What happens when an asphalt pavement ages?
Since the aggregate is basically unaffected by the interaction with oxygen, the aging occurs in the asphalt binder. Oxidation can occur at any point where asphalt binder is exposed to oxygen. It happens short-term during mixing at an asphalt mixture production facility, when the asphalt binder is exposed to high temperatures in thin films. It also occurs long-term during the service life of the pavement, when the asphalt binder is exposed to lower temperatures, but for longer periods of time. The Rolling Thin Film Oven (RTFO) and the Pressure Aging Vessel (PAV) tests approximate short-term aging and long-term aging in the laboratory, respectively.
The RTFO test simulates the aging in an asphalt mixture production facility by exposing thin films of asphalt binder to heat and air by rotating bottles in a vertical carriage (like a Ferris Wheel) placed in an oven operating at 163°C while periodically blowing heated air into the bottles as the asphalt binder “rolls” – coating the interior of the bottle. It is a relatively quick test, taking approximately 85 minutes from start to finish, producing aged residue that is often 2-4 times stiffer than the original, unaged asphalt binder.
The PAV test simulates aging that occurs in place in an asphalt mixture by using a slightly lower temperature (90-110°C) and elevated air pressure to force oxygen diffusion to occur in the previously RTFO-aged asphalt binder samples. Unlike the RTFO, it is a longer test, taking approximately 20 hours from start to finish, producing aged residue that is often 2-4 times stiffer than the RTFO-aged asphalt binder.
Figure 1 shows the progression in aging of three asphalt binders from the Strategic Highway Research Program (SHRP). What is important in Figure 1 is not only that the asphalt binder stiffness increases as it ages, but that the aging rate is different for different asphalt binders.
In addition to the asphalt binder getting stiffer, the phase angle also changes – decreasing as the asphalt binder ages. This is illustrated in Figure 2 where the modulus and phase angle are represented on a single graph (referred to as a “black space” plot). In Figure 2, the colored lines represent different aging conditions from unaged (far right) to highly aged (far left). The circles represent data at equal conditions. As can be seen by following the path of the circles, as the asphalt binder ages it becomes stiffer and also loses some of its viscous behavior – becoming more like an elastic solid and less like a viscous liquid. In other words, the asphalt binder gets stiffer and more brittle with age.
Understanding how asphalt materials age is important for us, as asphalt technologists, to properly assess the impact of aging on long-term pavement performance. We can’t stop the march of time, but we can better understand how aging occurs in order for us to mitigate its effects.