Asphalt mixtures are common building and construction materials. The formation mechanism of asphalt-aggregate interfaces is very critical in determining the performance of the mixtures. The effects of surface conditions and aggregate types on the adhesive bonding of the asphalt-aggregate systems have attracted significant research attention. In particular, moisture is often associated with various forms of damage, including reducing the bonding between the aggregates and the asphalt and the stripping of the binder from the aggregate surface when subjected to cyclic traffic loads. Considering the negative impacts of the moisture on the structural integrity and performance of asphalt mixtures, understanding the mechanism of the moisture-induced damages is vital in developing damage preventive measures.
Moisture damages have been extensively explored in the literature. Today, researchers can obtain new insights into the formation mechanism and microstructures of asphalt binders at micro and nanoscale using advanced testing methods like X-ray computed tomography. Even though these experimental methods provide reliable and effective methods for quantifying the bonding strength of asphalt aggregates, the experimental results are often susceptible to materials sources and specimen reparation methods used. Computer approaches, particularly molecular dynamic simulations, have been identified as promising solutions for studying the molecular interactions of asphalt-aggregate interfaces and their engineering properties. Nevertheless, the effects of surface moisture conditions and anisotropic mineral surfaces on the moisture-induced damages have not been fully explored.
Previous research revealed that surface moisture conditions exhibit considerable effects on the adhesion ability and coating of asphalt on aggregates, while the mineral surface affects its wettability and adsorption behaviors, and both influence the overall mixture performance. On this account, Lei Luo (Ph.D. student), Dr. Longjia Chu and Professor T.F. Fwa from Chang’an University used molecular dynamic simulations to study the effects of anisotropic mineral surfaces on the bonding behaviors of asphalt-aggregate in the presence and absence of moisture. They aimed at developing an evaluation criterion for assessing the moisture susceptibility at different moisture contents. The work is currently published in the journal, Applied Surface Science.
In their approach, the interactions of different asphalt-aggregate systems were extensively explored at the microscopic scale (Fig.1). The atomic models used for the simulations comprised a 12-component asphalt model together with two minerals, calcite and α-quartz. The wettability properties and moisture susceptibility were determined by using an improved energy ratio taking into account the residual adhesion between the aggregates and asphalt in the presence and absence of moisture. The feasibility of simulation results was validated by comparing them with existing experimental data.
Results showed that the anisotropy of the mineral surfaces significantly influenced the moisture susceptibilities and bonding properties of asphalt mixtures. The presence of concentrated hydroxyl groups on hydroxylated α-quartz surfaces increased its surface hydrophilicity and reduced its resistance to water-induced damages. The interaction between the α-quartz surfaces and water was stronger in hydroxylated α-quartz surfaces than in un-hydroxylated surfaces attributed to the hydrogen bond interactions. Additionally, a strong interaction was observed between the freshly-cleaved calcite surfaces and water molecules.
In summary, the authors performed molecular dynamics simulations to study the effects of moisture and mineral surfaces on the adhesion bonding of asphalt mixtures. Anisotropic mineral surfaces significantly influenced the bonding properties of the asphalt mixtures. The improved energy ratio for qualifying the moisture susceptibility identified freshly-cleaved calcites surface as the main susceptibility contributor compared to α-quartz. The simulation results were in good agreement with experimental data, further confirming that the asphalt-aggregate debonding process does not require external energy. In a statement to Advances in Engineering, the authors stated that the simulation work provided better understanding of asphalt mixture interfacial interactions and moisture damage mechanisms and would facilitate the development of high-performance asphalt paving mixtures.
Luo L, Chu L, Fwa T F. Molecular dynamics analysis of moisture effect on asphalt-aggregate adhesion considering anisotropic mineral surfaces. Applied Surface Science, 2020, 527: 146830.