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Photo:  Asphalt rutting on hwy
Photo:  Asphalt showing fatigue cracking
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Rutting and Fatigue Specifications for Asphalt Binders
(Sponsored by Wisconsin Highway Research Program)

Researcher by
Rodrigo Delgadillo, Kitae Nam, Prof. Hussain Bahia
Dept of Civil & Environmental Engineering

Two 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.

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