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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Both air-cooled blast furnace slag (ACBFS) and expanded blast furnace slag can be used as a conventional aggregate in embankment or fill. They are generally considered by many specifying agencies to be conventional aggregates and require minimal processing to satisfy conventional soil and aggregate engineering requirements.(1,2,3)
At least seven states have reported the use of blast furnace slag in embankment construction. They include Indiana, Kentucky, Maryland, Michigan, Missouri, New York, and Ohio.(4)
Some positive features of ACBFS and expanded blast furnace slag include their low compacted density (which results in reduced dead weight load, reduced lateral pressures, and favorable transportation costs on a volumetric basis), high stability and friction angle, ability to stabilize wet, soft underlying soils at early construction stages, and the almost complete absence of settlement after compaction. Expanded blast furnace slag can also provide some structural strength because of its cementitious properties.
Leaching of sulfurous compounds from ACBFS has been reported. (See references Control Inc. for Environmental Protection Agency, May 1978.,Contaminated Land Conference, Eastbourne, 1979.,November 1985. (5,6,7,8 9 and 10) Investigation and testing have shown that when ACBFS is placed in poor drainage conditions and is in extended contact with stagnant or slow moving water, the contact water can exhibit a high pH, aesthetically undesirable odors (sulfur-related), and yellow/green color.(7,11)
MATERIAL PROCESSING REQUIREMENTS
Blast furnace slag requires minimal processing to satisfy the physical requirements for use in embankments. Primary crushing is generally adequate to satisfy gradation requirements.
Leachate from the material should be checked to verify that it is not prone to discoloration. While aging delays the discoloration and sulfurous odor from ACBFS in contact with water, it does not completely eliminate this occurrence.(12) The following are recommended guidelines to minimize the likelihood of sulfurous leachate from ACBFS:
Some of the engineering properties of ACBFS that are of particular interest when ACBFS is used in embankment or fill applications include gradation, stability, compacted density, drainage characteristics, and corrosivity.
Gradation: The gradation and physical requirements of AASHTO M145-82(14)are readily satisfied by ACBFS.
Stability: Due to its coarse texture and angular shape, ACBFS aggregate has a high friction angle (40° to 45° ), has high bearing capacity (California Bearing Ratio (CBR) greater than 100), and demonstrates almost complete lack of postcompaction settlement.(15)
Compacted Density: The compacted density of ACBFS aggregates, which is in the range of 1120 kg/m3 (70 lb/ft3) to 1940 kg/m3(120 lb/ft3), is usually lower than that of conventional embankment or fill materials (earth and rock). The bulk relative density of expanded slag is difficult to determine accurately, but its loose unit weight is about 70 percent of that of ACBFS. Typical unit weights for expanded blast furnace slag fine aggregates are 800 kg/m3 (50 lb/ft3) to 1040 kg/m3 (65 lb/ft3) and from 560 kg/m3 (35 lb/ft3) to 800 kg/m3 (50 lb/ft3) for coarse aggregates.(16)
Drainage Characteristics: ACBFS and expanded blast furnace slag are nonplastic and free draining and are not frost susceptible.
Corrosivity: Blast furnace slag is mildly alkaline, with a pH in nonstagnant water mixtures in the range of 8 to 10. Despite the fact that blast furnace slag contains a small component of elemental sulfur (1 to 2 percent), the leachate is slightly alkaline and does not present a corrosion risk to steel (e.g., steel pilings).(17,18)
There are no standard specifications covering blast furnace slag use as embankment or fill material. The supplier may be required to satisfy moisture content criteria according to AASHTO T99.(19)
Design procedures for embankments or fill containing blast furnace slag are the same as design procedures for embankments or fills using conventional materials.
Material Handling and Storage
The same equipment and procedures used for handling and stockpiling conventional aggregates may be used to handle and stockpile ACBFS.
Placing and Compacting
ACBFS should be placed in a manner that allows free drainage and prevents ponding within or against the material. The same methods and equipment used to place and compact conventional aggregate can be used to place and compact blast furnace slag.
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,(20) T205,(21) T238,(22) and T239(23) test methods.
There is a need to resolve the aesthetic environmental concerns regarding the odor and discoloration of water because of ACBFS leachate by conducting research to determine the properties (e.g., pH, redox conditions, etc.) that result in the odor and discoloration encountered in the field.