Granular Base

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:

• Gradation – a wide range of aggregate sizes and gradations are used depending on the pavement type and the conditions to which the granular base and subbase will be subjected. The aggregate grading markedly influences the base stability, drainage (permeability) and frost susceptibility. Aggregates for use as granular base tend to be dense-graded with a maximum size of 50 mm (2 in) or less, while granular subbase can have a nominal maximum size commonly up to 100 mm (4 in). The percentage of fines (minus 0.075 mm (No. 200 sieve)) in the granular base is limited, for drainage and frost-susceptibility purposes, to a maximum of 8 percent, with up to 12 percent permitted in granular subbase.
• Particle Shape – the use of angular, nearly equidimensional aggregate with rough surface texture is preferred over rounded, smooth aggregate particles. Thin or flat and elongated particles have reduced strength when load is applied to the flat side of the aggregate or across its shortest dimension and are also prone to segregation and breakdown during compaction, creating additional fines.
• Base Stability – granular base should have high stability, particularly in a flexible asphalt pavement structure. Large, angular aggregate, dense-graded and consisting of hard, durable particles, is preferred for stability. For maximum base stability, the granular base should have sufficient fines to just fill the voids and the entire gradation should be close to its maximum density. However, while base density is maximized at fines content between 6 and 20 percent, load-carrying capacity decreases when the fines content exceeds about 9 percent. Stability also increases with the percentage of crushed particles and increasing coarse aggregate size.
• Permeability – since the granular subbase provides drainage for the pavement structure, its grading and hydraulic conductivity are important. The fines content is usually limited to a maximum of 10 percent for normal pavement construction, and 6 percent where free-draining subbase is required.
• Plasticity – the presence of plastic fines can significantly reduce the load-carrying strength of the granular base and subbase.
• Abrasion Resistance – particles should have sufficient strength to resist degradation or breakdown during construction, under compaction or under traffic.
• Resilient Modulus – can assist in providing design coefficients for multi-layered pavements by defining the relationship between stress and the deformation of granular base and subbase layers.

Table 8 provides a list of standard test methods to assess the suitability of conventional materials for use in granular base applications.

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.