Effect of cooling rate on thermal cracking of asphalt concrete pavements

Asphalt concrete material selection procedures rely mainly on specifying low temperature binder properties as criteria for thermal cracking prevention. However, the effects of cooling rate are not rigorously considered in these methods. This study examines the effects of cooling rate on accumulation of thermal stresses in asphalt pavements. Enhanced integrated climatic model (EICM) simulations have indicated that the lowest pavement temperature and highest cooling rate events do not usually occur simultaneously. In more severe climates, such as the Midwest USA, there are frequent occurrences of high cooling rate events in the range of 1 to 3°C/hour. Using fundamental viscoelasticity formulations and Boltzmann's superposition principle thermally induced stresses can be estimated with relative ease. This formulation utilizes low temperature viscoelastic properties such as creep compliance or relaxation modulus as an input and can be readily used as a tool to identify materials with high susceptibility to thermal cracking as a result of severe cooling rate. To verify the dependence of asphalt concrete's thermal cracking performance on cooling rates; five mixtures from the SHRP General Pavement Study (GPS) sections were studied. Thermally induced stresses at different cooling rates were computed for a pavement section by means of viscoelastic finite-element simulations. In addition, thermal cracking predictions were made using TCMODEL, which is the thermal cracking software used in the AASHTO Mechanistic Empirical Pavement Design Guide. For different cooling rates, the amount of induced thermal stresses was found to vary significantly for asphalt concrete mixtures produced with same Superpave PG binder grades. Mixtures having high stress accumulations according to the analytical solution were found to perform poorly in the field and were predicted to have poor performance in the finite-element and TCMODEL simulations. The results of this study indicate that when evaluating low temperature cracking performance, both cooling rate and lowest pavement temperature may need to be considered.