Alternative Friction Measurement for Concrete Pavements: CA PST vs. CFME Correlation Study

Abstract: The California Portable Skid Tester (CA PST) is a device used to measure pavement skid resistance, following the California Test 342 (CT 342). Between 2021 and 2023, California Department of Transportation (Caltrans) conducted a study to evaluate the correlation between the CA PST and Continuous Friction Measurement Equipment (CFME) for assessing skid resistance on concrete pavements. The goal was to identify alternative friction-testing devices for Caltrans and industry professionals, particularly for use on bridge decks and limited-access areas. Following an analysis of the initial findings, the Review Committee comprised of Caltrans and Industry members recommended additional testing to develop correlation between CA PST and CFMEs. This additional testing and data collection took place between May 16 and July 31, 2024.
Friction testing included tests at various Caltrans locations in Districts 3 and 4, utilizing the CA PST and two walk-behind skid testers (also categorized as CFMEs), specifically, the Micro GripTester (MGT) and T2GO-R, as outlined in ASTM E2340/E3304. Results revealed that ASTM E3304 lacks sufficient standardization for preparation, calibration, measurement collection, and friction value reporting, potentially affecting data consistency. Data from this additional testing in 2024 expanded the range of friction values from previous data collection, and provided 21 new data points, with 20 of these showing CA PST friction values below 0.30, a range less frequently observed in previous data runs.
In terms of usability and data processing, the MGT required less effort than the T2GO-R, although additional operator training may further reduce processing time for both devices. Both MGT and T2GO-R demonstrated high data usability rates, with 92% and 99% of collected data deemed usable, respectively. Both devices exhibited trends similar to the CA PST across all tested Portland cement concrete (PCC) surfaces, with MGT and T2GO-R showing high correlations with CA PST (R² values of 0.86 and 0.81, respectively). The R² values for both devices compared to historical studies, surpassing correlations found in a 1971 Caltrans study.
The study recommends considering the MGT and T2GO-R as potential CA PST alternatives due to their strong correlation, data retrieval success, and differentiation among surfaces with varying friction properties. Proposed acceptance criteria include a CFME friction threshold of 0.70 for a CA PST friction criterion of 0.30 and a CFME threshold of 0.80 for a CA PST criterion of 0.35. Suggested next steps include refining CFME test procedures, modifying ASTM or CT methods to improve standardization, and keeping up with new CFME technology advancements.
This study demonstrates that, under certain conditions, CFME devices may serve as reliable replacements for the CA PST, offering promising alternatives for friction measurements in California.