Hot mix asphalt (HMA) airfield pavements offer a special challenge to both designers and material suppliers.
HMA pavements at airports have for years been subjected to jet fuel spills and hydraulic oil leaks. These problems, combined with heavy loads and a wet environment often result in pavement damage.
Boston’s Logan International Airport was experiencing these problems when they began looking at specially designed HMA to meet their needs.
Fuel leaks and spills on aprons and runways were not only softening the pavement at Logan but also may have been contributing to rutting and cracking failures.
Airport officials took remedial action. In June 2004, approximately 1,300 tons of HMA were placed on portions of Taxiway N and Runway 4L-22R. Eight to ten inches of the old pavement was milled and the final 1.5-inch-thick P-401 surface mix contained a fuel-resistant polymer modified asphalt (PMA). The PMA met a PG 94-22 specification.
The mix gradation was the standard P-401 specification, but the target air void content for design was lowered to 2.5 percent. The design was more impervious to fuel spill penetration. The plant and paving crews were able to produce, place and compact the mix to meet all contract specifications without a problem.
Stripping Resistant HMA
About the same time, Logan was experiencing some premature failures of HMA due to moisture damage. These pavements were supporting loads up to 875,000 pounds for a Boeing 747 at maximum takeoff weight. Tire pressures up to 200 psi, along with aircraft turning sharply, taxiing slowly, braking or standing in queues during summer weather, provided ideal conditions for pavement distress.
Airfield pavements are designed according to Federal Aviation Administration (FAA) standards for a 20-year service life, but in reality Logan’s most heavily used pavements were lasting only 10 to 12 years.
Research at Worcester Polytechnic Institute concentrated on ways to use aggregate from the local supplier to find an “everyday” HMA mix.
Various polymer-modified binders with PG ratings of 70-28, 76-28, 82-28, plus the fuel-resistant binder by Citgo, were tested.
The original everyday mix was first used in 2001 for a repair on one of Logan’s busiest taxiways, and it is still performing well. The eventual everyday mix consisted of local aggregates, RAP (18 percent), a modified asphalt binder, and one percent hydrated lime. It has been used regularly since 2004.
HMA met the challenge. At Logan, HMA was fuel-resistant for specific applications and functioned as a workhorse mix for routine use.