Roadbuilding: Sustainable techniques & materials

Increased use of both warm mix asphalt (WMA) and recycled asphalt pavement (RAP) are redefining road building practices. As Europe and Africa regional manager for Caterpillar global paving, Joshua Meyer told iC, "I think advances in road building will come from how we use and treat material to make it a higher quality, rather than from radical developments in equipment design.

"I see a significant increase in recycling, partly driven by environmental issues and partly because of the ageing state of infrastructure in the developed world and the subsequent need for rehabilitation."

As well as recycled material, Mr Meyer believes WMA could virtually replace hot mix asphalt (HMA) in the near future.

"I believe that HMA will be phased out over the next three or four years. Certainly, it appears as though this is the direction that things are heading.

"We've seen trends come and go in the past. Sometimes they've gained momentum and then faded, but with WMA I think its here to stay and the question is more about whether it's the way of the future or just a step towards the future," he said.

Vice president of engineering at Astec in the US, Malcolm Swanson said that as well as a reduction in energy consumption, WMA is also easier to compact to a specified density and is more workable than HMA.

"Other benefits for contractors using WMA include less smoke and odour," he said. "And because WMA is produced at temperatures up to 70⁰ C cooler than HMA, less heat is needed at the burner so less energy is required and less CO₂ is produced."

Mr Swanson conservatively estimates that fuel savings of between +14 and +16% are attainable through the use of WMA in comparison to HMA and these savings can been boosted further through a reduction in compaction as a result of density being achieved more easily with WMA.

"In some cases we've seen contractors take rollers off a site using WMA simply because they were not needed," he said.

Global road competency manager at Volvo Construction Equipment, Dale Starry told iC that while HMA continues to be the predominantly used material in the US, the Federal Highway Administration is now becoming interested in WMA.

"The initiative in the US is to develop WMA technologies to replace HMA," he said. "This will save energy and be kinder to the environment."

In Europe too WMA is gaining in popularity. Ammann spokesman Lukas Jenzer said, "Producing asphalt at reduced temperatures is of interest to road builders. As well as the ecological, time and cost benefits, it also has the scope to extend the paving season.

"In December we developed the asphalt areas at our headquarters in Langenthal, Switzerland using WMA produced with a Universal NG 320 asphalt mixing plant operating at a temperature of 115⁰ C. Despite seasonally cool outdoor temperatures, the paving more than met expectations," he said.

Reclaiming

A key element in reducing the environmental footprint of road building is to use RAP in the production of WMA.

Mr Meyer told iC, "I don't think we'll ever get a roadway that is 100% recycled material, but I'd like to think that we can arrive at a situation where 100% of the material we take out is reused."

Ammann's Mr Jenzer said, "We are ready for the use of more RAP in asphalt production and we are keen for contractors to adopt this technique as they can save money by using material from old roads.

"We have developed our Contimix continuous mixing plants with the goal of reducing asphalt temperatures down from 180⁰ C to 110⁰ C so that contractors can save energy and still have a material that is suited to compacting."

Ammann believes the high output Contimix plants will be attractive to the US, South American and African markets. "Customer acceptance and authority approval are the big issues at present," said Mr Jenzer. "Only 2% of the asphalt plants we sold last year were for designated for handling RAP. That said, we believe that developments in asphalt mixing plants will be a main driver in the future."

At the Bauma exhibition, Wirtgen unveiled its W210 cold milling machine, an innovative model that tackles operating costs and environmental issues in the production of RAP.

Featuring a dual engine configuration, the W210 boasts a maximum output of 500 kW, although through selective use of the two diesel engines, the company estimates fuel savings of up to -25% will be achievable over comparable single engine variants.

For milling to a depth of 40 mm or less or for moving the machine on site, the W210 uses just one of its engines, while for milling to depths of up to 330 mm, both engines and the full 500 kW of power come into play.

The 28,1 tonne machine has a standard milling width of 2 m, although drums of 1,5 or 2,2 m are available as options.

Milling specialist at Wirtgen, Bernd Holl told iC, "In developing the W210 we carried out a full cost analysis looking at the milling process and fuel costs. Through this process we identified that milling machines could be more economical and much more environmentally sensitive.

"We established that milling equipment only operates at full power for about 10% of the time. This was the driver behind the adoption of the twin engine concept," he said.

Compaction

A significant aspect of WMA is that it's easier to attain compaction density than with HMA. Vice president of Madden Contracting in Los Angeles, US, David Madden said, "The largest advantage to a contractor is that you get almost a 100% guarantee that you'll get the required density on the road."

Beyond mix characteristics, achieving the desired density quicker is an area that is playing an important role in saving time and the environment.

Marketing manager at Hamm, Gottfried Beer told iC, "With our HCQ Asphalt Navigator, the asphalt temperature is included in the evaluation as an important parameter. As such, uneccessary rolling passes are avoided and resources are saved.

"Furthermore, quality is increased because over-compaction and under-compaction are systimatically prevented."

At the Bauma exhibition Hamm unveiled its GRW 280 pneumatic-tyred roller with an operating weight of between 10 and 28 tonnes. Designed with a ballast space beneath the operator's platform, the GRW 280's intelligent ballasting system means the weight distribution between the front and rear axles always remains constant, regardless of the amount of ballast loaded.

The GRW 280 has also done away with twin sets of controls and replaced them with a single operator's seat that can be moved and pivoted along the console and towards the machine's outer edges.

"The cab with an integrated roll over protection system (ROPS) is located in the centre of the machine so the operator has an excellent view of all the relevant points," said Mr Beer. "We have replaced the hydraulic cylinders of the front axle with swing arms between the front wheels and as a result the driver can see the outer sides of all the wheels through viewing channels integrated in the frame."

Also at Bauma Dynapac unveiled its 9.5 tonne CC384HF tandem roller featuring soft starting and stopping to prevent unevenness and cracking in the asphalt layers. "This allows even an unskilled operator to start and stop without leaving marks," said product marketing manager Lars Lundahl.

The CC384HF features a swiveling operator's platform and the absence of over-the-drum water tanks in combination with the asymmetric cab means the operator has continuous visibility of the drum surfaces, sprinkler nozzles and drum edges.

Available with Dynapac's compaction analyser for asphalt (DCA-A) as an option - a compaction analyser that provides data and documentation for quality analysis - Mr Lundahl believes the CC384HF fitted with DCA-A takes the guesswork out of compaction.

Alongside the new tandem roller, Dynapac launched a new three strong range of pneumatic-tyred rollers in Munich. The CP224, CP224W and CP274 are all built on a modular basis and share many components and sub-systems with other models.

With compacting widths ranging from 1.8 m to 2.3 m and maximum operating weights of between 21 and 27 tonnes, the pneumatic-tyred rollers are ideally suited to a wide range of finishing, sealing and soil compaction applications. "The highly accurate control system and the option of DCA-A means compaction can be completed in the optimum number of passes," said Mr Lundahl.

"With the optional temperature sensor and DCA-A, the new machines can achieve the optimum density and surface texture while providing full statistics and documentation," he said.

Bomag's fourth generation 9.8 tonne BW 174 AP-4 pivot-steered tandem roller features a new ergonomic cab and operating controls based on what it calls its 'active armrest system' combined with a hand-shaped travel lever.

"The combined seat and operating unit can be turned through 270⁰ and extended sideways," said spokeswoman Iris von Kirschbaum. "The control layout allows for intuitive operation, maximum safety and reliability."

Equipped with Bomag's Ecomode, tests have demonstrated fuel savings of over +20% as the active engine management system and intelligent sensor instantly calculate the power requirements of any given situation and provide only what's needed. "An additional benefit is that this reduces noise levels in the cab as well," said Ms Kirscchbaum.

LiuGong launched its CLG612HIII hydrostatic single-drum vibratory roller at Bauma. Suited to compacting base and sub-base, the roller features dual amplitude for better compaction performance across a wide range of operating conditions, while an optional density meter instantly records the degree of compaction on-line, resulting in quicker completion with fewer interruptions.

Product engineer for the CLG612HIII, Xingyou Xie said, "We are pleased to deliver this machine to broader markets this year. It has already proved itself in China and North America and we look forward to making it available worldwide."

Recycle

While the use of RAP and WMA in association with efficient compaction machines and methods can influence the bottom line, there is still some way to go before these methods gain general acceptance.

Mr Meyer told iC, "I'd like to think the Nirvana is that 100% of RAP will be useable right through to the warm paving of the surface course.

"There's always going to be a demand for virgin material, but I hope the levels will be dictated more by expansionary road construction rather than as a result of repaving existing roads."