Oxidation

From the moment that new asphalt is laid down, it begins a process of oxidation. When asphalt is exposed to oxygen, molecular processes begin that causes new polar (bonding) sites to be created. The increase in polar sites allows the molecules to shuffle about searching for bonds that help bring it to a thermodynamic stable state, or equilibrium. As this process of "self-assemblage" continues over time, the asphalt molecules bond tighter with the aggregate in the asphalt. This results in the pavement becoming stiffer and more brittle over time.

Variables that have the greatest amount of influence on the rate of oxidation are the oxygen exposure, time, and temperature. These are all directly related to rate of oxidation in that as these variables increase, so does the rate of oxidation.

Oxidation is important to understand because it results in the asphalt becoming brittle, with significant loss of elasticity, bringing about a significant increase in the probably of failure.

This process can be observed over several years through the color of the pavement itself. New asphalt is a dark black color. Over time and oxidation progresses, the asphalt turns a lighter black, and eventually turns grey. Cracks develop during this process with fatigue and failure following.

The length of time that this process takes before reaching equilibrium can last from 5 to 15 years. This is the primary reason that the typical lifespan of most parking lots is within 20 to 30 years.

After the oxidation process has significantly stiffened the pavement, the load-bearing capability of the asphalt is reduced, resulting in asphalt fatigue, cracking, and ultimately failures. The failure rate for asphalt is significant accelerated once the asphalt has significantly oxidized.

A consistent regimen of planned sealcoating can help to reduce the rate of oxidation by as much as 75% if initiated within the first two years after the pavement was constructed. Sealcoating asphalt that has already oxidized will not bring back its elastic properties.