# Aggregate Specific Gravity

Aggregate specific gravity is useful in making weight-volume conversions and in calculating the void content in compacted HMA (Roberts et al., 1996[1]). AASHTO M 132 and ASTM E 12 define specific gravity as:

“…the ratio of the mass of a unit volume of a material at a stated temperature to the mass of the same volume of gas-free distilled water at a stated temperature.”

The commonly used “stated temperature” is 23° C (73.4° F). Given the structure of a typical aggregate particle, there are several different kinds of specific gravity. This section will first describe the structure of a typical aggregate particle and then discuss each type of specific gravity and its use.

### Aggregate Particle Structure

A typical aggregate particle consists of some amount of solid material along with a certain amount of air voids. These air voids within the aggregate particle (Figure 1) can become filled with water, binder or both (Figure 2). It takes a finite amount of time for water/binder to penetrate these pores, so specific gravity test procedures generally contain a 15 to 19-hour (for AASHTO procedures) or a 24-hour (for ASTM procedures) soak period for the purpose of allowing penetration into these pores.

 Figure 1. Dry aggregate. Figure 2. Wet aggregate.

Depending upon how aggregate voids are dealt with, calculated aggregate specific gravities can vary. If they are excluded entirely, then the specific gravity is that of the solid portion of the aggregate only, while if they are included entirely then the specific gravity essentially becomes a weighted average of the specific gravity of the solid aggregate and whatever is in its voids.

## Aggregate Specific Gravities

Generally, there are three different aggregate specific gravities used in association with pavements:

1. Bulk
2. Apparent
3. Effective

Footnotes    (↵ returns to text)
1. Roberts, F.L., Kandhal, P.S., Brown, E.R., Lee, D.Y., and Kennedy, T.W.  (1996).  Hot Mix Asphalt Materials, Mixture Design, and Construction.  National Asphalt Paving Association Education Foundation.  Lanham, MD.