Laboratory Aging and Annealing of Asphalt Binders by Microwave Radiation

Abstract

A household microwave oven with a frequency of 2.45 × 103 ±13 MHz and an output power of 1000 W is used to simulate short- and long-term aging of asphalt, and for annealing, prior to sampling, to remove steric (isothermal) hardening. A quartz petri dish holds the asphalt during microwave treatment, at atmospheric pressure. To simulate thin film oven (TFO) aging, a 10-g sample is microwaved for a total of 33 min. For 11 asphalts, the average difference in G*/sin δ, at limiting high temperature, between TFO and microwave aging is ±0.68 kPa. Simulating rolling thin-film oven (RTFO) aging is possible by microwaving for a total of 63 min. For 18 asphalts, the average difference in G*/sin δ, at limiting high temperature, between RTFO and microwave aging is ± 1.19 kPa. Aging by RTFO + pressure aging vessel (PAV) and TFO + PAV is simulated by microwaving for a total of 158 min. Microwave aging marginally underestimates long-term aging. For 18 asphalts, the average difference in intermediate temperature obtained after RTFO + PAV versus microwave aging is −2.8°C; for TFO + PAV versus microwave aging, the average difference is −2.7°C. Annealing for 2 hr at 150°C in a convection oven (conductive heating) is simulated by microwaving for a total of 27 min. The average difference in G*/sin δ between the two methods is ±0.24 kPa. Molecular size index (MSI, the ratio of first to second fraction from size exclusion chromatography) correlates with stiffness at low temperature. At intermediate temperature, tan δ remains practically unchanged until MSI reaches a minimum, then increases sharply. At limiting high temperature, G* does not correlate with MSI.