Tire pressure

March 14, 2005

Since the first load of Superpave hit the road in what seems like an eternity ago, contractors have been experimenting with various methods of laydown and compaction that will enable them to get the job done quickly and to spec.

Since the first load of Superpave hit the road in what seems like an eternity ago, contractors have been experimenting with various methods of laydown and compaction that will enable them to get the job done quickly and to spec.

Quickly—because many jurisdictions are paying big bonus money if the contractor can get on and off the job without raising the ire of the road-raging public. On spec—because these same jurisdictions are dishing out major rework and penalties for failure to get the job done right. One big penalty can put even the well-heeled asphalt man or woman in the poor house in a hurry.

Compactor manufacturers have been challenged more than ever by both these new contemporary mix designs and their contractor-customers who, in turn, are going out of their way to place them and roll them. The challenge: Any hot-mix asphalt technology that is as tough as Superpave to place and compact is doubtless tough enough to withstand more years of 18-wheelers and constant heavy traffic. The mix makers theory: The tougher the mix is to compact, the tougher it is to stand up to added compaction of heavy trucks and problems like rutting, raveling and prematurely failing before our taxpaying eyes.

Roller to the rescue

As is typical in this situation, the compactor manufacturers were asked to come to the rescue. It is doubtful that any other original equipment manufacturer (OEM) in the construction industry is faced with such chores. The primary problem with Superpave has always been the issue of the mix, once in place, becoming tender as it cools. While manufacturers have been faced with tender mix problems in the past, Superpave has definitely raised the bar on their technical expertise.

With Superpave immediately being tabbed as a time-sensitive mix, becoming more and more difficult to work with as it cooled, contractors were required to either add more equipment and labor to get the job done or seek help from the OEMs. Manufacturers went back to the drawing board and developed vibratory rollers that could roll faster and reach density at the same time. These high-frequency rollers enabled the contractor to step up his ground speed while maintaining the same number of impacts per foot that were found to meet density specs. These techniques have been proven to work well and are now quite established across North America. The new challenge is to further reduce the number of rollers and roller operators required to do the job.

Along the way, some other specific problems have cropped up. With profiling of bituminous pavements and thin-lift overlays of those pavements becoming a standard practice in some areas, another unique problem awaited an OEM solution. In many of these cases, depending on the specific mix design employed, it was discovered by core samples that the mix wasn’t necessarily being seated properly in the profiled grooves. No interlock meant that many times the pavement could delaminate under traffic and over a short period of time. Neither conventional vibratory double-drum rollers nor pneumatic tire rollers were able to consistently solve the problem.

The family approves

One innovative Washington state contractor, faced with both the Superpave issue as well as the thin lift over profiled surfaces issue, discovered a new concept roller while visiting World of Asphalt last year in Nashville. Woodworth and Co. of Tacoma is now using a unique vibratory pneumatic tire roller on a number of state DOT and other projects since purchasing the machine last fall. This 81-year-old, fourth-generation, family-run business meets a wide range of construction challenges, from underground and site work to grading and paving operations. During the work season, Tacoma-based Woodworth’s payroll swells to 180 people, including four paving crews that handle parking lots right on up to interstate projects.

“Washington is pretty much all Superpave work now,” claimed Skip Keely, general superintendent for Woodworth. Like Superpave mixes across North America, the challenges are no different in the Northwest. Since Washington also offers bonus incentives and dishes out penalties, Woodworth is always on the lookout for compaction equipment that meets density and other DOT demands fast. Profiled surfaces provide a special challenge for many paving contractors. According to Keely, “You place a thin-lift overlay on top of a profiled base on a night paving operation and you could be in for an exciting evening. Rapid mat cooling leads to tender zones that refuse compaction, so it’s imperative to get on and off quickly. What I see this new roller doing is massage the mix into the profiled grooves, something you can’t easily do with a steel drum or even static pneumatic.”

The combination of the vibration and the kneading action seats these mixes well. Woodworth and Co. is using the machine as a breakdown roller for these jobs. They follow the vibratory pneumatic with a vibratory double-drum and can meet specs in a couple of passes.

Keely is excited about his new roller and is looking forward to seeing it in action as the intermediate roller of a mainline paving train following a couple of double-drums.

“We’ve put this machine to the test in a number of different situations and it has handled them all well,” claimed Keely. “We are anxious to let it loose on more jobs in the spring.”

The new roller, a model GW750, was introduced by Sakai at World of Asphalt. Consistent with other advanced products recently brought to market by this OEM, the new combination of pneumatic tire and vibration is bringing much success to innovative contractors. The new concept employs a vibratory shaft housed within a series of specially designed rubber tires. The tires actually vibrate, with the combination of vibration and kneading effect giving both the density and finish desired.

According to Dave Brown, vice president of marketing and sales for Sakai, “Unlike any other roller type or brand in the world, this vibratory pneumatic tire roller gets consistent density throughout the thickness of the mat. This machine makes a tough mix like Superpave even more durable.”

With Superpave and other contemporary mix designs causing so many problems early on, the company’s engineers saw the new machine as a timely solution to a lot of issues that are currently causing problems on the jobsite. While many contractors employed static pneumatics in the Superpave rolling train, the pneumatics were just too slow to benefit a mix design that relied on speed to gain compaction and finish before the mat temperature dropped to the point of the infamous and tricky tender zone.

Not unlike double-drum steel vibratories, the new roller offers four different amplitude settings to suit the application. The roller can run static, like a conventional pneumatic, and offers a choice of four other outputs up to a high force equal to or greater than a 55,000-lb pneumatic. The machine itself weighs just over 20,000 lb with a rolling width of 77 in., so you can get a considerable amount of force out of a relatively small package.

With a year or so under its belt, this type of roller has found its way into a number of different rolling scenarios. Combined with a pair of 79-in. double drums on the front end and a balanced three-wheeler bringing up the rear, this four-roller train has had much success solving contemporary mix problems, with compaction and smoothness achieved before the tender zone and bonuses gained when it’s time to go home.

Another bonus with this machine: Contractors have always had problems seating a hot-mix overlay in a rough or profiled surface, with steel drums bridging the gaps. The kneading effect of the new machine combined with its vibratory and static components provides an excellent interlock between the new mix and the milled base.

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