Superpave asphalt pavements demand faster tandem vibratory rollers. Much of the equipment innovation has focused on increasing vibration frequency so machines can effectively compact mats while rolling faster. But some manufacturers are also expanding the choice of vibration amplitudes at various frequencies. Others have challenged the basic tenants of vibration, offering double-drum vibratory models that generate horizontal shear forces by rocking a drum forward and back instead of bouncing it up and down.
Forty-eight states allow or require Superpave mix designs, and well over half of all scheduled hot-mix projects at the state level are now designed using the system. The change in how asphalt roads are specified promises to create very durable pavements, but Superpave also serves up a mysterious problem called the "tender zone." As the new mat cools behind the screed, in the temperature range from about 245 F down to about 200 F, many Superpave-designed materials stubbornly refuse to compact.
"When the mat becomes tender, further rolling with steel-wheeled equipment is counterproductive until the mat cools below the tender zone. If a rubber-tired roller is not available, nothing productive can be done until the mat cools and stiffens at around 200 F," according to an Asphalt Institute report titled, What Have We Learned About Superpave. "Until we can determine the cause and resolve the problem, a couple of strategies are being used to successfully compact a tender mix. One option is to obtain as much compaction as possible with the breakdown and intermediate roller before the mat cools to the 'tender zone.'"
Breakdown and intermediate rollers must work close behind the paver. If vibratory rollers roll too fast, though, the distance between drum impacts starts to create a washboard surface in the mat. Industry standards generally recommend that a vibratory roller deliver 10 to 14 vibration impacts per foot of travel distance.
"As long as the vibratory compactor is operated at a speed consistent with this recommendation, the smoothness of the pavement will meet most smoothness specifications established by the agency overseeing the project," says Dale Starry, product-marketing manager for Ingersoll Rand compaction equipment.
In order to hit the mat 10 to 14 times per foot at faster rolling speeds, vibration frequency must increase.
In 2002, there were less than 10 tandem roller models on 63-inch or larger drums capable of vibrating at 3,700 vibrations per minute (vpm). Today there are 14. All of Sakai's tandem vibratory compactors can reach 4,000 vpm. Bomag Americas' 84-inch Bomag and Hypac models are also 4,000-vpm machines, as is Hamm's HD 110 HV on 66-inch drums. Dynapac has high-frequency versions of its three models on 66-, 77-, and 84-inch drums that can produce 3,800 vpm. High-frequency variations of Ingersoll Rand's large production rollers will go to 3,800 vpm, as will Caterpillar machines equipped with the optional VersaVibe system.
Of course, the goal is to compact a mat quickly, not just to vibrate faster. Amplitude, or vertical distance the drum travels, is also critical. Most high-frequency machines manage little more than about .01 inch of amplitude at their greatest frequency setting, whereas machines with vibration frequencies fixed around 2,500 vpm will deliver .04 inch of amplitude.
The most effective combination of frequency and amplitude is different for different types of asphalt mix and lift thicknesses. Ingersoll Rand's new HFA rollers offer a choice of eight amplitude settings, and the machine's SMART vibration system automatically raises vibration frequency to the highest possible value for the amplitude selected. On the DD-158HFA, for example, the top frequency in amplitude 1 is 3,400 vpm; in amplitude 5 it's 2,800 vpm; and in amplitude 8 it's 2,500 vpm. The SMART propulsion system automatically adjusts maximum rolling speed to provide the selected impact spacing in every amplitude setting for each unique vibration frequency.
Caterpillar's standard vibratory system offers five amplitude settings, but frequency is limited to 2,520 vpm. The VersaVibe option carries Cat into the high-frequency competition, and offers a choice of two amplitudes within both of the frequency settings. Cat's Automatic Speed Control allows the operator to dial in either the desired number of impacts per foot or the maximum rolling speed.
Integrating ground speed and vibration control on vibratory rollers has become a fairly common feature. Vibrating systems will stop automatically when ground speed slows to a certain pace, and restart when the machine accelerates again. Auto stop prevents the roller from compacting a divot into the surface when changing direction. Ingersoll Rand's SMART systems vary vibration frequency proportionally with ground speed so impact spacing does not change with ground speeds.
Bomag's optional Asphalt Manager, in its automatic mode, senses stiffness of the mat (based on energy reflected from the surface back into the drum) and automatically adjusts vibration energy. Like many production rollers, it measures and reports information such as effective amplitude, vibration values, and asphalt surface temperature. It's information that allows users to determine the number of passes needed to meet the density and quality requirements.
A roller's compaction-monitoring systems can be as valuable to preventing over-compaction as they are for improving production, especially with more large stone prescribed in Superpave asphalt-mix designs. Using the fine or continuously graded mixtures of the past, the rule of thumb was that lift thickness should be at least two times the maximum aggregate size. That leaves enough room in the lift for aggregate to move around during compaction. But the 2:1 ratio doesn't leave enough space for higher percentages of large stone to pack down. Too much vibration energy can fracture the aggregate in a thin lift of coarse-graded asphalt.
The Asphalt Institute recommends that coarse-graded mixtures and Stone Matrix Asphalt (SMA) be placed in lifts at least three times the nominal maximum stone size. States tend to spec thin lifts (3 inches or less) because each layer is an opportunity to improve smoothness. It's not generally a problem¡ªhigh-frequency rollers are at their most productive in thin lifts. But when SMA is specified thinner than the 3:1 ratio, contractors adjust rollers for lower amplitudes and keep an eye on the compaction meters to prevent cracking the stone.
Oscillating compaction has become one solution for gentler compaction. Hamm, Sakai and Bomag all have machines with some variation on the theme. Typical vibratory rollers generate vertical shear forces by vibrating up and down, unweighting the drum and bouncing back down in each cycle. Oscillating alternatives from Germany and Japan generate horizontal force by vibrating forward and back, keeping the roller's weight firmly on the ground at all times.
The two oscillating machines available in North America today combine technologies¡ªone traditional vibratory drum and one oscillating drum. Hamm's 20,200-pound model HDO90V sits on 66-inch-wide drums, and Sakai's 16,100-pound SW651N is a 58-inch machine. Sakai is analyzing North-American applications for another version, the SW651ND that is available in Japan, which has the ability to switch from oscillation to traditional vibration and back.
Bomag introduced Variomatic rollers a couple of years ago that had the ability to switch automatically from vertical to horizontal shear forces. But those models did not remain in the product line here. Bomag brought new, heavier models to BAUMA early this year with Variomatic II for distribution in Europe. Bomag Americas is currently testing one of those models and will soon decide whether or not to begin marketing it in North America in the spring.
The days of compacting all asphalt with one roller setup are probably gone. The Asphalt Institute recommends contractors build and evaluate test strips to be certain their equipment and rolling scheme will do the job. There's a very good chance that the many variations in today's tandem vibratory rollers will dominate Superpave projects. Depending on the makeup of the asphalt, the flexibility being built into vibratory systems should make them effective for breakdown, intermediate and final compaction.
|Average Tandem Vibratory Roller Costs|
|Operating Weight (lbs.)||List Price||Hourly Rate*|
|* Monthly ownership cost (based on list price) plus operating expenses, divided by 176 hours|
|Source: EquipmentWatch.com, phone: 800/669-3282|
|Less than 4,000||$20,197||$10|
|4,000 to 10,999||$45,654||$22|
|11,000 to 24,899||$117,293||$52|
|24,900 and over||$163,630||$68|
|High-Frequency Heavyweights (by weight)|
|Model||Operating weight (lbs.)||Drum width||Vibration freq. range (hertz)||Centrifugal force range (lbs.)||Gross power (hp)|
|* Multiply hertz times 60 to get vibrations per minute|
|This table of rollers over 8 metric tons excludes nine models with vibration frequencies of less than 50 hertz (3,000 vpm).|
|Dynapac CC322||18,300||66"||51||10,390 - 25,280||75|
|Ammann AV95E||19,200||63"||35 - 50||11,690||86|
|Hamm HD 90 HV||20,062||66"||42 - 53||14,850 - 17,100||131|
|Hamm HDO 90V||20,172||66"||42 - 50||11,925 - 17,100||127|
|Bomag BW266||20,600||66"||57 - 63||27,580 - 32,950||130|
|Hypac C766C||20,600||66"||33 - 57||17,950 - 26,375||125|
|Ammann AV95N||20,900||63"||35 - 50||11,690||86|
|Ammann AV95T||20,900||63"||25 - 50||1,570 - 11,690||86|
|Ingersoll Rand DD-90HF||21,705||66"||48 - 63||12,340 - 38,340||110|
|Bomag BW161AD-4 HF||21,826||66"||45 - 60||27,225 - 36,000||131|
|Terex TV-1700||22,047||66"||42 - 50||20,475 - 21,150||110|
|Caterpillar CB-534D Versa Vibe||22,050||67"||42 - 63||18,570 - 22,234||130|
|Hamm HD 110 HV||22,707||66"||42 - 67||27,675 - 28,800||131|
|Dynapac CC422||22,930||66"||51||15,700 - 30,960||125|
|Dynapac CC422HF||22,930||66"||50 - 63||16,630 - 26,070||125|
|Sakai SW800||22,930||67"||42 - 67||10,580 - 27,120||109|
|Bomag BW278||23,500||78"||57 - 63||30,368 - 37,099||130|
|Hypac C778B||23,500||78"||33 - 53||22,375 - 31,150||125|
|Caterpillar CB-534D Versa Vibe||24,917||79"||42 - 63||18,570 - 22,234||130|
|Ingersoll Rand DD-110HF||25,360||78"||42 - 56||15,920 - 36,650||125|
|Dynapac CC522||26,130||77"||51||15,700 - 30,960||125|
|Dynapac CC522HF||26,130||77"||50 - 63||16,630 - 26,070||125|
|Hamm HD 120 HV||27,514||78"||42 - 53||35,100 - 41,850||131|
|Sakai SW850||27,560||79"||42 - 67||13,010 - 33,290||109|
|Dynapac CC622HF||27,785||84"||51 - 64||17,310 - 31,025||125|
|Ingersoll Rand DD-125HF||28,043||84"||53||9,890 - 35,190||148|
|Bomag BW284||28,425||84"||60 - 67||34,665 - 41,235||185|
|Hypac C784||28,425||84"||57 - 67||34,665 - 41,235||185|
|Sakai SW900||28,660||84"||42 - 67||15,210 - 38,800||166|
|Ingersoll Rand DD-130HF||29,640||84"||50||8,077 - 36,800||173|
|Hamm HD 130 HV||30,644||84"||42 ¨C 53||35,100 ¨C 43,650||131|
|Ingersoll Rand DD-158HFA||33,810||84"||42 ¨C 57||37,170 ¨C 44,120||185|