Fast-track panels keep planes flying at Sea-Tac

By BRIAN J. KITTLESON
Gary Merlino Construction

Like many airports with aging infrastructure, Sea-Tac International Airport, operated by the Port of Seattle, faced a serious dilemma. On one hand, the port could not close a runway for repairs because that would disrupt air travel throughout the Northwest, setting off a domino effect of travel interruptions that could have national and even international repercussions. On the other hand, some of the aging concrete panels on Runway 16 Right were deteriorating to the point where safety would soon become an issue. Concourses C and B, along with the interconnecting taxiway, had the same problem. The challenge was to keep the traffic flowing while reconstructing the damaged areas.

Runway 16 Right is 8,500 feet long with about 4,000 18-foot-wide by 20-foot-long concrete panels. The port's challenge has been to replace hundreds of these aging panels without interrupting air traffic. The port has replaced more than 620 panels since 1994 while keeping the airport open to traffic.

Fast-track construction a success for Sea-Tac

The port adopted an innovative fast-track approach to panel replacement, which was performed by Gary Merlino Construction of Seattle. The solution to the engineering challenge was fast-track construction technology stretched to the maximum. With two runways and minimal traffic between 11:00 p.m. and 6:30 a.m., the port allowed Merlino to shut down Runway 16 Right and replace panels during this overnight period. However, the contractor had to have the runway reopened by 6:30 a.m. the following morning. The obvious requirement was to get the concrete up to strength rapidly. But before workers could place the rapid-set concrete, they had to remove the existing panel, excavate down to the new subgrade, place and compact the new sub-base, and place the rapid-set concrete in time for the concrete to reach its required strength by the reopening deadline. The new panels were 18 inches thick rather than the 12- to 16-inch thickness of the old pavement Merlino replaced.

To gain time, Merlino sawcut the old panels the night before the panels were scheduled for removal. The old panels, even though sawcut, retained sufficient strength to support aircraft during the day. The next night, the old panels and sub-base were removed, the new sub-base prepared, and adjacent concrete drilled for new dowel bars. Before this shift ended, temporary panels were installed and the runway reopened before the first flight of the day. The following night, temporary panels were removed, new dowel bars installed and rapid-set concrete placed. With multiple small crews working on different panels, four panels a night were averaged.

Merlino spent numerous hours researching and brainstorming the most efficient methods to complete the project. The project was broken down into the following six key elements:

• Batching the rapid-set concrete.
• Developing a workable high-strength mix design.
• Removing the existing panels.
• Placing temporary panels.
• Placing the rapid-set concrete.
• Curing the rapid-set concrete.
• Batching the rapid-set concrete

Because the rapid-set concrete cures so quickly, Merlino opted for an on-site batch plant. The alternatives, either off-site batching or volumetric trucks, were ruled out because of the risk that the rapid-set would set in the truck. Using the volumetric trucks meant taking up to 40 minutes to place the concrete and with the rapid-set, cold joints could form in that time.

"It was critical to have an on-site batch plant," said John Rothnie, the port's program manager. "The batch plant was the key to Merlino's success. They've made the travel time very short."

Developing the right mix

Figuring out the right concrete mix involved an extensive trial-and-error process. Merlino learned quickly that it was not dealing with normal Portland cement. This was a different animal. The goal was to obtain 650 psi flexural strength in four hours or less so that airplanes could land on it without sinking into wet concrete.

Experimenting with numerous types of aggregates, water reducers, superplasticizers, air entrainments, various amounts of citric acid and rapid-set cement finally led to the right mix. Merlino also learned that crushed aggregate worked better than washed rock. After about a month of testing, a mix design was formulated that could produce over 650 psi in four hours.

The next step was a field test. The first field test was a failure, but after adjusting the mix design, the second field trial was a success.

Removing the old panels

The project required Merlino to remove the existing panels, excavate the existing sub-grade material and compact the existing soil to 100 percent modified proctor density. The specifications prohibited the contractor from breaking the existing concrete panels, so it devised a method of sawcutting around the perimeter of the panel to be removed and then cutting the panel into smaller pieces. The cut pieces were then drilled and mining anchors inserted. This allowed the use of a large forklift to lift the existing concrete panel out. After the concrete panel was removed, the sub-grade material was excavated and recompacted, and dowel holes were drilled into the surrounding concrete.

Placing temporary panels

In order to reopen the runway or taxiway by 6:30 a.m., the sub-base was compacted and a temporary panel was installed in the opening. The contract plans contained details for these temporary panels. Each one consisted of four 18-inch-thick sections.

Merlino modified the port's design by casting four screw jacks into each temporary panel. These jacks were needed in order to make adjustments to achieve a level surface with the surrounding pavement. Sometimes workers had to be sent out in-between flights during the day to adjust the screw jacks to maintain a level landing surface.

The following night, the temporary panels were removed and dowels attached in the surrounding concrete panels with epoxy. At the same time, the batch plant started mixing the concrete. Each new panel required 21 cubic yards of concrete.

Placing the rapid-set concrete

Early on, it was observed that if the trucks were filled with more than 6 cubic yards of rapid-set concrete, the concrete would build up on the fins of the truck. To avoid this, Merlino started using four trucks to pour each panel. Workers waited until all four trucks were loaded with rapid-set and checked the slump. As an additional quality assurance, they checked the slump on every truck just prior to placing the rapid-set mix. Then, two trucks discharged simultaneously, one from each side of the panel. Fritz packs and water were used to help eliminate any buildup in the truck drums. High-cycle vibrators helped ensure proper consolidation. Crews worked quickly to finish the panels immediately after the vibrating screed.

Curing the rapid-set concrete

Because rapid-set concrete generates a lot of heat, the freshly placed panels needed to be cured with water. Workers were able to attach garden hoses and sprinklers to a water truck and continuously soak the concrete with water for two hours after placement.

Multiple small crews speed the job along

On any given night, Merlino had multiple small crews working on the panels. While the sawcutting crew cut the panels for the next night's work, the removal crew began taking out the panels cut the previous night. They also removed the old sub-base. Then the drilling crew drilled the dowel holes in the surrounding concrete. Next, another crew compacted the new sub-base and installed the temporary panels. Meanwhile, a paving crew removed temporary panels installed the previous night and placed the new rapid-set panel.

Quality result proves value of rapid-set concrete

Beam samples were taken for every panel that was placed. Workers broke two beams at three hours and two more at four hours. Then, two more were broken at the time the runway was scheduled to open. Two more were finally broken after 28 days. The average flexural strength at the time the runway was opened to traffic was over 700 psi. The average 28-day strengths were in the 1,000 psi range.

Merlino has successfully completed over $50 million worth of work at Sea-Tac Airport since 1995. The contractor has replaced more than 620 concrete panels using over 13,000 cubic yards of rapid-set concrete. In 1998, Merlino was presented with the National Award for Excellence from the American Concrete Pavement Association for its work on the 1997 Airfield Improvement Project at Sea-Tac Airport.

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