Permeable friction courses improve safety on the road
The difference is dramatic. After a downpour, one pavement sprays water everywhere, obscuring the vision of motorists and making it harder for them to steer. On the other pavement — hardly a whisper of water comes off the roadway.
The latter pavement has a permeable friction course (PFC), a porous pavement layer that allows water to drain away from the roadway. This hot-mix asphalt surface course has a high air void content, which makes it highly permeable. PFCs improve safety in wet weather conditions, since the water on the roadway travels through the air voids and off the road, instead of over the surface, and have good friction and skid resistance. Other benefits are cleaner runoff water and less roadway noise.
“We successfully completed a project for the Texas Department of Transportation [TxDOT] on mix design of PFCs,” says Amy Epps Martin, a Texas Transportation Institute (TTI) associate research engineer and associate professor of civil engineering at Texas A&M University. “Now our focus is on the performance of these mixtures.”
The questions now are — how are PFCs working for TxDOT, how long do the benefits last and can we change the mix design to make PFCs last longer? TTI with the help of The University of Texas is working to answer these questions for TxDOT. The current four-year project, led by Epps Martin, is in its first year of research on PFC performance.
“PFCs are more expensive than conventional surface mixtures due to the high-quality aggregates and polymer-modified binders that are required,” says Cindy Estakhri, a TTI research engineer working on the project. “You also have the added cost of removing it at the end of its life in order to resurface the roadway. The PFC layer could potentially trap water if another surface were placed over it.”
The life expectancy of PFCs — and their benefits — are crucial questions. “PFCs tend to degrade over time,” says the TxDOT project director, Robert Lee. “The air voids fill with dirt and debris, and the surface gets compacted and worn by traffic. We need to determine how long the benefits of PFCs will last under different conditions so that long-range planners know what to expect.”
Researchers plan to study the performance of PFCs throughout Texas, under a wide range of weather and traffic conditions. At regular intervals, they will monitor the field performance of sections from previous and new construction. When they find a pavement with performance problems, they’ll identify the problem, take cores and try to determine why the pavement failed.
“We hope to be able to correct some issues in the design phase,” says Lee. “For example, we had some issues with pavements in Houston using tire rubber. The rubber was closing up the air voids in the PFCs. It’s possible that we can redesign the mix to solve that problem.”
The research project will ultimately produce a database of PFC performance, including functionality (noise reduction and permeability), durability and safety. This database will help researchers produce guidelines that TxDOT can use to design, construct and maintain its PFCs.
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