CHARACTERIZATION OF ASPHALTS CEMENTS BY THERMOMICROSCOPY AND DIFFERENTIAL SCANNING CALORIMETRY: CORRELATION TO CLASSIC PHYSICAL PROPERTIES

Abstract

Numerous thcrmoanalyttcal methods can be used to evaluate asphalt cements. Optical thermomicroscopy techniques such as phase contrast microscopy and polarized light microscopy have been used to directly observe the in situ crystallization of aliphatic, waxy fractions, and to monitor any changes in structure that occur with temperature. These visual changes are then correlated with enthalpy changes resulting from precipitation/dissolution as measured by Differential Scanning Calorimetry (DSC) to better understand their thermal behavior. Temperature cycling studies show the phenomenon of crystallization to be completely reversible. Finally, the Tg and % crystallizable fraction as quantitatively determined by DSC are compared to classic physical properties of the asphalt cement. The Tg compares favorably to the glass transition temperature as measured by a dynamic mechanical analyzer at low frequencies, but correlations with the Fraass Brittle Temperature are lower than expected. Asphalts with large amounts of crystallizable material as measured by DSC show significant aberrations from a rheological mastercurve typical of homogeneous viscoelastic fluids, and these results become more pronounced with time. These same asphalts also exhibit changing rheological behavior as the thermal treatment of the sample is varied prior to testing. suggesting that crystallization requires time and fluidity for molecular agglomeration to occur. Hence, asphalt can and docs exist as a beterogcncous, biphasic liquid at ambient temperatures when sufficient crystallizable fractions are present. DSC offers a rapid, accurate technique to quantify this phenomenon and predict its effect on asphalt performance