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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction

GRANULAR BASE

Application Description

INTRODUCTION

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.

 

MATERIALS

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.

Property Test Method Reference
General Specifications Graded Aggregate Material for Bases or Subbases for Highways or Airports ASTM D2940
Gradation Sizes of Aggregate for Road and Bridge Construction ASTM D448/
AASHTO M43
Sieve Analysis of Fine and Coarse Aggregate ASTM C136/ AASHTO T27
Particle Shape Flat and Elongated Particles in Coarse Aggregate ASTM D4791
Uncompacted Voids Content of Fine Aggregate (As Influenced by Particle Shape, Surface Texture, and Grading AASHTO TP33
Index of Aggregate Particle Shape and Texture ASTM D3398
Base Stability California Bearing Ratio ASTM D1883/
AASHTO T193
Moisture-Density Relations of Soils Using a 5.5 lb (2.5 kg) Rammer and a 12-in. (305 mm) Drop ASTM D698/
AASHTO T99
Moisture- Density Relations of Soils Using a 10-lb (4.54 kg) Rammer and an 18-in. (457 mm) Drop AASHTO T180
Permeability Permeability of Granular Soils (Constant Head) ASTM D2434/
AASHTO T215
Plasticity Determining the Plastic Limit and Plasticity Index of Soils ASTM D4318/
AASHTO T90
Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test ASTM D2419/
AASHTO T176
Abrasion Resistance Resistance to Degradation of Large-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine ASTM C535
Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine ASTM C131/
AASHTO T96
Resilient Modulus Resilient Modulus of Unbound Granular Base/Subbase Materials and Subgrade Soils - SHRP Protocol P46 AASHTO T274

 

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.

 

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