Optical characterization of temperature- and composition-dependent microstructure in asphalt binders

We introduce noncontact optical microscopy and optical scattering to characterize asphalt binder microstructure at temperatures ranging from 15 degrees C to 85 degrees C for two compositionally different asphalt binders. We benchmark optical measurements against rheometric measurements of the magnitude of the temperature-dependent bulk complex shear modulus vertical bar G*(T)vertical bar. The main findings are: (1) Elongated (similar to 5 x 1 mu m), striped microstructures (known from AFM studies as 'bees' because they resemble bumble-bees) are resolved optically, found to reside primarily at the surface and do not reappear immediately after a single heating-cooling cycle. (2) Smaller (similar to 1 mu m(2)) microstructures with no observable internal structure (hereafter dubbed 'ants'), are found to reside primarily in the bulk, to persist aftermultiple thermal cycles and to scatter light strongly. Optical scattering from 'ants' decreases to zero with heating from 15 degrees C to 65 degrees C, but recovers completely upon cooling back to 15 degrees C, albeit with distinct hysteresis. (3) Rheometric measurements of vertical bar G*(T)vertical bar reveal hysteresis that closely resembles that observed by optical scatter, suggesting that thermally driven changes in microstructure volume fraction cause corresponding changes in vertical bar G*(T)vertical bar.