Development of a New Method for Assessing Asphalt Binder Performance Durability (0-1872-1)

This project was a comprehensive study directed at developing an improved method of screening asphalt binders for long-term pavement performance. A new dynamic shear rheometer (DSR) function which correlates well with ductility was coupled with a new aging procedure in a tentative specification which should guard against failure caused by premature asphalt hardening and consequent fatigue cracking. The correlation was found for unmodified asphalts between ductility (at 15 °C, 1 cm/min) below 10 cm and the DSR function G'/(r)'/G'). The correlation was originally developed for DSR measurements at 15 °C and 0.005 rad/s. These conditions were shifted to 43 °C and 10 rad/s by using time-temperature superposition to produce a method that is easily accessible to standard laboratory rheological equipment and methods. The developed method is not adequate for modified asphalts. The aging procedure that is recommended uses the pressure aging vessel (PAV) apparatus but is modified by taking advantage of the higher average aging rate when the asphalt is aged in thiner films. This, combined with somewhat longer aging, results in more extended binder aging and thus a more rigorous test of durability than the standard PAV method. At the same time, the resulting rankings of aged materials is more representative of rankings that are obtained from aging at atmospheric air pressure and 60 °C. For modified asphalts, the results were complex. Generally for a given value of the DSR function, the ductility was better than indicated by the DSR function correlation for unmodified asphalts. Larger amounts of modifier produced increasing values of ductility for a given function value. This result was very asphalt dependent, however, so no general correlation could be found. As modified binders oxidize, the asphalt hardens and the improvement to ductility imparted by modifiers decreases. After enough aging, the improvement is gone and modified binders perform no better than their aged unmodified counterpart. A critical issue is whether the life extension produced by modifiers is life-cycle cost effective.