How user-friendly automation helps in the process. 

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, and they have the smoothness numbers to prove their point. In fact, Keating can consistently pave surface asphalt mats with International Roughness Index 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 for automated pavement depth control. The Lunenburg, Massachusetts-based contractor manages a total of seven paving crews and has been using non-contact skis to automate its asphalt pavers since 2000, says Mark Pillsbury, construction manager.

Consider this example of Keating’s smoothness performance. In paving a surface friction course on a 5.8-mile stretch of Interstate 195 near New Bedford, Massachusetts, 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 miles per hour 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,” says 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.” 

Best practices

Yet automation isn’t everything in paving. “You can’t rely on automation to correct your bad habits,” says 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 bringing their A-game -- from the company president to the state trooper on the job,” says Ricker. “Everyone has to understand and accept what we want to do.”

Pillsbury says project planning is “a crucial part of our success in achieving good IRI numbers.” You have to take into account lane distances, distances between bridges, end joints, changes in cross slope, and more. “With this plan we develop a strategy for equipment setup that will allow us to maximize the capabilities of our paving train,” says Pillsbury.

To monitor and improve crew performance, Ricker introduced a  Paving Crew Daily Training Summary, or checklist of best paving practices. Project information includes check-offs for items ranging from screed straight-edge check to screed preheated to mix delivery -- consistent or erratic (for the complete list see sidebar).

To successfully pave a highway with multiple passes, the first pass of the shift has to be laid out straight, says Ricker. “That’s what I call the payline,” he says. “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, says 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,” says 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 feet per minute will lead to higher deviations on an IRI graph -- meaning less smooth pavements. “If you go too fast, your ride will suffer,” says 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 2 inches above the height of the mat,” says dos Santos. “So if we’re laying a one-inch mat, we set the augers at 3 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,” dos Santos says. “Often we can raise the auger to give it more material in the auger chamber.” 

Non-contact skis

As for automation, Ricker says most asphalt contractors in northeastern 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, says 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. 

Good IRI numbers

One night last September, we visited a Keating crew paving a 1-inch open-graded friction course on the aforementioned stretch of I-195 near New Bedford. A Roadtec 2500 Shuttle Buggy was feeding hot-mix asphalt to a Caterpillar 1055D track-mounted paver.

Paving began at 8:00 p.m. Moving at 40 feet per minute, the crew paved about 6,200 feet at 16-feet wide on the first pass <\!m> the high-speed lane. In successive passes, the paving train placed two travel lanes, each 12-feet wide, and lastly an 8-foot-wide shoulder. For about 1,200 feet at one end of the stretch, the pavement narrowed and only required three passes instead of four.

Keating ran the center point of the Topcon skis 3 feet ahead of the tow arm midpoint. “That slows down the reaction of the screed,” says Ricker. One could notice barely any movement in the tow point cylinder as the paver moved along.

Frank dos Santos said the challenge of the job was to achieve the best ride possible. “The key to this job is to place the foundation accurately,” says dos Santos. “When you start your binder course, make sure it’s right on target. Start from the beginning using System 5 and it makes the job go better. And it’s very important to use consistent speed of the paving train.”

Using the Topcon System 5, Keating was able to improve the profile of the pavement in placing both the binder course and the friction course. Average IRI readings on the binder course read as follows over a 5-mile stretch of the project: 64.4 inches per mile, then 59.8, 55.0, and 65.2.

Notice how the average IRI numbers drop in paving the friction course, again with a Topcon System 5: 29.6 inches per mile, then 28.4, 37, 34.1, 37.5, and 36.5.

Similar improvements can be seen in the IRI numbers on Route 140, a project Keating paved near Freetown, Massachusetts, in 2006. For that project, Keating won a Quality in Construction Award from the National Asphalt Pavement Association. For the 2-inch binder course, the IRI numbers all averaged between 51.5 and 57.2 inches of deviation per mile. By contrast, the friction course numbers averaged between 27.8 and 31.9 inches/mile.

And on one project on Interstate 495, another contractor’s IRI numbers were in the hundreds of inches of deviation per mile. Keating was called in to correct the problem. By paving one 1-inch lift with the Topcon SAS system, says Ricker, Keating brought the numbers down into the 30s.

Recently Keating bought a Topcon SAS system for its Roadtec Model 700 milling machine. By getting smoother profiles under its pavements, Keating can pave smoother mats, Ricker says.

Automation only works well in concert with best paving practices, Ricker says. “Many crews in the industry try to push the envelope all the time,” he says. “Automation is designed for realistic, practical paving rates.”

Setting Up the System

To work properly, Keating’s Walt Ricker says a Topcon System 5 must be set up correctly for slope control. The first step is to place your smart level on top of a straight edge across the road. (A smart level is a level with an electronic read-out on it.) Mark the spot where you took the reading, then back the paver up and lower the screed down on the same spot. Next you null out the screed, or relax it, on the spot you read with the smart level.

Put the System 5 in the Survey mode, and the reading you get on the system should be the same reading you read with the smart level. You’re now ready to pave in the Automatic mode.

A crucial part of running non-contact skis, says Ricker, is the location of the balance point -- or center point -- of the ski. You must first locate the point on the paver that is exactly the midpoint between the forward tow point and the rear pivot where the tow arm connects to the screed. That’s the center of your tow arm.

Next you locate your non-contact ski relative to that midpoint of the tow arm. If you place the center of the ski exactly across from the midpoint of the tow arm, you’ll get an average reaction from the screed in response to elevation changes read by the ski. If you want a slower reaction time -- say you’ve got an irregular reference pavement -- you will place the center point of the ski ahead of the tow arm center.

“For more deviations in the binder course or subbase, I will move the ski forward to make the screed react slower,” says Ricker. “You don’t want your screed to be moving up and down too quickly, because your pavement won’t be as smooth then.”