Superpave Volumetric Mix Design

Asphalt Article December 28, 2000
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Superpave is an improved system for specifying hot-mix asphalt
(HMA) component materials, HMA mix design and analysis, and
pavement performance prediction. Implementation of Superpave,
when fully developed, should minimize or eliminate rutting,
fatigue cracking, and low temperature cracking.

As states
are beginning to experiment and/or implement Superpave, the need
for refining certain aspects of the method are becoming
apparent. Two are given here.

A Superpave gyratory compactor
(SGC) is used in the volumetric mix design method to compact the
specimen to Nmaximum gyrations. The bulk specific gravity (Gmb)
of the compacted specimens can be estimated by the computer
program used in SGC. However, the actual Gmb is different than
the estimated Gmb and has to be measured. A correction factor is
then obtained from the ratio of measured and estimated Gmb. This
correction factor is then used to obtain the corrected Gmb at
Ndesign and Ninitial gyrations, which are then used to calculate
the air voids content (or percentage of theoretical maximum
density) at Ndesign and Ninitial. Recent experience has shown
that the correction factor obtained at Nmaximum is not
applicable at Ndesign and, therefore, can give an erroneous air
void content at Ndesign. This means that the correction factor
must be obtained at Ndesign gyrations to ensure that 4% air
voids are actually obtained at the optimum asphalt content.

The Superpave volumetric mix design method has a table for
selecting the gyratory compaction effort corresponding to seven
categories of design ESALs and four categories of high air
temperatures. Therefore, the table gives 7 x 4 = 28 different
compactive efforts depending on traffic and climate. This table
needs to be shortened especially in view of inadequate
supporting data. Recent experience has also shown that the
gyratory compaction effort in this table appears to be
excessive.

The excessive compaction effort results in low
voids in the mineral aggregate (VMA), which may not meet the
minimum VMA requirements. This forces the mix designer to
coarsen the mix gradation excessively. Such mixes have been
observed to be permeable in some instances. It should be noted
that the minimum VMA requirements were established on 75-blow
Marshall compactive effort. In view of the preceding discussion,
there is a need to refine the table on gyratory compactive
effort.

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