Edited by Joanne Ray
If you want to earn state payment bonuses for smooth asphalt paving, it's better to be good than lucky. And to be good, your paving crew has to be patient enough to follow best practices, maintain a steady speed, and not "push the envelope" to the breaking point.
That's the gospel according to the paving and construction managers at P.J. Keating Co., and they have the smoothness numbers to prove their point. In fact, Keating can consistently pave surface asphalt mats with International Roughness Index (IRI) numbers that are one-fourth to one-third of the upper spec limit set by the Massachusetts Highway Department.
Keating owns nine Caterpillar asphalt pavers — all fitted with Topcon System 5 Sonic Averaging Systems (SAS) for automated pavement depth control. The Lunenburg, MA-based contractor manages a total of seven paving crews and has been using non-contact skis to automate its asphalt pavers since 2000 according to Construction Manager Mark Pillsbury.
Consider this example of Keating's smoothness performance. In paving a surface friction course on a 5.8-mile stretch of Interstate 1-95 near New Bedford, MA, the contractor's IRI numbers ranged from just 28.4 inches of deviation per mile to 37.5 inches per mile — based on a zero blanking band. By comparison, the state's upper specification limit is 115 inches of deviation per mile on highways with speed limits of 55 mph or greater. And the I-195 project was done with three different Keating crews.
"We're not only meeting, we're exceeding the state specifications," said Pillsbury. "The Topcon SAS is an automation tool that our crews utilize along with best paving practices, and automation is a vital part of the package that we bring to a project."
Yet automation isn't everything in paving.
"You can't rely on automation to correct your bad habits," said Walter (Rick) Ricker, a paving specialist who does field training for Keating. "We follow a systematic list of best practices that must be documented for every project in order for the full benefits of automation to be realized. Successful paving for smoothness takes the whole team to bring their A-game — from the company president to the state trooper on the job. Everyone has to understand and accept what we want to do."
Pillsbury said project planning is "a crucial part of our success in achieving good IRI numbers." Lane distances, distances between bridges, end joints, changes in cross slope, and more must be taken into account.
"With this plan we develop a strategy for equipment setup that will allow us to maximize the capabilities of our paving train," said Pillsbury.
To monitor and improve crew performance, Ricker introduced a "Paving Crew Daily Training Summary," or checklist of best paving practices. Project information includes checks for items ranging from screed straight-edge to screed pre-heated to mix delivery — consistent or erratic.
To successfully pave a highway with multiple passes, the first pass of the shift has to be laid out straight, said Ricker.
"That's what I call the pay-line," Ricker said. "What you do on that first pass dictates how you're going to do all the way across the mat."
Paver speed must be consistent to achieve maximum smoothness, said paving foreman Frank dos Santos. That means delivery of asphalt to the paver must be steady and consistent.
"I like to keep the hopper half full of material," said dos Santos. "And the head of material should be about half-way high on the augers. The left-hand auger and the right-hand auger speeds should be consistent. I like to keep them in the area of 28 to 30 rpms."
Keating has found that 45 feet per minute is a good consistent speed to produce super-smooth pavements. Company crews have experimented with paving speeds and have found that speeds of 60 to 70 fpm will lead to higher deviations on an IRI graph — meaning less smooth pavements.
"If you go too fast, your ride will suffer," said dos Santos. "If you run too fast, your rollers can't keep up. You outrun the rollers and your plant can't keep up."
Auger height should be based on the thickness of the mat being paved.
"We put the augers at about two inches above the height of the mat," said dos Santos. "So if we're laying a 1-inch mat we set the augers at three inches from the bottom of the auger to the top of the mat we're laying. We have found that with Superpave mixtures, which are coarser, we can adjust the auger height to optimize mat quality. Often we can raise the auger to give it more material in the auger chamber."
As for automation, Ricker says most asphalt contractors in Northeast states use non-contact skis, as opposed to drag skis, to control mat thickness. For mainline highway applications, Keating typically runs two Topcon SAS skis, one on each side of the paver. Four trackers are suspended 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 on each side of the paver.
The controller "does the math" by averaging the readings from the four 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.
Ricker says Keating likes non-contact skis because they're easy to change if needed. If the paving foreman looks ahead and sees an especially irregular reference grade or shoulder coming up, he can easily move the non-contact ski inward or outward from the paver to reference a smoother path. Moving a drag (contact) ski, by contrast, is much more difficult. You have to stop the paver, loosen several bolts, move the ski, and reset the controller.
"When using the left shoulder as a reference, which is normally a fairly irregular surface, Keating can achieve better IRI numbers in the left wheel path compared to the right wheel path," said Ricker. "That's because the paving crew can move that left non-contact ski inward or outward as needed to choose a relatively smooth reference grade."
By the same token, moving the paver is easier with non-contact skis. When you come to a transverse joint, or a bridge deck, you can simply raise the screed, move the paver or back it up, reset the System 5, and begin paving again. That process is much more cumbersome with a drag ski. Typically they must be removed and carried to the next location with a pickup truck.
|Daniel C. Brown is the principal of TechniComm.|