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H. Grothe, E. Brunet:
"Study of the Adhesion between Bitumen and Aggregates in Asphalt Mixes";
Bericht für OMV Refining & Marketing GmbH; Berichts-Nr. 1, 2009; 59 S.

The use of asphalt in road construction has been developed in Europe since the beginning of the 20th century. Asphalt is a mixture of mineral aggregates that are embedded in a matrix of bitumen. The quality of the roads depends on the adhesion between bitumen and aggregate. This project was initiated in order to improve the quality of the asphalt mixtures by investigating the adhesion between bitumen and aggregate.
During this preliminary study to characterize - or even quantify - the mechanisms of the adhesion between bitumen and aggregate, several cutting-edge analytical techniques have been tried out on bitumen, aggregates and asphalt to determine the set of techniques that brings the most interesting information on the subject.
The contact angle measurement has been employed for the determination of the surface free energy components of the bitumen in order to predict its adhesion to different aggregates. One mode of operation is the sessile drop method, i.e. measuring the contact angle of liquid probe droplets deposited on a bitumen surface. It has mainly been used due to its simplicity and reproducibility. However, the results showed large systematic errors. One inherent problem is the time dependent change of the bitumen surface, stemming from the fact that bitumen at room temperature is still a supercooled metastable liquid. This implies a certain mobility of the bitumen constituents and thus chemical changes at both the bitumen/substrate and the bitumen/air interfaces. Therefore, two strategies have been devised: 1. changing the temperature of the sample, which means influencing the time dependent changes, and 2. changing the substrate, which means influencing the interface between bitumen and the substrate.
Unfortunately, the temperature variation has not been possible with the standard sessile drop setup. Therefore, the pendant drop method has been applied, where a bitumen drop is formed hanging down from the tip of a needle situated in a climate chamber. By this method the surface free energy of the bitumen is determined, but not its polar and dispersive components. To predict the adhesion between bitumen and aggregate, the sessile drop method in conjunction with a suitable temperature control unit would be needed.
In the second strategy of changing the substrates a variation of the sessile drop method was used, in which the bitumen has been deposited on different polymers. This approach is promising, but to exploit it additional measurement time would be needed.
Among the characterization techniques used in this study, mass spectroscopy has turned out to be inappropriate for the analysis of bitumen. Raman spectroscopy itself also has not delivered interesting information on the bitumen structure because of interfering fluorescence of the asphaltenes, but it has led to the use of fluorescence microscopy, by which the organisation of the fluorescent asphaltenes in the bitumen has been observed. Similar surface morphologies have also been investigated by a scanning electron microscope using a cryostat for sample preparation (cryo-SEM).
As far as the two investigated aggregates (Loya and Fröstl) are concerned, X-Ray Diffraction delivered their phase composition. The surface morphologies have been visualized with the cryo-SEM and their specific surfaces have been determined by the BET method. The combined results led to the conclusion, that the adhesion properties of the Loya aggregate are superior to those of the Fröstl aggregate. This result has been confirmed by the immersion water test, which has proven the much better adhesion of the bitumen to the Loya aggregate.