The International Falls-Koochiching County Airport in Minnesota is located in one of the most severe climates in the U.S. The average annual air temperature is zero deg C, with temperatures reaching 38 deg C (100 deg F) in the summer and minus 40 deg C (minus 40 deg F) in the winter. Low temperature cracking has been a continual problem with numerous 25-mm to 100-mm transverse cracks along the airport runway. Excessive stresses from the harsh environment also have caused significant secondary longitudinal and transverse cracking in the pavement.
To address the demands of this environment, transverse expansion/contraction joints had been installed at approximately 15-m intervals. When it came time to resurface the runway, the Koochiching County Commission and the consulting engineering firm of Short Elliott Hendrickson Inc. (SEH) decided to use both FAA engineering recommendations and the technology developed by the Strategic Highway Research Program (SHRP) to select an asphalt binder that would withstand this difficult climate. SEH proposed the newly developed SHRP Superpave specifications as an alternate bid for the project. The Federal Aviation Administration (FAA) reviewed and approved the alternate proposal, which provided funding to allow the innovative technology.
SHRP's Superpave performance-graded (PG) binders are defined by the high and low pavement temperatures as well as testing methods designed to directly relate laboratory analysis with field performance.
For a pavement surface in this climate and for the stresses on this runway, Superpave software calls for a PG 52-40 asphalt binder. The PG 52-40 should be impervious to binder-caused rutting at pavement temperatures up to 52 deg C (126 deg F) and to thermal cracking at pavement temperatures down to minus 40 deg C.
To meet the Superpave requirements for this climate, the asphalt needed to be modified. A styrene-butadiene (SB) block copolymer chemically reacted within the asphalt was selected for this project. The binder, manufactured in Marshall, Minn., by Koch Materials Co. using patented Stylink technology, also met FAA's Engineering Brief 51. Because the modified asphalt is preblended and reacted, it is homogeneous and requires no agitation or special handling.
A separation specification was added to the Superpave binder specification to assure the SB polymer-modified asphalt was a homogeneous preblended material that did not require special processing equipment at the hot-mix plant site.
The binder also met the requirements for FAA's P-401-2.3, Modifications needed to P-401 when specifying Styrene-Butadiene Modified Asphalt.
The SB-modified asphalt does not alter any safety characteristics of conventional asphalt pavements. The typical procedures of grooving the surface or using a porous friction course to address surface drainage in airport runways can actually be enhanced.
Improved resistance to permanent deformation, fatigue cracking, loss of adhesion, thermal cracking and aging helps ensure the surface grooving will provide optimal performance.
Thor Einarson, airport manager at the International Falls airport says, "We have been impressed with the adhesion of the porous friction course and the lack of thermal cracking after a very tough winter. The surface is performing better than expected at this point."
The modified asphalt also can increase the performance of porous friction courses by reducing asphalt drain down, which can create excessive and insufficient asphalt contents in localized areas.
Mix and placement
The 6,500-lin-ft project was performed in August 1996 by Ulland Brothers Inc., Cloquet, Minn. The International Falls runway is a full-depth asphalt pavement, which required milling of a portion of the pavement and overlaying. The same binder was used for leveling, base and porous friction courses of the overlay.
The aggregate blend and binder course mix were designed to meet the FAA P401M specifications, using Marshall mix design procedures ASTM D1559, D2041 and D2726 (75 blow). The aggregate blend had a sand equivalency of 72.4, an L.A. Abrasion loss of 21.5%, 100% fractured faces, a soundness loss of 3.8%, and 0.1% Clay Lumps. The binder content was 5.5% Testing was conducted by Braun Intertec Laboratories of Eden Prairie, Minn., and Koch's lab in St. Paul, Minn.
The recommended mixing temperature was 255 deg to 262 deg F, and the recommended compaction temperature was 222 deg to 227 deg F. The wearing course was a porous friction course meeting P402M, with a recommended 6.3% asphalt content. Open-graded courses aid the draining of water from the runways, and are often used for airport surfaces. There was some concern about the use of an open mix in a climate where there is so much ice and snow. After the first winter and a low temperature of minus 42 deg F on Christmas morning, the surface is doing well, and reportedly the county is pleased with the ease of snow removal.
Ulland used two pavers on the airport runways to place the 3 1/2-in. pavement (1-2-in. leveling course, 1 1/4-in. base, 3/4-in. friction): a Cedarapids CR551 and a Cat AP1000, both with pick-up machines for paving. Belly dumps were used for the 45-mile haul. Five rollers were used, including a Dynapac CP30 pneumatic rubber-tired roller, a Roygo Rhino, two Dynapac CC42A vibratory rollers and a Cat CB-634C.
Dan Klun, the QC manager for Ulland Brothers says, "The P401 spec is always a challenge, and considering the modified AC, the haul distance, the mixing temperature, and the compaction temperature, I thought the density spec was going to be a nightmare. But the combination of properly constructed stockpiles, QA/QC, and quality workmanship resulted in full payment. The importance of uniformity in the material is paramount to being successful in airport construction."
In cooperation with the International Falls-Koochiching County Airport Commission, the modifier producer will conduct performance evaluations and pavement analysis for several years. These evaluations will focus on the pavement stress contributed by the severe International Falls climate, and the performance of the modifier in the environment.
The information also will be valuable in evaluating the effectiveness of Superpave binder specifications as they are being implemented throughout the country.