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

Abstract

Summary We introduce noncontact optical microscopy and optical scattering to characterize asphalt binder microstructure at temperatures ranging from 15°C to 85°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 |G*(T)|. The main findings are: (1) Elongated (∼5 × 1 μm), 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 (∼1 μm2) microstructures with no observable internal structure (hereafter dubbed ‘ants’), are found to reside primarily in the bulk, to persist after multiple thermal cycles and to scatter light strongly. Optical scattering from ‘ants’ decreases to zero with heating from 15°C to 65°C, but recovers completely upon cooling back to 15°C, albeit with distinct hysteresis. (3) Rheometric measurements of |G*(T)| reveal hysteresis that closely resembles that observed by optical scatter, suggesting that thermally driven changes in microstructure volume fraction cause corresponding changes in |G*(T)|.

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

“…Here we explore the connection between microstructural and mechanical hysteresis in a way that complements our previous study (Ramm et al ., ). In Ramm et al .…”

“…These observations broaden and strengthen the connection between microstructural and rheological hysteresis developed in Ramm et al . ().…”

“…Here, near‐IR optical microscopy has the advantages over Atomic force microscopy of rapidly probing microstructures well beneath the surface at elevated temperatures. We do, in fact, observe surface bee structures prominently in visible wavelength micrographs of room temperature bitumen samples (Ramm et al ., ). But bees play no role in the current study, except as one source of background light scatter.…”

“…In Ramm et al . (), we measured microstructural and mechanical properties during a thermal cycle, using a fixed dwell time of 5 min following each temperature increment . This delay enabled the sample to stabilize at each new temperature before measurements were made.…”

Mechanical Resilience of Modified Bitumen at Different Cooling Rates: A Rheological and Atomic Force Microscopy Investigation

“…After a slow cooling ramp temperature (1 • C/min, 1C), a rough surface morphology has been imaged at RT, constituted by irregular often-interlocking domains. As the sample is cooled at a faster rate (10 • C/min, 1D), the phase-contrast images reveal several small submicrometer crystalline structures resembling the 'ant-like' spots observed by Ramm et al [31], considered as metastable aggregates subjected to a kinetic rather than thermodynamic control. Addition of polyphosphoric acid (PPA) gives rise to surface structuring very similar to pristine bitumen.…”