Interactions of moisture and oxidative ageing mechanisms in paving binders: Towards improving durability of pavements

Extensive transportation infrastructure is crucial for enabling efficient transportation and fostering economic development. However, the operation of roadways contributes to escalating environmental and societal issues, including noise pollution and carbon emissions. To achieve a balance between economy, environment, and society, growing focus is being placed on enhancing the durability and sustainability of asphalt pavement in all aspects. A pivotal factor contributing to the deterioration of asphalt pavement performance is the degradation of bitumen properties, significantly affected traffic and climate conditions. The latter is primarily related to two key processes: oxidative aging and moisture in bitumen. Extensive studies have been undertaken to investigate the moisture and ageing behavior while large variations have been reported due to the complexity of these behavior, the diversity of bitumen types, and the variability of environmental conditions. Such discrepancies complicate the formulation of bituminous binders and mixtures that mitigates these issues.

The objectives of this thesis are centered on gaining a thorough understanding of the mechanisms behind moisture and ageing behavior in bitumen, working towards a fully coupled moisture and ageing behavior in bitumen and their impact on bituminous materials. To figure out the complex moisture and aging effects in bitumen, it is essential to initiate from a simpler scenario without involving too many variables and then gradually take into consideration more factors affecting the moisture and ageing processes.

Guided by this principle, the moisture diffusion behavior in bitumen and its effects on bitumen properties are initially investigated. To obtain fundamental insights into the moisture mechanisms, both experiments and molecular dynamics simulations are conducted for mutual interpretation and corroboration. Results show that the moisture in bitumen consists of three states: free water characterized by a constant diffusion coefficient and a saturated concentration, immobile water bonded to polar functional groups
with a maximum concentration dependent on the number of these available groups, and water clusters, whose diffusion coefficient is significantly lower compared to free water and varies with cluster size.

The effects of ageing on the physicochemical and rheological properties of bitumen derived from diverse crude oil sources are evaluated and the physicochemicalrheological relationship is elucidated. The direct analysis of specific parameters and the chemometric analysis of full curves obtained from various chemical, physical, and rheological tests are combined for more accurate and extensive characterization. Results provide new insights into the chemo-mechanical relationship of bitumen, demonstrate the high capability of comprehensive information embedded in FTIR spectra and GPC curve in characterizing various rheological properties, and guide the selection of key physicochemical and rheological parameters for the evaluation of bitumen degradation. To characterize the impact of ageing on the moisture diffusion behavior in bitumen including moisture diffusion coefficient and absorption amount, the moisture transport in aged bitumen is measured through dynamic vapor sorption. The key factors determining moisture diffusion coefficient and moisture absorption are fractional free volume and polarity of bitumen. The changes of these two factors with increasing ageing level integratedly lead to increased activation energy for the diffusion of free water and reduced enthalpy of its adsorption. Simultaneously, the ageing kinetics of bitumen films are affected by the presence of moisture. The presence of moisture can inhibit oxidative reaction through the competitive diffusion between oxygen and moisture while the removal of moisture accelerates it due to additional diffusing pathways created by the diffusion of water molecules, especially water clusters.

In conclusion, this thesis provides in-depth insights into the understanding and evaluation of moisture diffusion and oxidative reaction in bitumen, as well as their respective effects on bitumen properties. The effects of moisture on the ageing process and that of ageing on the moisture behavior elucidated in this thesis establish a robust foundation for the modelling of fully coupled moisture-ageing effects in bituminous binders. These findings provide valuable guidance for the design, construction, and maintenance of porous asphalt pavement aiming at improved pavement durability and sustainability.