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1 This report presents the experimental, analytical, and computational research conducted under NCHRP Project 09-54. This projectâs goals were to (1) develop a long-term aging procedure for asphalt mixtures appropriate to the fabrication of performance test specimens and (2) develop an asphalt pavement aging model for mechanisticâempirical (ME) pave- ment design and analysis. Original component materials and field cores from in-service and test track pavements from eight states and the Province of Manitoba, Canada, were used in this study. These pavement projects yielded a total of 18 different asphalt mixtures, includ- ing warm-mix asphalt (WMA). In order to track the aging level of the laboratory-aged mixtures and field cores, a com- prehensive study was first performed to identify efficient and accurate binder aging index properties (AIPs). Both chemistry-based and rheology-based parameters were included in the study. The logarithm of the binder shear modulus, log G*, was selected as the rheologi- cal AIP and the total absorbance under the carbonyl and sulfoxide infrared (IR) peaks was selected as the chemical AIP. Also, a sensitivity study was performed to evaluate the sensitiv- ity of the mixture dynamic modulus to changes in the binder AIP that are caused by oxida- tive aging. The sensitivity study provided thresholds by which to evaluate the significance of observed differences in asphalt binder AIPs in terms of asphalt mixture performance. As a rule of thumb, it was concluded that a 15% change in the binder dynamic shear modulus value would lead to about a 10% change in the mixture dynamic modulus value. Factors that were investigated as part of the selection of the aging procedure included: (1) loose mixture aging versus compacted specimen aging, (2) oven aging versus pressure aging, and (3) a 95°C aging temperature versus 135°C. The selection process was based on considerations of practicality, efficiency, and versatility. Loose mixture aging reduced the aging time significantly compared to compacted specimen aging. Also, significant aging gra- dients were found in the aged compacted specimens, which violated the representative vol- ume element requirement for performance test specimens. Pressure aging expedited the aging process; however, a much larger pressure aging vessel (PAV) than the binder PAV would be needed in order to age a sufficient quantity of loose mixture for the fabrication of performance test specimens. Aging at 135°C caused changes in the chemistry of the binder; such changes do not occur in the field. These chemical changes led to significantly different cracking perfor- mance results compared to the results obtained after aging at 95°C. Based on these findings, loose mixture aging in the oven at 95°C is proposed as the long-term aging procedure for the fabrication of asphalt mixture performance test specimens. Based on a limited study, the same recommendation is tentatively made for the laboratory long-term aging of WMA mixtures. A comprehensive aging model for asphalt mixtures in pavement systems should account for both binder oxidation kinetics and diffusion. In this study, a rheology-based kinetics S U M M A R Y Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction
2 Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction model was developed for loose mixture aging based on an existing chemistry-based kinetics model that was initially developed for binder aging. The rheology-based kinetics model was verified using isothermal and non-isothermal laboratory aging conditions. This kinetics model can predict the binder dynamic shear modulus as a function of aging temperature and duration. The kinetics model and laboratory experiments demonstrate that the labora- tory aging duration that is required to match a given field condition is independent of the material-specific kinetics. A climatic aging index (CAI), based on a simplification of the developed kinetics model, was developed to prescribe the required laboratory aging durations to reflect different pave- ment temperature histories at different locations and depths. The field aging levels obtained from field cores were compared against loose mix aging rates at 95°C in the laboratory to determine the aging duration that is required to match field aging. This CAI study resulted in a procedure that calculates the laboratory aging durations to match the field aging at any pavement depths for any locations. This procedure was used to develop a series of laboratory aging duration maps to match 4, 8, and 16 years of field aging at depths of 6 mm, 20 mm, and 50 mm below the pavement surface as examples. Pavement temperature as a function of pavement depth was predicted using Enhanced Integrated Climatic Model (EICM) data. The predicted pavement temperature history was coupled with the kinetics model to predict field aging throughout the pavement depth. The kinetics model was calibrated successfully (R2 = 0.75) against field core measurements of log G* at a depth of 20 mm, which represents a reasonable depth for the evaluation of the bulk behavior of surface layers and avoids the effect of ultraviolet (UV) oxidation at the pavement surface. These results suggest that the kinetics model can be extended in the future to replace the Global Aging System (GAS) model to improve the prediction of changes in asphalt binder properties with oxidative aging in pavement performance prediction software (e.g., Pavement ME Design). The calibrated kinetics model tends to under-predict aging near the pavement surface and over predict aging at depths below 20 mm, which matches intuition in the absence of a diffusion model and consideration of UV oxidation for the pavement near-surface. These results highlight the need for the development of a diffusion model that considers the morphological properties of asphalt mixtures in order to predict field aging more accurately. The new model accounts for mixture-specific kinetics parameters that can be determined directly from loose mix aging; however, it still requires cumbersome mixture preparation and corresponding binder extraction and recovery. An empirical model thus was developed to determine loose mix aging kinetics parameters from Universal Simple Aging Test binder aging. The developed empirical model facilitates the determination of mixture-specific kinetics parameters and eliminates the need for binder extraction and recovery. Within pavement performance prediction frameworks, the predicted changes in binder properties as a result of oxidation must be translated to the corresponding changes in asphalt mixture properties. Therefore, as an example application, three common asphalt mixture dynamic modulus predictive models [i.e., Witczak, Hirsch, and North Carolina State Uni- versity (NCSU) Artificial Neural Networks (ANN)] were used to predict the dynamic mod- ulus values at different aging levels, and the prediction results were compared against the measured dynamic modulus values. Significant errors were found, which suggests the need to adjust the existing models so that they can predict aged mixture properties based on inputs from binder AIPs. Finally, suggestions are made for future research.
