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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
The term stabilized base, presented in this section, refers to a class of paving materials that are mixtures of one or more sources of aggregate and cementitious material(s) blended with a sufficient amount of water, that result in a mixture having a moist, nonplastic consistency that can be compacted to form a dense mass and gain strength. This class of base or subbase materials is not meant to include the stabilization of soils or aggregates using asphalt cement or emulsified asphalt.
Soil-cement (or cement-treated base) is probably the earliest example of a stabilized base material. Roller-compacted concrete, which is similar to, but more granular than soil-cement, is another type of stabilized base product. Probably the most frequently used type of stabilized base materials are the basic lime (and/or cement)-fly ash-aggregate family of mixtures, which can use many different combinations of reagents(s) and aggregate(s) together with coal fly ash. Calcium chloride has also been used to a lesser degree in some of the warmer regions of the country for mechanical stabilization of dense-graded aggregate base courses.
The purpose of a stabilized base or subbase layer is to provide a transitional load-bearing strata between the pavement layer, which directly receives the wheel loadings of vehicular traffic, and the underlying subgrade soil. Stabilized base or subbase materials may be used to provide support for either flexible or rigid pavements, but are more frequently used with flexible pavements.
Stabilized base or subbase materials are either mixed in place at the job site, or are mechanically combined in a mixing plant and transported to the site. These materials are spread evenly in loose layers on a prepared subgrade or subbase using either a blade-grader bulldozer, spreader box, or paving machine. Once the material has been spread, it is then densified by means of conventional rollers of compaction equipment.
The components of a stabilized base or subbase mixture include aggregate, cementitious materials, and water.
Aggregates comprise the major portion of stabilized base. Normally, between 80 to 95 percent by weight of a stabilized base or subbase mix may consist of aggregates. A wide range of different types and gradations of aggregates have been used in stabilized base and subbase mixtures. These include conventional aggregate sources, such as crushed stone or sand and gravel, and other aggregate materials, such as blast furnace slag, recycled paving materials, and bottom ash or boiler slag from coal-fired power plants. Reclaimed pavement materials have also been successfully recycled into stabilized base and subbase mixtures, as have some marginal aggregates. Aggregates used should have the proper particle size, shape, gradation, and particle strength to contribute to a mechanically stable mixture.
The key to strength development in stabilized base or subbase mixtures is in the matrix that binds the aggregate particles together. The strength of the matrix is affected by the cementitious material used in the mixture. The amount of cementitious material in a stabilized base or subbase mix usually ranges from 5 to 10 percent by weight of the mix, but may in some cases comprise as much as up to 20 percent by weight if a lighter weight aggregate is used.
A number of different cementitious materials have been successfully used to bind or solidify the aggregate particles in stabilized base or subbase mixtures. The material that has been most frequently used is Portland cement
In some parts of the United States, mainly west of the Mississippi River, fly ash from the burning of sub-bituminous coal is widely available and, because it exhibits self-cementing characteristics when mixed with water, it can be used by itself with no other cementitious material to bind aggregate particles together.
Coal fly ash, produced during the combustion of bituminous coal, is frequently used in stabilized base mixtures. Since this type of fly ash is a pozzolan, the mixtures in which it is used are often referred to as pozzolanic stabilized base (PSB) mixtures. Pozzolans are materials composed of amorphous siliceous or siliceous and aluminous material in a finely divided (powdery) form (similar in size to Portland cement particles) that will, in the presence of water, react with an activator to form compounds possessing cementitious properties. Pozzolan activators are alkaline materials that contain calcium and magnesium compounds present in sufficient amounts to chemically react in the presence of water with the silicate and aluminates in the pozzolan. Descriptions of various kinds of pozzolans and their specifications are provided in ASTM C618.
In PSB compositions, the fly ash is usually used in combination with either lime, Portland cement, or kiln dust, plus water, to form the matrix that cements the aggregate particles together. When used with a chemical reagent, this type of fly ash normally comprises between 10 and 20 percent by weight of a stabilized base or subbase mix. When used with lighter weight aggregates (such as coal bottom ash), the percentage of fly ash may be as high as 30 percent or more.
MATERIAL PROPERTIES AND TESTING METHODS
Aggregates used in stabilized base and subbase mixtures play a major role in determining the quality and performance of stabilized base and subbase mixtures. Aggregate materials used in these types of mixtures must be properly graded and possess good to adequate particle shape, strength, and integrity.
AASHTO, in conjunction with the Association of General Contractors (AGC) and the American Road and Transportation Builders Association (ARTBA), has published a Guide Specification for Pozzolanic Stabilized Mixture (PSM) Base Course or Subbase (see reference section). This guide specification recommends quality requirements for aggregates.
The following is a list and brief comments on some of the more important properties of aggregates that are used in stabilized base and subbase mixes:
Standard test methods typically used to assess the suitability of conventional aggregate materials for use in stabilized base and subbase applications are listed in Table 24-10.
Table 24-10. Stabilized base and subbase aggregate test procedures
Cementitious materials used in stabilized base and subbase mixes must be capable of reacting to bind the particles of aggregate together into a stable mass that is able to support imposed wheel loadings and resist the deteriorating effects of climate and water. Some of the more important properties of cementitious materials used in a stabilized base application include:
Table 24-11 provides a list of standard laboratory tests that are presently used to evaluate the mix design or expected performance of cementitious materials for use in stabilized base mixtures.
The most important properties of fly ash (or other pozzolans) used in stabilized base mixtures include:
Table 24-11. Cementitious materials test procedures.
Table 24-12 lists applicable test methods that contain criteria for evaluating the suitability of fly ash for use in stabilized base and subbase mixtures.
Table 24-12. Pozzolan test procedures.
STABILIZED BASE OR SUBBASE MIXTURES
The mix proportions for a properly designed stabilized base or subbase mixture are determined in the laboratory during mix design testing. To perform properly in the field, a well-designed stabilized base or subbase mixture must be properly compacted and be capable of developing sufficient strength and durability to meet or exceed design criteria. Properly designed stabilized base or subbase materials should be evaluated for the following properties:
Table 24-13 provides a list of standard laboratory test methods that are used to evaluate the mix design properties and/or performance characteristics of stabilized base or subbase mixtures.
Table 24-13. Stabilized base and subbase material test procedures.
REFERENCES FOR ADDITIONAL INFORMATION
AASHTO/AGC/ARTBA Guide Specification for Pozzolanic Stabilized Mixture (PSM) Base Course or Subbase. American Association of State Highway and Transportation Officials, Washington, DC, 1988.
AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC, 1993
Lime Stabilization Construction Manual. National Lime Association, Arlington, Virginia, 1980.
Materials for Stabilization. American Road and Transportation Builders Association, Washington, DC, 1977.
Soil-Cement Construction Handbook. Portland Cement Association, Skokie, Illinois, 1995.
Soil Stabilization in Pavement Structures. A User's Manual. Volumes 1 and 2. Federal Highway Administration, Report No. FHWA-IP-80-2, Washington, DC, 1980.
Stabilization and Pavement Recycling. American Road and Transportation Builders Association, Washington, DC, 1979.