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.

Extracellular bone material can be characterised as a nanocomposite where, in a liquid environment, nanometre-sized hydroxyapatite crystals precipitate within as well as between long fibre-like collagen fibrils (with diameters in the 100 nm range), as evidenced from neutron diffraction and transmission electron microscopy. Accordingly, these crystals are referred to as ‘interfibrillar mineral’ and ‘extrafibrillar mineral’, respectively. From a topological viewpoint, it is probable that the mineralisations start on the surfaces of the collagen fibrils (‘mineral-encrusted fibrils’), from where the crystals grow both into the fibril and into the extrafibrillar space. Since the mineral concentration depends on the pore spaces within the fibrils and between the fibrils (there is more space between them), the majority of the crystals (but clearly not all of them) typically lie in the extrafibrillar space. There, larger crystal agglomerations or clusters, spanning tens to hundreds of nanometers, develop in the course of mineralisation, and the micromechanics community has identified the pivotal role, which this extrafibrillar mineral plays for tissue elasticity. In such extrafibrillar crystal agglomerates, single crystals are stuck together, their surfaces being covered with very thin water layers. Recently, the latter have caught our interest regarding strength properties (Fritsch et al. 2009 J Theor Biol. 260(2): 230–252) – we have identified these water layers as weak interfaces in the extrafibrillar mineral of bone. Rate-independent gliding effects of crystals along the aforementioned interfaces, once an elastic threshold is surpassed, can be related to overall elastoplastic material behaviour of the hierarchical material ‘bone’. Extending this idea, the present paper is devoted to viscous gliding along these interfaces, expressing itself, at the macroscale, in the well-known experimentally evidenced phenomenon of bone viscoelasticity. In this context, a multiscale homogenisation scheme is extended to viscoelasticity, mineral-cluster-specific creep parameters are identified from three-point bending tests on hydrated bone samples, and the model is validated by statistically and physically independent experiments on partially dried samples. We expect this model to be relevant when it comes to prediction of time-dependent phenomena, e.g. in the context of bone remodelling.

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.

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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.

Awareness that natural, financial, and energy resources are scarce goods has increased. Thus demand is growing for infrastructure that is not only of high quality but also efficient. Efficiency, in this case, aims to optimize cost and energy consumption over the complete life cycle of a structure. The objective is to build long-lasting infrastructure with low maintenance demands and with high recycling potential after it has reached the end of its service life. For bituminous bound materials, the aging of asphalt binder has a crucial impact on durability and recyclability. Because asphalt binder is organic by nature, the thermal and oxidative aging processes affected by chemical and structural changes occur when asphalt mixes first are produced and applied and continue over the course of their service life. Increasing stiffness and brittleness of the binder make pavement more prone to thermal and fatigue cracking. The interdisciplinary research project reported here worked toward a better understanding of the physicochemical fundamentals of asphalt binder aging, as well as of the impact of binder aging on the mechanical properties of asphalt binder and asphalt mixes. Through extensive chemical and mechanical analyses, a new model was developed to explain the aging process comprehensively (i.e., on the physicochemical and mechanical levels). Aging can be determined mathematically by micromechanical modeling. With the model presented in this paper, changes in asphalt binder as a result of aging (i.e., increasing brittleness and stiffness) can be explained.