In 1998 the MDOT changed its method of designing "hot mix"
for Maine roads. The change involved a switch from the "old"
Hveem design method to the "superpave" design method.
Generally, this results in a paving mix which is coarser, has less
sand, and less liquid asphalt. It does " lay down"
differently than the "old" B,C or D mixes.
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Superpave stands for "SUperior PERforming Asphalt PAVEment" and
was developed through a Federal government program whose goal
was to relate characteristics of hot mix asphalt to pavement
performance.
The result was the Superpave system for the design and analysis
of asphalt mixes.
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Superpave raises expectations of a "super" pavement
compared to the conventional mixes. In reality, it is a better
mix which combines the best design methods in the industry
and
relates them to real life conditions.
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Maine DOT has accepted this system of designing asphalt pavements
and almost all designs in 1998 were under this new system.
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MDOT placed about 650,000 megagrams of Superpave in 1998 and
mostly successfully. Rate of failure is about same as the conventional
mixes and mostly due to construction practices or poor bases.
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Superpave mixes should be much better than the previous "Hveem-designed" mixes,
as long as drainage and base conditions are adequate.
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Density and rideability is the goal of a quality Superpave mix.
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There is no such thing as a Superpave equivalent for DOT's "skinny
mix" (maintenance mulch).
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There is no such thing as a State-approved B, C, or D mix anymore.
(they may be available but they are probably the plant's local
recipe).
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All towns should require a State-approved mix design for its
hot mix pavements.
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Although not exact matches, the "old" designation
of a D mix is similar to a 9.5 mm mix; a C mix is similar to
a
12.5 mm mix; and a B mix is similar to a 19 mm mix.
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The "mm" designation relates to the nominal maximum
size of the aggregate (not the layer thickness !!)
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Superpave must be put down thicker than the conventional mixes.
Generally, the minimum compacted lift thickness of a Superpave
mix should be about 2-1/2 to 3 times the "mm" designation.
(ie. min. thickness of a 19 mm mix is about 2 inch (50 mm)).
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Superpave mixes have higher quality aggregate, very little sand,
and less liquid asphalt binder, making them more rut-resistant
and durable, as long as they are applied correctly.
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Any pavement is only as good as the underlying material and
whether good drainage exists. A super clean surface must be ready
prior to paving.
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Compaction is even more critical with Superpave mixes and must
be started earlier than normal and have the rollers close behind
the paver. Densities should be 92% to 97%.
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The liquid asphalt binder used is now a "performance-graded" binder
and is specified as PG 64 -28. The numbers relate to the average
maximum and minimum temperatures in Celsius which the
mix can withstand. In Maine and other New England states, PG
64 -28 is the most common liquid asphalt used in Superpave
mixes.
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Prices for Superpave should not be much different than the previous
mixes. The extra benefits of Superpave will far outweigh any extra
tonnage costs.
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No special equipment is needed for Superpave laydown.
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MDOT does QC/QA testing on Superpave mixes and most towns
will not require this testing. "QC" is quality control and
is what a producer does to its product to track its quality. "QA" is
what the buyer does to track the mix quality.
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Towns should require some testing of the mix either by the contractor
or a private testing form. Gradations, asphalt contents, and densities
should be performed for every 1000 tons.
