag aggregates are free draining and are not susceptible to frost.

Volumetric Instability: Steel slag has a potentially expansive nature. Volume changes of up to 10 percent or more can occur during the hydration of calcium and magnesium oxides.

Tufa Formation: Drainage from steel slag aggregates can result in the formation of tufalike precipitates, which are powdery deposits that consist primarily of calcium carbonate (CaCO3). Such deposits have clogged drainage paths in pavement systems.(4)



Properly processed steel slag aggregates can readily satisfy gradation requirements and the physical requirements of AASHTO M147(8) and ASTM D2940.(9) It is recommended that steel slag be tested for expansion potential in accordance with ASTM D4792.(10)

Granular base containing steel slag should be designed so that it is well drained (no standing water) and adequately separated from water courses to prevent immersion. Pavement joints should be sealed to minimize the ingress of surface water into the steel slag granular base. These provisions are recommended to minimize the potential for leaching of free lime or dolime that may be present in these aggregates, causing tufa deposits.

Conventional AASHTO pavement structural design procedures can be employed for granular base containing steel slag aggregates.



Material Handling and Storage

The same general methods and equipment used to handle conventional aggregates are applicable for steel slag.

Stockpiles of processed steel slag aggregate, however, should be maintained in a wet condition prior to delivery to the job site. The period of aging in wet stockpiles should be established by process-control testing to satisfy deleterious components (petrographic examination and ASTM D4792 expansion testing) criteria. Until process-control testing indicates that the steel slag aggregates are suitable for use in granular base, it is recommended that additional aging and reprocessing be required.

Placing and Compacting

The same methods and equipment used to place and compact conventional aggregate can be used to place and compact steel slag. Care is required to avoid placing the material below grade and in locations where it is likely to be immersed in water (to avoid volumetric instability and tufa formation). A good groundwater drainage system is recommended when steel slag aggregate is used to allow free drainage and to prevent ponding within or against the steel slag.

Quality Control

The same field test procedures used for conventional aggregate are recommended for granular base applications when using steel slag. Standard laboratory and field test methods for compacted density are given by AASHTO T191,(11) T205,(12) T238,(13) and T239.(14)



There is a need to establish standard methods to assess the suitability of steel slag aggregate for granular base applications and to develop guidelines for the use of steel slag aggregates in this application. Improved testing methods are needed to establish the potential for tufa precipitate formation.



  1. Collins, R. J., and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, DC, 1994.

  2. Feldman, R. M. Tufa Precipitation and Its Effect on Drainage of Highways. Report, Kent State University, July 1981.

  3. Gupta, J. D., W. A. Kneller, R. Tamirisa, and E. Skrzypezak-Jankun. Characterization of Base and Subbase Iron and Steel Aggregates Causing Deposition of Calcareous Tufa in Drains, Not Dated.

  4. PIARC. Marginal Materials. State of the Art Report, Permanent International Association of Road Congresses, Paris, 1989.

  5. Kneller, W. A., J. Gupta, M. L. Borkowski, and D. Dollimore. "Determination of Original Free Lime Content of Weathered Iron and Steel Slags by Thermogravimetric Analysis," Transportation Research Record 1434, National Research Council, Washington, DC, 1994.

  6. Narita, K., T. Onoye, and Z. Takata. On the Weathering Mechanisms of LD Converted Slag (in Japanese). Koba Steel Ltd., Japan, 1978.

  7. Farrand, B. and J. Emery. "Recent Improvements in the Quality of Steel Slag Aggregate," Paper Prepared for Presentation at 1995 Annual Meeting of the Transportation Research Board, Washington, DC, January, 1995.

  8. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Aggregate and Soil-Aggregate Subbase, Base and Surface Courses," AASHTO Designation: M147-70 (1980), Part I Specifications, 14th Edition, 1986.

  9. American Society for Testing and Materials. Standard Specification D2940-92, "Graded Aggregate Material for Bases and Subbases for Highways or Airports," Annual Book of ASTM Standards, Volume 04.03, West Conshohocken, Pennsylvania, 1996.

  10. American Society for Testing and Materials. Standard Specification D4792-95, "Potential Expansion of Aggregates from Hydration Reactions," Annual Book of ASTM Standards, Volume 04.03, West Conshohocken, Pennsylvania, 1996.

  11. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.

  12. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.

  13. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.

  14. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.


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