LINEAR AND NON-LINEAR VISCOELASTIC BEHAVIOUR OF BINDERS AND ASPHALTS

In order to predict the engineering performance of a material, it is necessary to understand its stress/strain behaviour. This is usually accomplished by means of laboratory tests under controlled stress or controlled strain conditions. As bituminous materials are viscoelastic in nature, their performance must be characterised with test methods and analytical techniques that account for time (or rate) of loading and temperature. In addition, it is usually advisable to confine the characterisation of a bitumen to its linear viscoelastic (LVE) response (small strains) to simplify the mathematical modelling of the material, as non-linear response, particularly for viscoelastic materials, is extremely difficult to characterise in the laboratory and model in practical engineering problems. This thesis describes research into the linearity limits of various unmodified (penetration grade) and modified (both process modification and polymer modification) bitumens as well as the linearity limits of asphalt mixtures incorporating these binders. The linearity limits for the binders have been determined by means of stress sweeps at various temperatures and frequencies using a dynamic shear rheometer (DSR). The linearity limits and dynamic mechanical properties of the conventional and modified asphalt mixtures have been determined by means of a purpose built dynamic, direct tension-compression, servo-hydraulic testing apparatus. The linearity results show that for sensibly engineered modified binders (softer base bitumens with higher modifier contents) there is no significant narrowing of the linear range. The process modified, plastomeric (EVA PMB) and conventional binders show a strain dependent LVE criterion between 2% and 6% at low temperatures (high stiffness and intermediate to low phase angles) as well as a stress dependent LVE criterion between 1.5 and 7 kPa at high temperatures (low stiffness and high phase angles). The thermoplastic rubbers (SBS PMBs) show, in addition to the low temperature strain criterion, a high temperature (low stiffness) polymeric-based strain dependent LVE criterion between 50% and 150%. The asphalt mixtures studied in this thesis consisted of dense bitumen macadam (DBM), hot rolled asphalt (HRA) and HRA mortar mixtures with conventional 50 pen bitumen, radial and linear SBS PMBs, EVA PMB and multigrade bitumen. Cylindrical specimens of the mixtures were subjected to sinusoidal loading conditions at temperatures between 10 and 40°C under stress sweep conditions. The results indicate that the linearity limits of behaviour for the asphalt mixtures are strain dependent and found to be between 20 and 100 microstrain for the DBM and HRA mixtures and between 50 and 150 microstrain for the HRA mortars. Additionally, upon establishing the LVE limit, the viscoelastic properties of the bitumens and asphalt mixtures, in their linear region, were evaluated by means of frequency sweep tests at various temperatures and frequencies. The results show that, in the low frequency region (high temperatures), the rheological behaviour of the SBS PMBs differs from that of the other bitumens. The DBM and HRA mixtures all show very similar rheological characteristics over the temperature and frequency range used in this study. With regard to the HRA mortars, due to their higher binder and air void volumetric proportions, their stiffness values tend to be approximately half that of the DBM and HRA mixtures. Finally, the relationship between binder and mixture stiffness was investigated in the linear viscoelastic region, for different unmodified and modified binders and asphalt mixtures. Measured asphalt mixture stiffness was compared with selected empirical and theoretical mixture stiffness prediction methods. In general, the bitumen to mixture complex modulus relationship was independent of temperature. However, although a straight line could be established for the EVA PMB and linear SBS PMB binders, the results show that the binder to mixture stiffness relationship exhibits temperature dependence for mixtures produced with these two binders. The results show that there are large differences between the stiffness predicted by the empirical and theoretical methods and those measured experimentally.