September 2000: A better way to pave was “Under the Microscope” as Australian contractor Pioneer Road Services conducted initial performance trials of its HIPAC asphalt compactor. Raymond Rollings, a civil engineer with the U.S. Army Corps of Engineers’ Waterways Experiment Station geotechnical laboratory, called the HIPAC “the most revolutionary thing in asphalt compaction since vibratory rollers were introduced.”
The HIPAC relies on a heated, thick rubber belt mounted on rollers. This rubber belt creates a large footprint, allowing the compaction process to take place over a longer period of time. According to its designers, this process compresses air out of the mix and gives a better bond between the asphalt mastic and the aggregate.
In the fall of 2000, Rollins and Mike Nunn from the Transport Research Laboratory, United Kingdom, were in Australia to observe and verify trials of the prototype with Ian Rickards, Pioneer technical manager and HIPC inventor. Pioneer, in the meantime, was beginning to look for a manufacturer willing to develop the HIPAC concept further and produce it in commercial quantities.
Today: Rickards says not much has happened with the HIPAC concept, although new life is being breathed into the project. A shift in Pioneer’s business focus mothballed the HIPAC prototype for most of the last two years. But recently the company began field-testing again and even put the unit into regular paving operations to acquire more performance data.
“You must remember that the HIPAC prototype was built to prove the principles of heated-belt compaction,” Rickards observes. “But the machine has limitations, most notably the available belting and the use of a retired tractor unit. The maximum belt width currently available to us is approximately 4 1/2 feet across. So we need three passes to cover a typical lane width. Our initial hypothesis was that better compaction could be achieved by slowing the HIPAC down and making the machine longer so compaction load duration could be extended.”
But, Rickards says, he and Pioneer forgot a crucial point. “Air is compressible,” he notes. “So while we can achieve good compaction in a single pass, better compaction can be achieved if two or more passes are applied. I think that after the first pass the entrained air in the mat – which is compressed – in part escapes into the atmosphere (as the rubber belt moves on), and part of it coalesces into larger discrete voids. Subsequent passes of the compactor ‘squeegees’ the mat and expels much of the remaining entrained air.”
Still, the HIPAC prototype has proved the concept’s basic principles, Rickards says. “A commercial model will have to be wider and more agile to cover a lane width in two passes,” he says. “On high production work two machines could probably be used more economically than a conventional paving train while yielding better, more controllable compaction.”
Gene Wortham, materials and construction engineer, Idaho State Department of Transportation, recently visited Australia to observe the HIPAC in action. Wortham says he believes the HIPAC compaction method is valid, but still not definitively proven. “I think it’s a good idea, and I think the HIPAC could eventually help contractors win incentives for smoother, faster compaction jobs,” he says. “But until more data is acquired on density, pavement smoothness, permeability, maintenance costs, production rates and other factors of the HIPAC’s use on sustained plant-mix paving projects, the process has yet to prove itself against conventional rolling compaction methods.”
