Aging and rejuvenators: Evaluating their impact on high RAP mixtures fatigue cracking characteristics using advanced mechanistic models and testing methods

Fatigue cracking is a major distress in asphalt mixtures. It is highly dependent on many factors including aging. During mixing and construction this is referred to as short-term aging, and aging during the service life of the pavement is referred to as long-term aging. Using larger amounts of Reclaimed Asphalt Pavement (RAP) in new paving mixtures presents a concern that the resultant mixtures may be prone to more fatigue cracking during the service life of the pavement. This is due to the asphalt binder in the RAP being significantly aged. In new paving mixtures, this already aged binder will be exposed to additional short and long-term aging. To alleviate the effect of the aged RAP binder on the cracking susceptibilities of new asphalt mixtures, generally a softer binder is used. However, several studies have indicated that asphalt rejuvenators can allow more aged binder to be incorporated into mixtures than a softer binder alone. The objectives of this study were to evaluate the effect of long-term aging on the fatigue characteristics of high-RAP mixtures modified with rejuvenators using the conventional fatigue test and recently developed mechanistic models and tests. Also the results from these fatigue tests were compared to see if they provided similar performance trends in regards to each other. Four tests/models were used: Four Point Flexural Beam Fatigue Test, HMA Fracture Mechanics Model, Simplified Visco-Elastic Continuum Damage Model (SVECD), and the Semi-Circular Bending (SCB) Test. The results indicated that the long-term aging in accordance with the AASHTO R 30 specification (five days aging at 85°C) did not have a significant effect on the fatigue characteristics of the high RAP mixture with and without rejuvenators. Comparison of the fatigue test rankings suggested that they did not show universal agreement in the rankings of the mixtures. Also, the repeated load cyclic tests (beam fatigue and SVECD) exhibited a wide range of performance compared to the constant rate fatigue tests and analysis (HMA-FM models and SCB). The reason for these variations is not known at this time and requires further study. Field trials are needed to determine the actual ranking trend that should be obtained in the laboratory tests. This may be the only way to separate and evaluate the quality of the results obtained by these methods.