Novel Combined Degradation Index for Long-Term Performance Comparison of Epoxy and Asphalt-Based High-Friction Surface Treatments

High-friction surface treatments (HFST) are crucial safety measures that improve skid resistance and reduce accidents on high-risk roadway sections. While traditional epoxy-based HFSTs are commonly utilized, they have limitations, such as compatibility with existing pavements, expensive construction and removal costs, and durability issues owing to substrate pavement conditions. This study presents a comprehensive experiment-based investigation into the long-term degradation behavior of innovative asphalt-based flexible HFST alternatives versus traditional epoxy systems. Under controlled polishing conditions, laboratory testing was carried out to compare the performance of both systems. The experiment program included systematic polishing investigations up to 140 K cycles on bauxite and rhyolite aggregates of two different gradations. The performance was evaluated using coefficient of friction (COF), mean profile depth (MPD), and a new combined degradation index (CDI = COF × MPD), which integrates both micro- and macro-texture degradation. CDI validity was supported through consistency with multi-speed friction measurements and unsupervised k-means clustering (silhouette score = 0.549), which identified statistically distinct CDI ranges representing low, moderate, and high degradation states based solely on laboratory measurements. To characterize degradation progression, multiple models were evaluated, with Weibull-based functions providing the best fit (average R 2 = 0.95) relative to exponential (0.90) and power decay (0.76) models. Extrapolation to 300 K cycles indicates that epoxy systems maintain CDI values within the low-degradation range throughout the extended test duration, while asphalt-based systems typically transition into the moderate- or high-degradation ranges between 150 K and 200 K cycles. Among the asphalt systems, PG88-16 highly polymer modified (HPM) with coarse calcined bauxite retained CDI above the moderate-degradation threshold through approximately 245 K cycles (≈82% of epoxy’s >300 K performance window), though with lower absolute CDI values. Asphalt-based HFSTs show promising adaptability to varied pavement conditions and offer practical advantages in constructability, substrate compatibility, and cost. Although epoxy systems show higher absolute durability in laboratory testing, certain asphalt-based configurations deliver performance levels that may be adequate in applications where both systems exceed typical HFST design demands. The validated CDI framework establishes a robust, laboratory-based methodology for evaluating HFST material degradation; however, further work is needed to calibrate CDI ranges for field conditions and broader pavement applications.