What does the term "hardening" refer to in the context of asphalt aging?

In the context of asphalt aging, the term "hardening" refers to the stiffening of the asphalt binder over time. This phenomenon occurs due to oxidative reactions and volatilization of lighter fractions within the asphalt, which leads to an increase in the viscosity of the binder. As asphalt ages, it loses components that contribute to its flexibility and reduces its ability to flow, resulting in a harder and stiffer material. This hardening process can affect the performance of the asphalt in pavement applications, leading to cracking and decreased durability if not properly managed.

The other options do not accurately represent the term "hardening." For instance, adding more aggregate would not describe hardening but rather adjusting the mix design. Similarly, an increase in binder flexibility would contradict the concept of hardening, as would the notion of adding chemicals to maintain workability, which addresses short-term handling qualities rather than the long-term aging characteristics of asphalt.