The idea of vibrating soil or asphalt to increase its density is almost as old as diesel-powered construction. But on certain of its compaction machines Hamm has engineered a different approach. Instead of pounding the surface with short, high intensity up-and-down movements of the drum, Hamm has developed an oscillatory system that vibrates the drum in the horizontal plane.
“In an oscillation roller you have eccentric weights rotating in the same direction, but they are 180 degrees out of phase,” says Bruce Monical, marketing manager at Hamm. “Instead of the drum bouncing up and down, it rocks back and forth. Instead of vertically impacting material, it massages it.”
The result is that the soil or asphalt gets compacted, but with some key differences.
“For one, you never overcompact,” says Monical. “One of the dangers of asphalt when it starts to cool is the aggregate doesn’t move. If you are vibrating it with an up-and-down motion you can break your rock and create an air void, which later becomes a pothole.” But the back-and-forth oscillation will continue to increase the density, without fracturing the aggregate, regardless of temperature. “We’ve actually had density go up in asphalt mixes as cool as 110 degrees,” he says.
This kinder, gentler compaction action has proven useful in several applications. Asphalt joints and seams are a big beneficiary, Monical says. “With a traditional vibratory roller, you can’t vibrate the joint,” he says, “because you have to put a little bit of the drum on the cold side.” As a result, many road design standards have different, lower density specs for the joints – a compromise you need not make with oscillating vibration.
Concrete bridge decks also stand to gain, Monical says. “Before oscillating compaction you could never vibrate asphalt on a concrete bridge deck because that would drive those vertical impact forces into the concrete and break the bridge.” The same holds true for compacting asphalt or soil over old water or sewer lines, or alongside buildings where the foundation may suffer if it were vibrated too aggressively.
Hamm offers the oscillating technology on three sizes of asphalt compactors: 59 inch, 66 inch and 78 inch. It’s also offered on two sizes of soil compactors: 66 inch and 84 inch. On the asphalt machines the rear drum is the oscillating drum and the front drum is a traditional vibratory drum. “If the mix is hot and viscous you can hit it with the vibrator drum,” Monical says. “But when the material starts to cool and you want to have more control over the density, you turn the front drum off and do all the work with the back drum.” With the push of a button on the soil machines the eccentrics on the front drum can be positioned in-line for up-down vibration or out of phase for oscillation.