The physicochemical influence of the inorganic phase on the aging and performance of asphalt pavements in Finland

Abstract

The need to understand the reasons behind the deterioration of asphalt pavements led to the research focused on the relationship between molecular interactions within this composite material and its macroscopic properties. The aim of the work was to establish the risks arising from the interaction between bitumen and the inorganic compounds, namely fine particles passing the 125 μm sieve. The first case study, the forensic analysis of failed asphalt pavement, Ring Road II in the Helsinki Metropolitan Area, revealed that a substitution of the asphalt concrete component occurred between the mix design and production, mass by mass. The mineral filler used contained fly ash in lieu of using pure limestone as was designed. This work investigated how physicochemical properties, such as the altered density and chemical composition of the filler, affect the properties of asphalt during recycling in Finland. The iron compounds located on the surface of the filler grains, in addition to the differences in the volume of the reinforcing phase, correlated well with the increase in the softening point of asphalt mastic. Additionally, the interaction of iron rich fillers with deicing chemicals resulted in higher chloride accumulation in the inorganic fines of asphalt concrete during service life. During the recycling of asphalt concrete, the accumulation of chlorides could prove dangerous, if those inorganic compounds catalyzed the bitumen oxidation. The in-situ laboratory study of the oxidation reaction of bitumen demonstrated how the presence of inorganic salts alters the mechanism of reaction. The iron (III) chloride was confirmed as a catalyst of oxidation, while a typical deicing chemical, calcium chloride, left the reaction mechanism and its rate unaltered. What is more important, the novel time-resolved infrared spectroscopy analysis provided insight into pure bitumen oxidation as well, revealing that the reaction at high temperatures occurs step-wise i.e., thiol oxidation followed by ketone formation. Controlling the reaction by blocking the thiol oxidation via interaction with copper successfully hindered the ketone formation. The novel approach to asphalt concrete engineering on a molecular level is promising, but the effect of such control on the physical properties of asphalt pavements still needs to be confirmed. The effect which oxidation of bitumen, followed by blending with fresh organic material, has on the molecular weight distribution, change in polarity, and rheological characteristic was studied in laboratory conditions to simulate four cycles of reuse. Increasing molecular weight distribution correlated well with an increase in the elastic response of the material. The loss of the ability to dissipate energy by flowing was postulated to cause the declining performance in the cold climate regions.