An analytical framework to assess the chemical changes in polymer-modified bitumen upon natural and simulated ageing

Abstract

Polymers play a crucial role in modifying the mechanical and rheological properties of bituminous construction materials. One of the most employed polymers, particularly in road engineering, is styrene–butadiene–styrene (SBS). However, long-term interactions of the SBS composite mixture with solar radiation and atmospheric gases result in material deterioration. Despite its prevalent use, the specific contribution of the polymer in these processes remains insufficiently understood. To address this gap in knowledge, we developed an analytical framework to precisely determine the chemistry of SBS-modified bitumen. Unmodified and SBS-modified samples were aged according to different ageing procedures to analyse thermal and thermochemical influences. These samples were then compared to genuine samples obtained from construction sites/test fields. Infrared spectroscopy revealed the formation of carbonyls and sulfoxides during all ageing procedures. The highest increase was observed in the field-aged sample. High temperatures during laboratory ageing affected the sulfoxide formation, as decomposition became a limiting factor. Comparing unmodified and SBS-modified samples showed that the oxygen uptake was similar during all ageing procedures, indicating that the presence of SBS did not impact the oxidation susceptibility. Two-dimensional nuclear magnetic resonance (NMR) spectroscopy facilitated the quantification of SBS substructures, showing a loss of 21% olefinic and 25% allylic SBS groups during field ageing. Aromatic SBS structures remained unaffected. The addition of ozone and nitrogen dioxide led to a degradation of 5% olefinic and 10% allylic structures during thermochemical ageing, while the variations during thermal ageing were less than 3%. Field ageing did not alter the aromatic structures in the bitumen phase in the SBS-modified binder but revealed significant decreases in the unmodified sample. Diffusion-ordered NMR spectroscopy showed that the average molecular size of the polymer halved during field ageing. Multiple ageing cycles were required to replicate this trend in the laboratory. However, extending the ageing time and adding reactive oxygen species reduced the number of aromatic structures in the bitumen phase, while no alteration occurred in the field. This study introduces the most precise framework to evaluate the chemical state of SBS-modified binders. The developed framework can be applied to monitor degradation, determine the lifetime and validate ageing technologies.