April 7, 2011

From the air, the Berkeley County Airport whitetopping project in South Carolina seemed like a simple enough job. Construction crews would overlay an existing asphalt runway, which was in dire need of repairs, with 9 in. of concrete.


From the air, the Berkeley County Airport whitetopping project in South Carolina seemed like a simple enough job. Construction crews would overlay an existing asphalt runway, which was in dire need of repairs, with 9 in. of concrete.

But on the ground, it proved to be a little more difficult. Despite extensive planning and preparation, the contractor and engineers had no way of knowing what was lurking a few feet underneath the runway. However, even when the proverbial monkey wrench was thrown into the project, the contractor, APAC-Ballenger, adjusted on the fly to get the airport open on schedule.

Before crews could first break ground though, they had to know what material they would be working with. Greg Dean, the airport programs director for the American Concrete Pavement Association (ACPA) Southeast, said the project was bid with two alternatives to evaluate designs that contained either concrete or asphalt as the final surface layer.

Since the original runway was made of asphalt, Dean said alternative bids would not have been considered a few years ago. But the current economic climate changed all that.

“Five, 10 years ago, the automatic fix was that if you’ve got an existing asphalt pavement, you just put back asphalt,” he told Roads & Bridges. “And now you’ve got some different thinking out there that makes it advantageous to bring concrete overlay strategies to light.”

Specifically, the uncertainty surrounding future transportation funding and skyrocketing energy prices made concrete more appealing, even though it would cost slightly more upfront. Asphalt prices tend to follow oil, and as oil prices have soared, the material also has become more expensive. On the front end, Dean said concrete is a slightly pricier option. With the Berkeley County project, the winning concrete bid cost about 5% more initially. But, according to Dean, the benefits come with concrete’s longer lifespan, which typically lasts a minimum of 20 years. In many cases, Dean said performance data show that concrete pavements outlive their initial 20-year lifespan.

Time for a facelift

After the winning bid was chosen, APAC-Ballenger started working on Nov. 15, 2010, to reconstruct the airport’s lone runway, which was 4,350 ft long and 75 ft wide. The total cost, including drainage and runway lighting work, was $3.77 million, with the vast majority of that—95%—coming from the Federal Aviation Administration’s (FAA) Airport Improvement Program.

The pavement was in bad shape. Although it had been crack-sealed recently, the runway still had numerous open cracks. While it was extended in 2003, no major construction work had been done since 1984, according to Matt DuBose, a project engineer with WK Dickson. Additionally, significant portions of the subgrade were saturated. In fact, the FAA said the runway had actually failed beyond the point of rehabilitation.

David Roe, project manager for APAC-Ballenger, said his team had worked at larger airports, such as Charlotte Douglas Airport in North Carolina and Hartsfield-Jackson Airport in Atlanta. In many cases, they had to work under live air traffic. But thankfully, crews did not have to do that this time, since planes were rerouted to neighboring airports during the job.

“We were glad that the airport was able to be closed down,” Roe told Roads & Bridges, “Because that’s rare in this day and age.”

An unwelcome surprise

APAC-Ballenger started off with some light milling to correct some of the subgrade and cracking issues, since the old asphalt essentially served as the base. Roe said they knew there were drainage and saturated subgrade issues going into the project, but crews were not ready for what they found during excavation.

“What caught us by surprise was the existence of an under-drain system that was put in here probably 20 years ago,” Roe said. “But that seemed to slip by everybody.”

None of the records or plans that the team had, which stretched back several decades, ever mentioned an under-drain system beneath the runway. Even the county engineer was unaware of the system. To make matters worse, the system was not operational. So rather than taking water away from the runway, it simply collected water.

The problem lay in what to do with the old system. Fortunately, the contractor had already planned on installing a new under-drain system. The cost to backfill the old under-drain trench was too expensive, so crews made the best of a bad situation and built the new system around the old.

“We basically just rolled with the punches as best we could and shifted our proposed under-drain out a foot or two,” DuBose told Roads & Bridges.

Although the unexpected discovery did delay work somewhat, it ultimately did not throw off the schedule too much. DuBose credits much of the smooth turnaround to the team’s preparation and planning.

“The only thing that made it a fix that we could come to quickly and successfully was the fact that our new under-drain system was planned,” he said. “The fact that that was there really gave us some latitude on what we could do.”

Once the new system was in place, it was tied into a pump system that drained water off the airport’s property. And even before the project was finished, the new system showed positive results. DuBose said it quickly helped drain the subgrades, which is especially important since the entire airport is in a region of South Carolina known for having poor drainage and a particularly high water table.

From there, most of the paving work was fairly standard. Roe said the milling presented a concern at times, since the existing runway was fairly thin. Most of it averaged only 4 or 5 in. thick, and in order to correct some of the cracking and subgrade issues, crews had to mill more than an inch in certain places.

Paving the runway

The entire airport was closed for the project, but the heavy trucks, weighed down with mix, stressed some parts of the runway too much. As a result, Roe said they had to cut out small sections of the runway, excavate it, subgrade out and then replace it.

After Ballenger finished milling, it divided the 75-ft-wide runway into four paving lanes: two outer 121?2-ft lanes and two inner 25-ft lanes. A standard paving train, including a Gomaco 2800 slipform paver, then added 9 in. of concrete in alternating lanes. Because the alternating lanes had different widths, the paver had two separate pans, one set at 25 ft wide and the other at 121?2 ft wide.

Crews used a standard P-501 cement and fly-ash mix design for runways with No. 4 and No. 67 stone and natural sand. The mix itself came from Charleston Concrete Co.’s central mix batch plant, which was only 12 miles from the jobsite and delivered in ready-mix trucks. Once on-site, the trucks discharged on grade in front of the paver.

The mix itself was designed to achieve 650-psi flexural strength in 28 days. Ballenger made test beams to break at two, three, seven and 28 days to gauge the concrete’s strength at the time. The two- and three-day test beams were made to ensure that the mix met the 550-psi minimum strength requirements for paving equipment and construction.

At press time, the contractor had not broken all of the 28-day test beams, but Roe said the maximum design strength had been achieved in most of the beams after 14 days. Samples also were taken throughout the paving process to test for slump and air content. To ensure that the pavement met all specifications, APAC-Ballenger and WK Dickson each had a testing firm examine the samples.

Acceptance criteria were based on flexural strength, thickness and smoothness. In addition, cores were taken to measure thickness, and a 16-ft straightedge was used to test for smoothness. Based on the PWL method of estimating percentage of material within specification limits, Roe said the project met all the acceptance criteria.

Originally, the project was scheduled to wrap up in mid-February. But crews were given an extension after the FAA decided to upgrade the runway lights to LEDs. At time of publication, the runway was scheduled to be finished by April 14.

Positive results

Although the runway was not officially open yet, DuBose said the project has already been a huge success. For one thing, it was initially supposed to be two separate projects: a runway paving job and a grading and drainage job. But APAC-Ballenger and WK Dickson combined the two for one fiscal year. Not only that, they bid the combined projects under budget. Perhaps even more important, DuBose said that because both projects were done at the same time, the airport was shut down only once.

“We’re proud of being able to help the county get that done and get them a good facility all at once as opposed to piecing it together over a couple of years.”

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