The Chemistry & Physics of Bitumen & Asphalt

Synopsis

Understanding the physical properties of bitumen and their relations to subsequent asphalt performance has been one of the most fundamental areas of research in recent years. It is critical that end users of bitumen have a knowledge of the relationship between properties and performance so that the latter can be maximized. However, the link between the chemical and physical properties of bitumen has, perhaps been less well established. One of the reasons for this is the difficulty in carrying out chemical analysis on bitumen. Several models have been developed, which seek to address the structure of bitumen on a macroscopic scale and subsequently relate this to overall physical properties. The classical 'micelle' model of asphaltene clusters surrounded by a maltene phase is one that has gained almost universal acceptance, yet unequivocal evidence for this model is limited. Models based on molecular weight or charge have also been developed. Another model (BISOM) addresses the solubility parameters of the components in bitumen and seeks to assess the level of interaction between individual species.

Precise determination of the individual chemical species contributing to these larger structures is practically impossible, although groups of molecules with similar characteristics can be defined. These are usually described as saturates, aromatics, resins and asphaltenes, with the relative stability of an individual bitumen.

On an elemental level the chemistry of bitumen is dominated by carbon with contributions from hydrogen, oxygen, nitrogen, sulphur and some metals.

The chemical nature of bitumen can be influenced by a number of processes but is most affected by heat, oxygen and, to a lesser extent, ultra-violet light. Changes in chemical composition will occur when bitumen is mixed with aggregate in an asphalt plant and during its service life. Some species will be lost, while others may react together. The extent of these chemical changes will influence the changes in physical properties.

This lecture aims to explain the chemistry and physics of bitumen on a number of levels:

• structural chemistry and relation to physical properties
• complexity of chemical species with bitumen
• the chemistry of ageing
• the chemistry of adhesion

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CPD Info

SCI Members attending this meeting are able to claim CPD points.

Additional Info

Speaker

Dr Ian Lancaster 
UK Technical Manager
Nynas