Glass transition and physical hardening of asphalts

Glass transition and physical hardening was studied in straight-run paving asphalt binders. Two methods, modulated differential scanning calorimetry and dynamic mechanical analysis, were utilized in this study.;Kinetic nature of the glass transition was observed in studied asphalts. The glass transition temperature, which represents the transition, was found to be a quantity dependent on observation time and thus meaningless without observation time being specified. The glass transition of asphalts was found to be very broad on the temperature scale due to complexity of the chemical composition. Asphalts were found to be multiphase systems, with glassy amorphous, non-glassy amorphous and crystalline domains existing between approximately 10 and -45°C.;Physical hardening was observed in asphalts at broad range of temperatures. Physical aging, i.e. structural relaxation of the glass, was identified as a major process contributing to physical hardening.;Direct effect of crystallization was rather insignificant in the temperature range of glass transition. However, the presence of crystals was suggested to affect the molecular mobility of the amorphous phase and thus increase the hardening rate and also extent the phenomenon to higher temperatures outside the normal glass transition range. The concept of rigid amorphous phase was offered.;The effect of the physical hardening could generally be reversed upon heating to higher temperature. Although for semi-crystalline asphalt, temperature higher by 50°C than the isothermal storage temperature, was found not to be sufficient to successfully reverse the hardening.;Effect of thermal stress on the hardening rate was studied. It was found that the imposed stress was either not significant factor affecting the asphalt hardening or the imposed stress was too low to affect hardening rate significantly.;Rheological model able to capture the dependence of relaxation times on the isothermal storage time, reference temperature and temperature was derived from Williams-Landel-Ferry equation and successfully applied to studied asphalts.