Superb smoothness in North Dakota: careful mixing and strategic scheduling result in a successful runway

Concrete Construction, August, 2006 by Jane Greer

Hector International Airport in Fargo, N.D., is the largest commercial airport in the state, serving two commercial carriers, six fixed-base operators, air cargo, and a North Dakota Air National Guard base supporting F-16 fighter planes. Hector's 2005 Runway 18-36 reconstruction was the largest runway project in North Dakota history.

Fargo, located in the Red River Valley, has fat clay soil that typically has a frost depth of 5 feet or more in open areas and under pavement. Ulteig Engineers, Fargo, designed the reconstructed runway as a 17-inch-thick plain concrete pavement resting on 6 inches of "econocrete" and 8 inches of crushed concrete base. Econocrete is a low strength (750 to 1200 psi in 28 days) concrete mix that was used as a stabilized base.

The project requirements were a smooth, durable concrete runway, no loss of airport service, maintained safety, and completion in time to allow flight-checking of the instrument landing systems so the information could be published before Thanksgiving. If it hadn't made the deadline, Hector International would not have had an instrument approach until mid-January and there would have been many unhappy holiday travelers.

A wet spring slowed construction and paving had to be shut down for a week because of cement shortages. In spite of these and other obstacles, Ulteig and Shafer Contracting Co, Shafer, Minn., exceeded the project expectations. The runway was opened to aircraft on time. There were no on-the-job injuries. Airport service was maintained--in fact, the passenger numbers for 2005 were greater than those for 2004. And the project earned close to maximum incentives for work quality.

Runway 18-36 is exceptionally smooth. "Three factors were key to achieving such smoothness," said Ulteig aviation sector leader Steve Synhorst, "designing the mix to match the paver, mixing consistently, and keeping a constant head of concrete in front of the paver."

Mix Design. Sharer worked with Midwest Testing Laboratories, Fargo, to develop an optimized aggregate gradation mix that used 1.5-inch aggregate and was compatible with Shafer's paving equipment. The mix flowed through the paver efficiently, filled all voids, held a perfect vertical edge behind the paver, and earned 93 percent of the available bonus for strength and thickness.

Consistent mixing. Sharer implemented its own quality control plan in addition to Midwest Testing's required independent quality control plan, and also built a portable concrete batch plant at the jobsite. This helped them make immediate adjustments, producing a more consistent mix. The plant was a Rex Model S double-drum that produced 8 cubic yards per minute. To further ensure a consistent mix, Sharer used four bins for optimized mix production and used Shilstone aggregate blending techniques. The mix was so consistent that no edge slump boards were required at any time and bull floating behind the paver was minimal.

Nonstop paving. Sharer achieved virtually nonstop paving by placing a constant supply of consistently mixed and monitored concrete in front of the paver. This reduced stops in the paving operation, required less finishing, and produced a smoother finish. Most of the concrete and econocrete paving was done with a Gomaco slip-form paver.

Strategic scheduling. Nonstop paving wasn't Shafer's only strategic scheduling practice. The contractor teaches its paving foremen--and requires them to use--the practices outlined in the IPRF's Best Practices for Airport Portland Cement Concrete Pavement Construction. Strategic scheduling was an important factor in achieving maximum pavement smoothness.

Placement of the in-pavement light cans was nearly 100 percent accurate as a result of strategically scheduled paving. The centerline light cans were in the fourth paving lane, 2 feet off the third paving lane. Sharer paved the third paving lane first to allow the maximum amount of time to set and align the cans to match grade.

Strategic scheduling made maturity monitoring unnecessary. The Federal Aviation Administration usually doesn't approve maturity monitoring for runway pavement, but they allowed it for the Hector project in order to accelerate the paving schedule. But as it turned out, Sharer didn't need to use maturity monitoring because the strategic paving schedule gave pavements time to reach full strength before being put into service.

Measuring smoothness. The project's smoothness specification include the following:

* No surface deviations in excess of 1/4 inch when tested with a 16-foot straightedge placed in any direction.

* Deviations between 1/2 and 1/4 inch are to be corrected by grinding. Before grinding, the pavement is subjected to an aircraft ride analysis to determine the severity of the deviation.

* The Air Force has smoothness criteria for the runway 200 feet before and after each aircraft-arresting barrier: the pavement is subject to a special tolerance of no deviation in excess of 1/8 inch when tested with a 12-foot straightedge placed longitudinally every 5 feet across the runway. This special tolerance ensures that the tailhook of an aircraft won't bounce before engaging with the cable.