A durable bitumen is required for the pavement to perform during its design life. Therefore, the bitumen must be resistant to changes in properties over time.

Bitumen composition has its greatest effects on aging. The main physical property change that leads to failure is hardening (consistency change) to embrittlement. This is a result of a combination of oxidation and volatility. Aging produces polar species that can form structures (figure 1).

Aging Mechanisms

1) Oxidation

As bitumens age, they incorporate oxygen at reactive sites of one of two different types:

heteroatoms
benzylic carbon groups
The hetero atoms in an unaged bitumen may be polar or non-polar; for example, aliphatic sulphur is non-polar, but becomes a sulphoxide group when oxidized. This is weakly basic and so can participate in polar associations. Similarly, carbon is non-polar but becomes a carbonyl group (which is polar) when oxidized.

Oxidation may be catalyzed by components in the bitumen and by some metal compounds in the aggregates. Recent work indicates that aggregates mainly affect only viscosity by affecting the associations of bitumen molecules. The level of carbonyl or sulphoxide groups is not affected.

The mechanism is free radical and is initiated by peroxide formation. Figure 2 shows a proposed reaction sequence. Figures 3 and 4 show common reaction products and a range of different bitumens.

The predominant materials formed in oxidation are oxygen polar functionalities. These are mostly ketones but dicarboxylic anhydrides, carboxylic acids, aldehydes and sulphoxides are also formed. The process may be followed by measuring carbonyl group increase using Infrared Spectroscopy.

The amphoterics are the most reactive in terms of reactivity and are capable of forming interlinked structures because they have more than one reactive site (see Figure 1). The other components are capable of being functionalized further which increases associations. The monofunctional materials, those with one oxidizable site, can act as chain terminators, limiting the degree of associations. Aromatic oils can be functionalized by oxidation to become part of the associated SEC I phase; this depletes the SEC II phase and decreases dispersion.

In terms of more conventional components, the reactivity ratio of the asphaltenes to resins to aromatic oils to saturated oils is about 40:32:7:1.

The SEC I fraction is more reactive than the SEC II fraction. Non-polar aromatics will convert to polar materials by oxidation.

The amphoterics have a large effect on the viscosity of the bitumen; the monofunctional polar molecules do not. This explains why carbonyl concentrations and viscosity increases have never correlated well with each other or with aging effects observed in the field. For example, a bitumen such as Californian increases in carbonyl content without increasing greatly in viscosity while Venezuelan bitumen increases in viscosity with small carbonyl increases. This is because Venezuelan bitumen can form extensive associations with little aging, and Californian bitumen cannot.

The amphoterics have a very large effect on the viscosity of the bitumen; the monofunctional polar molecules do not.

This does not mean necessarily that Venezuelan bitumen performs poorly. The oils phase also contributes to properties and, if highly aromatic, maintains a balance of flexibility with high viscosity. This is discussed below.

2) Other Mechanisms of Aging

Bitumen aging is a complex process in which several processes occur at once.

Oxidation occurs simultaneously with loss of lower molecular weight volatiles. This loss rate depends on the concentrations, temperature, and environment.

Because the loss rate is dependent on conditions, volatility is important mainly in handling bitumen at elevated temperatures.

The effect on the bitumen is two-fold: Loss of the dispersing phase allows increased associations through a reduction in dispersion and through an increase in average molecular weight. Bitumen must have a suitable consistency for application and the variation of consistency with temperature and shear must be appropriate. Aging this plays a vital role in the performance of bitumen at any time during its life.