Runway Paving to Keep Cargo Flying
High
Cement-treated subgrade and base strengthen
300,000-sq-yd concrete runway project at new FedEx airport terminal
at Fort Worth, Texas.
 by
Wayne Adaska - director of public works for the Portland Cement
Association
 When
the new FedEx hub at Alliance Airport in Fort Worth, Texas, opens
in late 1997, the clearinghouse for packages shipped between the
West Coast and the southwestern U.S. will include more than 300,000
sq yd of reinforced concrete pavement underlain by a cement-treated
base and cement-treated subgrade.
Unlike
most airport terminals, there are no passengers to load and unload;
instead there are packages. In the hours just after midnight, FedEx
planes each day fly into these hubs carrying cargo. Workers then
unload the packages, sort them by their destination and reload the
planes with the sorted cargo in two to four hours.
The
Alliance facility will be the smallest of FedEx's hubs even though
it has the long-term potential to expand into the company's second-largest
distribution center.
The
initial plan for the FedEx distribution center was for 50 acres
(242,000 sq yd) of paving at the Alliance Airport in North Fort
Worth, Texas. Additional scope was added later in the project with
the potential for another 50 acres of paving. The actual total paving
is about 330,000 sq yd or about 70 acres.
Five pavement areas will be part of the project:
- Aircraft taxi lanes will support fully loaded aircraft arriving
and departing. There will be 50 acres of taxi lane, aircraft gates
and concrete paving between all the areas. This area is designed
to have 9 in. of cement-treated subgrade, 9 in. of cement-treated
base, and 14 in. of portland cement concrete surface.
- The ramp area where aircraft are parked and serviced will support
loading and unloading of the aircraft. The pavement is designed
so any aircraft that FedEx has or anticipates having can park
in any position on the ramp. It is designed for the heaviest aircraft.
There are 242,000 sq yd of aircraft paving. Huitt-Zollars Inc.
designed a pavement that includes 9 in. of cement-treated subgrade,
9 in. of cement-treated base, and 14 in. of concrete.
- The truck terminal area is where loaded tractor trailers come
in to the facility to the sort building. This pavement must support
a high level of repetition with heavy loads of highly channelized
traffic. Truck paving is 10 in. of jointed-reinforced concrete
pavement over 6 in. of cement-treated clay subgrade.
- The container storage area has a high volume of turnover in
a 24-hour period. Essentially this is where empty freight containers
from the aircraft are stored. Pavement design in this area is
still under consideration, but most of those areas will be similar
to the truck-paving section.
- Employee parking has not been designed. But engineers say this
will most likely be a concrete pavement.
In
designing the project, FedEx considered a long-range outlook in
working with engineers. FedEx had two concerns. It was looking for
a dependable sorting system and a dependable aircraft pavement with
a long design life, according to Huitt-Zollars the project engineers.
Designers
used a 90-day, 750-psi, third-point loading design for the concrete
taxi way and ramp pavements. The third-point loading design was
used rather than a center-loading design because it is thought to
be a more conservative design.
Rone
Engineers, the geotechnical and materials testing firm on the project,
chose a cement-treated base and cement-stabilized clay subgrade.
The cement-treated subgrade was less expensive and stronger than
the lime alternative, says Charles Jackson, P.E., vice president
of Rone Engineers, Inc.
Jackson
says the soil's bearing capacity will be improved, allowing overall
cost saving on the project. The cement-stabilized clay subgrade
provided a good all-weather working platform for construction and
weather durability under construction traffic.
Other
considerations that went into designing the pavement were keeping
aircraft wing tips level, providing a separate drainage system for
taxi lanes and gates, and making sure there was no swelling of the
soils or pavement heaving.
Level
wing tips aid in the loading and unloading of aircraft. The slope
from nose to tail of the aircraft can be no more than .5%.
In addition to the contractor providing level paving, the design
incorporates tethers into the aircraft pavement to load and unload
the wide body aircraft. Crews tether the nose gear when it is nearly
empty to keep the nose from lifting up when loads are in the rear
of the aircraft. Eye hooks are embedded in the concrete for this
purpose. Straps are attached to the aircraft and concrete mass and
dowels in the pavement resist uplift forces of up to 50,000 Ib.
Drainage
in the taxi lane and gate areas accommodate refueling the aircraft
and deicing at every gate. These chemicals played an important role
in selecting the joint sealant. Designers chose a neo-prene joint
sealant for its longevity and for jet fuel resistance.
The
difference between this concrete pavement and other airport pavement
is the vast area of concrete and FedEx's requirement that it be
essentially flat in gates and the method of drainage on the site.
The pavement has a 50-year design life because FedEx wanted to look
at value costs of the project over time.
The
gates are on a separate drainage system. Those areas drain to systems
with oil/water separators and underground containment areas. FedEx
will treat the first flush of drainage to remove the oils and fuels.
Paving contractor, Duininck Brothers, Grapevine, Texas, constructed
the subgrade, base and concrete paving. Concrete was batched on
site. Most soils in this area of Texas are clay with a high plasticity
index (PI) of about 40 to 50. "The soils here are about the
worst you can build an airport on," says Rone Engineers' Jackson.
"However, cement reduces swelling in the clays and the cement
gives the subgrade its needed strength."
 For
the subgrade, Duininck pulverized the soil prior to spreading cement.
Crews then spread dry port-land cement and blended it with the soil
using a single-shaft CMI pulver mixer. They added water to bring
the blended soil and cement mixture up to optimum moisture content.
Crews followed with two passes with the pulver mixer, mixing and
compacting the layer. Specifications called for the processed material
to be pulverized to 100% passing the \Vi sieve and 60% passing the
No. 4 sieve. Compaction was specified to be a minimum of 95% of
Standard Proctor.
According
to Ronnie Rone of Rone Engineers, the firm established a maximum
plasticity index of 12 (in-situ Pi's were as high as 38), as well
as a strength of 250 psi for the treated subgrade. A cement content
of 7% by dry weight of soil produced the desired result. Tests with
other commonly used stabilizers failed to produce even 100 psi in
the laboratory using as much as 9% addition of the material.
When
a seam of highly plastic mont-morillonite clay was encountered during
construction, project engineers adjusted the cement factor in the
field to 9%, which was retained for the duration of the subgrade
treatment operations. At this level, strengths of some of the less
plastic stabilized soils exceeded 500 psi after the cement processing.
Select
granular material was mixed with cement and water in a pug-mill
to produce the cement-treated base material. After mixing, the soil-cement
was placed in dump trucks and spread on grade using a jersey spreader.
Soil-cement
was compacted using vibratory steel-drum rollers. Strength of the
cement-treated base was in the 1000 to 1200 psi range, compared
to a 750 psi, 28-day strength requirement. Concrete paving followed
completion of the cement-treated base.
"There
were no problems except for the weather," said Kyle Duininck,
project manager. "We placed about 2000 cu yd of concrete a
day with a Goma-co 3000 slipform paver. The work area was wide open
and that made the job easier. We had a tight grade tolerance, /2%
cross slope for drainage, but it was no problem."
Holnam,
Inc., Midlothian, Texas, supplied the cement. Construction began
in May 1995 and support structures are scheduled for completion
next year.
Airport
paving project movers and shakers
Key Players in the Alliance Airport paving project
include
o Architect: Taylor/Gardner Architects, Memphis
o General contractor: Austin Commercial, Dallas
o Engineers: Huitt-Zollars Inc., Fort Worth, Texas
o Paving contractor: Duininck Brothers Inc., Grapevine, Texas
o Geotechnical engineer: Rone Engineers Inc., Fort Worth, Texas,.and
o Owner: FedEx Corp., Memphis
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