There’s no question that density specifications on today’s high-production paving projects are getting harder and harder to achieve. Strict smoothness requirements, tight project timelines, variations in mix quality, changing weather conditions and various other factors can all seem to conspire against you. Your ability to overcome these obstacles and still meet the numbers can mean the difference between getting the bonus or incurring a penalty upon completion.
High-frequency vibratory rollers have emerged as an important weapon in the fight to achieve specs and still maintain the necessary production rates. In fact, they often set the pace on high-tonnage projects.
“We pave depending on the speed of the rollers,” says Rick Ricker, paving specialist at P.J. Keating Co., Lunenburg, MA, and a trainer with Team Paving Consulting. “If I’m going into battle, I want to know there’s a good chance I am going to get maximum speed out of my whole [paving] train... With the high frequency, if I’m having a good day and it’s going well, I can max out the equipment and get my production.”
Staying within the envelope
When compacting hot-mix asphalt, there is a limited temperature envelope during which the asphalt maintains a certain amount of pliability, notes Bruce Monical, marketing manager, Hamm Compaction Division, Wirtgen America. If the paver is moving fast, and the rollers can’t keep up, you lose your window of opportunity to effectively achieve density.
“It all boils down to time and temperature,” Ricker agrees. “The sooner we can get to the mix - the hotter it is - we’re going to achieve a greater density with that material.”
According to Dale Starry, director of strategic technology, Roadbuilding Division, Volvo Construction, successful paving operations require a careful balance between all of the elements of the paving process. “You want the paver and compactor to be able to process all of the mix that you can produce and deliver to the jobsite,” he states. “You don’t want an imbalance one way or the other, because it will cause you to be inefficient, less productive and less profitable.”
For example, say you have a paver that’s capable of 250 tph, but a roller that’s only capable of 100 tph. You would need three rollers to keep up with that one paver, Starry indicates. “Because of simple coordination, it may not be possible to have three rollers in the breakdown and intermediate rolling zones and achieve the air void removal before the mix has cooled,” he says.
“So ideally, what you would want is a roller with greater capability.”
This is where a high-frequency machine comes in. “You want to stay close to a certain temperature range, and a higher frequency compactor allows you to impact more often per foot traveled,” says Bill Stalzer, product manager - asphalt products, BOMAG Americas. “That allows higher production without causing a washboard because you’re trying to hurry.”
Faster rollers for fast pavers
Vibration frequency is the number of rotations/vibrations per minute of the eccentric (unbalanced weight) that can be achieved within the roller drums. “High frequency” is generally accepted as 3,600 vpm and above, though it varies depending on the manufacturer.
A roller’s vibration frequency determines how fast it can travel and still maintain the necessary impacts to achieve density. As a general rule of thumb, optimal impact spacing is viewed as 10 to 12 impacts per linear foot.
A high-frequency roller is specifically designed to achieve optimal impact spacing at higher rates of travel. For example, a 3,800-vpm roller can maintain 10 impacts per foot at 380 feet per minute (fpm), vs. 240 fpm for a 2,400-vpm model. This enables it to keep up with faster pavers and still meet density specs.
“If you have a fast paver speed, you have to have a roller that can also operate pretty fast and still maintain impact spacing,” says Terry Humphrey, training manager, Caterpillar Paving. “The high-frequency compactor can operate at a higher working speed and still maintain good mat quality.”