Fundamental Properties of Asphalts and Modified Asphalts, III

SUBTASK 2-2.2. SUPPORT OF THE MECHANISTIC-EMPIRICAL PAVEMENT
DESIGN GUIDE
Statement of Problem
The global aging system is a series of models that attempt to predict the change in binder
viscosity in hot-mix asphalt (HMA) pavement with time and depth. These models are an integral
part of the NCHRP 1-37A Mechanistic-Empirical Pavement Design Guide. The models were
developed from an extensive database consisting of capillarity viscosity, and penetration and
softening point measurements converted to viscosity, over a broad range of temperatures [Mirza
and Witczak 1995]. Christensen and Bonaquist [2006] modified the global aging system models
to make use of rational rheological measurements and binder master curve parameters while
maintaining consistency with the original models. They noted in regard to a part of their study,
which employed the global aging system, that the results should be considered approximate,
since “there are many questions concerning the accuracy of the global aging system.”
WRI, also in 2006, evaluated how well the GAS models predicted the actual aging that occurred
at the FHWA/WRI Arizona validation site [Farrar et al. 2006]. One of the main findings of the
study was that oxidative aging occurring at the Arizona site was substantially greater than
predicted by the GAS models, particularly in the top 13 mm of the pavement. Al-Azri et al.
[2006] in an extensive investigation of selected Texas highway pavements concerning
unmodified binder aging reported the level of hardening reached in pavement binders
significantly exceeded estimated values calculated by the global aging system both at the
pavement surface and at 125 mm below the surface.
More specifically, the global aging system was developed based on laboratory tests of
conventional “S” binders. These linear or straight-line Class “S” asphalts were non-modified,
and the authors of the global aging system (Mirza and Witczak) advised the global aging models
should not be used for modified asphalts, Class “W” (waxy) or Class “B” (blown) asphalts. The
S, B, and W designations were originally developed by Heukelom [1973] who observed the
linear and nonlinear relationships of asphalts using the Bitumen Test Data Chart developed in the
1960’s.
In addition: (1) the global aging system does not address photo-oxidation effects on the surface,
(2) the air void adjustment factor is based on limited data and considered “optional,” (3) the
global aging system does not take into account sealer/rejuvenator, chip seals, or open graded
friction course effects on pavement aging, and (4) the global aging system is not based on
“fundamental” binder properties.
Approaches
The accuracy and possible modifications to the global aging system are being studied at WRI by
analyses of data and materials from the FHWA/WRI validation sites and laboratory aging
(RTFO and PAV). Analytical tests applied include infrared spectroscopy and dynamic shear and
bending beam rheometry of unaged and aged materials. Master curves of unaged and aged
asphalts and age based shift factors are being studied to quantify age related changes.
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