ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 357

100

Max density
line

Nom
max
size

Max
size

Control point

0

0.075 0.3 2.36 4.75 9.5 12.5 19.0

Sieve size, mm (raised to 0.45 power)

FIGURE 9.27 Superpave gradation limits for 12.5 mm nominal

maximum size.

Design Aggregate Structure After selecting the appropriate aggregate, binder,
and modifiers (if any), trial specimens are prepared with three different ag-
gregate gradations and asphalt contents. There are equations for estimating
the optimum asphalt content for use in the specimens prepared for deter-
mining the design aggregate structure. However, these equations are empirical
and the designer is given latitude to estimate the asphalt content. Specimens
are compacted using the Superpave gyratory compactor (Figure 9.23) with a
gyration angle of 1.25 degrees and a constant vertical pressure of 600 kPa
(87 psi). The number of gyrations used for compaction is determined based
on the traffic level, as shown in Table 9.9.
As shown in Table 9.9, the Superpave method recognizes three critical
stages of compaction, initial, design, and maximum. The design compaction
level corresponds to the compaction that is anticipated at the comple-
tion of the construction process. The maximum compaction corre-
sponds to the ultimate density level that will occur after the pavement has

N des

N max

T ABLE 9.9 Number of Gyrations at Specific
Design Traffic Levels

Traffic Level (10 6 ESAL*)

6 0.3 0.3 to 3 3 to 30 7 30

N ini 6 7 8 9

N des 50 75 115 125

N max 75 100 160 205

*ESAL is the 18,000-lb equivalent single axle load. It is a
design factor used in the design of pavement that
considers both traffic volume and loads (Huang 2004).