The logic behind the combination roller, or "combi," can't be faulted. A vibratory steel drum, usually in the front, provides dynamic forces for achieving compaction of asphalt lifts from the bottom up. At the back, four smooth pneumatic tires knead the mat from the top down, yielding a closed texture that reduces permeability. The combination roller, since it brings together two major types of compaction technology, sometimes is cited as providing "the best of both worlds."
The only problem, however, is that the concept is not universally effective over the broad range of compaction applications.
"A single vibratory drum may not provide the compaction forces needed, especially in a highway-type paving environment," says Bruce Monical, marketing manager for the Hamm Compaction Division. "And having rubber tires only in the back is not as effective as a pure pneumatic roller, which would normally carry more weight."
According to Steve Wilson, manager of marketing services/product manager for Bomag Americas, steel and rubber each have desirable qualities: Steel removes marks in the mat and leaves a smooth surface, and rubber tires work material into depressions and indentations that the steel drum may bridge across.
But, that said, Wilson's opinion is that contractors can get "the best of both worlds" from a combination roller only on a small scale. The machine, he says, typically will not deliver the high production that many contractors need.
Dale Starry, Ingersoll Rand's product manager for compaction, agrees. For the majority of contractors, he says, the combination roller likely would be a disappointment if they think that two machines can be replaced with one. In high-production situations, says Starry, the combination simply can't achieve the same level of productivity as separate double-drum and full-pneumatic machines.
If you haven't yet flipped to another page in the magazine, thinking that the combination roller isn't for you, at least hear the remainder of Starry's thought:
"But for some contractors who appreciate the machine's versatility in certain applications," says Starry, "the combination roller can be very popular."
According to Hamm's Monical, those applications may include patching and small projects that don't require high production.
"Alley ways, residential streets, cul-de-sacs, parking lots, patching main roads and repairing utility cuts are applications where these machines come into play," says Monical. "In these applications, the machine provides all the capability needed to achieve maximum compaction results, without having multiple machines on site."
Wilson notes another situation in which the combination roller may be of value:
"If job specifications call for a rubber-tired machine on the job," he says, "the combination can often fulfill that requirement. If so, then the contractor need not have a full pneumatic on site. But on large projects, where the requirement might be for a 20-plus-ton machine, the inspector may not be so easily satisfied."
Also, says Starry, contractors who do a fair amount of work on steeper slopes may sometimes select the combination, because its four driving rubber tires provide more tractive effort than does a steel drum.
"Where traction and gradeability are of importance to the contractor," he says, "the combination performs better than a double-drum vibratory."
Chip sealing is yet another application in which some contractors and governmental agencies have tried the combination, says Starry. The reason, he says, is that the rear tires can "marry" to the contour of the existing surface, which typically is not uniform from edge-to-edge.
And one final thought about combination-roller applications. At least one manufacturer says its machines can be used to compact granular or mixed soils, crushed aggregates and cement-stabilized materials. If that's true, then the versatility of the combination further expands. Potential concerns, however, would include cutting or abrading the smooth tires, thus subsequently marking asphalt mats, and whether the smooth tires would provide enough traction, compared to the dedicated soil compactor's tractor-type tires.
When the concept of the combination roller originated in Western Europe, says Starry, manufacturers of the day typically used only two smooth tires at the back. Although the tires applied forces similar to those of a full pneumatic, he says, the machine frame separated the tires so widely, that achieving uniform coverage required an excessive number of side-by-side passes.
Today's combination-roller manufacturers address that deficiency, says Starry, by spacing the machine's four tires closer together. Two side-by-side passes with a combination normally achieve the same coverage as one pass with a full pneumatic, which has rear tires that track between the front tires.
The basic design of the combination roller is essentially that of a double-drum vibratory roller that has had its rear drum removed and substituted by four pneumatic tires. (Ammann offers its AV/H Series machines as combinations with the wheels—two driven pairs—installed either at the front or back of the machine.)
Most combinations use a separate tank and spray system (in addition to the drum's water spray system) for applying release agents to the tires. (But never fill that tank with diesel fuel, say manufacturers. Diesel fuel degrades both the mat and the tires.) Usually, if the tires are sprayed with a release agent when paving begins, material pickup is prevented until the tires get hot. Once the tires are at mat temperature, further application of the release agent usually is not required. But that advice is not universal.
"If the tires are seasoned, and you get them clean and warm," says Bomag's Wilson, "then the initial application of emulsion usually will keep them from picking up the more conventional mixes. But if you get into specialized mixes—polymers and rubberized asphalt, for instance—that tend to be sticky, then you may need a constant spray."
Also, according to Dynapac's Tim Springer, some manufacturers offer optional shields that can be installed around the tires to more effectively retain heat, thus producing more uniform temperatures and more uniform compaction. Routine maintenance for the combination is essentially the same as for double-drum models, says Springer, except that the tires should be checked frequently for cleanliness and for damage that could mark the mat. And, of course, he says, proper and uniform tire inflation is critical.
Likewise, the criteria for selecting a combination roller are similar to those for choosing a particular double-drum model.
Matching performance specifications—such as weight, rolling width, vibration frequency, amplitude and total centrifugal force—to satisfy the majority of applications is a key factor in selecting a machine, says Vibromax America's Tom Meyer. But equally important, he says, is choosing a machine with ample operator comfort and convenience features. Actually, the selection of combination rollers is quite extensive. We counted 38 distinct models (from eight manufacturers) that range in operating weight from 3,527 to 46,350 pounds, and in drum width from 35 to 84 inches.
As far as operating a combination effectively, your best advice is to consult the manufacturer and explain your specific applications. Generally, most say to lead with the vibratory drum for initial breakdown, and to finish the rolling process with the drum to remove tire marks. But, says Dynapac's Springer, mix design may sometimes determine how the machine is used most effectively. For example, he says, the steel drum may best be utilized leading into harsh mixes, while the pneumatic tires may be best used leading into more tender mixes.