Back in the late 1960s-early 1970s, when production-line vibratory asphalt rollers first hit the streets, paving contractors and public works officials had a tough time buying into the theory that slower was better.
When static machines ruled the roost, faster was always better. Regardless of whether the method specs called for tandem static rollers, three wheelers or pneumatic-tired rollers, it always made good sense to roll faster — for productivity and profitability. The faster contractors got from point A to point B, the better.
Vibratory compactors changed all that. The theory behind vibration brought new terms like centrifugal force, amplitude, vibration frequency and impact spacing into the mix — terms that contractors either had a difficult time accepting or just plain didn’t bother to try to understand.
In actuality, the new technology was quite simple. It vibrates the hot mix asphalt (HMA) at the same time as it is rolled over. As was proven with soil compaction, this vibration helps to reposition the aggregate and decrease the voids, reaching density faster.
To achieve this goal, the HMA must be rolled at a ground speed that will produce somewhere between 10- to 12-impacts per foot as you roll along. (Figure A) Any slower speed may damage the mat. (Figure B) Any faster will spread the impacts out too far and will not achieve density and also may create a washboard effect depending on the mix and ambient conditions. (Figure C)
Vibratory compaction of asphalt slowly but surely became the norm.
The transition to this new procedure was not always a smooth one. The new machine technology ran head-on into new asphalt technology — unique mixes which at times behaved poorly under compaction. Mixes designed for airport runways under Federal Aviation Administration (FAA) jurisdiction, for example, can pose a problem. In order to market vibratory rollers, manufacturers had to solve these problems, sometimes with special rolling patterns — the FAA mix, for example, had to cool after placement and be rolled hundreds, even thousands, of feet behind the paver.
For whatever technical reasons, certain combinations of gradation and ambient conditions prevent compaction of the HMA within certain temperature ranges. The term “tender mix” came into being and is still used today. During this temperature range, it is not possible to meet density and smoothness specs. The problem gained media attention over the past few years as Superpave mixes gained popularity.
Other contemporary mixes have offered similar problems. These mixes typically give the contractor a temperature window in which to work. Within this window, required density and finish can be achieved. In the case of Superpave, the mix is compactible from the temperature it comes out of the paver, down to about 240 degrees, after which the tender zone begins.
Initial response from the manufacturers was to simply add on another roller or change to a wider drum roller or both. Some “fixes” included adding a specific combination of static rollers through or after the tender zone. These changes simply did not permit the contractor to produce a successful job within the budget allocated by the project. Additional machines meant additional money for both equipment and labor.
With their model SW800 and SW 850 double-drum vibratory asphalt rollers, Sakai introduced a vibration system that runs at 4,000 vpm. This meant that contractors could now increase the ground speed of its rollers, maintaining the 10- to 12-impacts per foot rule. For example, a roller vibrating at 3,000 vpm must roll at 3.41 to 2.84 mph (5.5 to 4.6 km) to achieve an impact spacing of 10- to 12-impacts per foot. (Figure D)
Increasing the vibration frequency by 10 percent to 3,300 vpm allows the operator to increase the ground speed 10 percent, thus enabling the work to be done 10 percent faster. Thus, a 10-percent gain in productivity. Increasing that figure to 4,000 vpm, results in a 33-percent gain overall. The result of placing a 4,000 vpm on a Superpave job can be seen in Figure E.
Roller drum width also is a critical factor. While a typical 12-ft. (3.7 m) wide mat requires three widths of a 66-in. (167 cm) roller to cover the surface and only two widths of an 84-in. (213 cm) roller will do the same job. (Figure F).
For more information, 302/323-0500.
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