Bitumen ageing

The ageing process involves chemical changes, which affects the physical properties. This results for example in an increase in viscosity and a decrease of endurable stress at lower temperature. The exact chemical mechanism for ageing is still unknown, but on the level of fractions it can be stated that aromatics decrease while asphaltenes increase. Further the oxygen content increases, indicated by increasing ketone and anhydride formation, monitored by infrared spectroscopy.

Atmospheric oxygen may play an important part during asphalt production, but during service atmospheric radicals have a greater potential for oxidation, due to the high activation energy of diatomic oxygen molecules.

Most radicals and ozone are increased by air pollution. Traffic could be identified as a main source and is spatial very close. Also radiation (UV) affects bitumen, but has the main effect on the surface. Radicals like hydroxyl radicals, single excited oxygen, hydroperoxy radicals , nitric oxide and their corresponding in water (raindrops) dissolved acids can diffuse from the surface into the bulk. This lead to an oxidation gradient. In road construction the oxidation of the surface layer is limited by the erosion of the road surface, reaching at some point a dynamic equilibrium.

Different mechanism and staged can be distinguished.