ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 348

9.8 Asphalt Concrete

Asphalt concrete, also known as hot-mix asphalt (HMA), consists of asphalt
cement and aggregates mixed together at a high temperature and placed and
compacted on the road while still hot. Asphalt (flexible) pavements cover ap-
proximately 93% of the 2 million miles of paved roads in the United States,
while the remaining 7% of the roads are portland cement concrete (rigid)
pavements. The performance of asphalt pavements is largely a function of the
asphalt concrete surface material.
The objective of the asphalt concrete mix design process is to provide
the following properties (Roberts et al. 1996):

1. stability or resistance to permanent deformation under the action of
traffic loads, especially at high temperatures
2. fatigue resistance to prevent fatigue cracking under repeated loadings
3. resistance to thermal cracking that might occur due to contraction at
low temperatures
4. resistance to hardening or aging during production in the mixing plant
and in service
5. resistance to moisture-induced damage that might result in stripping of
asphalt from aggregate particles
6. skid resistance, by providing enough texture at the pavement surface
7. workability, to reduce the effort needed during mixing, placing and
compaction

Regardless of the set of criteria used to state the objectives of the mix de-
sign process, the design of asphalt concrete mixes requires compromises. For
example, extremely high stability often is obtained at the expense of lower
durability, and vice versa. Thus, in evaluating and adjusting a mix design for
a particular use, the aggregate gradation and asphalt content must strike a fa-
vorable balance between the stability and durability requirements. Moreover,
the produced mix must be practical and economical.

9.9 Asphalt Concrete Mix Design

The purpose of asphalt concrete mix design is to determine the design as-
phalt content using the available asphalt and aggregates. The design asphalt
content varies for different material types, material properties, loading levels,
and environmental conditions. To produce good-quality asphalt concrete, it
is necessary to accurately control the design asphalt content. If the appropri-
ate design asphalt content is not used, the pavement will lack durability or
stability, resulting in premature pavement failure. For example, if not enough
asphalt binder is used, not all the aggregate particles will be coated with as-
phalt, which will result in a less stable and less durable material. Also, if too