By: David Banasiak
Last summer, when
most people were asleep, workers for Allied Asphalt Paving,
Elgin, Ill., were on the job paving runway 9R-27L at O'Hare
International Airport in Chicago. When you're doing a pavement
rehabilitation job on a runway at the world's busiest airport
you're bound to run into some unusual problems. Bob Hart, senior
project engineer, HNTB Corp., Chicago, explains one, "All the
work had to be done at night, because for operational reasons,
the city of Chicago Department of Aviation can't shut a runway
down at O'Hare during the day."
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."
Runway
lighting
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."
Resurfacing work
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."
Mix design
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."
