Superpave System Works
The Bottom Line:
Superpave System Works
by
By Thomas W. Kennedy
Technical Director, SCSC
In sorting out many of the controversies regarding Superpave,
it becomes evident that there are many driving forces, such as:
- Those who recognize that the system is not perfect.
- Those who want to destroy or take it apart for whatever personal
- reason.
- Those who have vested interests in the elimination of Superpave or a part of Superpave.
- Those who always feel there is more to be done and, until done, we cannot accept Superpave, and
- Those who believe Superpave is wrong.
Thus, some of the forces are well-meaning and some are self-serving
and at best it is difficult to determine which is which.
It should be remembered that the system seems to be working, when
it is not violated; and I hope we do not want to return to yesteryear.
Maximum density line
There are many ways to draw the maximum density line. SHRP selected
the method of connecting the origin and the maximum aggregate
size (when maximum size is defined according to SHRP Superpave)
because the available data (developed by the Asphalt Institute)
indicated that VMA responded to the line and increased as the
sum of the absolute values of the deviations from the line increased.
For the majority of the mixes evaluated to date this appears
to be true (the Asphalt Institute, South Central Superpave Center,
and possibly others). I have not seen data to the contrary, although
it is often rumored that it exists. I am also sure there are
exceptions.
If it is not true, the definition can be revisited, but not until
substantial data is presented and analyzed when the realities
and defects of the mix experiment are known.
It must be emphasized that the relationship between the sum of
deviations and VMA are aggregate system dependent. Thus, if a
different aggregate is substituted and the gradation is held constant,
the VMA will probably change. A given sum of deviation does not
produce the same VMA for all aggregate mixtures. There is minimal,
if any, correlation between sum of deviations and VMA for a wide
range of aggregate mixtures using pooled data.
Restricted zone
The restricted zone has nothing to do with VMA except to prevent
a gradation following the maximum density line. The restricted
zone eliminates many rounded, dirty, gap graded, very fine sand
gradations and helps assure the development of internal structure
and resistance to permanent deformation.
The width can be refined, but the elimination of the restricted
zone (without a well-thought-out replacement) opens the door to
the gradations which have exhibited poor performance. I might
note that the applicability of the binder specifications is then
questionable.
It has been reported that various states and other groups have
said that their mixes, which they feel are very satisfactory and
perform well, violate the SHRP gradation recommendations. Often
on further questioning these mixes are performing for periods
of 5 to 8 years rather than 15 to 25 years. The point is that
we have become so accustomed to early failures that short service
has become acceptable.
Gradation and VMA
The gradation requirements do not insure a minimum VMA. VMA requirements
are not easily satisfied and may require aggregate changes other
than gradation. It has been suggested that the gradation and
VMA requirements automatically force the gradation above the restricted
zone. Why someone would suggest this is a mystery to me since
it is almost always not true.
I would recommend that gradations for high volume, high performance
highways always pass below the restricted zone and that only gradation
for "secondary" highways be allowed to go above the
restricted zone. This is a life-cycle cost question.
In general
The maximum density line, VMA, etc., are concepts which are very
important to the performance of asphalt mixtures. They
are estimated on the basis of certain assumptions which may not always be met. Thus there are exceptions.
Will the gradation requirements always produce the absolutely
best mix? Probably not; but to date the Superpave mixtures are
performing well. I am sure that there are mixes which violate
the Superpave requirements which may perform well or at least
are judged to perform well. I would suggest, however, that we
can find more poorly performing mixtures using the old system,
than with the new system.
In summary, I would move slowly on these issues. Superpave was
not thrown together without a lot of thought, evaluation, and
discussion. To begin dismantling Superpave can assure failure.
In addition, failure to move ahead with implementation and its
use because of the fear of a possible problem or because everything
is not absolutely "perfect" will definitely assure failure.
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