When it comes to compaction — whether it’s soil or asphalt — many contractors rely on past experience with similar materials to set amplitude and frequency, the number of passes and roller speed. Yet, a range of factors can affect how quickly and efficiently density is achieved on a particular surface, including changes in ambient temperature or weather, moisture content in soil, mat temperature for asphalt, mix quality, experience of the roller operator, etc.
Manufacturers of large rollers have recently introduced drum and onboard control technologies designed to minimize the effect of such variables. The goal is to transform the compaction process into less of an art and more of a science.
Greater frequency plus a smart drum
High-frequency asphalt rollers have grown dramatically in popularity. Dale Starry, product manager at Ingersoll-Rand, attributes this to an increase in average paving speeds. Higher frequencies (3,800 to 4,000 vpm) enable the rollers to maintain the same impacts per foot at faster rolling speeds. This in turn speeds up the entire paving process and, in some cases, may eliminate an extra roller on the job.
One of the challenges associated with higher frequency has been the potential for increased bearing wear in the drum. “The life of the eccentric bearings that support the unbalanced weight inside the drum is reduced by the factor of the speed of rotation,” Starry explains. “If it spins at 2,500 vpm, bearing life might be 10,000 hours. If it spins at 4,000 vpm, the life might be 2,000 hours.”
In order to achieve acceptable bearing life, some manufacturers have been forced to reduce the load on the bearings by reducing amplitude. “That means that the high-frequency compactor can’t compact layers as thick as a standard compactor of five years ago,” says Starry.
Ingersoll-Rand has addressed this problem with the introduction of SMART drum technology, a system that automatically adjusts roller frequency to the amplitude setting. “In the lowest frequencies, it has the ability to have very high amplitude like we used to have,” says Starry. “But at the highest frequencies, we limit the amplitude and the machine does it without the operator’s attention.”
Currently, this feature is available on the DD-118HFA, DD-138HFA and DD-158HFA (high frequency and amplitude) models, with a fourth model to be added later this year. Other SMART technologies include a propulsion system for automatic control of rolling speed; a water system with automatic shutoff and regulation of water flow; and a start system that prevents vibration of the trailing drum on cold or hard pavement surfaces.
Another recent advancement in drum design is oscillation. Available on the Hamm HD O90V and HD O120V rollers, oscillation compaction works by “massaging” asphalt in a forward/backward motion.
“In a standard vibratory drum, you have a center-mounted eccentric shaft. That out of weight eccentric spins and causes the drum, in essence, to bounce up and down,” explains Bruce Monical, marketing manager for the Hamm Compaction Division of Wirtgen America. “Oscillation uses two eccentric shafts with the eccentrics turning in the same direction, but they are 180 degrees out of phase with each other. Instead of the drum bouncing up and down, it rocks back and forth.”
Because the drum never leaves the mat, there are no vertical shear forces that could break the aggregate, asserts Monical. “It allows the operator to compact continually without a concern of over compaction or breaking the aggregate,” he states. He also contends that asphalt temperature is less of a concern due to the elimination of the “hammering” movement of the drum.
“As the temperature starts to cool, traditionally, in North America, we’ll use either a pneumatic rubber-tire compactor or a tandem steel roller with the vibration off,” says Monical. “In the case of oscillation, you can run the tandem steel, but you can still oscillate and run those vibratory horizontal shear forces. This should be able to draw your densities up quicker because you have a vibratory effect in the asphalt.”