This severely limited the time with which the contractor had to work on the runway and it posed design challenges for the engineering design firm on the projectÑHNTB Corp. "The challenge at O'Hare is to design something which can be built within the time available," states Pat Kane, aviation services department head with HNTB.
To make matters even more challenging, the contractor did not have many hours at night to work, because paving couldn't begin until after 10 p.m., and all work had to be finished by 6 a.m. otherwise liquidated damages were leveled. "If we were not done by six there was a $15,000 fine. If we weren't done by 6:15 it was another $10,000 fine," states Bob Housholder, paving superintendent, Allied Asphalt. This meant that all equipment and crews had to be off the runway at that time.
Kane adds, "By 6 a.m. they had to finish their temporary striping and get their equipment out of the way so that planes can begin using it. Between 10 p.m. and 6 a.m. they had to do all their mobilization, preparation, do the resurfacing work, construct tapers, do temporary striping and have everything up and running by 6 a.m.
"Of the eight hours they had to work only half of the time may have been used for paving. Sometimes they could not start by 10 o'clock because of planes coming in."
Because the largest aspects of the job involved electrical work on the runway, the prime contractor was an electrician. His task was to replace in-pavment lighting first installed in 1977.
There were some technical issues to be resolved in the placement of the new lights. Kane explains, "The old system was taken out with a 30-in. diam core drill. They drilled around the light, took it out, put in a new one, put a plate over it, then paved over the light. After the paving was done they cored down to the plate and adjusted it up to the final grade."
A drainage system for the lights also had to be designed because of their tendency to fill with water. Kane explains, "These lights tend to get condensation in them. Essentially they're a bucket in the pavement and water leaks or condenses into them. Then the water freezes, then it will condense some more, then freeze some more. We have had some problems with the ice pushing the lights out. Our solution was to install a series of drains in the lights. We put the conduit, light bases and drain pipe deeper than before, hopefully below the normal frost line."
Even though the majority of the project involved electrical work, there was also an overlay of the runway, which used Superpave criteria.
Hart describes what the job entailed, "It was an overlay project of approximately 9,000 ft of a 10,000-ft runway, with a polymer-modified asphalt. The depth was approximately 4 in. at the center and 3 in. at the edge. The shoulders were also overlaid. There were some areas of milling particularly in an intersection with runway 14L-32R and there were some areas where we had to match existing grades."
Housholder sums it up, "What we did was remove up to 4 in. and replace it with a polymer asphalt."
The polymer asphalt is very tough allowing the surface to be more tolerant to temperature changes. The added strength also should provide longer periods between resurfacing jobsÑan important bonus for an airport that can not afford to shut down its runways. While there were benefits to using the polymer mix there also were additional challenges.
Kane explains, "Most asphalt used at O'Hare will have a stability of at least 2,000 psi. The stability we got at O'Hare on this project was in the 4,000 psi range, which is what we wanted. When we sawed it to groove the runway, the saw-cut edges were very sharp. There is always a problem at airports with the grooves picking up rubber from the airplane tires, and these grooves have to be cleaned. Usually when the runway is open these grooves will be very sharp but within days or weeks, because conventional asphalt has a lower stability, they will soften up and they won't have the problem. But the polymer asphalt at O'Hare was so hard, city personnel had to clean more rubber out of the grooves, and more often than they were used to.
"The polymer asphalt also is intended to fight stripping. It is very highly bonding. Because of this, it took longer for it to weather off the exposed aggregate to achieve the expected surface friction numbers. Now, I understand, the measured friction numbers are improving, the grooving is fine, and the rubber accumulation rate has leveled off. We built something that is tougher so it apparently takes a little bit longer for it to break in. We're hoping that instead of a five-to seven-year life span for the asphalt surface and grooving we can double it."
The mix design had to stand up to rigorous specifications. Hugh Chapman, sales manager, specialty products, for Seneca Petroleum Co., Crestwood, Ill., which supplied the binder, explains, "The spec was a stringent aspahlt mix P-401 specification modified. It was designed to carry aircraft traffic weighing in excess of 60,000 lb, with tire pressures greater than 100 psi. So they went with a modified mix using a polymer. The polymer specification was a task force 31 spec for a mac 20 HD. That gave it a 30% increase in stability. It graded out to a Superpave grading of a 76-28 but it was just barely a 28, so we would technically say it was a strong 76-22."
The aggregate, supplied by Vulcan Materials, McCook, Ill., needed to be larger than usual. The aggregate structure consisted of 3/4-in. top-size material and a very high level of manufactured sand.
The design allowed Allied Asphalt to compact the mat right behind the paver. This meant that planes could use the runway immediately after paving. Over 48,600 tons of hot-mix asphalt was used on the project with placing and compacting done at 310 deg F. In order to place maximum tonnage the contractor used two pavers each placing 25-ft wide mats. Six vibratory rollers compacted the 50-ft wide mats directly behind the pavers.
The project was finished last August and according to Hart, "the pavement is holding up quite well."