Critical Behavior of Asphalt Mixtures Undergoing Glass Transition and Physical Hardening

In this study a device was developed that simultaneously tests two asphalt mixture beams; one unrestrained, and the other with restrained ends. The unrestrained beam is used to measure the one dimensional change of length with temperature, and consequently the glass transition temperature (Tg) and coefficients of expansion/contraction above and below the Tg. The restrained beam is used to capture the induced thermal stress buildup due to prevented contraction in the sample. The device, referred to as the Asphalt Thermal Cracking Analyzer (ATCA), was used to show how asphalt mixtures undergo isothermal contraction and physical hardening, and the consequences of this behavior for the buildup of thermal stress. A prediction model for the rate of physical hardening at different temperatures and conditioning times was developed based on a creep viscoelastic model, and was used to predict changes in binder stiffness and relaxation rate during isothermal conditioning. The model was then extended to mastics and a conversion factor was derived to shift results for applicability to asphalt mixture thermal strain calculation.;The physical hardening model of binders was coupled with relaxation modulus master curves and glass transition measurements to propose a model for the calculation of thermal stress buildup in mixtures as a continuous function of conditioning time and temperature. The model was validated using an elaborate set of experimentally derived input properties for a number of mixtures at different cooling rates.;An experimentally calibrated multiphase micro-mechanical model for asphalt mixture undergoing thermal shrinkage and glass transition was also developed in a finite element platform. The results where compared to experimental data measured using the ATCA device, which allowed measuring strain and stress build up during cooling.;The results of this study indicate that physical hardening of binders has important effects on the thermal stress and thermal strain accumulation in mixtures during cooling cycles. Failure to consider this time dependent behavior, which varies among binders, could lead to failure in predicting performance. The existing modulus and strain used in predicting thermal cracking of asphalt pavements need revisions to integrate a function for the time dependent changes.