Five bituminous samples were carefully studied by confocal laser scanning microscopy using 488 nm excitation radiation and observing 500–530 nm of emission. The images revealed the microstructure of bitumen. The influence of the admixture of mineral aggregates concerning the microstructure was tested. For the minerals, no significant influence was found. For understanding the origin of fluorescent signals, the samples were separated into asphaltenes and maltenes and analyzed with fluorescence spectroscopy. Although former works have assumed the origin of fluorescent emissions in bitumen to be found in the asphaltene fraction, the asphaltenes produce little to no emissions, but the maltenes exhibit strong fluorescence in the observed spectral region. For deeper insight, fractionation of the bitumina into the SARA fractions by chromatographic column separation was necessary. The fluorescence spectra of these fractions were analyzed and revealed the aromatics and resin phases to be the only components capable of sufficiently intense fluorescent emission. This is a strong argument for a complex internal microstructure consisting of a mantle of aromatics surrounding an inner core.

During ageing the chemical composition of bitumen changes, which is reflected by the change in the distribution of bitumen fractions. By measuring the spectrum of fractions, and by correlating fractional changes during ageing within bitumen with the corresponding spectra, a connection between bitumen’s age and the fluorescence spectrum can be drawn. While the age of bitumen increases, its fluorescence emission intensity decreases. Therefore non-aged, laboratory aged (rolling thin film oven test (RTFOT) and RTFOT + pressure ageing vessel) bitumen and bitumen extracted from asphalt slabs from a test field after different time periods were investigated. The test field confirmed the connection between intensity and ageing time, further the development of an oxidation gradient over the vertical cross section of the asphalt slabs could be observed. A modified RTFOT was conducted to study the effects of different temperatures and operating times on the fluorescence emission spectrum, as well as the influence of air (containing traces of atmospheric radicals) by conducting the test with nitrogen. Even under nitrogen atmosphere the intensity decreased, which is assigned to internal reactions.