Asphalt concrete -- also known as asphaltic concrete -- is a mixture of gravel or aggregate with crude oil residue to serve as a binder. Although asphalt concrete is the technical name, laypeople typically refer to this mixture as asphalt. The unit weight of asphalt concrete varies depending on the aggregate and is an important property of the paving material.
The specific gravity of asphalt concrete is the ratio of its density to the density of water -- in other words, the density of the asphalt concrete divided by the density of water. Two factors impact the specific gravity of the mixture: the specific gravity of the asphalt binder -- which is usually provided by the manufacturer -- and the specific gravity of the combined aggregate, which can be calculated as the average of the specific gravities of the materials it contains.
The unit weight of asphalt concrete is equal to the weight of a given sample divided by its volume. Unit weight is just another way to measure density; measurements of specific gravity can easily be converted to unit weight and vice versa. Engineers sometimes refer to the maximum unit weight as the solid density, so density is sometimes described as a percentage of the maximum unit weight. While this nomenclature may sound a little confusing, bear in mind they're all just different ways to talk about density.
As the asphalt congeals, tiny unfilled pockets called air voids can form. A high void content can accelerate water damage and cracking; a low void content can lead to the formation of ruts. The void content is inversely proportional to the density. Engineers determine the void content by comparing the density to the theoretical maximum unit weight or density for that particular mixture. The asphalt's density will increase somewhat because the weight of steady traffic makes the asphalt slightly more compact.
There are multiple ways to measure the relevant parameters for a given asphalt mix and determine the target density. Engineers may compact the asphalt in the lab, for example, and use the resulting figure as a target density -- the density they want to achieve to ensure the correct properties for the paving material. In some cases, engineers may be unable to achieve the target density even after repeated efforts. If so, they may set a new target value, but this approach is generally a last resort.