What One Million Polishing Cycles Reveal About Long-Term Friction Safety

BATT’s Groundbreaking Million-Cycle Test

In early 2024, the asphalt specialists at the BATT Lab in Richmond, KY, set out to challenge a long-standing assumption: that only dolomitic limestone aggregates can provide durable surface friction under traffic when compared to Kentucky aggregates containing silica and other hard minerals. Under the direction of Phil Blankenship, PE, MSCE, and led by Lab Operations Manager Zack McKay, BATT conducted the first known one-million-cycle aggregate polishing test using the Three-Wheel Polishing Device (TWPD).

Historically, Kentucky’s reliance on limestone has been based on legacy data from quarry ledges, correlated with skid numbers obtained through locked-wheel or SCRIM testing. An open question is whether newer or alternative aggregates can also provide adequate long-term friction performance.

Traditionally, answering this question required constructing a highway test section and waiting 5–10 years for natural polishing under approximately 10 million vehicle passes. To accelerate the evaluation, three-wheel polishing (TWP), combined with Dynamic Friction Testing (DFT), offers a promising laboratory approach.

How many polishing cycles are sufficient to represent field performance?

NCAT research suggests 100,000 to 150,000 TWPD passes may approximate typical traffic exposure, since the test is always conducted wet (a worst-case condition) with a contact pressure of 87 psi per tire — equivalent to a semi-truck tire footprint — multiplied by three. Still, caution is essential when applying this benchmark. If the polishing requirement is underestimated, an aggregate may appear acceptable in the lab yet lose friction prematurely in service. Unlike pavement cracking, which can often be managed, inadequate surface friction is a direct safety hazard. Because little work has been done to calibrate this method, BATT undertook a study to refine the approach while assisting a local aggregate producer and quarry operator.

The Million-Cycle Approach to Aggregate Wear

BATT’s Million Cycle TPWD test ran 24/7 for three months, requiring 270 hours of machine time and duplicate slabs to reduce variability and improve confidence in the results. What it revealed was striking: Even some of the best dolomite limestone surface declined over time, while the silica-based materials continued to resist polishing under extended loading. These findings challenge long-standing assumptions about aggregate wear and open the door to new thinking that could improve asphalt mix designs for long-term friction performance and durability.

While the silica-based material did not start with the higher friction values of the dolomite limestone, it maintained the friction longer than the limestone. Does this mean that dolomite limestone is not as good? No, rather, it means that we should be able to use the silica-based aggregate or any aggregate combination as defined in the DFT where the polishing slope is flatter (not changing with continuous polishing).

We wanted to run this test to validate assumptions and assist research into accelerated lab testing to certify aggregate mixes based on lab properties instead of waiting years to see aggregate wear in roadways. The main goal is always to improve the safety of asphalt pavements.

Beyond Rutting: A New Role for Aggregate Testing

While traditional tests like the Hamburg wheel track test focus on rutting and moisture damage, they stop short of evaluating aggregate polishing—a key contributor to loss of friction and skid resistance over time. That’s what makes BATT’s million-cycle TWP test so significant: it extended well beyond conventional testing windows to simulate the long-term surface wear that aggregates experience under real-world traffic conditions.

Most traditional polishing or wheel tracking tests are limited to a fraction of that number (100,000 to 150,000 cycles as stated earlier), and they produce results that favor limestone in short-term performance. But BATT’s extended test showed that materials like granite and crushed gravel retain surface texture longer, potentially offering safer, more durable pavement surfaces.

While BATT does NOT recommend that million cycle polishing be used in everyday testing, the experiment proved useful in taking these mixes to the limit. One-million-cycle polishing can lead to other issues such as stripping of the mix that can cause variability in friction results.

But the stripping and friction loss we observed are real-world effects and is most likely what is happening on our pavements.

Locally Sourced Aggregates and the Case for Re-Evaluation

Limestone’s dominance in asphalt mixes isn’t solely based on lab test performance. It’s widely available — especially across the Midwest and South — making it a locally sourced, cost-effective option. It’s also easier to crush and tends to meet moisture susceptibility, stability, and gradation requirements with less processing. Combined with agency specs written around its characteristics, these advantages have made limestone the go-to aggregate, even when harder aggregates may outperform it over time.

Aggregate is one of the costliest materials to transport in highway construction and hauling it long distances adds both financial and environmental burdens. If a polish-resistant aggregate is available closer to the project site, it can reduce costs, lower carbon impact, and simplify logistics.

This raises an important consideration: some local materials may not currently be “approved,” yet they could still provide excellent long-term skid resistance if properly evaluated. BATT’s million-cycle polishing test raises important questions:

• Is it time to rethink aggregate acceptance criteria based solely on acid insolubility residue, and consider TWP and DFT as additional mix design tools?

• Should we place greater priority on polish-resistant local materials that may deliver longer-lasting friction, reduced maintenance costs, and improved roadway safety?

KYTC: Leading the Way in Friction Safety Research

BATT’s work aligns closely with an ongoing research initiative by the Kentucky Transportation Center (KYC) and funded by the Kentucky Transportation Cabinet (KYTC) to improve pavement friction and reduce roadway fatalities.

In 2021, KYTC/KTC launched a project focused on integrating friction testing into Balanced Mix Design (BMD). Track sections at NCAT (S7A and S7B) were designed to test medium- and high-friction mixes while validating both the TWP and the DFT. The project aims to correlate lab-measured friction values and polishing cycles to real-world traffic performance and build them into performance-based specifications alongside rutting and cracking resistance.

These tests strive to make friction a core design criterion, not just a performance afterthought — a goal that aligns directly with the implications of BATT’s million-cycle TWP that pushes the limits of the test.

Limestone’s Earlier Polishing Calls for Smarter Mix Design

The KYTC study also highlights a key geographic concern: sedimentary limestone is widely used in states like Kentucky, Missouri, Indiana, Ohio, Louisiana, and parts of Tennessee, where it polishes more quickly under traffic. In contrast, northeastern states often use granite or other hard materials that are more durable under traffic.

The combination of soft aggregates and high traffic volumes makes friction loss more likely — and more dangerous — in these regions. That’s why designing asphalt mixes that better resist polishing over time is essential. This also has implications for how RAP is classified and incorporated, since most modern mixes contain reclaimed materials. These findings highlight the increasing focus on the importance of Balanced Mix Design and other performance testing methods — especially when using RAP, additives, and regionally available aggregates — to ensure long-term safety and sustainability.

From One Million Cycles to Safer Highways

BATT’s one-million-cycle polishing test is more than a research milestone. If these findings are validated in further studies, they may spark a shift in how friction, polishing, and long-term durability are considered in asphalt mix design.

With BATT, KYTC, NCAT, and others leading the way, the future of asphalt design is moving toward data-driven, safety-focused solutions, where smart testing and long-term thinking determine which materials make it from lab to pavement.