Last fall, Ace Asphalt of Arizona Inc. used hot mix asphalt to pave the two straight-aways of a nearly 6-mile oval that measures 39 feet wide with banked curves at the ends.
The smoothness spec was exceptionally stringent, but Ace rose to the challenge, notes Josh Phillips, project manager.
The project called for the pavement smoothness at 4 inches of total deviation per mile from a 1/4 inch null (blanking) band on a high-speed profilograph. A typical Arizona transportation department spec is 6 inches per mile. Ace's average for this project: 1.1 inches per mile.
"If we could have gotten within 2 inches per mile, the customer would have been happy," says Phillips. "Previous to our resurfacing, the existing pavement measured 2.5 inches per mile in the wheel paths, and 8 to 10 inches per mile at the joints. So we smoothed it out significantly."
Phillips gives a great deal of credit for the smoothness to a Topcon System 5 Sonic Averaging System. Using that system, Ace ran two 30-foot-long non-contact skis, one on each side of the Ingersoll Rand paver. Four trackers are suspended from each ski.
The trackers have transducers that "read" the surface beside the paver, use it as a grade reference, and send messages to a 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.
Boost in production
Automating the pavement depth control not only produced a smooth pavement, it helped Ace boost production.
"Without it we could only pave at 175 tons per hour," says Phillips. "With the automated control we paved at 300 tons per hour, or about 45 to 50 feet per minute. You're able to set and maintain a certain speed.
"Once we set up that System 5, we didn't stop the paver until we reached the other end," he continues. "The system removes human error from the equation."
Ace placed the new pavement in two lifts of 1.5 inches thick each. For the first lift, the contractor laid an 11-foot-wide pass and two passes at 14 feet wide.
In the next lift up, Ace staggered the longitudinal joints by paving three 13-foot-wide passes. That helped smoothness, because the System 5 trackers never read a joint below to pave the second lift.
"When we started, the System 5 read grade off the paved shoulder on the outside and off the milled surface on the inside," says Phillips.
"Then after the first pass we could read off the newly-paved edge and the milled surface. We liked running the skis on both sides," he continues. "When you're paving a 1.5-inch course of asphalt, it's very difficult to eliminate any irregularities by just running one sensor off the end of the screed. By doing that, you just follow the joint where you're at. But by using a 30-foot non-contact ski on both sides, you average out your readings on both sides. Every time you make a pass it gets better and better."
Sonic feeder sensors
Ace also ran Topcon sonic feeder sensors on both sides of the paver.
Located at the ends of the screed, the sensors measure the distance from the sensor to the head of material in front of the screed. The goal is to keep a constant head of material in front of the screed. So in fact, the sensors automate that process by controlling the speed of the augers feeding asphalt.
"No matter how fast you pave, you always get the same amount of material at your screed," says Phillips. "We ran the feeder control sensor at first on one side, not the other, and it made such a big difference that we put it on both sides.
"The feeder sensor eliminates what we call 'chatter' in the mat," continues Phillips. "It's a ripple effect behind the screed. With the feeder sensor you don't get any of that."