The tendency for bitumen to harden under the influence of the atmosphere has been known and studied for many years. As many as 15 different factors that influence bitumen ageing have been identified.
The most important of these 15 mechanisms of bitumen hardening are:
- steric or physical factors
- exudation of oils.
This is considered to be the most important cause of hardening of bitumen in service. Like many organic substances, bitumen is slowly oxidised when in contact with atmospheric oxygen. Polar groups containing oxygen are formed, and these tend to associate into micelles (an aggregate of molecules in a colloidal solution) of higher micellar weight, thereby increasing the viscosity of the bitumen. Reaction of oxygen with bitumen molecules causes the formation of carbonyl species, resulting in larger and more complex molecules that make the bitumen harder and less flexible. The degree of oxidation is highly dependent on the temperature, period of exposure and the thickness of the bitumen film. The rate of oxidation doubles for each 108C increase in temperature above 1008C. Hardening due to oxidation has long been held to be the main cause of ageing, to the extent that other factors have been given scant consideration. However, it has been shown that, although other factors are generally less important than oxidation, they are measurable.