Fast, efficient and high-quality paving is the goal of any contractor using tracked asphalt pavers. And machines in the 29,000- to <40,000-pound classes certainly fit that bill. These paver models typically feature 8- to 10-foot paving widths. And there's a lot of overlap within these two classes: Units in both can handle commercial applications like parking lots and airports or mainline highway jobs, giving contractors a high degree of flexibility when it comes to selecting work.
In fact, according to Scott Wiley, product manager for Blaw-Knox, an Ingersoll-Rand company, selecting a paver from these classes generally boils down to projected applications and desired acquisition costs. He says that if you're going to be doing more larger-scale commercial and city work and won't be putting a high volume of tonnage through the machine, you probably can save some money up front by spec'ing a unit in the 29,000- to <35,000-pound class. "These units have less horsepower and they're lighter machines," Wiley notes. "While they will excel at commercial applications, they won't be as productive if you're targeting mainline highway jobs. If that's your priority and you're going to be running high material volumes through the machine, you should spec a heavier machine with higher horsepower since you're going to be pushing bigger trucks and heavier asphalt loads. In addition, the heavier machine will stand up a bit better to the wear and tear that highway paving and the sheer volume of the material it requires puts on the machine."
Machines in the 29,000 to <35,000-pound class with 8-foot screeds also can be hauled without permits, allowing for quick and easy transportation between jobsites, adds John Irving, vice president, Roadtec. "They can also maneuver and pave in tighter areas without sacrificing tractive effort or pushing power," he says. "This combination of traits makes them the ideal choice for parking lots, subdivisions, smaller roads and similar jobs."
Terry Sharp, solutions and services manager for Caterpillar Paving Products, adds another qualifier for pavers with 8-foot screeds: "There are new paver models which combine the horsepower and weight of the highway class units with the transport dimensions of the traditional 8-foot pavers. This combination of features provides both the productivity and durability needed for highway paving and the maneuverability and transportability required for commercial work. So for some contractors who need expanded versatility, the economics favor selecting a beefed-up 8-foot paver."
Higher speeds and smoother mats
Traditionally, there have been two different types of asphalt pavers in these classes: rubber-tire and tracked machines. Rubber-tire machines were the unit of choice for contractors concerned with putting high volumes of material down since they had higher paving speeds than pavers equipped with metal-track undercarriages. Rubber-tire machines also offered contractors a smoother ride than metal-track units – a boon during paving operations since a smooth ride translates directly to a smooth mat – and for operators when roading the machine between jobs. And because their drivetrains were not as complex as those of tracked machines’ rubber tire units were (and remain) cheaper to buy.
Metal-track machines, while not as fast or as smooth as their rubber-tired counterparts, were popular in many areas of the country because of the higher flotation and greater tractive effort they offered in soft or adverse bases. Based on these strengths and weaknesses, contractors simply selected the machine type that best suited their paving applications.
In 1996, however, the dynamic between metal tracks and rubber-tire pavers changed. That year Blaw-Knox introduced the industry’s first continuous rubber-track asphalt paver. The benefits of the new design were obvious: The new track system offered tractive effort and flotation on par with metal tracks combined with an extremely smooth ride and high travel speeds close or equal to those of rubber-tire pavers. In addition, rubber tracks give operators full counter-rotation ability and enhanced maneuverability with more available power. This allows them to easily move from joint to joint quickly, negotiate tight jobsites or road the machine to a nearby job instead of trailering it for transport.
Rubber-track pavers have grown in popularity every year since 1996. Today they outsell metal-track machines and rival rubber-tire units in terms of market share. But while rubber tracks appear to be the wave of the future, Bob Nittinger, product specialist, Dynapac Pavers, says there are still asphalt paving applications where metaltracks remain the undercarriage of choice. “If you’re in a spongy area,” he says, “an area that you shouldn’t be in, quite frankly, metal tracks and the extra tractive effort they offer can be a huge benefit.”
But rubber-track systems aren’t perfect. Among their current drawbacks are higher acquisition costs (compared to rubber-tire and metal-track machines) and track durability issues. On the positive side of the ledger, rubber-track units are proving to be easier and cheaper to maintain than metal-track machines, and the smoother, faster ride they provide means contractors can offset their higher initial costs relatively quickly by immediately achieving higher paving volumes with their new machines.
If trucks are properly timed, experienced crews and operators can rival the production of competitors using material transfer systems.
Track life steadily improving, but still a major cost
“The challenge facing rubber tracks’ acceptance in the paving industry has been the life of the tracks,” says Bill Rieken, paver application specialist, Terex Cedarapids. “That’s where every manufacturer has struggled because the longer we can make the track last, the lower that machine’s operating costs are going to be.”
“A rubber track machine initially is a greater cost than metal tracks,” says Steve McClellan, inside sales manager, Vogele America. “But over a period of time rubber tracks don’t inflict the same degree of wear and tear on undercarriage components that steel tracks do. And because they’re more productive machines, you should be able to counterbalance any additional initial costs of the rubber track with extra production and offset those costs.”
Rieken claims OEMs have gotten a good handle on track life, thanks to advances in undercarriage designs, improved tensioning circuits and proprietary features that help increase durability in the field. “But most of the credit for improved track life goes back to Bridgestone,” he says. “We now have a track that we expect to deliver four seasons or 4,000 hours of operation before it has to be replaced. And that is an important improvement because tracks aren’t cheap. Some contractors hear they can only expect 1,200 to 1,500 hours out of the tracks on some machines, and that’s a big concern to them. If they’re going to continue to use rubber-track machines, then extending the life of the track is a big item and a big priority for manufacturers.”
Operators also need to exercise caution when loading or offloading a paver from a trailer. “We had a contractor who was taking a paver down off a lowboy when it slipped off the ramp and slashed a track on the trailer, Nittinger notes. “Admittedly this was a freak accident, but it does highlight a downside to rubber tracks: You would not have that problem with a steel track.”
Sharp points out that advances in undercarriage and belt design have minimized many of the early concerns about rubber belt life durability. “Caterpillar has over twenty years experience producing rubber belt track undercarriages, starting with agricultural tractors and moving on to earthmovers and eventually asphalt pavers in 1997,” he notes. “Our experience shows that belt life is most adversely affected by the weight of large hopper inserts, by paving over milled surfaces, and by excessive maneuvering in commercial applications. A good track tensioning system eliminates the issue of tracks slipping off during turns.”
Still, says Wiley, the beauty of the rubber-track paver is that you can take on some new types of jobs that you weren’t able to do before. “If you’re worried about questionable bases – say Wal-Mart wants a parking lot paved in the middle of December – a rubber track machine can handle it and not tear up a soft or virgin base,” Wiley notes. “At the same time, you’ll still have the flexibility with the paving speed and the smooth ride to handle highway jobs and deliver a smoother mat to boot.”
Continuous paving a reality
The recent introduction of material transfer devices and vehicles has changed paving dynamics. The old “blow and go” days, followed by an idle paver waiting for fresh trucks may be fading into the sunset. These material transfer systems, advocates say, can allow near-continuous paving and a smoother final product.
“You can get real high production with a continuous paving process,” Rieken observes. “When you dump material directly into the paver you get a lot of production loss from that truck exchange. A transfer device or vehicle dramatically improves truck exchanges because a truck is emptied much quicker than if you’re dumping directly into a paver. And when that truck pulls away empty you have a full hopper rather than one that’s almost empty. So a transfer system allows American contractors to approach the mat quality European contractors get, while maintaining the high paving speeds that allow them to make more money.”
And because a paver stops and starts less, McClellan says material transfer systems can reduce or eliminate joints in the mat created when a machine takes a break.
At the same time, Rieken notes that ride quality and material density standards are becoming stricter across the country. “A transfer system will help you meet those tougher specs without sacrificing production,” he says.
Another nice benefit of a material transfer system is it eliminates the need for trucks backing into the paver. When a truck hits a paver and drops its load into the hopper, it has a tendency to push the screed down into the mat, scarring it. “They also eliminate asphalt spills in front of the paver,” Nittinger adds. “And an asphalt spill can stop a paving operation in its tracks.”
Vehicle versus device
Material transfer devices attach to and are guided by the paver. These machines must either windrow the material or are integrated with a truck dump. “With a windrow pickup attachment a large amount of surge capacity can be maintained ahead of the paver, allowing the paver to run at a constant speed,” Irvine explains. There is a disadvantage to this approach though. Irvine says the windrowed material can segregate and cool. “Also,” he says, “transfer devices with truck dump hoppers only provide minimal remixing and still have to deal with truck dumping or engaging with trucks.”
Another type of material transfer device is an integrated paver with a truck dump. This machine is attached to the paver, but has a truck-dump hopper for receiving the hot mix asphalt. It can enhance productivity, but Irvine warns that it carries many of the same disadvantages that using a paver alone does. “Even with shock-absorbing push rollers, you can still have trucks bumping into the paver and subsequent screed settling,” he points out.
The ultimate material transfer system is the material transfer vehicle – a separate, dedicated machine that allows a truck to unload very quickly. These vehicles can store up to 25 tons of asphalt mix, transport it and convey it to the paver. Because the material transfer vehicle handles all aspects of the material flow, the paver is free to concentrate on laying a mat, running non-stop and completely untouched by other equipment.
Beyond the enhanced productivity, material transfer machines are also designed to provide a better asphalt mix to the paver. They are equipped with augers in the bottom of the surge bin that act as remixing screws, blending the mix and eliminating temperature and aggregate segregation that may have occurred during truck loading or dumping. A rear conveyor can then feed the mix into the paver at a rate of 600 tons per hour.
According to Rieken, the main disadvantage to material transfer systems is cost. “The transfer vehicles in the marketplace right now have a very high cost of operation,” he says. “Material transfer devices have lower costs of operation, particularly modes that attach to the paver itself. And there is a learning curve with both material transfer vehicles and devices as crews adjust to a new paving method. They’ll have to rethink the way they stage trucks and manage equipment in the paving train. All of these machines and systems can fall short of their potential productivity goals if the proper training isn’t there.”