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KERCHER ENGINEERING, INC. “If a job is worth doing, it is worth doing right!” |
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Specifying Superpave and Del DOT Implementation As stated in Part 2 of
this 3 Part series, the
Superpave system
provides a new approach specifying component materials, mixture design
and analysis, and pavement performance prediction in order to
consistently
produce superior performing hot mix asphalt (HMA). Two key
components
of specifying Superpave is the performance graded (PG) asphalt cement
binder
and the nominal aggregate size. PG Binder Grades Superpave’s PG binders must able to perform over the full range of expected temperatures that the pavement is expected to experience over its design life. During the winter, it must be able to resist low temperature cracking and must remain sufficiently stiff during extended warm periods in the summer. The selection of the binder to ensure proper performance is based on two factors: pavement temperatures and traffic. First, the expected high and low pavement temperatures must be selected. Once, the temperatures are selected, adjustments can be made for traffic volumes or traffic conditions. Traffic Volumes Adjustments to the PG grade
of asphalt cement may need to
be considered
if the pavement will be subjected to heavy traffic volumes.
Table 1. Binder Adjustments for Traffic Volumes Traffic Condition Also, there may be a need for making adjustments to the PG grade of asphalt cement if slow moving or standing moving traffic is expected.
Table 2. Binder Adjustments for Traffic Conditions
Aggregate Size The Superpave system produces an asphalt concrete mixture that contains a high percentage of larger aggregate that is durable and angular, while minimizing the amount of fine aggregate in order to provide stone-on-stone contact. This high degree of larger, higher quality aggregate with stone-on-stone contact produces a strong, structural framework that is necessary to resist rutting. As such, aggregate size and gradation are critical to the long-term performance of the HMA. Fortunately, local agencies will not be involved in determining the gradation of the aggregate. However, agencies will specify the nominal maximum size of aggregate. Maximum nominal size is defined as one sieve size smaller than the first sieve to retain more than 10% of the aggregate sample by weight – it is not the maximum size aggregate. Superpave defines five maximum nominal sizes of aggregate:
Since the maximum size
aggregate in Superpave mixes is larger
than the
specified nominal maximum aggregate size, the old rule of thumb for
minimum
layer thickness of 2 times the maximum aggregate size is no longer
valid.
The minimum HMA layer thickness for Superpave mixes should be increased
to at least 3 times the nominal maximum aggregate size.
Conventional HMA
Superpave Figure 1. Minimum Layer Thickness
DelDOT Implementation PG Binder Grades DelDOT is currently using PG
64 –22 as the standard binder
grade.
Locations which have high levels of traffic are adjusted one grade
higher
for the high temperature to 70 –22. Locations which have a high level
of
heavy truck and/or bus traffic may be adjusted upward two grades for
the
high temperature to 76 –22.
Table 3. PG Binder Grades Nominal Aggregate Size For the wearing course (Old
System - DelDOT Type C
material), the
maximum nominal size aggregate is 9.5 mm for most situations and 12.5
mm
for high traffic roadways. The maximum nominal size aggregate for
the binder course (Old System - DelDOT Type B material) is 19 mm.
If a bituminous concrete base course (BCBC) is needed, the maximum
nominal
size aggregate is the same as for the binder course (19 mm).
However,
the asphalt concrete will be reduced.
Table 4. Nominal Aggregate Size
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