Selecting a Laboratory Loose Mix Aging Protocol for the NCAT Top-Down Cracking Experiment

The objective of this study was to select a laboratory loose mix aging protocol for the NCAT top-down cracking (TDC) experiment. Field aging of asphalt mixtures was characterized using cumulative degree-days (CDD), which was defined as the accumulation of daily high temperature throughout mixtures’ service life. Performance data from over 80 existing pavements showed that TDC typically initiated after a critical field aging condition of approximately 70,000 CDD. A laboratory experiment was conducted to select an aging protocol that was representative of this critical CDD. Materials used in the experiment were from five projects in Michigan, Washington, and Alabama. Four loose mix aging protocols were evaluated in terms of their effects on the rheological and oxidation properties of asphalt binders. Results from the dynamic shear rheometer (DSR), bending beam rheometer (BBR), and Fourier transform infrared spectroscopy (FT-IR) tests showed that the 24-h, 135°C protocol yielded the most significant level of asphalt aging, followed by the 12-h, 135°C protocol, 5-day, 95°C protocol, and 6-h, 135°C protocol, respectively. No significant difference in the oxidation–hardening relationship of asphalt binders was observed for mixes aged at 95°C versus 135°C. Among the four aging protocols, the 5-day, 95°C protocol was most representative of 70,000 CDD of field aging. Additionally, DSR and FT-IR results indicated that loose mix aging of 8-h at 135°C and 5 days at 95°C were likely to achieve an equivalent aging level; thus, the 8-h, 135°C protocol was recommended as an alternative protocol to simulate 70,000 CDD of field aging.