The impact of field ageing on molecular structure and chemistry of bitumen

Abstract

Bitumen is an important construction material applied as sealing material in roofing membranes and as binding material in roads. Due to its organic nature, it is prone to environmentally induced changes on the molecular up to the macroscopic-mechanical level, resulting in a limited service life. Various individual spectroscopic and chromatographic methods have been applied to elucidate specific aspects of the ongoing chemical processes. However, a general view and systematic chemoanalytical approaches are still missing. In this study, the impact of ageing during construction and paving and after 5-years on a test field was analyzed by looking at the chemical changes within bitumen and its polarity-based fractions. Elemental analysis showed doubling of the oxygen content on the binder level and molecular exchange of nitrogen-, oxygen-, and sulfur-containing structures between the fractions. The combined use of two-dimensional nuclear magnetic resonance (NMR) and fluorescence spectroscopy indicated the increase in size of aromatic systems by aromatization and/or dehydrogenative polymerization reactions. Fluorescence and infrared spectroscopy demonstrated the ageing sensitivity of nitrogen-containing structures, such as amides and porphyrins. Finally, in-depth analysis of the specific nature and fate of hydroxyl groups with 31P NMR spectroscopy after derivatization revealed increasing amounts of aliphatic carboxylic acids and alcohols with ageing. The outcome of this study aims at improving the understanding of ageing processes in bitumen and at showing new ways to use and combine analytical approaches for proper assessment of bitumen ageing.