“It becomes a logistical challenge for the plant,” Hutchins said. “Some truckers will be picking up a 9mm or 11mm mix for the surface course, while others will be picking up a 25mm mix for the binder course. They have to communicate with the material transfer vehicle operator so he will know into which machine to place the mix, either directly into the hopper insert of the binder course paver, or the conveyor which supplies the surface course to the end paver.”

This also necessitates two plants making two different mix designs. “Two different mixes will be ‘cooked’ at the same time,” Hutchins said.

The InLine paving process also requires a great deal of compaction at the screeds of the pavers. “Both pavers with have high-compaction devices,” Hutchins told Better Roads. “These will be tamping screeds, and the first paver – the middle machine in the train – will be placing HMA with a pressure bar screed. Typically for InLine, this will involve a Super 2100-2 paver with TP-2 screed that is putting down a thicker layer of binder. Behind that, most contractors will use a somewhat smaller machine like a Super 1600-2, as it’s putting down a thinner surface course, and these machines will have a single tamping screed that can lay a mat fairly wide.”

For the U.S. paving community that typically favors relatively high-speed placement of HMA pavements, slower two-lift HMA paving is a hard sell. “Two-lift paving is an elaborate set-up, with a lot to orchestrate, in our case three machines and two different plants,” Hutchins said. “Most of the type of work going on here – milling and resurfacing – does not justify a process this elaborate. But it holds potential for placing high-performance perpetual pavements on new alignment.”

Compactasphalt from Dynapac

Contemporaneous with Vögele’s InLine Pave is Dynapac’s Compactasphalt process. This two-lift process uses a single, large paver, modified with two hoppers and two screeds. It has the advantage of using just one paver, along with a material transfer vehicle.

Dynapac’s Compactasphalt system places two lifts of asphalt, hot-on-hot, using a single, large, modified paver and two compacting screeds. Photo courtesy of Dynapac

“The Compactasphalt technology is the one and only technology officially approved in the German regulation for road construction ZTV-Asphalt-StB07 as a regular paving method,” says Roland Egervari, product manager for Compactasphalt. “The regulation forbids that a second vehicle [paver] runs on the hot binder layer.”

Dynapac has improved its two-lift system and has released a new plant, the CM2500 Compactasphalt modular system. It features enlarged hoppers and a unique drive system, the firm says.

Designed to be used with Dynapac’s DF145CS paver, the CM2500 module’s 8.37-foot width is ideal for applying the Compactasphalt method on highways up to 24-foot wide, the firm says. The Compactasphalt 2500 features an enlarged hopper for binder material with a total capacity of 28 tons. Equipped with its own power unit, the module also has a 15-ton hopper for the top layer material.

This hopper features two augers, which allow the top layer material to be carried under the electrically-heated VB5100TV-E screed. A second screed, Dynapac’s VB805TV Plus-E high compaction screed, compacts the binder layer. Both screeds are synchronized in the case of slope and cross-slope changes.

The support frame is a minimally-modified serial paver that is predominantly used for standard construction methods, and which can be converted into a modular paver, when necessary, at low cost. This guarantees a high degree of utilization.

The use of a modular paver allows for the use of paving-improving additives or low-viscosity asphalts in the binder area. Only one crew is needed to operate the modular paver.

With the Compactasphalt process, both paving screeds are coupled in succession to the frame of the modular paver. This meets the identically-defined conditions whereby it is possible to achieve a consistent thickness of the upper layer, independent of the load-bearing performance of the hot base, Dynapac says.

While significantly different from the Vögele process, the Compactasphalt 2500 – like Vögele’s InLine Pave – simultaneously places both the wearing and binder courses – hot-on-hot – in a single pass. The result is a better interlocking of the courses that increases the durability of the layers. Furthermore, the residual heat of the hot binder course significantly increases the time available for final compaction.