A Boeing 747 jumbo jet can weigh more than 800,000 pounds at full capacity and have a takeoff speed of 180 miles per hour. Now imagine 350 aircraft of varying weight and speed operating on one runway daily.
With that kind of force hitting the pavement, the construction specifications for the runway must be strictly adhered to, and the contractor responsible for paving must do so with surgical precision.
If that isn't difficult enough, try to stay on time and under budget. Tilcon Connecticut Inc. did just that this past spring when it rehabilitated runway 6-24 at Bradley International Airport in Hartford, CT.
Tilcon Connecticut Inc. is a leading asphalt producer and contractor in New England. The company began in 1923 handling roadbuilding projects with only a steam shovel and has grown to become a statewide conglomerate of 23 locations and plants.
Tilcon produces not only asphalt but also crushed stone and ready-mix concrete. It has nine paving crews and assumes a majority of state contracts for the Connecticut DOT, such as highway roadbuilding and repair. One such state contract was Bradley International Airport.
"Runway 6-24 is the main runway at Bradley," says Richard Birge, manager of construction for Tilcon. "The backup runway 15-33 was open to air traffic, but both runways had to be shut down for three consecutive weekends while we milled and paved the intersections of the two. It was imperative we kept to the schedule."
Bradley is managed by ConnDOT and services more than 6 million passengers annually (2008 statistics), second only in New England to Boston's Logan International Airport. Runway 6-24 handles approximately 60% of all daily air traffic.
The Federal Aviation Administration (FAA) recommends rehabilitation of airport runways every 20 years. This $17 million project was funded through the FAA Airport Improvement Program and passenger facility charges. Renovation of runway 6-24 entailed not only resurfacing, but also upgrading of the water main line and installation of all new lighting along the runway.
The project began the last weekend in April with a drop-dead completion date of June 23, 2009. Like all state highway jobs, if Tilcon missed the completion date, ConnDOT would assess fines. To accommodate such a tight timeline, crews were scheduled around the clock with up to eight crews working each 12-hour shift.
"There were no rain dates in the schedule," explains Birge. "If it rained a lot, we had to change the hours worked. At times we had milling and paving going 24 hours a day."
Runway 6-24 is 9,500 feet in length and 250 feet wide. The project called for milling and repaving the runway with a 4-inch-thick mat - 62,900 tons - of FAA-approved hot mix asphalt. Tilcon's asphalt plant is 24 miles away from the airport, one hour by truck. Tilcon had a convoy of 30 trucks running back and forth during the day, and less during the night shifts. Approximately 1,800 tons of asphalt were used per shift.
"One very productive day we used 3,100 tons," says Birge.
Paving a wide berth
The specifications for paving and compaction were very stringent. Test strips were laid prior to the project start, which had to be tested for compaction density and quality of the asphalt.
"We did a couple test strips at our facility, making adjustments to see what gave the best result for texture and mat consistency," says Birge. "Then test strips at the airport were inspected by ConnDOT and the FAA before we could begin work."
The project called for each pass to be a minimum of 20 feet wide; however, it was Tilcon's choice to pave 25 feet wide. With the continuous work within an already tight schedule, Birge wanted to ensure as much efficiency on the job as possible.
"We paved across the runway in 10 passes that were 6,000 feet in length," explains Birge. "The less time you take to back up, the more efficient the job."
Tilcon used a Volvo PF6110 tracked paver with an Omni 318 screed. The screed has a standard paving width of 18 feet; however, Volvo and Tilcon installed four 1-foot extensions on each side that brought the total width of the screed to 26.3 feet. The extra width proved necessary for the joint matching.
According to Larry Spring, road paving specialist for Volvo Construction Equipment, the Omni 318 screed provides optimum density, especially at the joints.
"We added four 1-foot extensions on each side, all heated," he says. "Two were vibratory extensions, which provided better density of the mat."
Birge explained that the consensus was that a tracked paver was needed to achieve the tight specifications.
"Everyone felt a tracked paver would be most conducive to the job because it would provide the traction we needed," he says.
Birge and his crew spent four days at Tilcon's New Britain facility becoming familiar with the paver and running practice strips. Both Volvo and Tilcon's dealer, Tyler Equipment, had representatives on hand to provide support and training to the crew.
"There definitely was a learning curve with the machine because we don't normally pave 26 feet wide," he says.
No RAP allowed
The pavement on runway 6-24 needed to be flexible to withstand harsh winters, which is why asphalt was chosen over concrete. A special mix design called FAA P-401 is the only asphalt material approved for airports. Contrary to Superpave or Marshall designs, which include some milled or recycled asphalt, P-401 is composed of virgin materials.
"Runway 6-24 is 250 feet wide, which can accommodate any aircraft out there, so the asphalt can't rut," says Spring. "They need the best mix design to tolerate the high impact of an aircraft landing."
P-401 asphalt is a virgin mix of 3/8-inch stone and 6% PG 64-22 oil. The mix must comply with FAA regulations for shearing strength and consistency. Because of this, no recycled asphalt can be used in the mix. It needs to have excellent consistency, be extremely dense and have little to no air voids.
Percentage within limits
Just as precise as the mix design needs to be, compaction density requirements are also very stringent on airport paving jobs. Highway contracts usually require 92% compaction density; however, FAA stipulates that compaction of airport runways needs to be 96.3% and 93.3% at the joint.
Compaction was done with two Volvo DD138HFA double-drum asphalt compactors and a DD118HF double drum asphalt compactor.
The two DD138HFA compactors ran right behind the paver in an echelon formation for the initial breakdown compaction. Then the DD118HF followed for the finishing.
The Volvo DD118HF and DD138HFA compactors feature eight amplitude settings allowing for the specific amount of force needed depending on mat thickness and mix design. Spring explains the customization feature helps achieve proper density without over-compacting.
"Over-compaction will breakdown the stone in the mix design," says Spring. "If you over-compact and break the stone from 3/4 inch to, say, 1/2 inch, you lose the stability and strength of the pavement necessary to handle the airplanes."
The FAA requires a percentage within limits (PWL) of at least 96%. ConnDOT and the engineering firm for the project performed routine inspections to ensure that Tilcon maintained the required PWL. Five-inch core samples were taken after compaction, and density tests were performed at the ConnDOT laboratory. The PWL average for Tilcon on this project was 104.5%.
"PWL is required on a P-401 mix," explains Birge. "The material is tested at the plant, tested at lay-down and then at compaction. These results are put together to get the PWL value, and we are always at least 100%."
On time and under budget
Tilcon's paving train rolled along toward the deadline date without any showstoppers. Runway 6-24 opened at 12:01 a.m. on June 23, 2009. Connecticut Governor M. Jodi Rell hailed the success of the project, stating it was "on time and under budget."
Tilcon also won a recent bid to repave runway 15-33 at Bradley, which is a smaller project than runway 6-24. Work began on August 3 and is scheduled to be finished on October 3, 2009.
"Runway 15-33 will be easier," Birge says. "Only 41,000 tons of asphalt."