Influence of filler-to-total bitumen ratio on mastic properties and moisture susceptibility of bituminous mixtures

Abstract

The performance and longevity of asphalt pavements are significantly influenced by the quality of bituminous mastics, which consist of bitumen and fillers. While the mastic phase constitutes a smaller proportion of the overall asphalt mixture, it plays a crucial role in determining the mixture rheological, durability, cohesion and adhesion properties. The choice of bitumen, filler and their proportions impact mastic’s effectiveness. Whereas there has been considerable research focused on the interaction of neat bitumen with aggregates, there remains a significant knowledge gap regarding the role of mastics in asphalt mixtures, particularly in relation to stripping and ageing. Thus, this study sought to evaluate the influence of filler-to-total bitumen ratio on mastic physical properties and moisture susceptibility of bituminous mixtures. The research focused on characterizing the study materials, determining the effects of material type, proportion and short-term ageing path on mastic physical properties and the implication of mastics on stripping in bituminous mixtures. The results revealed that mastic stiffness increased with increasing filler-total bitumen ratio, but their temperature susceptibility decreased. The ageing path did not significantly impact mastic physical properties. Rock dust mastics were less resistant to stripping in comparison to the other filler mastics. The chemical-elemental properties of hydrated lime, mollusc shell fines and fly ash, which are categorized as active fillers, enhanced the adhesion of their mastics to the aggregates due to the chemical reaction between alkaline components (Ca+ ) in them and bitumen acidic components mainly carboxylic acids (-COOH) and phenols (-OH). The poor adhesion (high moisture damage susceptibility) of rock dust mastics was attributed to the acidic nature of rock dust filler owing to its high silica content. When carefully selected and controlled, alternative active fillers have the potential to improve the performance, durability and cost-effectiveness of bituminous mixtures in Uganda’s tropical environment. Their use aligns with local resource availability, climatic conditions and sustainability goals. However, quality assurance, specifications and capacity building need to accompany their adoption.