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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Steel slag can be processed into a coarse or fine aggregate material for use in dense- and open-graded hot mix asphalt concrete pavements,(1,2,3) and in cold mix or surface treatment applications.(4) Proper processing of steel slag and special quality-control procedures are extremely important in selecting steel slag for use in asphalt paving mixes. Of particular importance is the potential for expansion because of free lime or magnesia in the slag, which could result in pavement cracking if ignored. Steel slag use in paving mixes should be limited to replacement of either the fine or coarse aggregate fraction, but not both, because hot mix asphalt containing 100 percent steel slag is susceptible to high void space and bulking problems due to the angular shape of steel slag aggregate. Mixes with high void space (100 percent steel slag aggregate mixes) are susceptible to over-asphalting during production and subsequent flushing due to in-service traffic compaction.
Steel slag has been successfully used as aggregate in wearing course hot mix asphalt and in surface treatments in the United States and internationally. Its use requires proper selection, processing, aging, and testing to ensure that it will perform in accordance with intended design specifications.
At least 11 states (Alabama, California, Illinois, Indiana, Kentucky, Louisiana, Michigan, Missouri, Pennsylvania, South Carolina, and West Virginia) have evaluated steel slag use as aggregate in asphalt paving, and one state agency (Louisiana) has been involved in steel slag use in surface treatment.(5)
Some of the positive features of steel slag aggregates in hot mix asphalt include good frictional properties and stripping resistance, high stability, and resistance to rutting/plastic deformation. Use of unsuitable or improperly processed slag, however, can result in performance problems.
Extensive map cracking of pavements has been observed by a number of agencies in the United States (Illinois, Indiana, and Minnesota), Canada, Germany, and Japan. The cracking is related to the volumetric instability associated with free lime (CaO) and/or free magnesia (MgO) in the steel slag. Hydration of the free lime or magnesia results in expansion and cracking of the slag particle. When this reaction occurs in a pavement, cracks or popouts can occur. Recent problems reported in Ohio and Illinois relating to deterioration, ravelling, and the coarse texture of hot mix asphalt containing steel slag aggregate have resulted in some restrictions on steel slag use.(6)
MATERIAL PROCESSING REQUIREMENTS
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.
To control volumetric instability resulting from the presence of potentially hydratable free lime and free magnesia, only suitable high-quality furnace slags that do not contain significant quantities of unreacted lime and dolime should be used. Recent recommended options suggest that the coarse and fine steel slag aggregates be washed and contain less than 3 percent by mass of nonslag constituents, and less than 0.1 percent wood content. Further, it is recommended that no detectable soft lime particles or lime-oxide agglomerations be present.(7)
Crushing and Screening
In addition to cleaning, aging, and quality-control requirements, crushing, screening and magnetic separation are required to produce a suitable coarse or fine aggregate product. Consistency of gradation, unit weight, and absorption are important for steel slag aggregates to be used in hot mix asphalt, and the aggregate processor must ensure that this is achieved.
There is typically a large variation in the physical, chemical, and mineralogical properties of steel slags. This difference is dependent on the steel-making plant, steel-making process, specific furnace, steel slag processing, and storage strategies. For this reason, the use of steel slag aggregate must be considered on a specific steel-making furnace and processing basis, with recognition of the inherent variability of the slag production and the presence of potentially hydratable free lime and free magnesia.
Some of the properties of steel slag that are of particular interest when steel slag is used in asphalt concrete include gradation, specific gravity, durability, moisture content, absorption, frictional properties, and thermal properties.
Gradation: Steel slag aggregates used in hot mix asphalt must meet the same gradation requirements as conventional aggregate, as outlined in AASHTO T27.(8) For surface treatment, steel slag aggregates should satisfy the gradation and physical requirements for ASTM D1139.(9)
Specific Gravity: Due to the relatively high specific gravity (3.2 to 3.6) of steel slag, steel slag aggregate can be expected to yield a higher density product compared with that of conventional mixes. Bulk relative densities are 15 to 25 percent greater than most conventional mixes.
Durability: Steel slag aggregate is very hard and abrasion resistant. Steel slag aggregates display good durability with resistance to weathering and erosion.
Moisture Content: The relatively rough surface texture (deep pores) of steel slag increases the susceptibility of the aggregate to differential drying and potential retention of moisture in the hot mix. Moisture retention coupled with the presence of hydratable oxides could result in volumetric instability. To minimize drying requirements and the potential for hydration reactions, steel slag aggregate moisture content should be limited to 5 percent prior to use in hot mix asphalt. The moisture content of the steel slag aggregate after drying should be no greater than 0.1 percent.
Absorption: Steel slag has somewhat higher absorption than conventional aggregate. This can result in an increased asphalt cement demand. Asphalt cement extractability (in lab tests) can be more difficult than for conventional aggregate.
Frictional Properties: Experience in several countries with steel slag aggregates in hot mix asphalt suggests that very satisfactory frictional resistance can be anticipated. Polished stone values (PSV, high values desirable) and aggregate abrasion values (AAV, low values desirable) support the general finding that steel slag aggregate exhibits superior frictional resistance for highway pavements.(10)The high frictional resistance, as well as the abrasion resistance of steel slag aggregate, is advantageous in applications where high wear resistance is required, such as industrial roads, intersections, and parking areas subjected to heavy traffic.
Thermal Properties: Steel slag aggregates have been reported to retain heat considerably longer than conventional natural aggregates. The heat retention characteristics of steel slag aggregates can be advantageous for hot mix asphalt repair work during cold weather.
Some of the mix properties that are of interest when steel slag is used in asphalt concrete mixes include stability, stripping resistance, and rutting resistance.
Stability: Steel slag aggregate mixes combine very high stabilities (1.5 to 3 times higher than conventional mixes) with good flow properties.
Stripping Resistance: Steel slag mixes typically exhibit excellent resistance to stripping of asphalt cement from the steel slag aggregate particles. Resistance to stripping is most probably enhanced because of the presence of free lime in the slag.
Rutting Resistance: The high stability (1.5 to 3 times higher than conventional mixes) with good flow properties results in a mix that resists rutting after cooling, but is still compactable. Rutting resistance is advantageous for highways, industrial roads, and parking areas subjected to heavy axle loads.
Asphalt mixes containing steel slag can be designed using standard laboratory procedures. Mixes combining steel slag aggregate and conventional aggregates are usually designed volumetrically because of the significant difference in aggregate bulk relative densities.
The requirements of ASTM D5106(11) and ASTM D4792(12) outline recommended properties of steel slag aggregate for use in hot mix asphalt. Some agencies, such as the Pennsylvania Department of Transportation, have adopted additional specifications setting minimum aging periods for processed steel slag aimed at limiting the risk of expansive cracking of steel slag aggregate in hot mix asphalt. Internationally, Germany and Japan have comprehensive specifications for the processing of steel slag aggregates that, in addition to aging requirements and aggregate expansion testing, also include expansion testing of the hot mix asphalt incorporating steel slag.13)
Steel slag aggregate use is limited by many jurisdictions to either coarse or fine aggregate (but not both) to mitigate potential bulking problems (poor compactability and high void space). This problem can also be mitigated by blending the coarse or fine steel slag aggregate with conventional natural (more rounded) materials to facilitate the compactability of the hot mix.
A recent comprehensive assessment of steel slag aggregates in hot mix asphalt use was undertaken in Canada. This assessment resulted in the following recommended tests and performance specifications for slag aggregates:(13)
These specifications were recommended as a supplement to minimum aging requirements for stockpiles of processed steel slag aggregates.
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating steel slag in the mix.
Material Handling and Storage
The same general methods and equipment used to handle conventional aggregates are applicable for steel slag aggregates.
Stockpiles for the processed (crushed and screened) coarse and fine steel slag aggregates, however, should be maintained in a consistently wet condition before supply to the hot mix plants. The period of aging in wet stockpiles should be established by process-control testing to satisfy deleterious components criteria (autoclave soundness, ASTM D4792(12) expansion testing). The storage period should be a minimum of 3 and perhaps up to 18 months. Until such time as process-control testing (as outlined above) indicates that the steel slag aggregates are suitable for use in hot mix asphalt, it is recommended that additional aging be required.
Mixing, Placing, and Compacting
Steel slag aggregate moisture content and hot mix asphalt moisture content consistency are important to satisfactory mix production. It is recommended that the maximum moisture content of coarse and/or fine steel slag aggregates upon introduction to the hot mix plant not exceed 5 percent. The maximum moisture content of steel slag aggregate hot mix asphalt should not exceed 0.1 percent.
The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing steel slag.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing steel slag. Mixes should be sampled in accordance with AASHTO T168,(14) and tested for specific gravity in accordance with ASTM D2726(15) and in-place density in accordance with ASTM D2950.(16) Cold mix/surface treatments using steel slag should satisfy conventional cold mix/surface treatment materials requirements given by ASTM D1139.(9)
Several issues pertaining to the use and subsequent recycling of steel slag aggregates in hot mix asphalt still need to be resolved. Although the incidence of map cracking associated with the use of steel slag aggregates is widely acknowledged, there is a need to determine the extent to which this problem undermines the structural integrity of the pavement. Limited investigations suggest that structural performance is not adversely affected; however, more long-term monitoring data are required to verify influence of the cracking on pavement life expectancy.
Since map cracking is recognized as a performance problem, but is not directly addressed by ASTM and AASHTO standard methods, there is a need to establish standard methods to assess the suitability of steel slag aggregates for hot mix asphalt use and for surface treatment. Also, long-term field performance data are required to assess the performance of surface treatments containing steel slag aggregates.
There is a need to assess the recyclability of steel slag in asphalt pavements incorporating reclaimed asphalt pavement and containing steel slag aggregates. There is presently a concern that recycling steel slag aggregate pavements that have previously exhibited volumetric stability (map cracking) problems could result in similar problems in the recycled mix.(17)