22 OXIDATION Oxidation is the natural process of organic compounds in asphalt binder breaking down in the presence of oxygen. It is characterized by loss of the black color of asphalt. Oxidation is a natural process that starts to occur the moment asphalt is mixed. Asphalt binders are made of organic molecules that react with oxygen in the air. This reaction is similar to how an apple or banana browns when exposed to air. As oxidation progresses, asphalt becomes more susceptible to cracking. These factors affect the rate of oxidation: • The rate of oxidation doubles for every 10°C increase in temperature. • The more oxygen that’s available to react with the asphalt binder, and the longer it is available, the faster the binder will oxidize. • Sunlight accelerates the oxidation of asphalt binders. Oxidation can be slowed down by: • Using true warm mix where the temperature is reduced during application instead of hot mix asphalt. • Applying a surface treatment layer over the asphalt surface to protect it from sunlight and isolate it from oxygen. This is more effective when done early in the lift of the asphalt. Tips on responding to oxidation: • Most surface treatments won’t restore the flexibility of oxidized asphalt pavement, but they will inhibit further oxidation. • A rejuvenator should only be used on HMA that has at least 5% air voids. If voids are less than 5%, the rejuvenator may fill the voids, resulting in an unstable mix. • Rejuvenator will penetrate deeper in the asphalt if best applied in warm weather (over 70°F).
RkJQdWJsaXNoZXIy ODQxMjUw