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1 C H A P T E R 1 Introduction Background Porous Friction Course (PFC) mixtures are designed with an open aggregate structure to yield high in- place air voids (i.e., typically between 15 and 20 percent). The open aggregate structure (i.e., high air voids) of the PFC allows rainwater to drain horizontally through the layer toward the edge of the pavement structure, thereby improving pavement surface friction, reducing the potential for hydroplaning, and reducing splash and spray from vehicles. The open structure of the PFC also creates a negative, sound- absorbing texture, thereby reducing pavement noise. PFC mixtures are typically placed as thin layers on top of conventional pavements to improve safety, especially in wet weather conditions, due to their ability to drain rainwater effectively. Use of PFC surfaces was found to reduce the number of accidents by 85 percent in 213 pavement locations prone to wet weather accidents in Japan (Takahashi, 2013). PFC surfaces were also found to help reduce accidents by 76 percent and fatalities by 100 percent in Louisiana (King et al., 2013). This safety improvement supports the continuous use of PFC mixtures in the United States. PFC mixtures have also been used as surface layers for quieter pavements because of their negative, sound-reducing texture. They were found to reduce the noise level generated by passing vehicles from 3 to 6 dB when compared to conventional dense-graded asphalt surfaces (Cooley et al., 2009) and between 5.5 and 10.5 dB as compared with Portland cement concrete (PCC) surfaces (Kandhal, 2004). Several countries in Europe reported that their main motivation for using PFC mixtures was to control pavement noise (Kandhal, 2002). In spite of their multiple benefits, use of PFC mixtures has declined in the United States over the years. A survey conducted as part of NCHRP Project 01-55 (Watson et al, 2018) showed that only about half of 41 responding SHAs were using PFC mixtures. In addition, the agencies that did use PFC felt that their designs were not adequate to maintain the expected service life of PFC mixtures. The primary distress that reportedly caused premature failure was raveling, which is the progressive loss of aggregates from the surface downward to the interface with the underlying layer due to the repeated abrasion caused by traffic. To address the premature failure of PFC mixtures, a proposed performance-based mix design procedure for PFC was developed in NCHRP Project 01-55, Performance-Based Mix Design of Porous Friction Courses, and described in NCHRP Research Report 877 (Watson et al., 2018). The procedure includes performance tests to be conducted during the mix design process to evaluate the resistance of a PFC mixture to raveling, moisture susceptibility, and asphalt draindown (with optional tests for cracking and rutting) while maintaining an air void structure that would provide water drainage from the surface. Objective The objective of NCHRP Project 20-44(18) was to assist SHAs in implementing the proposed performance-based mix design procedure, verify if the thresholds proposed in the procedure could be achieved, and refine the procedure if needed.
2 Scope of Work To accomplish the project goals, the project team worked with three state highway agencies:, Georgia (GDOT), South Carolina (SCDOT), and Alabama (ALDOT) Departments of Transportation. Participation of these agencies included: (1) conducting PFC mix designs and (2) constructing the designed PFC mixtures on their projects.
