Aggregate Base Course is often called an unbound pavement layer or granular base to distinguished it from layers bound with asphalt (flexible pavement) or cement (rigid pavement). Deep, unbound aggregate layers are extraordinarily strong when properly compacted and confined. In fact, some of NASA's heaviest rockets and the Cape Hatteras Lighthouse were moved over ABC layers.

Bound layers such as Hot Mix Asphalt and Portland Cement Concrete have cohesion and tensile strength in the top layers to resist the forces from traffic. Unbound layers, on the other hand, can be pulled apart if put under substantial tensile forces. This is why they are used lower in the pavement structure where the ABC is confined and principally must resist compression and shear.

In 2001, the National Cooperative Highway Research Program published NCHRP Report 453 on “Performance-Related Tests of Aggregates for use in Unbound Pavement Layers.” It identified aggregate properties and tests best related to good ABC performance in pavement structures. A base that is too thin or of poor-quality can lead to pavement failures. Flexible pavement (HMA) failures include rutting, fatigue cracking, longitudinal cracking, depressions, corrugations and frost heave. Rigid pavement (PCC) failures include pumping, faulting, cracking, corner breaks and frost heave.

Of the properties studied regarding pavement performance, NCHRP named these six as the most important for ABC: shear strength, stiffness, toughness and abrasion, durability, frost susceptibility and permeability. Many potential tests were evaluated in a laboratory study with a range of aggregate sources and performance histories from seven states. The aggregates tested were traprock, dolomite, limestone, sandstone, granite, crushed gravel, pit-run gravel, and glacial sand and gravel.

Quality, long-lasting pavements should be designed with thick ABC layers, as they are in Europe. Thick layers provide the support, durability and protection properties needed for the climate, subgrade and traffic loadings. The AASHTO “Guide for Design of Pavement Structures” is the basis for most pavement designs in the United States; a new guide is expected this year. Compaction, strength and stiffness are needed to assure the base will stand up to heavy truck-traffic loads. Drainage is a critical issue. For dense-graded and poorly drained bases, strength and stiffness must be assured under saturated conditions. Unsaturated strength and stiffness properties can be used where permeable bases are used with positive drainage.

Other aggregate properties also are needed. These include toughness and abrasion resistance to minimize degradation during handling, construction and aggregate particles working against each other in the base under traffic. Particle durability is needed to resist freezing and thawing. It is important to include at least one ABC layer, in northern climates, that is free of frost heave susceptibility. That is, a layer is needed to interrupt the rise of capillary water from the subgrade that contributes to ice lens growth or base saturation.

ABC is a critical component in designing pavement systems that will perform under the projected growth of heavy traffic loads throughout North America for many decades.

Richard Meininger, PE has been active in developing aggregate standards for decades. He is past vice president of engineering for both the former National Aggregates Association and the National Ready Mixed Concrete Association.

Test Methods Best Related to Base
Performance for Selecting and Constructing ABC

Screening Tests: Sieve Analysis, Atterberg Limits (PI), Moisture-Density (Proctor), Flat and Elongated Particles, and Uncompacted Voids to measure aggregate angularity

Shear Strength: Triaxial tests (wet/dry) for resistance to rutting and shoving

Stiffness: Resilient Modulus (wet/dry) to minimize deformation under loads

Toughness and Abrasion Resistance: Micro-Deval test

Durability: Magnesium Sulfate Soundness as a measure of freeze-thaw resistance

Frost Susceptibility: Tube Suction test, evaluating the capillary draw of the fine phase