Without exception, the crews in charge of asphalt pavers and compactors have to hustle to get their jobs done and open the roads to traffic. In response to public pressure to minimize time spent with lane closures and traffic disruption, contractors recently have increased the production rates of their asphalt paving trains.
To achieve specified densities, however, it is commonly accepted that an asphalt mat needs 10 to 14 impacts per foot from the vibratory compactor as it rolls along. So to move faster, the roller's frequency, or vibrations per minute, must increase.
“Rollers used to work at 2400 vibrations per minute, and now you have highway compactors that do 4000 vibrations per minute,” said Chuck Deahl, national accounts manager for BOMAG Americas Inc. “We must control impacts per foot, which is directly related to smoothness and density.”
Virtually all roller manufacturers in the U.S. market are removing the art from asphalt compaction and making it more of a science. BOMAG Americas, the Hamm Compaction Division (Wirtgen America), Ammann America, and Sakai all offer some version of intelligent compaction, which either automatically reduces—or permits the operator to reduce—compactive effort as the mat reaches specified density as read by the machine.
“Intelligent compaction is a system for measuring the stiffness of hot mix asphalt as you compact it, and for documenting stiffness and temperature on the go,” said Deahl. “Then we use that information for the roller to make decisions about compactive effort.”
BOMAG calls its intelligent compaction system the Asphalt Manager. In initial passes over the mat, the breakdown roller will vibrate the drum vertically on the uncompacted asphalt. As the roller continues to work and the mat approaches final density, the Asphalt Manager will “tell” the roller to move the vertical force gradually through an angle until, at final density, the drum is vibrating horizontally to the surface. Typically an intelligent compaction system only applies to one drum on a roller, Deahl said.
Hamm uses a system of two shafts with oscillating weights that rotate in the same direction around the center shaft. But the two are 180 degrees out of phase with each other, said Bruce Monical, Hamm marketing manager. The result is a rocking force that moves the drum back and forth as it rolls along. “It's similar to rolling a rolling pin out and back, out and back,” said Monical. “It doesn't hammer the asphalt vertically; it allows the loose material to gently fill the air voids.”
By virtue of the Hamm drum's physics, the roller will exert less force on the asphalt as it approaches final density. “Once the mat is compacted, the energy is taken up by the rocking of the drum; it just skims across the surface,” said Monical. By contrast, a vertical impact on densified asphalt will break aggregates on the surface, he said.
Hamm offers oscillating technology on three models of asphalt rollers and two soil compactors. The three double-drum asphalt models are a 59-inch wide drum, a 66-inch wide unit, and a 78-inch wide machine.
Ingersoll Rand automates several phases of asphalt compactor operation, but not the compactive effort based on an onboard density reading. “In our view the whole area of automated density control is under development,” said Ray Gallant, marketing manager for compaction. “We are exploring other compactor technologies that will be incorporated in the future and will further automate machine functions.”