ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 356

Aggregate Selection Aggregate properties under the Superpave mix-design
system are described as either consensus or source properties. The follow-
ing consensus aggregate properties are required:

coarse aggregate angularity measured by the percentage of fractured faces

■

fine aggregate angularity (AASHTO TP 33) (see apparatus in Figure 5.5)

■

flat and elongated particles (ASTM D4791)

■

clay content (ASTM D2419)

■

Specification limits for these properties depend on the traffic level and
how deep under the pavement surface the materials will be used, as shown
in Table 9.8. In addition to these properties, highway agencies may consider
other factors that are critical to the specific local conditions. These are called
source properties and may include Los Angeles abrasion (see apparatus in
Figure 5.6), soundness, and deleterious materials. Source properties are de-
fined at the local level; consensus properties are defined at the national level.
Aggregate used in asphalt concrete must be well graded. Superpave rec-
ommends using the 0.45 power chart discussed in Chapter 5. The gradation
curve should go through control points specified by Superpave. Figure 9.27
shows the gradation requirements for the 12.5 mm (1/2 in.) nominal-sized mix.
To control segregation, aggregates are sorted into stockpiles based on
size. The designer of an asphalt concrete mix must select a blend of stock-
piles that meets the source, consensus, and gradation requirements.

Binder Selection The binder is selected based on the maximum and minimum
pavement temperatures, as discussed earlier. In addition to the specification
tests, the specific gravity and the rotational viscosity versus temperature re-
lationship for the selected asphalt binder must be measured. The specific grav-
ity is needed for the void analysis. The viscosity-temperature relationship is
needed to determine the required mixing and compaction temperatures. The
Superpave method requires mixing the asphalt and aggregates at a tempera-
ture at which the rotational viscosity of the asphalt binder is
and the compacting temperature corresponds to a viscosity of 0.280

20 Pa # s

0.170 ;

30 Pa # s. ;

T ABLE 9.8 Superpave Consensus Aggregate Properties

Course Fine

Aggregate Aggregate Sand Flat and

Angularity Angularity Equivalency Elongated

Design Level (% min) (% min) (% min) (% min)

Light Traffic 55/- — 40 —

Medium Traffic 75/- 40 40 10

Heavy Traffic 85/80* 45 45 10

*85/80 denotes minimum percentages of one fractured face / two or more fractured faces