By John Davis
For years, domestic and international airport maintenance engineers used coal tar sealers for airport aprons and alleys because it effectively resisted jet fuel spills and oil-based spills.
Now, there is fuel-resistant asphalt that is a viable alternative to coal tar sealers.
Before the use of fuel-resistant asphalt (FRA), airport maintenance engineers had to use coal tar sealers because traditional asphalt sealers could not effectively resist jet fuel and other oil-based fluids and leaks from aircraft.
Because aircraft fuels are refined crude oils, they are chemically compatible with asphalt pavement. Consequently, fuel spills and oil-based leaks caused softening of the asphalt binder that in turn caused deterioration of the asphalt pavement surface.
Over the years, coal tar sealers were frequently applied to hot mix asphalt (HMA) airport pavements because they were less compatible with oil-based fuels and consequently more fuel-resistant. Although these coal tar sealers were highly effective in protecting HMA pavements, their use has been limited because the thermal expansion of coal tar sealants is different than asphalt pavement and the coal tar sealants crack in two or three years making pavements require frequent re-sealing.
History of FRA
FRA was first developed in the Netherlands and has been used at several international airports, including Kuala in 1996, Cairo IAP (International Airport) in 1997 and Aden IAP in 1999. FRA was first used in the U.S. at La Guardia Airport in 2002 and then as a fuel-resistant apron application at Boston Logan IAP in 2004 and in 2005.
FRA was used successfully in 2006 at Douglas International Airport in Charlotte, North Carolina and to rehabilitate the Florida DOT Agricultural Inspection Station on Florida I-10. At all these facilities the FRA mix has shown excellent in-service performance in terms of resistance to fuel spills and petroleum deposits.
Through the applications of fuel-resistant asphalt on airport and other facilities, a reliable FRA mix design has emerged. It includes highly modified polymer asphalt, a ½ inch P-401 mix, 50-blow Marshall design and low air voids.
FRA at Bob Sykes Airport
The purpose of the recent Bob Sykes Airport project in Crestview, Florida, was to rehabilitate the existing, oxidized surface course of the entire north apron by milling the surface and replacing it with fuel resistant asphalt.
Bob Sykes Airport is a general aviation facility approximately 25 miles north of Elgin Air Force Base. It is owned and operated by Okaloosa County. The airfield consists of one runway supported with taxiways and parking aprons. The traffic at Bob Sykes is a mix of small, medium and large military and commercial aircraft—from single engine props to 747s. The airport has a major repair facility that brings in all sizes of aircraft for repair.
The rehabilitation of the north apron, completed in the fall of 2011, consisted of milling one-to-two inches of the existing asphalt surface. After milling, the contractor placed a P-401FR fuel resistant hot mix asphalt surface course on the apron. The mix for the surface course included a 12.5 mm (1/2-inch) nominal maximum size aggregate with a PG 82-22 asphalt binder. GCAC/HALCORP of Houston supplied the asphalt binder.
The fuel-resistant asphalt surface was between 1.5 and 2 inches thick. The acceptance protocol for the surface course was 96 percent of maximum theoretical density. A one-year warranty was required but it really wasn?t necessary because the pavement passed the stringent acceptance specifications.
The project worked well due to four key factors:
- The mix design called for a fine aggregate so the mix would be tight.
- The highly modified binder together with the 2.5 percent design air voids made a dense and waterproof mix.
- The use of superfine aggregate in the mix made good compaction possible.
- Stringent compaction was required to ensure acceptance.
“We designed a generic fuel resistant mix specification for the surface course,” says Bob Boyer, project consultant and mix design engineer. “But it was more than a mix design specification, it was a system.”
Boyer says that normally a good road pavement is designed with 4 percent air voids, but this surface was designed with 2.5 percent air voids. “The voids were smaller so we could get more impermeability. Then we specified 96 percent compaction, or 96 percent theoretical maximum density.”
“Besides designing for a 2.5 percent air voids and getting field compaction of 4 percent air voids, we used a highly modified binder, PG 82-22,” said Boyer. “All together, the design, the binder and the compaction requirements, provided a truly impermeable fuel-resistant asphalt pavement.”
Testing fuel-resistant asphalt
The PG 82-22 binder was tested for elasticity, creep recovery, cracking and deformation. The FR 50-blow Marshall specimens were soaked in kerosene for 24 hours and measured for weight loss, which was around 2.5 percent. The Marshall specimens were tested for stability and flow. All the tested specimens maintained their fuel-resistant properties.
“You get excellent stability with this mix,” says Ken Murphy of Asphalt Technologies, Inc., who prepared most of the binder and Marshall specimens, as well as conducting the quality control testing.
“The pavement with the FRA mix looks great,” says Murphy. “You’ll see no more spider-cracking like we did with coal tar.”
Both the design and construction of the Bob Sykes Airport apron with FRA was a success.
“We think highly of the mix design and of the entire construction procedure,” says Boyer, “and we hope to make it a standard specification.”
Boyer and others are working with the military?s Uniform Facility Guide Specification (UFGS) to designate a prototype specification for all military installations. He adds that the Federal Aviation Administration (FAA) is also considering a fuel-resistant specification for airports.
“It’s really a breakthrough,” continues Boyer. “I’ve been working on coming up with an asphalt specification to take the place of coal tar for 35 years. Now we have data and proof that fuel-resistant asphalt works.”
Boyer says that the significance of the fuel-resistant mix to the asphalt industry could be large. “We finally found an asphalt mix that satisfies the requirements of airport aprons. It won?t shrink and crack after a few years and it is safe for the environment and workers.”
GCAC’s David Cutting also thinks there is a market for FRA. “If we can’t use coal tar anymore at certain locations around the U.S., we have to have a replacement. I think this FR asphalt is a good one. It?s environmentally safe and, because it’s asphalt, it works well with the existing asphalt pavement.”
“You get no damage with this fuel-resistant asphalt,” says Ken Murphy. “I think that engineers in the military and the FAA will pick up on it.”
FRA = cost savings
Alternate bids were taken by engineering firm AVCON Inc. for the north apron rehabilitation project at Bob Sykes Airport in Okaloosa County, Florida. One bid included PG76-22 asphalt with a coal tar sealant. The alternate bid was to use PG82-22 Fuel-Resistant (FR) asphalt on the apron without a sealant.
When the two bids were compared, the PG82-22 initial cost came in 19 dollars higher per ton than the PG76-22 with a coal tar sealant.
The owner, Okaloosa County, and the engineering consultant, AVCON Inc, chose the PG82-22 FR asphalt because the apron would not experience cracking and the frequent re-applications that were necessary with a coal tar sealant.
Okaloosa County believed that, although the initial cost was higher, in the long run the apron would not have to be re-sealed several times and it would save the airport time and money.
“The coal tar sealer is just a thin layer of sealant that begins cracking two years or so after application,” says John Collins, AVCON’s Project Manager for the North Apron Rehabilitation Project. “Even a small crack in a coal tar sealant will allow fuel spills to enter the pavement structure and degrade the asphalt. So we had to reseal every few years with coal tar.”
By choosing the PG82-22 FR asphalt, the owners put it down once and don’t reseal, thus saving money.
“As professional engineers, we have to look for the best solution for our client. We believed that the PG82-22 FR asphalt was the best and most economical solution for the long haul,” said Collins.
John Davis is a Contributing Editor for Asphalt magazine.