Multi-criteria Framework to Evaluate the Oxidative Aging Resistance of Bitumen Binders

A multi-criteria framework, using both traditional and specially-designed long-term aging protocols, were utilized in order to discriminate binder formulations with respect to aging resistance due to oxidation. Two protocols were explored: a more traditional type of extended PAV under standard temperature and pressure conditions for durations up to 72 h, and an ambient pressure precision oven test developed by the Western Research Institute (WRI) under work with the Federal Highway Administration (FHWA). This second protocol ages 100 µm thick films at 70 °C for durations up to 84 days. By utilizing thin films, various complications due to oxygen diffusion effects are nearly eliminated. The methodology uses several indicators from infrared spectroscopy and rheology to fit an advanced oxidation kinetics model, also developed by WRI with the FHWA. This oxidation model allows determination of rate constants for the first (fast) and secondary (slow/constant) reactions, and also the amount of reactive material in bitumen. In this study, the overall aging resistance methodology relied heavily on changes in rheological behavior on the whole bitumen temperature range. Special emphasis was put on the mid- to low-temperature rheology, to determine if the modifiers reduce the non-load related surface cracking potential through the determination of various rheological aging sensitive parameters including ΔT_c, the rheological index (R), Black space parameters and crossover frequency and temperature. Overall, the impact of binder formulation on oxidative ageing resistance was clearly observed and confirmed through this multi-criteria approach which allowed clear discrimination between various formulations and the efficiency of modification with respect to given oxidation indicators. Results showed that it was possible to differentiate between additives that chemically retard oxidation from additives that merely act to soften the bitumen.