Bitumen surface microstructure evolution in subzero environments

Bitumen is a widely used material employed as a binder in pavement engineering and as a surface sealant in construction. Its surface microstructure and microscale properties have been shown to be temperature-dependent, with effects manifesting themselves on surface composition and texture, including the formation of the visually striking catana 'bee'-like structures. Despite the importance of a good performance of bitumen in subzero environments (<0°C), the behaviour of bitumen surface texture and composition at cold temperatures, affecting cracking, degradation and road icing, has received practically no attention. In particular, such knowledge is relevant to world regions experiencing long periods of subzero temperatures during the year. Employing advanced atomic force microscopy combined with infrared spectroscopy (AFM-IR) and an environmental chamber, we demonstrate the ability to characterise surface structure and composition with nanoscale precision for a broad range of temperatures. We show that cooling bitumen to subzero temperatures can have several interesting effects on its surface microtexture, nanotexture and composition, especially on its three surface domains, catana, peri and para. We found that the para domain coarsens and extends to form an interfacial transition domain (characterised by increasing surface roughness with peri domain composition) between the para and peri domains. We show that the catana and peri domains have a similar composition, but have different mechanical and chemical properties compared to the para domain. The essential findings of this work improve our understanding of the behaviour of bitumen in subzero environments, aiding us in our quest towards attaining better road and sealant performance.