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Cold in-place recycling with Wirtgen Group machines in San José

San José | California, USA

The Californian city of San José has been using cold recycling for the rehabilitation of pavement for the past ten years. The method developed by Wirtgen is stipulated in invitations to tenders for projects. The engineering contractor MCK Services, Inc. deployed a Wirtgen cold recycler during the rehabilitation of the Monterey Road. The W 380 CRi removed 42 km of damaged pavement to a depth of 10 cm, mixed the milled material with foamed bitumen and offloaded the mix to a Vögele paver. The SUPER 2000-3i then paved the cold recycling layer. This was followed by a 5 cm surface layer with mix containing a proportion of granulated rubber. Hamm rollers took over from there and compacted the new pavement.

550 kilometres of pavement in San José already rehabilitated with the cold recycling method

‘Like many other cities, we are always looking for ways to make our tasks more efficient and effective. At the same time, we make every effort to work with as little environmental impact as possible’, says Rick Scott, Deputy Director, Department of Transportation, Infrastructure Maintenance Division in San José, explaining the reasons for using the cold recycling method. Frank Farshidi, Division Manager at San José’s Department of Transportation, is also convinced that cold recycling is the way to go: ‘The method is unique. The existing asphalt package on roads is recycled in situ without having to truck it away from the site. The lead contractor can conduct the project as a rolling construction site. They recycle 100% of the old pavement, which is much appreciated on our part. Particularly because our key priorities are saving energy and sustainable, green construction. This technology fulfils our targets on both counts. Over the past seven years, we have rehabilitated almost 550 km of pavement in San José with the cold recycling method.’

On the Monterey Road project site, the engineering company MCK Services worked with a W 380 CRi cold recycler. This Wirtgen cold recycler is designed and built for milling off pavement, mixing the milled asphalt material with one or more binding agents like cement or lime, bitumen emulsion or foamed bitumen and repaving with the resulting mix in a single pass, and has a mixing performance of up to 800 t/h. Another characteristic feature of the CR series of machines is their ability to completely replace asphalt packages (Full Depth Reclamation, in short, FDR).

A common sight in San José: a W 380 CRi rehabilitating the pavement of an urban freeway to a depth of 10 cm.

Sustainable pavement rehabilitation by cold recycling with foamed bitumen

Cold recycling with foamed bitumen as a binding agent is a globally established method that is becoming increasingly attractive as a pavement rehabilitation option for municipal civil engineering departments and contractors. It permits the paving of flexible and durable base layers. These create the perfect foundation for the final asphalt layer with reduced thickness.

Foamed bitumen is produced from road-grade bitumen at approx. 175°C using state-of-the-art technology. The precisely-metered addition of the binding agent to a mineral aggregate mix was carried out in-place by microprocessor-controlled injector systems onboard the Wirtgen W 240 CR(i) and W 380 CR(i) cold recyclers. This means that the BSM mix is produced on the spot and enables projects to be conducted as a rolling construction site.

The foaming process takes place in the expansion chambers of the injection bar in the milling and mixing unit of the Wirtgen cold recycler, where air and water at a pressure of around 5 bar are injected into bitumen at a temperature of between 160ºC and 180ºC.

42 km of pavement with an area of 205,000 m² recycled and replaced

Cold recycling with pre-spread cement and pavers

When rehabilitating pavement with the cold recycling method, cold recyclers require the support of a range of other machines and equipment. In many cases, a binding agent spreader pre-spreads cement and a tank truck delivers the water required for the mixing process. This was also the case in San José. Laboratory investigations prior to the start of the project had revealed, that a combination of the binding agents foamed bitumen and cement would be required to ensure long-term stabilisation of the Monterey Road.

The mix required for the project was formulated with MCK Services’ own laboratory equipment. Each cold recycling project site is different and has its own special requirements with regard to the composition and grading curve of the asphalt granulate. The laboratory-scale WLB 10 S foamed bitumen plant from Wirtgen produces the required foamed bitumen in the same way as the CR deployed on the construction site. It enables the determination of the right proportions of water, cement, foamed bitumen and, if necessary, other additives, required to ensure the best properties of the bitumen stabilised material (BSM).

The W 380 CRi offloads the freshly produced cold recycling mix to a Vögele SUPER 2000-3i paver for immediate paving.

Vögele pavers pave the cold recycling layer

At the project site, the Wirtgen W 380 CRi cold recycler removed the asphalt package for rehabilitation together with the pre-spread Portland cement and mixed it with a precisely metered quantity of foamed bitumen. The parameters were as follows: the damaged pavement was removed to a depth of 10 cm and blended in the variable mixing chamber with the addition of 2.5% foamed performance grade PG 64-10 bitumen and 1.5% Portland cement.

The mix was then offloaded to the Vögele paver with the Wirtgen cold recycler’s belt conveyor for immediate paving. The SUPER 2000-3i is a 10-foot (3 m) class paver, which was deployed here in combination with a VR 600 paving screed. It paved the cold recycling mix with exactly the same thickness as the previously removed pavement layer.

Hamm compaction technology then took over and assured optimal evenness and surface quality.

High-tech from Hamm ensures a perfect finish

The layer was then compacted by Hamm rollers following behind the paver. Here, a type GRW 280i pneumatic tyre roller worked statically, and a team of type HD+ 120i VV HF, HD+ 110i VV HF and HD+ 90i VV HF tandem rollers worked with high-frequency vibration in both drums (as indicated by the type suffix VV HF).

Before preliminary approval of the rehabilitated section for use by traffic, MCK Services sealed the surface with a fine mist of bitumen emulsion known as a fog seal and rolled in a layer of chippings. The gentle touch of a Hamm HD 14i VT combination roller with a smooth drum and pneumatic tyres took care of the latter of these two tasks. The paving of the surface layer was performed at a later stage of the project.

The recycling method with integrated rear loading

When recycling with integrated rear loading, cement is first pre-spread where it’s needed. The milling and mixing rotor of the recycler granulates the asphalt layers. At the same time as the cement is mixed in, water, bitumen emulsion or – as in the case of San José – foamed bitumen – are sprayed into the mixing chamber by injection bars fed from connected water, emulsion or bitumen tanker trucks. Pavers lay down and pre-compact the new homogeneous construction material. Rollers then carry out the final compaction.

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‘The method save a lot of time and energy, and cuts costs, too. And we don’t have any disposal costs.’

Rick Scott, Deputy Director,
Department of Transportation, San José

Win-win situation for taxpayers and the environment

‘The method save a lot of time and energy, and cuts costs, too. We don’t have to transport truckloads of asphalt away from the site, and we can re-use the mineral aggregates we recover. And that means we have no disposal costs’, said Rick Scott, speaking for the Department of Transportation. ‘That you can get the traffic flowing again in such a short time is one of its countless benefits for road users, too.’

The method has also convinced John Moffat, Vice-President of MCK Services: ‘Cold recycling with state-of-the-art machines like the W 380 CRi saves the taxpayer a lot of money. Over the past ten years, MCK has recycled close to 2.3 million square metres of pavement. In comparison with conventional rehabilitation methods, cold recycling cuts our costs by around 40%. On this project alone, we recycled about 205,000 square metres of existing pavement and were able to re-use 100% of the material we removed.’

As cold in-place recycling significantly reduces materials logistics to and from sites, Moffat believes it will dominate the future of road rehabilitation: In San José, we needed only five trucks on the site instead of the usual thirty, so our carbon footprint was appreciably smaller.

‘We originally reckoned with a timeframe of seventy working days for the completion of the project, but it actually took us less than thirty. That also means that delays and inconvenience for road users were much less serious than they would have been with the original project timeframe.’

Richard Look, Senior Construction Inspector
Department of Transportation, San José

42-kilometre rehabilitation project in San José

The object of the project was the rehabilitation of a 42 km section of urban freeway. ‘We originally reckoned with a timeframe of seventy working days for the completion of the project’, said Richard Look, Senior Construction Inspector at the Department of Transportation, San José, ‘but it actually took us less than thirty. That also means that delays and inconvenience for road users were much less serious than they would have been with the original project timeframe. We couldn’t have wished for a better contractor for the project. MCK Services has an enormous amount of experience and expertise with cold recycling, a reliable team and top-class machines and equipment. You can actually sense the close collaboration between the contractor and their machine manufacturer, the Wirtgen Group. And that is a real advantage.’

‘When you look at an older stretch of road rehabilitated by cold recycling back in 2011, you can see that it will still be good years from now’, said Frank Farshidi, Division Manager Preventive Maintenance at San José’s Department of Transportation. ‘Even after ten years, there is no surface cracking, and we have no problems at all with the quality of the road surface. The core samples we take to check stability and long-term durability also confirm the good impression you get at first sight. In view of our positive experiences, I can only recommend other cities and communities to consider the cold recycling method.’

‘In view of our positive experiences, I can only recommend other cities and communities to consider the cold recycling method.’

Frank Farshidi, Division Manager
Department of Transportation, San José