California
vs. Nevada Polymer-Modified Asphalt Binders:
by
Chris C. Gerber and Roger Schlierkamp, Twining, Inc.
Caltrans and NDOT have each created
polymer-modified asphalt binders that meet PG “plus” specifications that differ
in their properties, but is there really a difference between the two? The
California Department of Transportation (Caltrans) and the Nevada Department of
Transportation (NDOT) use different polymer-modified asphalt binders. Since the
implementation of the Superpave performance grade (PG) system, several state
highway agencies, including Caltrans and NDOT, have experienced good
performance with polymer-modified hot mix asphalt (HMA) mixtures and have
created the PG “plus” specifications to enhance the performance of flexible
pavement in harsh climates.
Caltrans uses a polymer-modified
asphalt binder, PG64-28PM, in certain projects throughout the state. They use
PG64-28PM polymer-modified asphalt binders with their dense-graded asphalt
mixtures. The “PM” extension indicates that the binder meets the Caltrans PG
“plus” specifications which include the standard Superpave PG system plus
maximum phase angle on RTFO binder at the high critical performance temperature
and elastic recovery on RTFO binder at 77°F. NDOT uses the polymer-modified
asphalt binder PG64-28NV on all roadways in the northern part of the state. The
“NV” extension indicates that the binder meets the NDOT PG “plus”
specifications which include the standard Superpave PG system as well as
properties such as toughness and tenacity on original binder at 77°F, ductility
on original, and RTFO binder at 40°F.
In order to test their differences,
the asphalt binders were supplied by two manufacturers, Paramount Petroleum
Company and Valero Marketing and Supply Company. They each provided a blend of
PG64-28PM and a blend of PG64-28NV asphalt binder.
The HMA mixtures were designed using
the Marshall design method per the agency’s specifications. The optimum binder content was selected at 4%
air-voids and then used to determine the corresponding values for Marshall
stability and flow, VMA, and VFA for the appropriate relationships. The optimum
binder contents were 5.0% by total weight of mix for all evaluated mixtures.
The aggregate source was consistent for all mixtures, and the same gradation
was used. Laboratory tests were then performed for the following modes of
pavement failure: moisture damage, fatigue cracking, permanent deformation, and
thermal cracking. The tests revealed interesting results. For moisture damage,
the PM and NV binders displayed excellent resistance by displaying excellent
stiffness after 6 freeze-thaw cycles. For fatigue cracking, the NV binder
displayed significantly better resistance under repeated flexural bending. For
permanent deformation resistance, the NV binder exhibited more stability.
Lastly, for thermal cracking resistance, the NV binder displayed the lowest fracture
temperature and intermediate level of fracture stress, making it more resistant
to thermal cracking compared to the PM binder.
The results of the laboratory tests
show that the two different binders are in fact different. The PM mixture was
not as stable as the NV mixture.
Although the majority of California does not experience the extreme
temperatures that Nevada does, it is important to be familiar with other
products and standards. We are now aware of this other binder should we come
across a situation where this binder would be beneficial to use.
Reference:
Hajj, Elie Y., Sebaaly, Peter E., Schlierkamp, Roger W., Cortez, Edward.
“Laboratory Evaluation of PG64-28PM Polymer-Modified Asphalt Binders.” Western
Regional Superpave Center. January 2011.
About Twining: For over 100 years, Twining has been building a reputation for quality, reliability and expertise providing geotechnical, materials testing and construction inspection services. With five state-of –the-art laboratories and over 250 inspectors across California, Twining has the resources to meet changing needs of complex construction projects from multi-building hospital campuses to interstate highways. Twining employs some of the industry’s most well known construction materials experts who perform research and consult with regulatory agencies to shape the future of construction standard practices. Find out more about Twining innovations and services at www.twininginc.com or follow us on facebook at www.facebook.com/TwiningInc.
