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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 that can be used in granular base applications. The high stability of ACBFS aggregates can be especially useful in construction over soft ground. Its ability to "lock up" in granular base applications provides good load transfer when placed on weaker subgrade. The lower compacted unit weight of blast furnace slag aggregates relative to conventional aggregates results in a higher yield (greater volume for the same weight).
ACBFS has been successfully used as granular base or subbase material in many parts of the United States. At least seven states (California, Illinois, Indiana, Michigan, New Jersey, New York, and Pennsylvania) include ACBFS in their conventional granular base or subbase specifications.(1)
Some of the more desirable features of ACBFS in granular base applications include its ability to stabilize wet, soft underlying soils at early construction stages, good durability, ability to be placed in almost any weather, extremely high stability, and almost complete absence of settlement after compaction. Additionally, the high insulating value of blast furnace slag granular bases can be used to minimize frost heaving.
When ACBFS granular base is placed in poor drainage conditions, or other situations where it may be in extended contact with stagnant or slow moving water, sulfurous, discolored leachate may result. This leachate can result in sulfur-related odors and typically exhibits a yellow/green tinge, resulting from the presence of free sulfur and sulfur dioxide and a high pH.(2)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
ACBFS is readily processed using conventional crushing and screening equipment. The highest stabilities are obtained using crushed material with the largest maximum size that can be handled without segregation (generally 19 mm (3/4 in) to 38 mm (1-1/2 in)).
Leachate from the ACBFS aggregate should be checked by the processors to verify that it is not prone to discoloration and/or odor. Although aging delays the discoloration and sulfurous odor problems from ACBFS in contact with water, it does not eliminate them.(3) Recommended guidelines to minimize the likelihood of sulfurous leachate from ACBFS granular base are outlined below:
Some of the engineering properties of ACBFS aggregates that are of particular interest when ACBFS is used in granular base applications include gradation, compacted density, friction angle, bearing capacity, freeze-thaw resistance, weathering, hardness and impact resistance, abrasion, and drainage characteristics.
Gradation: Blast furnace slag should be crushed and screened to produce a material that satisfies the grading requirements of granular base and subbase specifications, such as AASHTO M147.
Compacted Density: The compacted density varies with size and grading of the slag, method of measuring, and bulk specific gravity of the slag. The compacted density of ACBFS ranges from 1120 kg/m3 (70 lb/ft3) to 1940 kg/m3 (120 lb/ft), which is somewhat lower than that of conventional granular materials. Allowance for this differential should be considered during design and in specifications in order to ensure equal volume irrespective of the type of aggregate used.
Stability: ACBFS angularity and high friction angle (40° to 45° ) contribute to high bearing capacity (California Bearing Ratio (CBR) greater than 100).
Freeze-Thaw Resistance: ACBFS aggregates display good durability with resistance to freeze-thaw weathering and erosion.
Hardness and Impact Resistance: The hardness of slag as measured by Moh’s scale is between 5 and 6, corresponding to durable igneous rock.(6) However, ACBFS is quite brittle and prone to breakdown when subjected to impact loading.(7)
Abrasion: AASHTO M147(5) requirements for Los Angeles Abrasion loss for granular base aggregates are typically waived for ACBFS since no correlation between the Los Angeles Abrasion test loss for slag in laboratory tests and degradation in field applications has reportedly been observed. For this reason ASTM has deleted this test for slag in its specifications (e.g., ASTM D692, D1139), and major slag-using states, such as Michigan, Ohio and Indiana, do not use this test procedure with ACBFS.
Drainage Characteristics. ACBFS granular base is free draining and is not frost susceptible.
The use of ACBFS aggregates in granular base applications is covered by conventional aggregate specifications in many jurisdictions. ACBFS aggregate can normally meet the requirements of AASHTO M147.(5)
Structural design procedures for granular base containing blast furnace slag are the same as design procedures for conventional granular materials.
Material Handling and Storage
The same equipment and procedures used for conventional aggregate may be used to stockpile and handle conventional aggregates as appropriate for ACBFS. However, greater care is required when handling and stockpiling blast furnace slag aggregates to avoid brittle fracture that can result in excessive fines generation.
Placing and Compacting
Procedures should be employed to ensure uniform gradation and layer thickness. Good uniformity is obtained by combining the coarse and fine aggregates with optimum water for compaction at the blending plant just prior to placing. The material should be graded and placed in a manner that allows free drainage and prevents ponding within or adjacent to the material.
The same test procedures used for conventional aggregate are appropriate for ACBFS. Standard field and laboratory tests for compacted density and field measurement of compaction are given by AASHTO T191(9), T205(10), T238(11), and T239(12) test methods.
There is a need to resolve the aesthetic and environmental concerns regarding the odor and discoloration of water due to ACBFS leachate by conducting research to determine the properties (e.g., pH, redox conditions, etc.) that result in the discoloration and odor encountered in the field.