WesTrack, FHWA's hot-mix asphalt (HMA) performance related specification
test facility in Nevada, passed its first significant trafficking
milestone in mid-September with the application of one million
equivalent single axle loads (ESAL's). The accelerated loading
effects are already beginning to show on the pavement. The application
of more than one million ESAL's, from April to September 1996,
has resulted in severe rutting in several of the test sections.
Several of the sections have rutted more than 25 mm. The question
is what can we learn from the performance of these sections.
WesTrack is an oval track made-up of 26 test sections with 21
different variables. The variables include asphalt content, gradation,
and roadway compaction. Over two years 10 million ESAL's are
to be applied to the track. This could be compared to a typical
pavement with a 15 year design life carrying 50 to 75 million
ESAL's. The track was designed to have all the sections fail
in the 2 year life of the experiment. Of the 26 sections placed
only four would meet Superpave mix design and roadway compaction
requirements.
Two mixes were designed using Superpave Volumetric Mix Design
procedures to evaluate the effect of construction variables on
pavement performance. Both mixes had a maximum aggregate size
of 19 mm. There was a coarse mix, with a gradation that plotted
below the .45 power maximum density line, and a fine mix, which
plotted above the .45 power maximum density line. The mixes placed
on the track were produced from a crushed gravel and local natural
sands. The mixes were designed to meet the Superpave Volumetric
Mix Design requirements for a traffic level of 3 to 10 million
ESAL's.
To evaluate construction variables on performance, the mixes were
placed at the optimum asphalt content, 0.7 above and 0.7 below
the optimum asphalt content. The road way compaction was also
varied. The mixes were compacted at 4, 8, and 12 percent in-place
air voids. To evaluate gradation changes on performance, the
fine mix had an additional 3% dust or minus 0.075 mm material
added to it. This mix was also place with varying asphalt content
and roadway compaction.
While all the in-place data on the mixes is not yet available
we can make some determinations on what is happening based on
the available information. For rutting the mixes were significantly
under designed. The application of one million ESAL's in the
first summer of operation at the track could be compared to a
roadway having to carry 50 to 75 million ESAL's in a typical design
life. The aggregate requirements for a 50 to 75 million ESAL roadway
are significantly higher than for a 3 million ESAL roadway. For
50 million ESAL's the coarse aggregate angularity requirement
is 100% 2 fractured faces with a strong recommendation that the
material should be 100 quarried stone. For Both Superpave mixes
the coarse aggregate angularity was 99/96 with smooth surfaces
for a proportion of total aggregate surface. Both mixes meet
requirements for fine aggregate angularity (FAA). The FAA was
45 for the fine mix and 48 for the coarse mix.
The current philosophy is that coarse graded mixes should perform
better under heavy loading conditions than fine graded mixes.
The preliminary performance at the test track indicates otherwise.
This raises the question why is the fine mix performing better
than the coarse mix to date?
The fine mix met all requirements for a 50 to 75 million ESAL
Superpave mix except for coarse aggregate angularity and VMA.
VMA is required for durability, fatigue resistance and moisture
sensitivity. Low VMA can help a mix improve rut resistance at
the expense of durability. In a fine graded mix the stiffness
is developed in the fine aggregate. An angular fine aggregate
will give the mix the rut resistance and the coarse aggregate
is just a filler used to reduce the asphalt content of the mix.
Mixes made of all fine aggregate will have extremely high asphalt
contents. For the WesTrack fine graded mix the fine aggregate
angularity was 45 quite good even though some natural sand was
used in it.
The coarse mix also meet all the requirement for a 50 to 75 million
ESAL Superpave mix except for coarse aggregate angularity and
VMA. For a coarse graded mix the stiffness is developed in the
coarse aggregate. The WesTrack coarse mix was produced from a
crushed gravel with a some rounded surface area. For a 50 to
75 million ESAL roadway this stone can cause significant problems,
which is exactly what happened. When the extremely heavy traffic
was place on a mix made up of aggregate with some smooth surfaces
that aggregate started to move.
Coarse mixes with quarried stone are known performers. The Europeans
have been using coarse mixes under heavy loading conditions for
many years. These mixes are known as Stone Mastic Asphalt (SMA).
The SMA mixes have been used in the U.S. for the past several
years with great success. The key to success of SMA mixes is
the aggregate requirement of 100 percent crushed faces, cubical
quarried stone be used. The Dayton stone used at WesTrack may
have had 99% one fractured face and 96% two fractured faces under
a liberal interpretation of fractured faces. However, under a
strict interpretation of fractured faces as defined in the latest
ASTM specification these numbers were dramatically reduced. Under
the ASTM procedure the fractured faces for the Dayton stone was
only 86/73, significantly less than that required for a 50 to
75 million ESAL roadway.
We have learned many things from the performance to date at WesTrack.
When evaluating a performance criteria such as rutting several
factors have to be taken into account. Traffic under all existing
design systems is intended for a long term period 15 to 20 years.
For an accelerated loading test the variations in conditions
has to be included in the design process. To truly evaluate the
rut resistance of a mix in an accelerated test facility the rate
of application of load has to be taken into account. To evaluate
rut resistance it may have been best to design the WesTrack mix
for 50 to 75 million ESAL's.
In the mix area we have learned that with the proper care in design
and construction fine graded mix may be placed on roadways carrying
heavy loads. The fine mixes at WesTrack were produced with too
much asphalt for a 50 million ESAL roadway but are exhibiting
reasonable performance with only 10 mm of rutting over one summer.
This performance would not be acceptable under real world conditions
but with a reduction in asphalt content and a minor adjustment
to gradation to increase VMA a mix may be produced that will perform.
Fatigue response is still a question, but we should have some
answers to this by next year.
The coarse graded mixes have not performed. These failures have
highlighted the significance of the need for high quality stone
for performance. The need for coarse aggregate with 100 percent
fractured faces, on high volume roads, is evident from the results
at WesTrack. Large stone alone will not provide rut resistance.
A high quality quarried material is needed to carry the traffic.
WesTrack was constructed to evaluate the effect of construction
variables on performance. We still have many questions left to
be answered from the results at the track. How much effect does
the variability of asphalt content have on performance, additionally
many compaction questions still need to be answered.
Over the next year hopefully we will have the answers to many of our questions on performance in relation to construction variables. We should also have several questions on the performance of Superpave mixes answered. The future is bright for asphalt as a paving material into the 21st century.
If you have any questions please contact John D'Angelo, Binder
ETG Chairman at 202-366-0121 or John.DAngelo@FHWA.DOT.GOV.
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