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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Aggregates are used in granular base and subbase layers below the driving surface layer(s) in both asphalt concrete and Portland cement concrete pavement structures. The aggregate base layers serve a variety of purposes, including reducing the stress applied to the subgrade layer and providing drainage for the pavement structure. The granular base layer is directly below the pavement surface and acts as the load bearing and strengthening component of the pavement structure. The granular subbase forms the lowest (bottom) layer of the pavement structure, and acts as the principal foundation for the subsequent road profile, provides drainage for the pavement structure, and protects the structure from frost.
Granular bases are typically constructed by spreading the materials in thin layers of 150 mm (6 in) to 200 mm (8 in) and compacting each layer by rolling over it with heavy compaction equipment.
Aggregates used in granular base and subbase applications generally consist of sand and gravel, crushed stone or quarry rock, slag, or other hard, durable material of mineral origin. The gradation requirements vary with type (base or subbase).
Granular base materials typically contain a crushed stone content in excess of 50 percent of the coarse aggregate particles. Cubical particles are desirable, with a limited amount of flat or thin and elongated particles. The granular base is typically dense graded, with the amount of fines limited to promote drainage. Granular subbase is also dense graded, but tends to be somewhat more coarse than granular base. The requirement for crushed content for granular subbase is not required by many agencies, although provision of 100 percent crushed aggregates for base and subbase use is increasing in premium pavement structures to promote rutting resistance.
MATERIAL PROPERTIES AND TESTING METHODS
The granular base and subbase generally make up the greatest thickness of the pavement structure, and provide both bearing strength and drainage for the pavement structure. Hence, proper size, grading, shape, and durability are important attributes to the overall performance of the pavement structure. Granular base and subbase aggregates may consist of durable particles of crushed stone, gravel or slag capable of withstanding the effects of handling, spreading, and compacting without generation of deleterious fines.
Some of the more important properties of aggregates for granular base and subbase include:
Table 24-8 provides a list of standard test methods to assess the suitability of conventional materials for use in granular base applications.
Table 24-8. Granular aggregates test procedures.
REFERENCES FOR ADDITIONAL INFORMATION
AASHTO Guide for the Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC, 1993.
The Aggregates Handbook. National Stone Association (R. Barksdale, Editor), Washington, DC, 1991.
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