Fog seal to protect new pavements, rejuvenate old ones
In his presentation on fog seals at the February 15, 2007, Minnesota Pavement Conference, chemist and pavement consultant Gayle King said "I've been working with civil engineers all my life, and never do I remember 95 percent of them agreeing on anything. So when I tell you that 20 of 21 civil engineers surveyed thought fog seals were cost-effective, that's amazing!" Nonetheless, King also pointed out that fog seals are banned in many states due to deadly accidents apparently caused by fog seals that reduced tire-to-pavement friction.
Figure 1: A core taken from a four-year-old pavement
King showed that the root cause for much of the raveling and cracking of pavements is asphalt oxidation. He used results of shear rheometry on an Arizona highway to explain the problem. A core, illustrated in Figure 1 (see page 4), was taken from a four-year-old pavement.
The bottom section of the core—about 1/2-inch thick—was found to have about the same performance grade (PG) as the original asphalt; in other words, virtually no aging had occurred. But the top 1/2 inch of the core had aged 3.5 PG grades. "We found that the asphalt binder at the top wasn't just getting harder," King said. "It was also getting more brittle—so brittle that it couldn't flow at lower pavement temperatures to relieve stresses." In other words, its m-value—its ability to flow in response to stress—had been reduced.
"That changes the way we need to think about protecting asphalt," King said. He recommended that we target oxidation as the enemy. Preventing oxidation in a new pavement will prolong its life. Reversing oxidation in an old pavement—i.e., increasing its m-value—will rejuvenate it.
King said fog seals can perform these important tasks, but he cautioned that we need to develop a more sophisticated approach to using fog seals. "A fog seal is not a fog seal is not a fog seal!" King has been researching a variety of fog sealing applications and products in a five-year study that was scheduled for conclusion in August of 2007.
Fog seal to prevent oxidation right after chip seal
King strongly recommended fog seals on several types of new pavement surface—especially chip seals. He showed a chip- sealed pavement that had been fog-sealed only on one lane. The lane with the fog seal had no measurable chip loss, but the lane without fog seal showed a great deal of raveling and plow damage. "If you're not losing chips to raveling or plows and your customers aren't complaining about wind shield damage," King said, "you don't necessarily need to fog seal. But you can take a little bit of the emulsion out of the chip seal application and put it down on top as a fog seal instead, so it doesn't cost much more, and it may save you a lot in terms of the length of time your chip seal lasts before you have to come back and fix it." The blacker color of the fogged surface also looks more like hot-mix and can offer safety advantages as well, particularly when applied to shoulders.
King enthusiastically recommended one particular product for fog seal over chip seals: "the Minnesota special: CRS-2P," as he nicknamed it. CRS-2P is a polymer-modified emulsion developed by Mn/DOT's Tom Wood. King explained that diluting CRS with water doesn't work: "That will break it, and it won't come out of the distributor, and you've got a mess." But if CRS is diluted with the soap from which it was made, King said "it dilutes very well, it applies very well, and it only costs 13 cents per square yard so you can afford to fog seal all your chip seals." He compared that with some of the "high- priced spreads"—specialty emulsions that cost as much as $1.60 per square yard. He concluded that "fog seals can be very cost-effective if you know what you're doing."
King especially recommended fog seal over chip seal on shoulders, pointing out that, because shoulders receive little traffic, they don't become as highly densified as travel lanes. This means shoulders absorb more oxygen, which eventually leads to raveling. He recommended applying the fog seal heavily on a shoulder so that it is very black. That helps drivers at night to easily see the edge of the travel lane. But he expressed caution at the idea of fog sealing the travel lanes themselves because of potential friction loss. King recommended experimentation on a small section before proceeding with a large project.
Fog seal over open-grade friction courses (OGFC)
King also recommended fog seals over Superpave coarse mixes and open-grade friction courses (OGFCs). In both of these pavement types, because there is more exposed surface and greater permeability, the pavement is more susceptible to oxidation and ages faster than other types. "Aggregate raveling caused by oxidation probably starts in three years if it's a straight OGFC and in six years if it's polymer-modified," he said. "But a regularly scheduled fog seal program can eliminate the problem."
However, King said the fog seal on these more permeable surfaces must be different from the type used on a chip seal: "On an OGFC or a coarse-mix Superpave asphalt, you're not doing it to avoid oxidation; you're doing it either to rejuvenate the asphalt or re-stick the aggregate with a less brittle binder to avoid raveling. Because we're doing the fog seal for a different reason, the emulsion needs to be different." The same factor that makes Superpave coarse mixes and OGFCs susceptible to oxidation —their permeability—also makes them good candidates for fog sealing because the fog seal emulsion can more easily and more deeply penetrate into the pavement. This means the friction problems associated with chip seals are avoidable and that it is possible to restore m-values at greater depths in the pavement. For these pavement types, he recommended polymer-modified emulsions because the high polymer content helps to lock down the aggregate.
Don't fog seal over SMA asphalts containing rubber
King recommended extreme caution when fog sealing stone-mix asphalt (SMA; also called gap-graded asphalt) and asphalts containing crumb-rubber. Because an SMA has very low permeability, a fog seal emulsion cannot penetrate into the pavement, thus producing a dangerously slippery surface. And he noted that some of the asphaltic materials in a fog seal can cause crumb-rubber to swell.
Developing fog seal testing methods
King's research includes assessments of several test devices and methods, including:
- A pavement permeameter developed by the National Center for Asphalt Technology
- A "ring test" for emulsion infiltration
- A portable seismic pavement analyzer—essentially a mini-FWD
- A bending beam rheometer test for thin mixture specimens, as developed by Professor Mihai Marasteanu of the University of Minnesota
- A dynamic shear rheometer torsion bar test for mixtures
- Several friction testing methods, from portable lab procedures to full-scale skid trailers
He said friction values change as a fog seal ages. "When applying a fog seal to a dense HMA surface, friction may be 30 percent lower on the first day; by the second day you may have a 20 percent loss; a month later it may be only 10 percent; and nine months later, you're typically back to the original value." Sanding immediately after the emulsion application will significantly improve early friction.
Detailed evaluations of testing devices and all other aspects of King's research are on a Web site hosted by the National Center for Pavement Preservation at www.pavementpreservation.org/fogseals.
Can research overcome well-founded fears?
It remains to be seen whether King's research will be enough to convince justifiably skittish officials to repeal their fog seal bans. But King's research shows that, with a greater understanding of how fog seals work and more care in matching products to conditions, fog seals offer great promise in the fight against pavement oxidation.
—Richard Kronick, LTAP freelance writer