Cover Story: Tracked asphalt pavers

|  June 12, 2007 |

It’s not exactly a revolution. But contractors engaged in highway-class asphalt paving with units 40,000 pounds and over are definitely experiencing significant changes in the way they do business. The old “blow-and-go” approach that defined this paving niche for so many years is now giving way to a slower, more methodical paving method with a new emphasis on smoothness. These changes are being driven at the DOT level, with smoothness bonuses replacing production-oriented awards.

“A lot of the states are giving a financial incentive for smoothness because studies have shown smoothness gives you a longer lasting road,” says Bill Rieken, paver application specialist, Terex Roadbuilding. “So there’s a financial advantage for everyone with a smoother road. The rewards available to contractors now for giving a smooth ride are a huge incentive to have the right equipment on the job and to use it correctly so you can guarantee that extra money at the end of a project.”

The challenge for OEMs to meet these new demands is daunting. “Mainline paving is as competitive as ever,” notes Don Lamb, North American sales manager, Roadtec. “Contractors are paving more tons annually at lower margins than ever before. Manufacturers must produce paving equipment that will surpass ride, density and segregation specifications at tonnage rates that guarantee profitability to the contractor.”

To achieve these new goals, OEMs and contractors are fine-tuning paver models in their lineups. Modular designs are allowing contractors to quickly adjust to different types of jobs. Contractors find it crucial to have pavers that quickly adapt to different mixes, bases and lane widths.

“Efficient production with pavers in this class is defined as anywhere from 2,000 tons to more than 5,000 tons placed in one day,” observes Bob Nittinger, Eastern regional manager, Dynapac. “This is a pretty wide range of production. The determining factors will be plant capacity and truck cycle times. And because the logistics aspect of paving is now zeroing in on truck cycle times, paving speeds have slowed down. But the tonnage being placed has not.”
Clearly, slower paving doesn’t mean a more relaxed pace for contractors and crews.

Sustaining a productive continuous paving process is vital for achieving good profilograph scores. To that end, many contractors rely on windrow paving or material transfer devices and vehicles to establish continuous paving. Rieken mentions one more critical component to this equation that helps improve mat quality, but it’s one some contractors often neglect to use. “It’s important for contractors to use a correctly sized and shape hopper insert,” he explains. “Inserts give the contractor additional surge capacity to help with truck exchanges. Also, a correctly sized and shaped insert creates a live bottom action that ensures the uniformity of the asphalt temperature, particle distribution and mat density.”

A material transfer device can positively boost the continuous paving process, Lamb says. “Our Shuttle Buggy MTV stores and constantly blends asphalt to ensure consistent production,” he says. “Because the buggy is not attached directly to the paver, the problem of trucks bumping the machine and marring the mat is eliminated. If an insert is placed in the paver to further increase the amount of asphalt available to the paving train, paving operations using Shuttle Buggies will actually see a reduction in the number of trucks required to maintain consistent production.”

Hot mix segregation is a major headache for contractors targeting smoothness bonuses, Nittinger says. Over time, poorly segregated mats look bad and can degrade significantly. The way around this problem, he says, is by spec’ing the proper auger drive system for your paver. “At Dynapac, we’ve settled on the outboard auger drive system as the best method for eliminating hot mix segregation problems,” he notes. “Tamper screeds, used in conjunction with outboard auger drive systems, have totally eliminated the centerline segregation problem, delivering a denser, more homogeneous, smoother pavement.”

At the same time, optional notched wedge joint makers are improving longitudinal joint densities, further contributing to a mat’s overall smoothness. Centerline segregation, also known as the “skunk line,” is affected by outboard auger drive systems as well. “The skunk line is a focal point for highway inspectors,” Nittinger notes. “It occurs when material starts building up on the center chain case driving the auger and slat conveyor. If you want a uniform, non-segregated, homogeneous, smooth mat, then an outboard auger drive system can definitely solve your skunk line.”

Ingersoll-Rand, manufacturer of ABG and Blaw-Knox paver lines, is focusing on paver feeder systems as well. “We’ve got four independent feeders and monitors – be they sonic or paddle systems,” explains Brodie Hutchins, marketing manager, highway class paving products, Ingersoll-Rand. “On the Titan and new 5000/6000 Series pavers, there is a feeding monitor at each conveyer on an Ingersoll-Rand paver and at each auger as well. We’ve redesigned the augers so the conveyer is completely open all the way across the back. These changes let you keep a consistent head of material all the way throughout the auger channel during paving operations, which delivers a consistent flow of material to the screed.”

Terex Roadbuilding’s answer to the problem of temperature control and segregation problems has been the introduction of its remix anti-segregation auger system. “Our pavers will accept either conventional slat conveyors or optional remixing augers,” Rieken says. “The remix system gives you the ultimate in thermal and material re-blending so that you have not only uniform mat texture, but you have a uniform temperature too. You can’t get a smooth ride if you’ve got temperature differentials occurring in your mat – and it’s not always a visible problem. Our remix augers do an excellent job of minimizing all those temperature, material and segregation problems.”

Refined screed technologies make a science of smoothness
Paver screeds are receiving greater scrutiny as well. “Tamper-type screeds are becoming the premier choice for pavers in this class,” Nittinger says. “This type screed with the proper accessories has drastically improved the quality of pavement being placed by increasing the initial density out of the screed 92 to 94 percent.”

“I see the tractors themselves in the years to come becoming more of a carrier of the screed,” says Hutchins. “Most of a successful paving operation is really about the screed anyway. Making the tractors more flexible is hugely important now. You want a tractor to be able to do base work one day and production work the next. But if you want to go out the day after that and do DOT work, then the focus turns to the unit’s screed and how flexible and modular it is. You want to have a screed that you can literally unbolt and change out with a different one quickly and easily, be it a dual- or single-tamping screed, a vibrating or fixed screed, or a front- or rear-mount unit.”

Heated screeds are nothing new, of course. But Hutchins thinks electrically heated screeds will gain popularity over propane- or diesel-fired units. “We’re beefing up generator capacities on the tractors to handle the extra load electrically heated screeds create,” he notes. “But it’s worth the investment because electric screeds offer so many benefits.”

Among them, Hutchins says, is that electrical power is inherently cleaner and simpler to work with than diesel or propane. In addition, the temperature of an electrically heated screed can be controlled more precisely and it has a faster heat-up time than competitive systems.

“More than 90 percent of our pavers now go out with electric screeds,” Rieken adds. “And we’ve had good up-time and reliability with them. We’re now adding a 34-kilowatt, electrically heated version of our Stretch 20 screed to our lineup. Good weight is still important for a screed because initial density is so critical for smoothness. So is good compaction behind the screed. But the addition of a durable electric screed is vital in helping contractors achieve the smoothest mat possible.”

Are steel tracks obsolete in large asphalt paving applications?
The majority of pavers in this class are now sold with rubber-track drive systems, yet another innovation that is bolstering mat smoothness. “The advantage of the rubber track is obviously it doesn’t scuff the matt near as much as steel tracks do,” Rieken explains. “Rubber tracks are forgiving in adverse terrain while offering almost the same travel speeds as rubber-tired pavers. Even better, they are extremely smooth in operation.”

The challenge for rubber-tracked pavers has been the life expectancy of the track itself, Rieken admits. “But we’re now routinely exceeding 4,000 operating hours with our tracks,” he says. “We think good tensioning is the secret and developed our Smart Track Tensioning system to always have optimum track tension whether you’re going forward or reverse, or whatever load is against the paver. We’ve also got a de-track system that stops the paver before track damage or complete loss of a track occurs.”

Like any drive system, rubber tracks aren’t perfect. They are costly to repair when compared to a rubber-tire paver. And caution must be used when backing some machines up because turning while backing or hitting a curb could result in track misalignment. The total replacement of a rubber track is also more expensive when compared to steel track replacement costs.

“If you’re doing straight overlays, a rubber-tire paver is by far your best choice,” notes Scott McClellen, product manager, Vogele America. “If you only have one paver in the 10-foot size, then a rubber-track machine can do both overlays and new construction. And they can go from the end of a pass back to the beginning faster than a steel-track machine, which certainly adds to their overall productivity.”

Steel tracks still have their place in paving operations. “The old-style steel-track paver with pads is still a horse,” Nittinger says. “It is still the best system for digging its way through tough ground conditions and going where other pavers can’t. But the bounce it produces contradicts the smooth finishes today’s jobs demand and the travel speed is very slow. The steel track may travel up to 4 mph compared to the rubber track with travel speeds up to 12 mph.”

But if you’re paving widths 24 to 28 feet wide, Nittinger thinks steel tracks are a better fit, “particularly if you’re placing a crushed stone base or working in a soft, spongy area,” he says. “Because paving is application driven, there’s still definitely a need for the steel track workhorse.”

Barber-Greene BG-2455C
Features include:
· Mobil-Trac System undercarriage
· High travel speeds
· 174-horsepower Caterpillar 3116TA diesel engine
www.cat.com

Caterpillar AP-655C & AP-1055D
Features include:
· High ambient temperature cooling system
· Cat C7 engine with ACERT technology
· Mobil-Trac undercarriage
www.cat.com

Dynapac F 30C & F 30CR
Features include:
· 200 hp Cummins Engine
· Electric Heated Screed with additional outlets for night lighting
· Outboard auger Drive system with sonic feeders and the ability to reverse augers for placing shoulders and medians.

Ingersoll-Rand ABG Titan 326, Titan 8820, Titan 525 & TITAN 6110
Features include:
· Easily converts from asphalt to roller-compacted concrete, cement-treated base and other types of concrete paving
· 243-horspower Deutz diesel engine
· Accommodates a variety of screeds
www.road-development.irco.com

Roadtec RP 195
Features include:
· Full counter-rotating track capability
· 200-horsepower John Deere 6068T turbo diesel engine
· Automatic grade and slope control system
www.roadtec.com

Terex Cedarapids 562, 462 Remix & 562 Remix
Features include:
· Turbocharged Cummins QSB diesel engine
· Smartrac self-tensioning track system
· Crawler-mounted, three-point suspension
www.terexrb.com

Terex Cedarapids 562, 462 Remix & 562 Remix
Features include:
· Turbocharged Cummins QSB diesel engine
· Smartrac self-tensioning track system
· Crawler-mounted, three-point suspension
www.terexrb.com

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