Correlated Time-Variation of Bulk Microstructure and Rheology in Asphalt Binders

We use near-infrared dark-field optical microscopy to probe time variation of the density of naturally-occurring, sub-surface microstructures in PG 64-22 asphalt binders following a rapid heating (cooling) increment |ΔT| = 20 C from initial temperature T₀ = 10 C (50 C). We compare these microstructure variations with time variations of the magnitude |G*(T,t)| of the bulk complex shear modulus measured for identical sample conditions with a Dynamic Shear Rheometer (DSR). The main findings are: (1) Microstructure density (inferred from intensity I(T,t) of near-infrared optical scatter) and |G*(T,t)| both continue to change appreciably long after measurable changes of binder temperature cease. Moreover, delayed time variations in I(T,t) and |G*(T,t)| (2) correlate closely with each other; (3) evolve on three distinct time scales — several minutes, ∼ 1 hour, > 1 day; (4) are more pronounced after a cooling step (ΔT = -20 C) than after a heating step (ΔT = +20 C); and (5) account for hysteresis in I(T,t) and |G*(T,t)| curves observed during heating-cooling cycles.