Laboratory investigation of bitumen based on round robin DSC and AFM tests
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In the past years a wide discussion has been held among asphalt researchers regarding the existence and interpretation of observed microstructures on bitumen surfaces. To investigate this, the RILEM technical committee on nano bituminous materials 231-NBM has conducted a round robin study combining differential scanning calorimetry (DSC) and Atomic Force Microscopy (AFM). From this, methods for performing DSC and AFM tests on bitumen samples and determination of the influence of wax on the observed phases, taking into account thermal history, sample preparation and annealing procedure, are presented and critically discussed. DSC is used to measure various properties and phenomena that indicate physical changes such as glass transition temperature (T g) and phase transition such as melting and crystallization. In the case of existence of wax, either natural or synthetic, it can further indicate the melting point of wax, that could be used to determine wax content. The results from seven laboratories show that T g temperatures obtained from the heating scans are more repeatable and easier to obtain in comparison to the cooling scans. No significant difference was noted for T g’s obtained from the first and second heating scans. AFM is an imaging tool used to characterize the microstructures on a bituminous surface. Using AFM three phases in the materials with wax could be distinguished. The changes in the phases observed with AFM for increases in temperature were correlated with the DSC curve, and it could be established that the so called “Bee” structure disappeared around the melting peak in the DSC curve. Thus, this research has confirmed the relation between the microstructures on a bitumen surface and the wax content.
KeywordsBitumen Asphalt DSC AFM Wax Multiphase material
The authors would like to thank the members of RILEM technical committee 231-NBM for their cooperation and input and Nynas and Q8 for supplying the materials. The contribution of W. Grimes, T. Pauly, F. Turner and J. Forney at WRI, P. Izdebski and B. Fischer and C. Walder from Empa, P. Wedin from Nynas, C. Nijssen-Wester from KPR&T and C. Petiteau from IFSTTAR is greatly appreciated.
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