Energy Bars

June 20, 2005

High-energy compaction at the screed was proving essential for a contractor to achieve compaction of 99% on an ongoing runway resurfacing project at Homestead Air Reserve Base in southern Florida.

High-energy compaction at the screed was proving essential for a contractor to achieve compaction of 99% on an ongoing runway resurfacing project at Homestead Air Reserve Base in southern Florida.

The 10-month project to upgrade runway and taxiway facilities began this spring, and will continue through four phases this year for the client, the U.S. Army Corps of Engineers. It includes mill and overlay of asphalt, installation of a new runway and taxiway lighting system and threshold lights and a new electrical vault. Of the $15 million contract, some $4 million will be for electrical work. The two-mile runway remains open during construction.

The runway is two miles long and 300 ft wide, and has a 75-ft-wide concrete “keel” section down the center to absorb takeoffs and landings from the era when Homestead hosted the old Strategic Air Command and heavy B-52 bombers. That is staying in place, with spall, crack and full-depth repairs where necessary. But outside the concrete section, from an existing 4-in.-deep asphalt section, Sunmount is milling 2 in. and replacing it with 2 in. of dense-graded asphalt mix in the 112-ft-wide sections that flank the PCC insert.

“A lot of the old air force bases have that 75-ft-wide concrete keel section to take the beating,” said John Hajdasz, project manager, Sunmount Corp., Roanoke, Tex. “Then, it’s less expensive to place asphalt over aggregate base courses outside the keel section.”

The existing hot-mix asphalt (HMA) surface, which dates from the early 1960s, is aged and weathered, and was cold-milled by Sunmount using its own machine. “We did the cold milling using one of our Wirtgen W 2000 cold milling machines, some 12,500 tons in Phase I, toward a total of 50,000 tons,” Hajdasz said. “The machine was very dependable, working at a rate of over 200 tons per hour.”

Just under a hundred

The Corps of Engineers spec for this pavement included a Superpave-style performance-graded (PG) liquid asphalt in an older-style Marshall mix design. “It’s a COE design, 3?4-in. minus aggregate mix, with PG 76-22 binder,” Hajdasz said. “It’s a 75-blow mix which conforms to Section 02749, Hot Mix Asphalt for Airfields. It’s a tough design to meet and get compaction. The mix contains a high asphalt content, 6.2%, but the crushed limerock used has a high absorption requiring added asphalt.”

The mix contains about 51% screenings (manufactured sand size and below), with about 40% of the mix containing 3?4-minus stone, and rest somewhere in between, Hajdasz said. The mix is compacted to 99% into a dense, high-performance asphalt lift (COE specs call for 98 to 100% density, lab molded). “We’re looking for in-place densities of 99% on the mat, and 97% on longitudinal joint densities. You can get to 99% with other mixes,” Hajdasz said, “but with this type of oil and stone the COE is getting superior density and long-term performance. On highways you can get more open-graded designs that can minimize road spray during rain and reduce hydroplaning; here the Corps is looking for strength.”

After milling the surface was broomed and sprayed with a water truck, followed by a tack coat when dry. Then the mix—from an on-site plant—was placed by a Vögele Super 2100 paver with a high-density screed. German-sourced Vögele pavers combine a tamper bar and two pressure bars to bring about a high degree of compaction at the screed.

“That paver screed has a tamper bar and pressure bars which help us get density at the screed quicker, reducing the rolling required,” Hajdasz said. “Coming out of the paver the mix will be at 88 to 90% compaction. But a conventional paver won’t have a tamper bar or pressure bars, and will place mix at a maximum of 82% compaction.” The extra 8 to 10 points is a significant advantage for the contractor. Without question, all things being equal, the screed is the ideal place to apply compaction, European contractors have found. Energy is needed to move the HMA through the paver as it exits the truck, while the paver pushes the truck. Energy also is needed to place the asphalt at a uniform depth and width and density and smoothness, thus the energy for compaction is best applied right at the screed, not after the asphalt mix has been placed.

Each day four cores are taken at random from the mats, and four random cores from the joints. These are analyzed in an on-site Sunmount lab which has been certified by the COE. “Sunmount performs quality control, and the Corps of Engineers performs quality assurance, periodically checking our results,” Hajdasz said.

“Pizza” slicing

Getting good compaction on longitudinal joints is a long-standing dilemma for the industry and the work this year at Homestead ARB is no exception. With highways, poorly constructed longitudinal joints can lead to raveling and degradation of the asphalt, but it’s much more critical on airfields, where the raveling can create high-speed foreign object debris (FOD) which can damage jet engines and aircraft.

“In-between passes we use a ‘pizza wheel’ attached to a motor grader, which cuts back the edge of the pass 6 in. prior to the next pass,” Hajdasz said. “It creates a vertical joint, as opposed to your normal sloughed-down edge.” The idea is to cut back the irregular, loose, existing edge to the portion of pavement where full compaction is achieved, and place the joint there. After cutting the edge the grader blade moves the loose HMA over about 12 ft, where skid-steer loaders or backhoe-loaders pick up the material and place it in small dump trucks. “The game plan is to get a uniform density across from mat to mat. We cut the mat while it’s still 150° and will place the next lift the same day or the next.”

Breakdown rolling is achieved with an 84-in. double-drum articulated roller, and finish rolling by one or two pneumatic rollers. “The double-drum will bring it up to 92 or 93, and we get our last five or six percent with the rubber-tired rollers,” Hajdasz said.

The HMA comes out of the plant about 350°F and exits the paver at 300°F. The mix exhibits a tender zone not unlike Superpave mixes. “You have to get on it with the breakdown roller right behind the screed,” Hajdasz said. “You then have to give it a little time to cool—down to about 240°—before you get on it with the pneumatics, or they will pick material on the tires.” The time frame was established using test strips and the roller operators are experienced enough now to tell when the mat is right for their work.

“The paver is working well,” Hajdasz said. “We’re running a joint-matcher automation system on one side, and on the new side, a Topcon automation system with 30-ft contactless ski. It’s up in the air, with one sensor ahead, one at the screed, and one at the back. They sense the profile of the ground and sends the information back to the controls to enhance smoothness.”

That’s good, because the specs require smoothness. “We have a profilograph spec and a straightedge spec that have very tight tolerances,” Hajdasz said. “Having the automated system helps us avoid problems later on.”

The 12-ft straightedge spec measures transversely and requires smoothness within 0.03 of a foot, about a quarter of an inch, under the straightedge. The profilograph index spec is 7 in. per mile maximum with a blanking band of 0.20 in., but Sunmount’s experienced crews and the Vögele paver were getting less than 1 in. per mile. Unlike some DOT projects, there is no bonus for smoothness.

Smoothness also was enhanced by the use of a material transfer vehicle, which isolated the paver from bumps caused at the paver/truck interface, minimizing mat irregularities and thermal mix segregation.

The Super 2100 is paving 17-ft widths. “Another good thing about the Vögele is that even with the screed tamping bar and pressure bar function, it has an adjustable width,” Hajdasz said. “It has a 10-ft main screed with two 5-ft hydraulic extensions, so we can pull this mat at 17 ft, turn around and pull something else at 12. That’s an advantage with this line of paver.”