As an asphalt binder ages, its viscosity increases largely due to oxidation.

• A. True
• B. False
• C. Decreases with age
• D. Unrelated to oxidation

Answer: (A)

Aging of asphalt binder leads to higher viscosity because the material undergoes oxidation over time. When binder is exposed to air and heat, oxygen reacts with the hydrocarbons, forming new oxygen-containing groups such as carbonyls and sulfoxides. These chemical changes promote stiffening and sometimes crosslinking in the molecular structure, making the binder flow more slowly. At the same time, lighter, more volatile components can evaporate or diffuse away, further reducing the material’s ability to flow. The result is a binder that’s harder and less workable, especially at lower temperatures. In practice, aging is what pavement engineers model with tests that simulate long-term exposure, and the observed increase in viscosity is a direct consequence of those oxidation-driven changes.