JFK - under the microscope

High-profile runway resurfacing tests advanced milling control technology

The decision was made to disconnect the laser receivers on three of the Roadtecs and have them rough-mill the existing surface to an inch and a half above finished subgrade. It was reasoned that since the machines were going to cut the last lift with the Millimeter GPS+ anyway, the rough mill accuracy was not as critical. Three machines cutting without the lasers actually put Intercounty ahead of schedule, and two other Roadtecs fine-milled where possible at any given time.

The FC-120 field controller also mounted on the pole listed the elevation in feet and the cut in thousandths of an inch. The fine-milling machines were set at 18 inches (minus-1.50 feet) and the bulk-milling machines were set at 16¼ inches (minus-1.36 feet).

Intercounty reports that the hardness of the top of the existing asphalt layer made milling off an average of six inches in one pass difficult, too. After the rough start, Intercounty was hitting the daily production goal of 2,000 by 100 feet over several days in the second week of work.

Under conventional elevation-checking methods, reference stations were marked every 25 feet on a pass, typically along both edges of a pass. Elevation checks utilized tape, stringline, and reference marking on the milled surface. Millimeter GPS+ allowed for on-demand elevation checks — in varying locations within a pass — using GX-60 control boxes and the rover.

Continuous Accuracy

GPS may not be any more accurate than manual at station, but between stations, it is. It performs like a virtual stringline, calculating smooth transitions from station to station. And it eliminates the occasional blown grade.

The system is intended to take the human error out. The two-stage milling process proved to be almost the opposite of paving with asphalt, where one puts down a base and binder course and then comes back and levels the surface with a consistent final lift. The more consistent the lift depths are, the better the ride on the final surface is.

One of the biggest advantages of the system’s accuracy is prevention of overmilling. If overcut, it’s necessary to go back and pave and then mill again. That costs time and money.

The entire area was milled without a single mark on the ground, which is roughly 8,000-plus shots in a 25-by-12-foot grid over 11,000-plus feet. The mark-out costs associated with 16,000 shots is actually insignificant compared with the potential downtime resulting from waiting for the marks to be made, due to the large penalties enforced for not finishing the job on time.

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