Features Roads & Paving Technology
Twice as HOT

The concept is simple enough. By placing and compacting at screed two
layers of hot mix asphalt – binder and surface – in one continuous and
simultaneous process, a more durable, bonded road surface is created.

Getting updates on hot-on-hot or two-lift paving was on everyone’s short list at ConExpo 2011, including ours. Is Canada ready to move from concept to reality?

Coming soon to a roadway near you? Dynapac’s CM2500 Compactasphalt hot-on-hot paving system will be running trials in Ontario later this summer. Shown here is the CM3000 larger unit, with (from right to left): MF 300 mobile feeder, binder or intermediate layer hopper, CM3000 module with surface binder (being filled), and paving unit with both binder and wear layer screeds.

The concept is simple enough. By placing and compacting at screed two layers of hot mix asphalt – binder and surface – in one continuous and simultaneous process, a more durable, bonded road surface is created. This concept is now reality in parts of Europe, where use may not yet be widespread, but is growing. Now agencies on this side of the Atlantic are looking at it with more interest, thanks in part to some very encouraging test results from four-year-old trials in the southern United States. Aggregates & Roadbuilding readers can even look forward to at least one Canadian trial later this summer. Here’s a recap of what’s available, and why we should be taking a closer look.

The systems
Currently the focus is on two players with their own ideas of what makes the right hot-on-hot paving train: Dynapac, which is part of the Atlas Copco Group, and Vögele of the Wirtgen Group.

Dynapac: The company’s Compactasphalt modular line-up boasts two different models – the CM3000 and new, smaller CM2500. In both cases the system starts with a mobile feeder capable of feeding both binder and wear material bins, followed by a large binder hopper (31 tons in the CM2500), the Compactasphalt module with a smaller wear binder (17 tons on the CM2500), a standard Dynapac paver that can be disconnected for use in conventional work, and a pair of synchronized screeds, the first a high-compaction screed for the binder, the second designed to create the multi-layer interlock. Final compaction follows immediately.

Vögele: The paving supplier’s InLine Pave process uses machinery that can also be used for conventional work, and differs from Dynapac’s approach in that its final wear layer paver travels on the semi-compacted binder layer. The train comprises three machines: an adjustable MT 1000-1 mobile feeder, a SUPER 2100-2 IP paver for placing the binder course, and a SUPER 1600-2 for paving the wear surface. In between the binder and wear pavers sits the AB 600-2 TP2 Plus binder screed, fitted with two pressure bars. Binder material is compacted to a high enough density that the final paver can travel on the binder ahead of final compaction by conventional rollers.

Visit www.rocktoroad.com for videos of both systems in action.

Pros and cons
Proven technology is readily available from competing suppliers, but why should the Canadian roadbuilding professional spend time and energy scoping it out? Possible advantages include reducing the volume of costly wear material required (by up to 50%), elimination of the tack coat used between two cold layers in conventional paving, the possibility of reducing paving time (and thus traffic interruptions), and the ability to create a more efficient process over time (roll both layers at once, a single crew and a single move in and move out, etc…). For the road system owner – and in Canada that would be you and me – the biggest possible gain is in increased durability and lower life cycle costs.

The latter appears to be more than just wishful thinking or clever marketing, at least according to the National Center for Asphalt Technology (NCAT) down in Auburn, AL. The NCAT test track is a 2.7-km loop divided into 46 specific 61-metre test sections, each dedicated to individual test projects (a PDF 10-yr report of the various tests and results is available at www.rocktoroad.com ). The loop is then treated to 16-hr a day, five-day a week heavy-truck traffic for a real world accelerated wear test. Since 2006, one of those sections has been a two-lift roadway paved by an earlier Dynapac hot-on-hot system.  Despite over 30 years’ worth of typical rural interstate traffic (20 million equivalent single-axle loads – ESALs), results are still encouraging. Here’s an extract from the 10-yr. report.

“This section has a 9.5 mm nominal maximum aggregate size (NMAS) OGFC surface layer on top of a 12.5 mm NMAS OGFC layer. Both OGFC layers were placed with a special paver built specifically to simultaneously place two HMA layers. After four years the twin-layer OGFC surface continues to be the smoothest, quietest and most effective section at eliminating water spray on the track.”

The next step
If the concept is simple, applying it in the field will likely not be, at least not initially. Attitudes about change aside, logistics will be a challenge for some, as the system consumes a lot of two different types of asphalt simultaneously, taxing your plant network, your hauling crew, and your supervisors. Yet mastering logistics is part and parcel of today’s roadbuilding sector, and for the more skilled will be manageable.

The approach must also pass muster with the various provincial transport agencies, winning the hearts, minds, and approval of a relatively conservative sector of the economy.

In both groups, expect to see some pioneers embrace this technology in the coming few years. For its part, Dynapac expects to have a full-scale trial of its Compactasphaltsystem running in the later part of this summer in Ontario. Stay tuned to Aggregates & Roadbuilding, and www.rocktoroad.com , for more.

Scott Jamieson is the editorial director of Aggregates & Roadbuilding magazine. He has over 20 years’ experience covering such fields as off-road equipment and roadbuilding.