Correlated time-variation of bulk microstructure and rheology in asphalt binders

We use near-infrared dark-field optical microscopy to probe isothermal time variation of the volume fraction of naturally-occurring, subsurface microstructures in PG 64-22 asphalt binders at temperature T=30 degrees C, following a rapid heating (cooling) increment |T|=20 degrees C from initial temperature T010 degrees C(50 degrees C). We compare these microstructure variations with isothermal time variations of the magnitud of the bulk complex shear modulus measured for identical sample conditions with a Dynamic Shear Rheometer. The main findings are: (1) Microstructure volume fraction (inferred from intensity I(t) of near-infrared optical scatter) and |G(t)| both continue to change appreciably long after measurable changes of binder temperature cease. Moreover, delayed time variations in I(t) and |G(t)| (2) correlate closely with each other; (3) evolve on three distinct time scales - several minutes, approximate to 1 h, >1 day; (4) depend on binder aging; (5) are more pronounced after a cooling step (T=-20 degrees C) than after a heating step (T=+20 degrees C); and (6) account for hysteresis in I(t) and |G(t)| curves observed during heating-cooling cycles.