In these days of high asphalt costs, you're probably more concerned than ever about asphalt yield. Especially if you're paid by the square yard, you don't want to lay the mat extra thick just to be sure you meet the spec.
That's what they believe at Allan A. Myers, a company of American Infrastructure, Inc., and they did something about it. They purchased a Millimeter GPS system from Topcon, and they figure it will pay for itself, in reduced asphalt waste, in about a year and a half.
"The Millimeter GPS controls the elevation of the uncompacted mat and gives us assurance that we're maintaining a consistent mat thickness during high-production paving," says Scott J. Styfco, Myers' roadway manager for a 5-mile asphalt paving project on the Pennsylvania Turnpike near King of Prussia. "All of our blacktop is paid for in square yards, so if we lay it too thick, it comes out of our pocket."
Under a $168-million contract, Myers is widening the stretch of Interstate 276 to six lanes from four. Last year, without a system attached to the paver, Myers paved the outside shoulders and slow lanes on both sides of the highway.
"Last year we had a tendency to lay the material a little bit thicker, to make sure we weren't going too thin anywhere," says Styfco. "This year, with Millimeter GPS, we can control the mat thickness better, so we don't waste any asphalt and we don't have any cases of insufficient mat thickness." This is one of the first applications of a Millimeter GPS system on an asphalt paver, say Topcon officials.
Myers is running an average of 2,500 tons per night, up to 4,500 tons per shift, with the plant making about 400 tons of hot mix per hour. The contractor paves at night because that's when the plant can dedicate itself to the turnpike project.
Last December through the winter, Myers tore out the existing four inside lanes of turnpike pavement. The work zone is a 58-foot-wide clear space that will contain a new travel lane, passing lane and inside shoulder for both the westbound and eastbound sides of the turnpike. Jersey barrier separated the work zone from traffic, which now could run on the outer two lanes on each side.
The old pavement consisted of 10 inches of concrete topped with 5 inches of asphalt. A Roadtec RX 700 milling machine removed the asphalt, and Myers used a hydraulic hammer to break apart the concrete. A Caterpillar 345 excavator picked up the chunks of concrete and loaded it into trucks.
Next, Myers removed the old drainage works and installed new drainage pipe — mostly 18- to 30-inch-diameter high-density polyethylene pipe. By late March, crews were grading the subgrade with dozers and motor graders. Sub-base came next, and for that Myers crushed the old concrete, then laid it down in a 6-inch-thick lift of base rock under the new pavement.
The pavement section consists of 20 inches of full-depth asphalt. The bottom lift, or base course, is made of 4 inches of asphalt-treated permeable base — 3/4-inch stone mixed with 2.1-percent asphalt binder. Moving upward, the next three lifts total 11 inches of bituminous concrete base course (BCBC). That is topped by a 3-inch binder course and a 2-inch surface course.
For paving, the Myers crew runs one Cedarapids paver on each shift and maintains another in a backup role. Crews lay out a 6,000-foot-long work area with six paving lanes and access for trucks at one point. Both pavers can accept the Topcon Millimeter GPS system, which Myers uses for every asphalt lift except the last two, for which the crew changes over to a Topcon Sonic Averaging System (SAS).
Topcon's SAS system uses non-contact skis, often one suspended from each side of the paver. Trackers are hung from each ski. The trackers have transducers that "read" the surface, use it as a grade reference, and send messages to a Topcon System 5 controller. The controller averages readings from the trackers. That way the length of the ski averages out the highs and the lows in the reference grade or pavement. In turn, the controllers send messages to the hydraulic valve that controls the paver's tow-point cylinders, which automatically raise or lower the screed. The skis "take out the highs and lows" of the reference grade and produce a more consistent basis for pavement depth.
Styfco credits the system with helping lay the asphalt base that produces a high-quality riding surface in the last two courses. "The SAS is specifically designed to give you ride quality, whereas the Millimeter GPS system is designed to give you elevation and slope control," says Styfco. "Once you have good elevation and slope established, you start with a solid base, correctly placed. Then you're going for ride."
He's got rideability numbers to prove his point. "Utilizing the Millimeter GPS system on the lower four courses of asphalt contributed to attaining an average IRI of 70 on the binder course this year," says Jeff Dremel, project engineer. "The Millimeter system helped achieve a much better quality ride than we got last year using manual control methods." A recent IRI reading was 62, compared to the specified number of 70.
Myers owns four Topcon transmitters for the Millimeter GPS system, which works much like a regular GPS system. Myers loads a control file and a surface file (3-D model) into the control unit on the paver, which also has a GPS mast with a receiver. When the receiver 'sees' where the paver is located in northing and easting directions, the control box 'knows' what the design elevation and cross-slope should be and controls the screed hydraulics accordingly.
For initial paving passes, Myers uses the system and slope control to set elevation. On the adjacent pass, the crew uses the system to control one side of the mat, and a joint matcher to match the previously-placed mat. For the third pass, the paver is controlled the same way.
"Before we had the Millimeter GPS system, we had to pull stringlines between stakes every 50 feet to check grade and make manual adjustments for the next lift," says Styfco. "But with Millimeter GPS we are certain that we place the right amount of asphalt in every lift."