Session: 13-15-01: Mechanics of Soft Materials I

Paper Number: 167173

Investigating the Effect of Asphalt Film Thickness on Aging via Nanoindentation 

Asphalt film thickness is a critical parameter in Hot Mix Asphalt (HMA) design and performance, as it affects not only the pavement’s resistance to aging and moisture damage but also its overall durability. Traditional methods of determining film thickness often rely on indirect calculations based on aggregate surface area and the percentage of binder, as outlined in various design manuals and literature. While these techniques have provided valuable insights over several decades, they do not directly measure the film coating on individual aggregate particles or track changes over time. This gap in direct, time-dependent measurement has limited our understanding of how thin binder films evolve under real-world aging conditions.

In this study, we address that gap by employing a nanoindentation approach to measure asphalt film thickness in a more direct and localized manner. The fundamental concept behind nanoindentation is straightforward: a fine indenter tip presses into the asphalt binder layer until it encounters a significantly stiffer substrate, which in this case is the aggregate particle. The load–displacement curve recorded by the nanoindenter exhibits a distinct change in slope when the tip transitions from penetrating the soft asphalt binder into the harder aggregate. We identify this inflection point as the boundary marking the thickness of the asphalt film. By carefully calibrating the maximum load and loading rate, we ensure that the indenter does not exceed the binder layer thickness significantly, thus allowing us to pinpoint the film thickness accurately.

Using this procedure, we tested HMA samples both before and after controlled oxidative aging in a draft oven at 85 °C. Initially, the unaged specimens revealed an average asphalt film thickness of around 13 micrometers (µm), although there was some variation due to natural differences in local binder distribution. After two days of oven aging, the average thickness dropped to about 4 µm, highlighting a swift reduction in the binder layer. By the fourth day of aging, the film thickness approached 2 µm, suggesting that most of the shrinkage and hardening of the binder occurs relatively early in the aging process. This rapid decline likely stems from the loss of lighter, more volatile components in the asphalt, coupled with the formation of increasingly polar and stiff chemical groups through oxidation. These findings confirm that nanoindentation is a viable method for directly assessing asphalt film thickness and illustrate the importance of early aging in determining the long-term performance of HMA pavements. Controlling or mitigating this early-stage binder hardening could significantly enhance pavement service life by maintaining a thicker, more flexible film for a longer period. Ultimately, this work underscores the need to refine mix design strategies and explore additives or treatments that can slow the oxidation process, ensuring longer-lasting and more cost-effective road surfaces.

Presenting Author: Hasan Faisal Colorado State University Pueblo

Presenting Author Biography: Dr. Hasan Faisal is a researcher and educator specializing in nano- and micro-scale rheology with applications in transportation materials and sustainable infrastructure. As the Director of the Civil Engineering Technology (CET) program at CSU Pueblo, he integrates cutting-edge research into curriculum development and student mentorship. His expertise includes asphalt binder characterization, polymer-modified materials, and advanced rheological analysis at the microstructural level.

Authors:

Hasan Faisal Colorado State University Pueblo
Rafiqul Tarefder University of New Mexico
Christopher Beascochea Colorado State University Pueblo