and Fatigue Specifications for Asphalt Binders
(Sponsored by Wisconsin Highway Research Program)
Kitae Nam, Prof. Hussain Bahia
Dept of Civil & Environmental Engineering
of the main distresses of asphalt pavements are rutting and fatigue.
Rutting is characterized by permanent deformation of the pavement.
It generally develops during the hot seasons, when the asphalt
is softer. It can be identified by ruts on the wheel path. Fatigue
is characterized by longitudinal cracks on the wheel path in the
first stage, and by alligator cracking in an advanced damage stage.
It occurs due to many of load repetitions (heavy trucks) that
causes the pavement to deflect until it cracks. For improving
the rutting and fatigue performance of an asphalt road, selecting
a good quality asphalt binder is a must. The current specifications
for asphalt binders (SUPERPAVE specifications) have critical gaps.
Binders are not ranked properly in terms of their contribution
to rutting resistance or fatigue resistance. A new proposal for
rutting and fatigue evaluation was presented in a project sponsored
by the National Highway Research Program (NCHRP 9-10 project).
Two new parameters were proposed for improving the characterization
of rutting and fatigue performance of asphalt binders. The rutting
parameter is called Gv (viscous component of the creep stiffness)
and the fatigue one is called Np20 (number of cycles for achieving
20% damage of the asphalt).
The purpose of the current research carried out at the University
of Wisconsin Madison is the development of new specifications
for asphalt binders based on this new parameters. The research
project is divided in two parts. The first one consists on laboratory
testing of a number of asphalt binders currently used in Wisconsin
and the development of new specifications. The second part corresponds
to the field validation of the findings from the first part. The
first part is already finished. A total of 19 different asphalt
binders, that included non modified asphalts and asphalt modified
chemically and with polymers. The results from the testing were
used together with pavement analysis programs to predict the rutting
and fatigue that a pavement would have when each of the tested
binders are used. This way, the binders were ranked according
to the predicted performance and specification limits were proposed.
For the second part of the research, a number road projects recently
constructed (2003 and 2004) have been selected for field validation
of the specifications. For the chosen projects, samples of asphalt
binder and asphalt mixture were taken from the field and tested
in the lab. The performance and the development of the distresses
in the pavements will be monitored during the following years
to correlate the lab results with the pavement performance.