The Power of PFC

Permeable friction course improves safety by putting water build-up at bay

By Cathleen Draper

In the 1970s, Texas began to implement what it called “plant mix seal”— a thin, porous surface treatment crafted in a hot mix plant—on its roadways to increase friction and prevent hydroplaning.

Many of these plant mix seals performed well, but some lasted only a few years before raveling began. Once they began to ravel, the surface rapidly degraded, resulting in pavement failure long before it was expected. The costs of replacing the plant mix seal often outweighed the benefits to the state’s roadways and drivers, and Texas backed off its use. By the late 1980s, few states continued to use plant mix seals as a surface treatment.

The state of Georgia, however, carried on. Between the late 80s and the 1996 Olympics, held in Atlanta, the state made significant improvements to plant mix seal, adding cellulose fibers and polymers to prevent the mix from raveling. Many states began to take notice of the quality improvements.

Dale Rand, then a TxDOT employee in the materials and test division, now executive director of the Lime Association of Texas, got the chance to travel to Georgia and analyze this new-andimproved asphalt that Georgia called “open-graded friction course.” The mix went by several diff erent names, including “popcorn mix” because of its high air void structure. When Texas began to use it again in the late ’90s, it coined the name permeable friction course (PFC)—and it stuck.

PFC is exactly what the name suggests: a permeable form of asphalt, designed specifically to allow water into the pavement, where it then drains off underneath the topmost layer. It’s implemented for a myriad of reasons on Texas roadways, but most importantly, as a safety measure to reduce wetweather accidents.

“On conventional asphalt pavements, if water stands on the roadway surface, hydroplaning can occur, causing the vehicle to essentially float on the water,” Rand said. “This condition results in zero traction and significantly increases the odds of wet-weather accidents.”

Rand worked for TxDOT from 1987 until 2013. He’s seen PFC utilized on many projects, but the most successful in his opinion is its use on two- or four-lane undivided highways. Most wet-weather accidents occur on horizontal or vertical curves and intersections, he explained. Texas has numerous undivided farm-tomarket (FM) or ranch-to-market (RM) roads, where opposing traffic negates curves at relatively high speeds. On rainy days with low visibility and high risk ofhydroplaning, it’s a recipe for disaster.

“I always give the example, if you are driving 50 miles an hour in one direction, and somebody else is driving 50 miles an hour in the opposite direction, and they come into your lane causing a head-on collision, neither of you are likely going to survive,” Rand said. “That’s equivalent of a vehicle going 100 miles per hour hitting a solid a wall.”

With PFC use on these types of roads, wetweather accidents are less likely to occur.

What Is Permeable Friction Course?

Almost all asphalt pavements are designed to prevent water from permeating the pavement and reaching the base and subgrade below. “With typical hot mix, you want it to be compacted and have about 4 percent air voids, which is not enough to let water into the pavement. It sheds the water off,” Rand said.

Unlike other asphalt paving mixes, PFC typically has between 18 and 22 percent air voids, allowing water to seep through to the underlying layer, where it then drains off to the side.

Usually, PFC is comprised of a single size aggregate, “maybe a half inch to threeeighths of an inch,” Rand said—roughly the same size as a marble.

“The open structure of PFC is what allows the water to drain through it,” he continued. “For traditional dense-graded asphalt mixes, you normally utilize various aggregate sizes. You take a coarser rock and then mix it with intermediate and finer rock, including some sand, and you compact it tightly together.

Though PFC today is an evolution of the plant mix seal of the ’70s and ’80s, it’s still used as a surface layer on roadways. It’s generally implemented on interstate highways and high-volume FM and RM roads.

“PFC works best on high-speed roadways where traffic is constantly moving and more channelized, but it is not ideal when there is a lot of stopping and turning action and traffic motions that you typically have in an urban area,” Rand said. Hard stops and turning vehicles can cause the treatment to ravel more quickly.

Where PFC is Placed

PFC performance is classified into three categories: functionality, durability, and safety. Functionally, PFC effectively reduces roadway noise thanks to its lower texture and surface roughness. According to a 2013 Texas A&M study, PFCs have the lowest tire/pavement noise level compared to all types of hot mix asphalt (HMA) surfaces. The average level for typical pavements is 101 dBA, but for PFC, it’s 98.1 dBA—a “significant” reduction of almost 3 dBA.

“In some of the more populated areas, instead of putting up sound walls, which are very, very expensive, TxDOT would use PFC to quiet the roadway,” Rand said. He’s also witnessed a number of projects, including in Houston and San Antonio, where concrete roads are covered with a layer of PFC to make it quieter.

Because it’s so thin—one to two inches— PFC is easy to compact, and crews can uniformly and quickly pave long sections of roadway.

PFC is durable, too. Due to the addition of polymers, fi bers, and additives, such as lime, the asphalt has increased resistance to rutting, and modern PFC can last well past 10 years without having to repave.

However, PFC isn’t the easiest to repair. It’s difficult to patch, and though there are specialty patching mixes out there, they’re not common. It’s also not conducive to hand work.

“You can’t really rake it in place very easily,” Rand said. “It’s for straight, continuous paving, not for stop-and-go paving. Cul de sacs and things like that are not ideal roads for PFC.

When water flushes into PFC, sediment can build up, decreasing its permeability over time. “That void structure, you need some rain and high-speed traffic to keep it flushed out,” Rand noted.

It also maintains a higher upfront cost. The price of high-grade polymer asphalt, cellulose fibers, and lime add up. A higher asphalt content—typically more than 6 percent, compared to 5 percent in conventional mixes—contributes to that higher initial cost.

PFC also requires hard, durable, highfriction aggregates, such as SAC A. But in Texas, not all aggregate sources provide that. In East Texas, aggregate is hard to come by and must be shipped from Central and North Texas or Oklahoma.

“You’d love to use it in some of those areas in East Texas that get more rainfall. But they’re having to ship that aggregate a long way,” Rand said. “Any time you have to ship materials a long way, the cost goes up significantly.”

He justifies the cost in terms of the reduced number of wet-weather accidents and fatalities.

Safety First

Rand has been a longtime advocate for the use of PFC as a safety measure.

Its coarse surface texture makes for better ride quality and vehicle handling, and its ability to drain water from the roads reduces splash and spray, prevents hydroplaning, and improves pavement marking visibility.

“If you’re driving down the road and you’re not on PFC, and then you get on PFC, it looks like it’s not raining,” Rand said. PFCs also have superior texture and skid resistance compared to other pavement types. Research shows that high skid resistance mixtures reduce wet-weather accidents by 54 percent and overall accidents by 29 percent.

One roadway left an impression on Rand: FM 1431, a winding, Hill Country route northwest of Austin that’s prone to wetweather accidents. When Rand drove through it, he noted all the crosses that lined the side of the road, and he began to push for a solution.

That solution was PFC. “It was a major safety improvement in the industry that needs to be used in the right places,” Rand said. FM 1431 was one such place.

In 2003, the Cedar Park Police Department in Travis County, where FM 1431 begins, reported 23 wet-weather accidents on the road. But after overlaying it with PFC, the number of accidents dropped to just two in 2004—despite twice as many rain days that year.

“It’s another tool in the toolbox,” Rand said. “It’s an important one. It doesn’t need to be used everywhere, but it certainly has its place.” Since then, it’s been used on roads across Texas, resulting in significant reductions in fatalities.

“Th e reality of it is, when it’s your loved ones driving on that roadway…” Rand trailed off . “If my daughter is driving at night in the rain, and I know she’s driving on an undivided road, I hope she’s driving on a PFC mix.”