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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Air-cooled blast furnace slag (ACBFS) is considered by many specifying agencies to be a conventional aggregate and can replace both coarse and fine aggregates in asphalt paving applications. ACBFS, however, is more absorptive than conventional aggregate and therefore has a higher asphalt cement demand. It also has a lower compacted unit weight than conventional mineral aggregates, which results in a higher asphalt pavement yield (greater volume for the same weight).
At least 17 states have specifications covering the use of ACBFS as aggregate in surface course hot mix asphalt. They include Alabama, Colorado, Florida, Illinois, Indiana, Kentucky, Louisiana, Michigan, New York, Ohio, Pennsylvania, Tennessee, Texas, Utah, Virginia, West Virginia, and Wisconsin.(1) Hot mix asphalt containing properly selected and processed ACBFS aggregates demonstrates good frictional resistance in pavement surfaces, good stripping resistance, and high stability.
The use of ACBFS aggregates in surface treatment applications has been accepted by many of the same jurisdictions that incorporate ACBFS aggregates in hot mix asphalt. Surface-treated pavements incorporating ACBFS aggregate demonstrate good friction resistance, good resistance to stripping, and fair wear resistance. However, the resistance of ACBFS to impact is not very high and the material can break down under heavy traffic conditions. Such aggregate is better suited to surface treatment applications on light traffic pavements.
Variability in the iron production process can result in a lack of consistency in the physical properties (gradation, specific gravity, absorption, and angularity) of ACBFS. This lack of consistency has occasionally contributed to hot mix asphalt performance problems, such as flushing due to high binder content (too rich), ravelling due to low binder content, and high fines to asphalt ratios (too lean).(2)
MATERIAL PROCESSING REQUIREMENTS
Material Quality Control
Hot mix asphalt and surface treatments require aggregates that exhibit consistent physical characteristics and quality. Special attention is required to address inconsistent physical properties of some ACBFS. More rigorous quality control is required in the selection and processing of ACBFS aggregates than conventional aggregates.
Crushing and Screening
Conventional aggregate crushing and screening operations are used to process ACBFS for use as an aggregate in asphalt concrete.
Some of the engineering properties of ACBFS that are of particular interest when ACBFS is used as an aggregate in asphalt concrete include gradation, compacted density, absorption, abrasion, and freeze-thaw resistance.
Gradation: Blast furnace slag should be crushed and screened to produce aggregate that satisfies the gradation requirements for hot mix asphalt as specified in ASTM D692(3) for coarse aggregate and AASHTO M29(4) for fine aggregate. For surface treatments, ACBFS aggregate should satisfy ASTM D1139(5) gradation specification requirements.
Compacted Density: The compacted density of ACBFS ranges from 1120 kg/m3 (70 lb/ft3) to 1940 kg/m3 (120 lb/ft3), which is somewhat lower than that of conventional aggregates. Allowance for this differential should be considered during design and in specifications in order to ensure equal volume irrespective of the type of the aggregate used. Asphalt mixes incorporating blast furnace slag aggregates should be designed volumetrically.
Absorption: The vesicular (porous) surface texture of ACBFS contributes to higher absorption than conventional aggregates. High absorption leads to an increased asphalt cement requirement (up to 3 percent more by weight of mix), resulting in an increase in the cost of the ACBFS paving mixes relative to conventional mixes. This is offset somewhat by the higher yield (volume per mass) of ACBFS paving mixtures because of the lower unit weight of the mix.
Abrasion: Although the resistance to degradation of ACBFS aggregates as measured by the Los Angeles machine (6)is not particularly favorable (because the sharp edges break off), field performance has been satisfactory. Consequently, the ASTM testing requirement for degradation of ACBFS aggregates has been deleted from standard specifications for hot mix and surface treatment aggregates (see ASTM D692 and ASTM D1139). Major slag-using states, such as Michigan, Ohio, and Indiana, do not have Los Angeles Abrasion loss requirements for ACBFS.(7)
Freeze-Thaw Resistance: ACBFS displays good resistance to freeze-thaw weathering.(8)
Some of the properties of asphalt paving mixes that are of particular interest when ACBFS is incorporated into the mix include stability, frictional properties, resistance to rutting, and resistance to stripping.
Stability: The angular shape and high friction angle (40° to 45° ) of crushed ACBFS contributes to good lateral stability when ACBFS is incorporated into paving mixes. This is particularly beneficial where hard braking and acceleration are considerations.
Frictional Properties: One of the more notable features of asphalt concrete containing ACBFS is its high frictional resistance.(9,10) This is due to the rough, vesicular surface texture, high angularity, and hardness (Moh’s hardness of 5 to 6) of ACBFS. ACBFS aggregates have high polished stone values (PSV) tested according to AASHTO T279() and impart good frictional resistance to hot mix asphalt and surface treatments. (See references 6,7,8, and 12 .)
Rutting Resistance: Asphalt concrete mixes containing ACBFS aggregates combine very high stabilities with good flow properties, resulting in a mix that resists rutting after cooling but is still compactable.
Stripping Resistance: Due to its hydrophobic nature, ACBFS has a high affinity for asphalt cement (rather than water), resulting in excellent adhesive bond between ACBFS aggregate particles and asphalt cement and excellent resistance to stripping.
Asphalt mixes containing blast furnace slag can be designed using standard laboratory procedures.
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements containing ACBFS.
Material Handling and Storage
The same methods and equipment used to store or stockpile conventional aggregates are applicable to ACBFS. Due to its brittle nature, some care must be taken in handling ACBFS, since excessive handling could result in particle breakdown.
Mixing, Placing, and Compacting
Since ACBFS is more porous than conventional aggregates, longer retention time for drying at the hot mix plant may be required.
The same methods and equipment used for placing and compacting conventional pavements can be used for asphalt concrete containing ACBFS.
To minimize problems associated with the variable properties of some ACBFS aggregates, a comprehensive quality control testing program may be necessary to monitor the gradation, specific gravity, absorption, and angularity of ACBFS used in asphalt concrete.(12)
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing blast furnace slag. Mixes should be sampled in accordance with AASHTO T168,(13) and tested for specific gravity in accordance with ASTM D2726(14) and in-place density in accordance with ASTM D2950.(15)
There is a need to evaluate factors that contribute to the lack of consistency in the physical properties (gradation, specific gravity, absorption, and angularity) of ACBFS aggregates among individual slag producers and its effect on performance problems such as flushing and ravelling in hot mix asphalt. In addition, formal quality control procedures should be instituted to monitor the quality of specific ACBFS sources to assist in mitigating these problems.
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling