As a widely accepted concept, bitumen consists of four fractions that can be distinguished by their polarity. Highly polar asphaltene micelles are dispersed in a viscous phase of saturates, aromatics and resins (maltene phase). Different concentrations of asphaltenes in the bitumen result in a range of mechanical response properties. In an interdisciplinary study the impact of the maltene phase and asphaltenes on the linear viscoelastic behavior and the microstructure of bitumen were analyzed by creep recovery testing in a DSR and by atomic force microscopy (AFM). Therefore, bitumen was separated into the maltene and asphaltene fractions and artificial bitumen samples with different, pre-defined asphaltene concentrations were produced and investigated. It was found that the artificially produced, precipitated bitumen samples can be regarded as a representative, bitumen-like material in terms of mechanical behavior and microstructure. Asphaltenes play an important role in the typical viscoelastic behavior of bitumen being mainly responsible for stiffness and elasticity. Also, their concentration appears to be correlated to the occurrence and shape of the bee-like inclusions which can be typically observed by AFM.

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.

Bitumen is a widely used material, but its aging behavior is only understood at a macroscopic level as hardening and embrittlement over time. To assess bitumen aging behavior in the long run, the pressure aging vessel (PAV) testing procedure was developed. However, this procedure including high-pressure and high-temperature oxidation of the bitumen has not yet been understood on a molecular level. Here, a bitumen sample and its SARA fractions, i.e., saturates, aromatics, resins, and asphaltenes, were investigated in comparison with their aged samples to study changes of their chemical compositions. Negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry [ESI(−)] FT-ICR-MS was used to analyze samples. The effect of aging was characterized using the aromaticity equivalent (X c), double bond equivalent (DBE), and van Krevelen plots. It was found that aging induces reduction of condensed aromatic compounds to alicyclic and open chain aliphatic compounds, while small aromatic compounds have been found to be relatively stable (or altered only slightly). Abundant alterations were detected in unaged bitumen. These changes can be assigned to resins and asphaltenes as compared to saturates and aromatics. Overall, alterations of highly condensed compounds were found to be related to aging. Furthermore, molecular series of CHO, CHNO, and CHOS fragments were more susceptible to oxygenation in bitumen, aromatics, resins, and asphaltenes as compared to saturates. In addition, molecular changes in asphaltenes showed a significant difference from classical assessment with high content of condensed aromatic compounds. Rather, the most abundant compounds in asphaltenes appear to be more saturated and apolar.

No abstract

When it comes to describe the mechanical behaviour of hot mix asphalt (HMA), the change of the mechanical properties over time due to environmental impacts known as ageing has to be considered. Hardening and embrittlement of bitumen lead to a reduced resistance against cryogenic cracks and premature formation of fatigue cracks in bituminous layers. Within this work, the microstructure of bitumen is investigated by conducting static shear creep tests on artificially composed bitumen with asphaltene contents varying between 0 vol-% and 26.71 vol-% as well as on a paving grade bitumen 70/100. Both are considered in unaged and laboratory-aged (rolling thin-film oven test þ pressure ageing vessel) conditions to be able to identify and describe ageing effects. While the properties of the considered material phases of bitumen (identical to saturates, aromatics, resins and asphaltenes (SARA) fractions) seem to remain unaffected, an increase of the asphaltene content due to ageing can be observed. A micromechanical model is proposed that allows a prediction of the consequences of these microstructural changes on the mechanical behaviour of bitumen. Atomic force microscopy and environmental scanning electron microscopy images confirm a composition consisting of a micelle-like structure in a contiguous matrix, a structural representative volume element concept based on SARA fractions is suggested, extending an existing multiscale model for HMA. A very good accordance between model predictions and experimental results indicates the model's ability to reproduce as well as to describe the effects related to ageing.