Stiffness m-value and the Low Temperature Relaxation Properties of Asphalt Binders

The development of the Strategic Highway Research Program (SHRP) low temperature binder specifications was based on the observation that asphalt mixture creep stiffness at 2 hours correlated well with instances of transverse cracking. Extending the correlation to asphalt binder stiffness and using time-temperature superposition, a maximum stiffness value was imposed on the binders and a limiting temperature at which the stiffness at 60 seconds reached the limiting value was selected as a specification criterion. The slope at 60 seconds of the stiffness vs. time curve on a double logarithmic scale, the so-called “m-value” was introduced as an additional criterion based on the idea that a low m-value meant a slower relaxation of the thermal stresses, which was detrimental for the performance. This paper investigates the use of the “m-value” as a specification parameter for asphalt binder physical characterization. A comparison between m and the corresponding slope for the relaxation modulus is performed. The influence of m-value on thermal stress calculation performed in accordance to AASHTO MP1a is also investigated.