Oscillation Conquers Changed Density Specs

A Columbus, OH asphalt paving and pavement maintenance firm is using exclusive Hamm oscillation compaction technology to tackle jobs that are difficult with conventional compaction.

And it's being driven by the City of Columbus' shift to density specifications on asphalt paving projects: meet the new density specs without excessive compaction and disruption to thin lifts.

Decker Construction Co. prides itself on its quality asphalt pavements, and is a recurrent winner of quality pavement awards presented by Flexible Pavements of Ohio, the state asphalt association. The firm uses four paving crews and four stone crews, on jobs that range from large driveways, tennis courts, commercial parking lots, private roadways, public streets and airport runways. And its two Hamm HD O90V oscillation rollers help make it possible.

"We have used our Hamm oscillation rollers through three seasons," says Fritz Smith, vice president, Decker Construction Co., Columbus. "We acquired them primarily because the City of Columbus changed its specifications, and went to a density spec for the surfaces of all newly constructed city streets for subdivisions and new housing. We had a tough time getting compaction with the standard roller design, so we changed to the oscillation rollers, which do a much better job."

The switch has done more than meet specs; it's enabled Decker to reduce the number of rollers on a job. "The HD O90Vs are more efficient," Smith says. "We've eliminated a roller off the roller train, and have changed our whole way of thinking about the roller issue."

What is oscillation compaction?

Hamm's unique, non-aggressive oscillation technology is changing the way contractors work in North America. The oscillation technology compacts with a gentle rocking motion, not a vertical pounding, and can be a powerful new tool for boosting asphalt paving productivity and profits.

Instead of vibrating straight up and down, the Hamm oscillatory design gives the drum a horizontal or "rocking" force which keeps the drum in contact with the mat.

Hamm oscillation technology attains very high compaction numbers with minimal passes and no shattering of aggregate. And as Decker Construction found, it often permits contractors to remove one or more rollers from a job.

"I've been here 25 years, and have watched the whole operation," says Bill Snoke, asphalt superintendent. "The Hamm rollers are our preferred weapon of choice. To reach our density spec, we usually can get it in one pass. It cut our fuel use almost a third from what we were using before. It's really made a difference. Its user friendly and you can see really well out of the cab. It's probably our most asked-for piece of equipment by the operators."

The HD O90V rollers were useful on a recent airport job, where 98 percent compaction was required on a runway. "We were reaching the spec with our double-drum rollers, but they weren't as efficient," says Jonathan Apple, project manager. "The Hamm rollers got density in fewer passes, and with a lot less fuel and water usage. We were able to run five rollers instead of six."

"With the old roller, we might have had to hit it with two to four passes," Snoke says. "With the Hamms we can cut back to one pass. They speed everything up."

The rollers have also precipitated a change in how Decker's subdivision streets are compacted. "We were running a three-wheeler up front," Snoke says. "We are now running the Hamms up front, following a meeting we went to earlier this year that pointed out that the hotter you hit it, the better it is. We've always said that, but it's hard to get that into the operators' heads."

The smoothness of the suburban mats also impressed Snoke. "You don't have bumps like you do with a three-wheeler," he says. "We were using three rollers, and now we use two. It means you can cut down on labor expenses."

Change in city specs

Columbus specifications have changed from "tons per hour per roller", Smith says. "A certain roller would have so many tons per hour that it could compact, according to their science. Then they went to a density specification, where you had to get a minimum of 92 percent of the laboratory density, up to 97 percent, but with no number-of-passes requirement. We couldn't guarantee that we could do that every time, especially as the weather got colder, in the fall of the year, when it seems we do most of our work."

That's because Decker is called by developers to quickly pave subdivision streets before the season shuts down before winter. "As people start to panic, and realize it's almost winter, we're out there getting the last few subdivisions in before the season closes up, and conditions aren't suited for compaction."

The penalty in Columbus for not meeting the spec is that the contractor must supply an extended warranty. "That's something we didn't want to have to get involved with," Smith says. "And we were on the cusp of failure all the time, right at 92 percent. But we wanted to get up higher than that, up into the middle range."

Today, the mix specification remains in flux. "The whole specification is evolving as we speak," Smith says. "They are trying to come with a spec that will give them more longevity for their streets and highways. They're experimenting — sometimes at our expense, it seems — but we're working our way through it. And the HD O90Vs help assure that we won't have a problem."

Exacerbating the situation is the Portland cement concrete (PCC) pavement lobby, which is pushing the city to look more critically at HMA paving in its borders. "The lobby claimed HMA was not being tested enough, so the city decided to do compaction testing," Smith says.

Nonetheless, the high technology of the oscillation system in its twin HD O90Vs are helping Decker meet the challenge from PCC. "They do the job," Smith says. "We're happy with them. They are good machines and they have saved us money by not having to have another extra roller on the job."

In the process of changing specifications, the city went from a single spec of concrete base with a couple of inches of asphalt, to five different kinds of potential mat designs, Smith says. "On concrete, asphalt was a single lift of an inch and a half thick, on a variable surface. Before the oscillation rollers we had a tough time because we had variable lift thicknesses. It's not that we failed; it's just that we were on the edge of failure, too close to be safe."

Thin mats need oscillation

Other design specs include composite with stone and aggregate base and 1 1/4-inch surface course. "If you roll it too many times, it just breaks over and starts to destroy the mat," Smith says. "We were just getting to the 92 or 93 percent level, and then it would go the other way. With the conventional rollers, half a pass might be too much, but with the oscillating rollers, they seem to stop compacting the mat at the right time. When it gets as hard as it needs to get, it doesn't go in the other direction.

Don't ask me why, but I've seen it, and it's remarkable. We're also getting compaction in fewer passes, which means less time on the job."

Another danger of overcompaction is crushing of the aggregate within the mat, with telltale white streaks on the mat surface. "That can happen," says Snoke, asphalt superintendent. "But what we are having more of is our edges breaking off on us as we try to roll too much out. For that we will use an 8- to 12-ton straight roller on the last 6 to 8 inches."

More than just paving

Decker was started in Columbus in the 1950s by Ed Decker. It was acquired in the 1960s by Marble Cliff Quarries, and later by Medusa Aggregates. In 1981, Roger Apple and Fritz Smith — both employed by Marble Cliff — purchased the assets from Medusa and reincorporated in the name of Decker Construction Company.

Today, Decker operates within a five-county area in central Ohio, and specializes in a remarkable number of construction niches, including asphalt paving, asphalt maintenance, cold milling, roller-compacted concrete pavement, concrete construction, and new construction related to road widening, excavation and storm sewers. The firm completes approximately 350 commercial and public projects per year.

During the peak of the season, Decker employs approximately 150, with four paving crews, four stone crews, six maintenance crews, six concrete crews and four excavation crews.

And its Greensboro Corp. subsidiary manufactures and distributes a fast-setting, innovative flowable fill granular material that can be paved on in 30 minutes, called Flash Fill. It's used for utility cut repair and has the advantage of being easily excavated at any point.

Via its Apple-Smith subsidiary, Decker sells hot mix asphalt all year around, including the dead of winter, the only such plant open during the winter. "We produce asphalt in a limited way," Smith says. "We have a small batch plant — a one-ton plant — that we use for our specialty purposes, such as our patching operation, which has long-term contracts with the local utilities to do utility cut restoration. But as a result, we are the only plant in the Midwest, I think, that is open in the wintertime. We're open 365 days a year, and it enables us to do cut restoration without having to use cold patch, and having to come back and re-excavate it later. Instead we can do a permanent patch, whenever it needs to be done."

For peak season work, Decker obtains hot mix asphalt (HMA) from a variety of local providers. "Their plants are larger and more efficient than ours," Smith says. "We lay anywhere from 300,000 to 400,000 tons per year through three or four paving crews."

 

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