ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 349

much binder is used, aggregate particles may have too much “lubrication”
and may move relative to each other upon application of the load, resulting
in a less stable material. Typical design asphalt contents range from 4% to
7% by weight of total mix.
Before the Superpave mix design method was developed during the SHRP
program, there were two common asphalt concrete design methods: Marshall
(ASTM D1559), and Hveem (ASTM D1560). The Marshall method was more
commonly used than the Hveem method, due to its relative simplicity and its
ability to be used for field control. Both methods are empirical in nature; that
is, they are based on previous observations. Both methods have been used sat-
isfactorily for several decades and have produced long-lasting pavement sec-
tions. However, due to their empirical nature they are not readily adaptable to
new conditions, such as modified binders, large-sized aggregates, and heavier
traffic loads.
The Superpave design system is performance based and is more rational
than the Marshall and Hveem methods. Many highway agencies are imple-
menting the Superpave system.

■ 9.9.1 Specimen Preparation in the Laboratory

Asphalt concrete specimens are prepared in the laboratory for mix-design
and quality-control tests. To prepare specimens in the lab, aggregates are
batched and heated, according to a specified gradation. Asphalt cement is
also heated separately and added to the aggregate at a specified rate. Aggre-
gates and asphalt are mixed with a mechanical mixer until the aggregate
particles are completely coated with asphalt. Three compaction machines are
commonly used:
1. Superpave gyratory compactor
2. Marshall hammer
3. California kneading compactor
Regardless of the compaction method, the procedure for preparing spec-
imens basically follows the same four steps:
1. Heat and mix the aggregate and asphalt cement
2. Place the material into a mold
3. Apply compactive force
4. Allow the specimen to cool and extrude from the mold
The specific techniques for placing the material into the mold vary
among the three compaction methods, and the standards for the test must be
followed.
The greatest difference among the compaction procedures is the man-
ner in which the compactive force is applied. For the gyratory compaction,
the mixture in the mold is placed in the compaction machine at an angle
to the applied force. As the force is applied the mold is gyrated, creating
a shearing action in the mixture. Gyratory compaction devices have been
available for a long time, but their use was limited due to the lack of a