Vergelijkend AFM Onderzoek: Microstructuur van bitumen in relatie tot healing

In this report we present the background, the scientific and experimental approach and the results of AFM experiments performed on two different batches of bitumen. The specific bitumen researched in this project has also been studied in the context of the InfraQuest project ‘Pragmatisch Healing Onderzoek’. It has been known for quite some time already that bitumen posessess a microstructure at the typical length scale of micrometers. This can be shown experimentally by imaging the bitumen surface with Atomic Force Microscopy (AFM). As is the case for many other engineering materials (e.g. steel), the microstructure will manifest itself by the macroscopic mechanical response of the material; thus on the typical length scales where it performs its load bearing function in pavement structures. Together with the hitherto not precisely specified properties of the many available bitumen grades, this justifies further research into the origin and properties of this microstructure. Here we also anticipate that a better understanding about the origin and properties of the bitumen microstructure will lead to improved bitumen grades (material appraisal) and possibly to better criteria for selecting a bitumen for a specific application. In the context of this research first the objectivity of the AFM imaging technique has to be established. Therefore two independent laboratories (TNO and CiTG, TU Delft) have prepared and conditioned bitumen samples for the AFM. All samples have been prepared from the same batch of bitumen. Then each laboratory has imaged its ‘home made ‘samples’ as well as the samples prepared at the other lab. The results appear to be qualitatively identical. Thus one may conclude that the microstructure of bitumen is a reproducible quantity. It was also found that the (thermal) conditioning of the bitumen (prior to imaging) has a significant impact on the microstructure observed. One may conclude from this that the sample conditioning procedure is a very important aspect in the AFM imaging process. In other words: an AFM image of bitumen is meaningless, unless the conditioning procedure of the samples is reported extensively. The next step was to find the influence of temperature on the observed bitumen microstructure. Identical samples have been prepared by TU Delft and both laboratories have imaged the microstructure as a function of temperature. A similar observation as stated before has been made: the microstructures observed by both laboratories were very similar. Moreover, it has been observed that the microstructure gradually disappears when the temperature is raised. However, even at the highest (experimental) temperatures (70 °C) traces of the microstructure remain visible. Apparently the ordering process that governs the bitumen microstructure has an associated interaction energy in the order of 400 kB, i.e. 30-40 meV (kB, Boltzmann constant). It was also found that (chemically) reclaimed bitumen (from an asphalt test beam) does show a microstructure as well. Surprisingly however, it was found that the microstructure of harder bitumen grades disappears at lower temperatures compared to softer bitumen grades. This is against the intuition that in harder bitumen molecules are more tightly bound together than in softer grades, and that for harder bitumen the microstructure would ‘melt’ (disappear) at higher temperatures. The molecular mobility appears to be higher in harder bitumen grade, hence they are anticipated to be better ‘healers’. Macroscopic fatigue test have shown similar trends.Structural EngineeringCivil Engineering and Geoscience

Is Atomic Force Microscopy suited as Tool for fast Screening of Bituminous Materials? An Inter-laboratory Comparison Study

Bituminous binders are known to have microstructures at typical length scales of micrometers. This microstructure can be probed with Atomic Force Microscopy (AFM). Now that worldwide several research groups are reporting AFM results on bitumen, it is becoming important to improve the understanding of the reproducibility and objectivity of the technique for studying bituminous samples. When reproducibility and stability are proven, AFM can be a tool for asphalt professionals to rapidly screen bituminous binders. In this context two independent laboratories have developed a standard method for preparing and conditioning bitumen for AFM imaging. By means of an inter-laboratory comparison of independently imaged specimen, the reproducibility of microstructure measurements was investigated. A quantitative comparison on different microstructures was developed, and the consistency of independently obtained results was confirmed. The results from both labs were comparable: the microstructural properties were found to be randomly distributed within a 5% interval. Also the influence of temperature on the microstructure was demonstrated to be reproducible and consistent. With the increase of temperature, the microstructure gradually disappeared, however traces of the microstructure remained visible up to the highest measurement temperature of 60°C. It is concluded that given well defined sample preparation and measurement procedures, the microstructure of bitumen can be reproducibly imaged by AFM from room temperature up to temperatures where bitumen has become soft and too sticky to be probed by the same setup as used for lower temperatures.Structural EngineeringCivil Engineering and Geoscience