Investigating and Developing Fatigue-Healing Characterization of Asphalt Materials

Canada’s aging highway network, consisting of over 1.1 million kilometres of roads, is a vital component in ensuring the safe and reliable day-to-day movement of people and goods. Flexible asphalt pavements experience deterioration due to repeated traffic and environmental loading, and as a consequence, may require costly maintenance and rehabilitation treatments to remain functional over their service life. An alternative strategy to these reactive treatments can be found in the form of self-healing asphalt pavements. \nThe innovative strategies employed in self-healing materials are rooted in natural and biological processes. In the simplest sense, as these materials become damaged, a natural healing response allows them to restore their integrity or functional properties. Bitumen and asphalt materials have a very similar intrinsic healing ability which has been recognized since it was first observed in the 1960s. Asphalt material self-healing arises from the asphalt cement and its ability to fill microcracks caused by repeated small strain amplitude loading (i.e., fatigue). Intrinsic healing is influenced by internal factors (e.g., asphalt cement chemistry, aging level, and modification type) and external factors (e.g., moisture, UV exposure, rest period duration and temperature, etc.). To overcome the limitations of intrinsic healing, several extrinsic healing technologies have been used in literature including molecular interdiffusion techniques, structures containing healing agents and secondary self-healing polymer phases. In recent studies, healing is primarily characterized using destructive accelerated fatigue and fracture-based tests with rest periods, but the lack of industry-standard tests and terminology for both asphalt cement and mixtures leads to an often-ambiguous understanding of healing itself. This lack of standardization limits the capability of researchers to effectively characterize the intrinsic healing ability, but also develop new extrinsic healing technologies. The principal goal of this thesis is then to investigate current self-healing characterization techniques and develop new testing protocols for asphalt materials. \nThe work presented in this thesis uses a multiscale approach to the healing characterization of asphalt materials in collaboration with RILEM Technical Committee CHA-278 (Crack Healing of Asphalt Materials). Based on the initial healing study of unaged and aged asphalt cement containing a chemical warm mix additive using the linear amplitude sweep healing (LASH) test, it was evident that current generation DSR-based fatigue and healing characterization techniques experience measurement artifacts caused by geometry changes/ specimen flow under loading. As a result, a comprehensive evaluation of the linear amplitude (LAS) test and different failure criteria found in literature was conducted. These failure criteria were then supplemented with the complementary parameters: the electric torque inflection point, the peak normal force, and the flow strain amplitude (FSA) as determined from the novel DSR Visual Analysis workflow. From the analysis of the LAS test, it was shown that failure criteria strain amplitudes could be categorized as either peak or post-peak behaviour; the traditional peak shear strain amplitude was concluded to be the most conservative failure criteria for all aging levels. From the FSA analysis, it was demonstrated that the shear stress-strain peak correlated with the onset of specimen flow, thus, the peak value was selected as the maximum strain amplitude for the first phase end condition for subsequent healing tests. In cooperation with the RILEM CHA-278 Task Group (TG) 2a, a second version of the LASH test protocol was proposed and evaluated based on the recommendations of the LAS visual analysis. From various works in literature, the Pure-LASH or P-LASH peak-based analysis method was derived using fracture mechanics to model the restoration ability of several binders at different aging levels using the LASH V2 protocol. Test parameters such as rest period duration and aging level were found to not be statistically significant factors, but further analysis determined that “damage” was only observed when the first loading phase end condition was the peak shear stress strain amplitude (γpeak) as flow had already irreparably change the geometry of the DSR specimen. The general conclusion of these works was that geometric changes of the DSR sample during loading produce increasingly inaccurate measurements of post-peak data in amplitude sweep tests, thus, future work should take a greater emphasis on the characterization of pre-peak behaviour. \nAs a contribution to RILEM CHA-278 TG 2b, intermediate temperature fatigue tests with rest periods were conducted on a single asphalt mix at two strain levels. From the fatigue tests, a new fatigue model called the Intrinsic-VECD or iVECD linearization model was derived from the DGCB intrinsic damage model and simplified viscoelastic continuum damage model (VECD). The iVECD model was then extended to healing tests to separate “true” fatigue damage from common bias prevalent in accelerated fatigue testing of asphalt materials. From the iVECD model, several restoration/healing and damage indices were proposed: %Heal, %Recovery, %Restoration and Permanent Damage (%PD). Results of the iVECD healing indices indicated that the majority of recovery and healing occurs within the first 4 to 6 hours of the rest period. However, it was observed that increasing the strain level produces more permanent damage for the same loading duration. Finally, non-invasive ultrasonic measurements were coupled to these destructive fatigue tests and coda wave interferometry (CWI) was used to analyze the effects of multiple scattering due to damage and healing. Two windowing selection methods were proposed (i.e., a simplistic statistical method and an adaptive analytical method). Using both window selection techniques, it was demonstrated that CWI could capture the effect of both loading/unloading and was sensitive enough to clearly distinguish between different strain levels.

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