Self-healing of bitumen is a property that positively contributes to the sustainability, maintenance requirements and cost effectiveness of asphalt pavements. Ideally one would like to design an asphalt mix with a well-defined healing potential. Although substantial research efforts have been dedicated to the healing mechanism in bitumen, complete understanding of the fundamental mechanisms that govern the property of healing is still lacking. Here we investigate the manifestation of damage and healing of bitumen at the microstructural level. Three distinct bitumen grades are subjected to mechanical loading conditions, and the damage is investigated at the microstructural level by atomic force microscopy combined with finite element simulations. One of the bituminous phases appears to display visible signs of cracks, which are found to (partly) disappear at moderate temperature changes. Simulations of mechanical loading of experimentally derived finite element meshes are corresponding well with these experimental observations. Moreover, the simulations provide a measure of mechanical response, i.e. stiffness, of the material as a function of strain level. From this it is found that the microstructural cracks lead to diminished structural response properties, whereas after healing these properties are partly recovered. The experimental observations, together with the simulations, support earlier ideas that relate the phenomenon of self-healing in bitumen to their rheological property of thixotropy. Moreover, the work presented hints that the property of self-healing is governed by processes at the microstructural length scale.

The macroscopic mechanical response properties of bituminous materials originate from the mechanical properties at the microstructural level. From atomic force microscopy (AFM) investigations, it is evident that mainly two material phases are present in bitumen; these phases can be loosely associated with bitumen's chemical composition (i.e., crude oil origin). However, little is known about the mechanical properties of the constituent phases of bitumen. In this research, an AFM technique was used to obtain mechanical property maps of two bitumens. This technique can distinguish between phases and provide quantitative results. The mechanical properties at the nano-to micrometer-length scale govern the overall properties of bitumen when considered as a microscale composite material. A mechanics approach is followed to derive the composite modulus from the individual phase properties. Furthermore, the temperature dependence of mechanical properties is determined on heating the bitumens from ambient conditions. With an increase in temperature, the moduli of both phases decrease, whereas the phases become more adhesive. The results demonstrate a successful quantitative characterization of the mechanical properties of bitumen microphases and the subsequent coarse graining of these properties into composite mechanical response properties. These mechanical properties (i.e., stiffness and adhesion potential) are important input parameters for material design and modeling and will allow one to predict the macroscopic behavior of asphalt concrete according to fundamental quantities. Finally, a better understanding of the temperature dependence of microstructural mechanical properties can contribute to the understanding of the thermorheological properties of bitumen for optimal processing conditions and best performance.

Measuring properties at the nanometre scale such as topography, morphology and roughness within a production line becomes increasingly important for quality control and process monitoring tasks. In a production line, ground vibrations are transmitted to the sample and the inspection tool, corrupting nanoscale measurements by affecting the distance between inspection tool and sample. To enable nanometre scale measurements a mechanism is needed that keeps this distance constant. This paper describes the concept and experimental results of a metrology platform that tracks the sample for nanoscale inspection. The nano inspection tool is carried by the metrology platform and is artificially coupled to the movement of the sample by using a feedback controller. A one degree of freedom experimental setup was built for demonstrating tracking performance. The implemented closed loop control achieves disturbance rejection with a bandwidth of 410 Hz and reduces emulated on-site vibrations from ±500 nm down to ±9 nm, showing significant reduction of external vibrations.

Mechanical vibrations occuring in a production environment cause a relative motion between the sample and inspection tool that distorts measurements at the nanometer level. To overcome this problem, this paper proposes a metrology platform that maintains a constant relative distance to the sample by means of an H& inf;-feedback controller. Experiments in one degree of freedom show that the metrology platform can reduce vibrations as they occur in a production environment by one order of magnitude. Therefore, it enables in-line surface metrology at the nanometer level.