Compacting for Superpave Success

Asphalt Article December 28, 2000
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A number of state highway departments have begun using the Superpave
method to design their hot-mix asphalt (HMA) mixtures. In most
cases, these mixtures have been able to be produced, placed, and
compacted without significant problems. The mixtures are
relatively stiff and stable under the compaction equipment and
the required level of density can be achieved with normal
rolling procedures.

In other cases, however, use of the
Superpave mix designs has resulted in a "tender mix," one that
is difficult to properly compact. These mixes move and check or
crack under the applied compactive effort of the rollers. The
desired level of density is hard to obtain because the rollers
have to attempt to catch up with the mix that is moving both
longitudinally and transversely in front of them.

Superpave
mix designs

In most cases, the requirements for a Superpave
mix are very similar to the requirements for a Marshall mix
design. The primary exception, for what has sometimes been
called Superpave Level 1 or Volumetric design, is the use of the
gyratory compactor instead of the Marshall hammer to compact the
laboratory prepared specimens. For high levels of traffic, use
of the Superpave mix method typically results in a lower asphalt
content in the mix for a given type and gradation of aggregate
used in the mix.

Superpave mix design methods require the
use of volumetric calculations to determine the properties of
the mix. Similar to the Marshall mix design method, the air-void
content and the voids in mineral aggregate (VMA) content of the
mix are very important properties. The mix is typically designed
for a given air-void content and the VMA content is used along
with other mix characteristics to determine the optimum asphalt
or binder content for a given mix.

Because of the greater
compactive effort imparted to the laboratory mix by the gyratory
compactor compared to the Marshall hammer, the VMA value for the
Superpave mix is normally somewhat lower than the HMA mix using
the same aggregate type and gradation but compacted using the
Marshall hammer. For this reason, it is sometimes more difficult
to meet a minimum VMA value for a Superpave mix design compared
to a Marshall mix design. (VMA is not usually a specification
requirement for a HMA mix designed using the Hveem method.)

The original Superpave specification for aggregate gradation set
up a so-called "restricted zone" through which the combined
coarse and fine aggregate gradation was not allowed to pass. It
was thought that by keeping the aggregate gradation out of the
restricted zone, when plotted on 0.45 power gradation graph
paper, production of a tender mix would be avoided. The location
and the size of the restricted zone changes as the nominal top
size of the aggregate incorporated into the HMA mix changes.

Further, it was originally recommended that the aggregate
gradation be kept on the coarse (bottom) side of the restricted
zone. The reason for this suggestion is to increase the amount
of coarse aggregate in the mix and thereby increase the internal
strength of the HMA mix. A mix that has a gradation above the
restricted zone contains a greater amount of fine aggregate and,
thus, is theoretically less strong than the mix, which contains
more coarse aggregate.

Various state highway departments
investigated the aggregate gradations used in their HMA mixes
over the years. It was found that some mixtures that have an
excellent performance history under heavy traffic contain
aggregate gradations that would pass through the restricted
zone. Based on this information, the requirement that the
aggregate gradation shall not pass through the restricted zone
has been modified to a recommendation that the gradation should
not pass through the zone. This change means that HMA mixtures
that have a good performance history but have an aggregate
gradation that passes through the restricted zone can still be
used, which makes good common sense.

The Superpave mix

The Superpave mix design gradation method provides a method to
determine a so-called maximum density line. The maximum density
line in the Superpave methods is drawn from a point of 100%
passing on the first size larger than the nominal maximum size
sieve to the zero point.

Theoretically, an aggregate
gradation that approximates the maximum density line would
result in a HMA mix that has both a minimum VMA content and also
a minimum binder content. Such a mix would be very sensitive to
the amount of asphalt cement added to the mix. In addition, such
a mix would be very sensitive to any moisture remaining in the
aggregate during mix production at the asphalt plant. Thus, it
is recommended that a mix not be produced that has a combined
coarse and fine aggregate gradation that approaches a straight
line similar to the maximum density line.

It has been found,
however, that as the gradation of the combined coarse and fine
aggregate is moved away from the so-called maximum density line
on the coarse (bottom) side, that the VMA content of the mix is
actually reduced instead of increased. In turn, the density of
the mix is actually increased instead of decreased.

The
question has been raised, therefore, as to whether or not the
line called the maximum density line in the Superpave system is
really the true maximum density line. Indeed, it has been found
for many HMA mixes the minimum VMA value, the minimum asphalt
content, and the maximum density occurs with an aggregate
gradation that is just below the restricted zone-just where the
best mix is supposed to be designed.

Mixtures that are at a
minimum VMA content have a distinct possibility to be tender
during compaction. If the binder content of the plant produced
mix is slightly high, the mix may become unstable under the
rollers.

If the mix contains some residual moisture, again
the mix might be tender and move under the compaction equipment.
Even though the mix might meet all of the Superpave mix design
requirements, that mix might not be able to be properly
compacted on the roadway, in part because of the sensitivity of
the mix to an excess of fluids content-asphalt cement and/or
moisture.

It must be pointed out that tender mixes have
existed for years. Mixes that are produced at the minimum VMA
limit will be prone to tenderness whether designed by the
Superpave method, the Marshall method, or the Hveem method.
While most Superpave HMA mixtures are quite stable and stiff,
some of these mixtures, particularly those with gradations just
on the coarse side of the restricted zone, may be internally
unstable and very difficult to compact.

Compaction of stiff
mixtures

Superpave mixtures that are properly designed will
be reasonably stiff and stable and will require a considerable
amount of compactive effort in order to attain the required
degree of density. The mix will support the weight of the
compaction equipment directly behind the paver. If the mix is
placed at a temperature of 280 deg F or greater, the rollers
will typically be able to properly compact the mix before it
reaches a temperature of 175 deg F.

Most often, three
rollers are used-a breakdown or initial roller, an intermediate
roller, and a finish roller. For breakdown rolling, a vibratory
steel-wheel roller is most often used.

For intermediate
rolling, a pneumatic tire (rubber-tire) roller is generally
employed although sometimes a second vibratory roller is used.
Finish rolling is normally done with a static steel-wheel
roller.

The breakdown and intermediate rollers should stay
close to the paver. If the mix is stable, a bow wave will not
occur in front of the vibratory roller drum and the mix will not
exhibit any cracking or checking. With a relatively stiff mix,
the finish roller should also be close to the paver because
there will be minimal marks to remove from the breakdown and
intermediate rollers.

For very stiff mixes, or when a high
degree of density is desired, a pneumatic tire roller should be
employed for breakdown rolling. In this case, for intermediate
rolling, a vibratory steel-wheel roller follows directly behind
the pneumatic roller and finish rolling is again performed with
the static steel-wheel roller.

Because of the internal
stability and strength of the stiff mix, more compactive effort
may need to be applied in order to obtain a given level of
density (percent of the theoretical maximum density) but the mix
will not move under the compaction equipment during the rolling
process.

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