Polishing the Asphalt Surface: Tennessee Researches Aggregate Frictional Properties
BY AsphaltPro Staff
University of Tennessee report offers guidelines for aggregate polishness in asphalt surface mixtures
The final report titled “Investigating the Long-term Frictional Properties and Establishing Aggregate Polishness Guidelines for Asphalt Surface Mixtures in Tennessee” from the Department of Civil and Environmental Engineering at the University of Tennessee, Knoxville, explores innovative methods and technologies for evaluating the frictional behavior of aggregates used in asphalt, with a strong focus on Reclaimed Asphalt Pavement (RAP). This summary highlights the research findings related to technology application in assessing and improving asphalt aggregate performance, especially in terms of long-term friction and skid resistance.
Combining chemical and physical parameters provides a reliable method for estimating long-term pavement skid resistance.
The State of Flat and Elongated Aggregate Requirements for SMA
Context and Problem
The authors, Baoshan Huang, Jingtao Zhong, Kai Huang, Yuetan Ma and Pawel Polaczyk, explained the project was conducted in cooperation with the Tennessee Department of Transportation (TDOT) and the Federal Highway Administration (FHWA) under project number RES2023-15. They acknowledged highway safety is closely tied to pavement skid resistance, which largely depends on the frictional and textural properties of aggregates.
In Tennessee, where certain high-quality aggregates must be imported, ensuring long-term pavement friction is both a safety and economic concern. Although RAP has been used extensively in the state, its role in surface mixes has been limited due to unknown frictional characteristics. Traditional mix design methods don’t incorporate performance-based friction testing, which has prompted the need for research.
Technological Methods and Tools Used
This study employed a range of tools to evaluate the long-term frictional performance of asphalt aggregates, especially in mixes containing RAP.
X-ray Fluorescence (XRF) was used to measure silica dioxide (SiO₂) content—a key indicator of polish resistance. Testing focused on uniformly sized aggregate particles (passing the 3/8-inch sieve, retained on No. 4) to ensure consistent results. RAP samples were processed chemically (using Trichloroethylene, TCE) or mechanically (with a hammer) to expose aggregate surfaces. TCE treatment proved more accurate and consistent.
Aggregate Image Measurement System (AIMS) assessed surface angularity and texture, which reflect micro-texture and friction potential. Materials with higher SiO₂, especially slag and granite, retained their texture better under abrasion, supporting the link between chemical makeup and skid resistance.
The Micro-Deval (MD) test evaluated aggregate durability through rotational abrasion. Results confirmed that aggregates with higher SiO₂ content suffered less wear, correlating with better frictional performance.
To simulate traffic wear, the Three-Wheel Polishing Device (TWPD) polished asphalt specimens in controlled lab settings. This enabled assessment of how surface friction degrades over time.
Friction was measured with the Dynamic Friction Tester (DFT), which recorded the coefficient of friction (COF) at various speeds under wet conditions. DFT results were reliable for both lab and field use and are recommended for performance-based mix design.
The Circular Track Meter (CTM) measured macro-texture via Mean Profile Depth (MPD), complementing the DFT’s micro-texture data. Together, they supported the calculation of the International Friction Index (IFI).
The Locked-Wheel Skid Trailer (LWST) validated DFT results by measuring friction directly in the wheel path. While LWST values were often lower due to heavy polishing, they confirmed DFT’s broader applicability.
Lastly, the study tested gyratory-compacted pills as a practical alternative to large slab specimens. These “rings” provided similar friction and texture results, offering a cost-effective option for lab testing.
Overall, the integration of chemical analysis, image-based texture evaluation, and friction simulation tools provided a comprehensive system for assessing and designing asphalt mixes with durable frictional performance.
Key Findings
The study found that both slab and pill-shaped asphalt specimens yielded similar friction and texture results when measured with the DFT and CTM, making ring-shaped samples a practical alternative for lab testing. Accurate measurement of SiO₂ content using XRF depended heavily on aggregate size and uniformity; using particles retained on No. 4 sieves produced the most consistent results for both virgin aggregates and RAP. SiO₂ content showed a strong correlation with key friction indicators such as Polished Stone Value (PSV) and surface texture loss, confirming its usefulness as a predictor of polish resistance.
The DFT proved to be a reliable tool for evaluating friction in both lab and field settings. It tends to report higher friction values than the LWST because it measures a broader area that includes both polished and unpolished pavement zones, whereas the LWST measures directly in wheel paths. Field testing revealed consistent friction properties between left and right wheel paths across most segments, suggesting uniform polishing effects under traffic.
A multiple regression model combining silica-iron mineral content (SiO₂ + Fe₂O₃) with the percentage of polish-resistant aggregate successfully predicted terminal COF with high accuracy, explaining 88% of the variation. This indicates that combining chemical and physical parameters provides a reliable method for estimating long-term pavement skid resistance.
The deployment of advanced technology such as XRF, AIMS, TWPD, DFT and CTM in this research represents a leap forward in understanding and predicting the long-term frictional behavior of asphalt aggregates. These methods allow for quantitative, reproducible and efficient assessment of aggregate polish resistance and offer a framework for performance-based mix design that enhances safety and sustainability. Particularly, the strategic use of RAP can now be better managed through accurate chemical analysis, making this research instrumental in evolving Tennessee’s asphalt pavement specs.