Electric Heat Electric heat tends to disburse more evenly across the screed surface, as demonstrated in these infrared photos from Cedarapids. Even heating reduces sticking as well as warping of the screed.
- No segregation allowed
- Generating heat
- Average Paver Costs
- Basic Specs: Heaviest Track and Wheeled Pavers
- Caterpillar
- Vögele America
- Roadtec
- Cedarapids/Terex
- IR/ABG
- Marini America
- IR/Blaw-Knox
- Dynapac
The revolution that Superpave is bringing about in asphalt road design is the catalyst behind significant increases in the horsepower and weight of highway-class asphalt pavers. Finicky finish standards and compaction requirements are also forcing innovations in material-handling systems and screeds, as well as fanning the competitive fires of European paver manufacturers here in America. Environmental issues are driving a major shift in screed heating systems. Major features of most highway paver models have changed dramatically in less than five years.
As of last year, 48 states permitted or required Superpave. It's a performance-based mix-design system, taking into consideration local temperature extremes and traffic loads, to arrive at a durable, long-lasting asphalt mix that will hold up to tomorrow's traffic. Between 1998 and 2002, the number of scheduled projects designed using Superpave procedures grew three-fold—to 60 percent of all hot-mix-asphalt projects planned by states in 2002.
In addition to Superpave, new quality initiatives have touched every aspect of the paving process, but three departures from traditional design are having the greatest effect on pavers.
First, today's smoothness specifications encourage continuous-paving methods, such as use of material-transfer equipment to feed the paver with an uninterrupted stream of mix. Second, Superpave quality standards will tolerate very little segregation of the asphalt—either stone separating from binder or cooler mix from the hot stuff. And third, the need to compact Superpave within a defined temperature range is demanding higher densities closer to the screed, before the mix cools.
The point of continuous paving is to place more mix on grade without interruptions. If the paver never has to wait for material, the screed never has a chance to stop and settle into the mat. A growing roster of states is writing material-transfer vehicles (MTVs), and transfer devices that are pushed by the paver, into their highway-paving specs. As a result, paver manufacturers are selling more hopper inserts, which extend the hopper capacity. Several manufacturers have also increased the load-carrying capacity of their highway-class machines.
Paver manufacturers have also taken advantage of engine refits required to meet the Environmental Protection Agency's Tier II exhaust-emissions limits to increasing horsepower in 20-, 30- and 40-percent increments. More power is necessary, in part, to push heavier loads imposed by heaped hopper inserts and transfer devices.
Segregation is a dirty word in the Superpave vernacular because gobs of rapidly cooling mix that make it to the screed don't compact as well as hotter mix. Pavement studies prove it, infrared thermography detects it, and several states spec the MTV to prevent it. A transfer vehicle's augers blend the mix to keep the asphalt temperature uniform as it moves to the paver.
Manufacturers hoping to offer a lower-cost alternative to self-propelled MTVs have designed anti-segregation features into their pavers, and market lower-cost transfer devices or windrow pickups that are pushed by the paver. Perhaps the most aggressive paver modification is the Remix paver from Terex Roadbuilding. The design replaces conveyors in the hopper with a pair of counter-rotating augers to blend mix as it feeds the screed. Four of the Cedarapids 10-foot pavers offer Remix options.
Caterpillar has wired four independent hydraulic motors, each driving a conveyor or auger, to an electronic controller that monitors the head of material in front of the screed. The conveyors only work as hard as necessary to maintain a consistent head of material, eliminating the need for feeder gates and cutting down the amount of energy that would otherwise be wasted if conveyors and augers were allowed to work faster than the laydown rate requires.
Astec credits Roadtec's patented tilting augers with combating segregation that can occur at the center of the screed. And Dynapac mounts an independent drive at each of the outboard ends of its augers, which it claims completely eliminates centerline segregation.
Compaction studies indicate that the minimum mat temperature at which compaction density can be achieved is fairly high, so contractors on Superpave jobs are moving rollers closer to the screed. To get maximum compaction results where the mat retains its heat, the paving industry is also looking for screeds that can deliver density.
American manufacturers are responding with heavier machines, heavier screeds, and vibratory-screed choices that compact the mat. Import of European paver technology—where screeds are designed to compact very deep lifts of asphalt—has intensified. A notable portion of paver imports come to us by way of companies that are also marketing U.S.-designed pavers. Most recently, Caterpillar bought Bitelli (primarily to gain access to international paving markets, but the company took up Bitelli's North American marketing this month), and Ingersoll Rand has invested in ABG.
Wirtgen Group of Germany bought the U.S.-sourced Pro-Pav line, and is merging European paver technologies into pavers sold under the Vögele America name. Dynapac has been importing pavers for some time, and has recently expanded its line with rubber-tracked machines. And the ranks of European importers grew in the past year when Italian manufacturer, Marini, launched its paver line in North America.
European pavers tend to be heavy, and as noted, American paver models are bulking up. A distinction of pavers from overseas, which has not made significant inroads here, is the tamper-bar screed. Tamper bars are important for compacting the very thick base courses specified around the world, but they tend to slow paving speeds. Most imports offer screeds that extend to paving widths of more than 50 feet to reduce the number of seams in wide pavements.
European screed characteristics appeal to American contractors who do a lot of airport and speedway work. Those pavements are specified for great load-carrying capacity and impeccable smoothness, which calls for deep lifts and wide passes more like international contracts. But these pavers' weight is also proving effective in helping to meet density and smoothness standards on the road.
Occupational-safety issues are driving what appears to be an industry shift from diesel screed heaters to electric heat. Sensitivity to diesel fumes in the operator's environment created the fume-extraction system during the late 1990s. Since then, manufacturers have been improving electric heaters' performance and durability.
Heating elements and wiring have become more effective and durable. Larger generators heat the screeds faster than diesel heaters. Thermostats keep screed temperature constant, turning off electric elements when they're not needed and back on when the screed cools.
With more-powerful Tier II engines in place, product managers from several manufacturers agree that electric screed heat is the one technology that has had the greatest impact on paver sales over the past 24 months. They report that half of their screeds are shipped with electric heaters, and the percentage is growing.
Electrically heated screeds have begun to challenge the market dominance of diesel-heated screeds in much the same way that rubber tracks have been replacing steel tracks on machines. When you consider these market dynamics along with elemental increases in horsepower and weight, innovations to material-handling systems and screeds, and intensified competition from imports, it's easy to use the word "radical" to describe these last five years of change in highway-class asphalt pavers.
No paving contractor can afford to ignore this technological revolution. With virtually all states using Superpave, the asphalt industry has turned its communications resources to educating counties and municipalities. It's a process that will inevitably bring paver-changing road specifications to local contracts.
| Web Resources | ||
| Caterpillar www.cat.com |
Dynapac www.dynapac.com/index_main.asp |
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| IR/ABG www.ir-abg.com |
IR/Blaw-Knox www.road-development.irco.com |
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| Marini America www.mariniamerica.com |
Roadtec www.roadtec.com |
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| Terex Roadbuilding www.cedarapids.com |
Vögele America www.wirtgengroup.com |
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