The phenomenon of bitumen ‘bee' structures – bulk or surface layer – a closer look

ABSTRACT

Bitumen is the primary binder of asphalt covering most of the roads in the world. The origin of the primary oil, the refinery treatment, the specific chemical structure, and the natural or induced chemical modifications determine the mechanical properties of bitumen. Aging affects the time evolution of its thermo-rheological behaviour. Numerous studies have suggested that the particular local microstructure of bitumen affects its rheological properties including ‘bee’ – patterned structures. We have used scanning probe microscopy to distinguish between surface effects and bulk properties. Using scanning probe microscopy we conclude that the ‘bee’ structures exist at the surface only and do not occur in the bulk. In particular, they are not observed on freshly fractured surfaces. A special technique was used to chisel off the bitumen surface. The material relaxes and the ‘bee’ structures disappear from the surface. This proves that the ‘bees’ are formed as a thin surface layer. Improved recycling will thus have to rely on chemical treatment of aged and used bitumen rather than on intentional modifications to the surface microstructure which is merely a surface effect. The bitumen surface microstructure can nevertheless be seen as a fingerprint of the overall bitumen properties to a certain degree.

KEYWORDS:

• Bitumen microstructure
• surface layer
• bulk structure
• atomic force microscopy
• surface manipulation

Disclosure statement

No potential conflict of interest was reported by the author(s).