Low Temperature Characterization of Asphalt Mixtures by Measuring Visco-Elastic Properties under Thermal Loading

Low temperature thermal cracking is a common type of failure in asphalt pavement that occurs particularly in cold regions or locations with significant daily temperature fluctuations. The resistance of asphalt mixtures to low temperature cracking is generally influenced by the thermal contraction/expansion, visco-elastic, and fracture properties of the asphalt mixture. The accurate characterization of these properties is essential to the meaningful modeling of thermal cracking in asphalt pavements and, thus, the design of thermal cracking resistant mixtures. This paper describes a fundamental approach to determine the visco-elastic properties of asphalt mixtures from direct measurements of thermal stress and strain using a uniaxial test device recently developed at the University of Nevada, Reno. The relaxation modulus was computed in the temperature domain using linear visco-elastic constitutive equation, known as Boltzmann's superposition principle, from the measured, thermally-induced stress and strain during the test. Five distinct stages were identified from the relaxation modulus change with temperature: viscous softening, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages. The proposed approach was used to assess four hot mixed asphalt mixtures made from three aggregate sources with different mineralogy (Quartzite, Limestone and Rhyolite) and two different binder grades (PG64-22 and PG64-28) from different sources. It was found that the evolutions of thermal stress and strain are significantly influenced by the grade of asphalt binder and aggregate source. The relaxation modulus and the derived thermal visco-elastic properties were highly affected by the grade of asphalt binder.