Machine Matters: Smooth movers

When it comes to asphalt paving screeds most contractors think if it isn’t broke, don’t fix it. But with uncertain asphalt prices, changes in roadbed specs and more bonuses being offered for smoothness, now may be a good time to examine all the choices.

Screeds are categorized by extension type: fixed (bolt-on) extensions, or hydraulically extendable. Fixed extension screeds are used primarily on big highway jobs with wide lanes and long pulls. They are mechanically less complicated and less expensive than hydraulically extendable designs, but weigh less. “As the name implies, they pave at a fixed width, so it takes longer to change paving widths than with extendible screeds,” says Bill Rieken, paver application specialist for Terex Roadbuilding.

Hydraulically extendable screeds make it easier to change widths and are heavier than fixed-width screeds. They’re popular in applications where you have to maneuver around obstacles or change widths frequently. The hydraulic category is further broken down into those with extensions ahead of the main screed and those with extensions to the rear of the main screed.

Commercial class growing
The definition of what constitutes a commercial paving application is growing as heavier pavers enter the commercial market, Rieken says. Commercial paving can be divided into two groups: driveways and small parking lots; and larger parking lots, subdivisions, city streets and county roads.

For commercial class pavers with hydraulic extensions, many contractors prefer the front-mounted screed design, says Mike Lee, with LeeBoy’s research and development department. Both front and rear extension screeds are proven designs and do a good job, but their differences require different paving techniques, he says.

Both types are fine when you’re moving the extensions out, but with rear-mounted extensions you can trap asphalt between the end gate and the main screed, says Eric Baker at Roadtec. “When bringing the extensions in on a rear-mounted screed you have to do it slowly and use up the mix that’s trapped in there. With a front-mounted screed you either have a pre-strike off plate between the two extensions, or they’re narrower so you don’t trap as much material and you can bring them in faster.”

“The material stays more live on the front-mounted extension screed,” says John Sunkenberg, road industry manager, Volvo Construction Equipment. “A rear-mounted extension screed tends to hold material in that back pocket a little longer and can have a tendency to get cold.”

When it comes to short paving runs, the front-mounted screed extensions require less clean up work, says Bob Batty, senior engineer for pavers at Caterpillar. You don’t have that extra pile of unused asphalt that has to be raked into the mat that you have with a rear-mounted screed extension. “If you’re paving on the Interstate it really doesn’t matter. But if you’re doing parking lots and picking it up all the time, the front-mounted extension screeds eliminate a lot of handwork at the end, so you’re cutting time out of the job,” he says.

“The biggest knock on rear-mounted extensions is controlling that head of material when you’re pulling the extensions back in,” says Tom Chastain, product manager, pavers and planers for Dynapac. “That’s why every manufacturer runs sonics on the end plates – to control that head of material.”

Rear-mount extensions heavier
Rear-mounted extensions are typically the same depth as the main screed, says Baker. This gives you more even compaction and density and makes it easier to pave at wider widths because of the natural flow of the material to the ends of the extensions. Rear-mounted extensions will also be a bit heavier than front mounted, since there is more room for bigger, beefier components.

Having identical main and extension widths, the rear-mounted extension screed also offers a more uniform mat texture which can be critical in certain applications. “With commercial applications such as larger parking lots, bonuses can be tied the aesthetic appearance of the mat, so contractors will use the rear-mount extension screed,” says Rieken.

As a general rule, rear-mounted extensions are the screed of choice in mainline and highway paving, since in these types of jobs uniform density is more critical and the lane width is fixed. “If you’re a contractor doing a lot of high quality work, and getting smoothness bonuses, then you’ll be using a rear-mounted extension screed with vibration,” Batty says.

That’s not to say that mainline paving contractors avoid front-mounted extensions. “A lot of contractors buy a machine and pave mainline for a month, then go back to do two weeks of custom or commercial work,” Sunkenberg says. In most of those cases they’ll choose the front-mounted extensions so that they have that convenience available for the commercial work.

Rieken argues that contractors who prefer rear-mounted extensions will use them in both mainline and large commercial applications and develop methods to minimize the time it takes to change widths.

Electric heat
There’s no question more manufacturers are moving to electric heat and away from propane and diesel heat for screeds. Electric screed heaters give a more uniform heat, eliminating hot spots that can warp the screed. They also offer smoke-free start ups, Rieken says. You need about one kilowatt of electricity for every foot of screed length. Some commercial pavers run 16-watt screed heaters, but most run about 34 watts.

With screeds more than 30 feet wide diesel heat is still appropriate. On the small end of the scale, propane is also still a reasonable option. “Propane is less costly initially, and has a place, especially on smaller asphalt pavers such as those used for driveways or small parking lots,” Lee says.

Screed vibration
Many screeds used in the United States offer vibration to improve the texture of the mat and add a few extra percentage points of density. Not all mixes require vibration, however, and it’s not used on every job.

“I always recommend running vibration because you have the opportunity to gain a few percentage points of compaction without adding any additional labor or equipment,” Sunkenberg says. “Most contractors have it on out on the highway. If it’s a job where density isn’t being measured or it’s fairly easy to achieve, then they may not run it. On big jobs where the rollers may have a hard time keeping up with the paver, vibration can help them achieve density sooner.”

Vibration can also help with dry mixes by bringing fines and bitumen to the surface, creating a close texture without breaking the aggregate, Batty says. “With a vibrating screed you can fine tune it to the mix,” he says.

European flair
In the 1950s the federal government decided that the best way to get the Interstate system built in a timely fashion was to pave fast and shallow. In Europe and much of the rest of the world, however, asphalt paving specs call for deeper mats and slow paving speeds, says Brodie Hutchins, general manager, at V