Superpvae

An Executive Summary
of the Superpave System

he use of Superior Performing
Asphalt Pavement technology or
Superpave is increasing around the
country. Nearly every state in the US
is making some commitment to
implement part or all of Superpave
technology. From the point of view
of many in the highway community,
Superpave is the future of hot mix
pavements.
Superpave was created tomake
the best use of asphalt paving tech-

T

Superpave,
The Future of
Hot Mix Asphalt
Pavements
By Tim Murphy,
President, JFG Technical Center

nology and to present a system that
would optimize asphalt mixture
resistance to permanent deformation,
fatigue cracking and low temperature cracking.
The key parts of this process are
the Performance Graded (PG) system
for specifying theproperties of the
asphalt binder and the volumetric and
densification characteristics determined by the Superpave Gyratory
Compactor (SGC). The
system was developed and calibrated

for a wide range of applications—
from farm-to-market roads all the way
through high-volume interstate roads.

Superpave Versus Conventional Design
How does Superpave compare with
current technology? Theconventional mix design system, known as
the Marshall method of mix design,
primarily addresses the determination of asphalt binder content, while
Superpave addresses all the elements
of mix design. The primary elements
of Superpave volumetric design are:
• Selection of component
materials,
• Volumetric proportioning of
aggregate and binder, and
• Evaluation of the compacted
mixture.Selecting Materials
Selection of component materials
consists of determining the acceptability of mineral aggregates, asphalt
binders, and any modifiers for use in
a mix design. Traffic data is used for
asphalt binder selection, mineral
aggregate selection, and mixture testing criteria.
Aggregate gradation selection is
also a part of the Superpave system.
Superpave requires differentgradation limits for different types of mix-

tures. A helpful tool in the design
phase is the 0.45-power gradation
chart (Figure 1) that uses four control sizes and a restricted zone. The
control sizes used are:
Figure 1: Superpave Gradation for a 19.0 mm Mixture
100
90

Maximum Density Line

80
60

Control Point

50
40

Restricted Zone

30
20
10
0

.075 mm

2.36 mm

19mm 25 mm

Figure 2: The Sand Hump Phenomenon

Sieve Size (raised to 0.45 power)
50

• Maximum sieve size
• Nominal maximum sieve size
• 2.36-millimeter sieve
• 0.075-millimeter sieve.
The restricted zone is an area
surrounding the maximum density line
from the 2.36-millimeter sieve to the
0.3-millimeter sieve. Gradations should
avoid passing through the restricted
zone. The controlpoints,along with
the restricted zone,are used to control
the shape of the gradation curve.
The restricted zone is used for two
purposes. Gradations passing through
the restricted zone have been
observed to have problems meeting
some compacted mixture properties,
specifically the percentage of voids in
the mineral aggregate (VMA). This
mixture property is used as an impor-

RestrictedZone
Partial gradation
shown of a
typical aggregate
structure plotted
on the 0.45
power gradation
chart

% Passing

% Passing

70

tant indicator of mix durability.
The restricted zone also serves to
prevent gradations having a “hump”
around the 1.18 and 0.6-millimeter
sieves, as in Figure 2. A humped gradation is generally associated with a
high percentage of fine, roundedsand in the mix. In effect, the zone
restricts the use of a high percentage
of rounded sands. This is advantageous because excessively rounded
aggregates are generally associated
with poor shear resistance, a major
cause of rutting in asphalt mixes.

the trial gradations are compared
with mix criteria. Volumetric criteria
consists of
• percent air voids
• voids in the mineral...