Rheology involves the study and evaluation of the flow and permanent deformation of time-and temperature-dependent materials, such as bitumen, that are stressed through the application of a force. The fundamental rheological properties of bituminous materials including bitumen are normally measured using a dynamic shear rheometer (DSR), from low to high temperatures. DSR is a powerful tool to measure elastic, viscoelastic and viscous properties of binders over a wide range of temperatures and frequencies, provided the tests are conducted in the linear viscoelastic region. Therefore, the study of bitumen rheology is crucial since its reflects the overall performance of a flexible pavement. However, it is well known that the DSR also has limitations, where the measurements are exposed to compliance (testing) errors particularly at low temperatures and/or high frequencies. In addition, conducting laboratory tests are known to be laborious, time consuming and require skilled personnel. Therefore, this research is conducted to elucidate a better understanding of the rheological properties and modelling procedures of bitumens and bituminous binders.
Various materials such as unmodified bitumens, polymer-modified bitumens (PMBs) and bitumen-filler mastics, unaged and aged samples, are used in this study. An extensive literature review was undertaken to identify reliable models that can be considered as a valuable alternative tool to describe or fit the rheological properties of bitumen. These properties are commonly presented in terms of complex modulus and phase angle master curves, together with the determination of shift factor values at a particular reference temperature. In general, the complex modulus and phase angle master curves can be modelled using different techniques; nomographs, mathematical equations and mechanical models. However, the nomographs have become obsolete in recent years and tended to be replaced by the two latter models.
Those models are able to satisfactorily describe the rheological properties of unmodified bitumen. However, the observations suggest a lack of agreement between measured and predicted rheological properties for binders that contain a phase transition, such as found for highly crystalline bitumen, structured bitumen with high asphaltenes content and highly modified bitumen. An attempt was made to evaluate the validity of several mathematical equations and mechanical element approach using unaged and aged unmodified bitumens and PMBs database. It is observed that the Sigmoidal, Generalised Logistic Sigmoidal, Christensen and Anderson (CA), and Christensen, Anderson and Marasteanu (CAM) Models are able to satisfactorily describe the rheological properties of unmodified bitumens. Nevertheless, they suffer from the same drawbacks where the presence of highly EVA semi-crystalline and SBS elastomeric structures render breakdowns in the complex modulus master curves. Similar discrepancies are observed when one of the mechanical models (the 2S2P1D Model) is used.
To construct the master curves, different shifting methods are available. It is found that a numerical shift produced the best fit between measured and modelled data, followed by the Laboratoire Central des Ponts et Chaussées (LCPC) approach, William, Landel and Ferry (WLF), Modified Kaelble, Viscosity Temperature Susceptibility (VTS), Arrhenius and Log-Linear methods. A temperature range from 10 to 75oC is used in this study. It is worth mentioning that most of the methods are empirical and might not be applicable for all materials. Finally, the phase angle master curves must also not be neglected to yield a complete rheological properties of binders. The statistical analysis between measured and modelled data shows that the Fractional Model yielded the best correlation for a temperature range from10 to 75oC, followed by the Al-Qadi and Co-workers, CAM, CA and Kramers-Kronig relationships. An anomaly is observed between measured and descriptive data of the Kramers-Kronig relationship particularly at high frequencies and/or low temperatures. The Fractional Model is not considered suitable for practical purposes due to the high number of coefficients that need to be solved.
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