每日吃瓜

The Golden Quadrilateral is a gigantic motorway network stretching 5,846 km. It connects the Indian metropolises of New Delhi, Chennai, Mumbai and Kolkata. The four-lane sections are being rehabilitated and extended to six lanes, and Indian construction company L and T is carrying out this work as part of a public-private partnership (PPP) project on the south-eastern section, the NH-5 near Chennai.

L and T will operate the stretch for 15 years. The company has opted for technically advanced yet cost-effective rehabilitation using cold recycling technologies from the Wirtgen Group. L and T processed a total of approximately 142,000 sq m from October 2012 to April 2013 with a WR 2400 cold recycler and Hamm 3520 and 311 compactors. In the process, the company used the resources from the extremely worn road surface to carefully create a durably solid base course that only required covering with an asphalt layer of 4 to 5 cm thick.

The Union Ministry of Transport has been expanding the national motorway network for many years. The focus is on expanding existing sections and improving the quality of the existing roadways. In this context, the National Highways Authority of India (NHAI) also investigated the option of using cold recycling. The results were impressive from the technical, organisational and ecological viewpoints, prompting the authorities to recommend the use of this innovative road construction method.

One-stop planning, construction and operation
The contract for expanding the 43 km-long section between Chennai in Tamil Nadu and Tada in Andhra Pradesh was awarded to L and T Chennai Tada Tollway (L and T CTTL). The hundred per cent subsidiary of Larsen and Toubro Infrastructure Development Projects is responsible for the planning, construction, operation and maintenance of the stretch. Because L and T CTTL will operate the motorway as a licence holder for a period of 15 years after construction, the responsible parties were very keen to use a construction method that permits long-term economical use. And precisely that is ensured by in-situ cold recycling with the WR 2400 from Wirtgen.

Cold recycling: eco-friendly and cost-effective
Arashdeep Singh, cold recycling specialist at Wirtgen India says, ?There are many reasons why this method is so cost-effective. One of which is the low requirement for construction materials. In situ recycling requires only small amounts of new aggregates or asphalt. This was a crucial advantage in rehabilitating the NH-5 as there are no quarries in the Chennai region. ?Also, only small amounts of cement, water and bitumen are needed. This yields tremendous savings in transport costs.? Singh adds, ?The energy requirement for heating is also very low due to the materials being processed cold. Only the bitumen is processed on site at a temperature of 175?C. The cost-effectiveness and environment friendliness also convinced the responsible parties at Larsen and Toubro which is not surprising, as they rank among the top ten most eco-friendly companies in the world, according to the US magazine Newsweek.

The basis: initial evaluation
The NH-5 was not the first cold recycling project for L and T. Consequently, they knew what needed to be done: a thorough survey of the road surface condition is crucial to the success of in situ recycling projects. This task was assigned to the specialists of the Indian Institute of Technology at Chennai University of Technology. They conducted a visual inspection of the section and took samples from along the entire stretch of motorway. The results were a very mixed bag. In some places, the pavement was still fit for traffic, but about 12 km displayed massive damage to the entire bituminous surface: crazing as well as block, transverse and longitudinal cracks were discovered, as were deep ruts.

30,000 vehicles each day
The cause for the heavy localised damage was readily identifiable: in 2012, some 30,000 vehicles used this stretch every day, approximately 30 per cent of which were heavy goods traffic to and from the busy ports of Chennai and Ennore. All transport to Northern India is routed from those ports via the NH-5. In the light of these baseline conditions, the planners decided on a design with 84 million Equivalent Standard Axle Load (ESALs) that can meet the most challenging demands. They also stipulated a minimum useful life of 20 years.

Recycling set in action
As soon as the formula for the cold recycling mix was finalised, it was time for the Wirtgen WR 2400 recycler to take centre stage. This machine granulates the road surface with a robust 2.40 m wide milling and mixing rotor, mixing the granulate with binding agents and other additives. In this way, it transforms a ramshackle road surface into a high-quality and hard-wearing base course in a single pass. And it achieves this with minimal energy and transport costs. Indeed, no other road construction process uses resources so economically.

The addition of the binding agent and other additives is an extremely simple task. On the NH-5, a grader in front of the recycler spread the crusher dust over the road surface. An SW 16 MC binding agent spreader from Streumaster then spread the cement uniformly across the area. Directly behind this, a powerful WR 2400 recycler rated at 420 kW (563 BHP) pushed a bitumen and water tanker ahead of it. These were coupled to the recycler with push rods. Behind the wheeled machine, various heavy-duty Hamm compactors compacted the prepared base course. In this way, the WR 2400 recycled some 4,000 sq m per day with a working depth of 26 cm and a typical speed of 7 m/min.

Precise addition of binding agent
The WR 2400 is a true all-rounder. Depending on equipment version, it can process soil or road surfaces with water, bitumen, cement or lime. ?On the NH-5, it was equipped with a foamed bitumen system. This produces foam consisting of hot bitumen, water and air in special-purpose expansion chambers. The requisite quantities of additive are precisely metered via the control system to produce foam with the specified properties. This is then injected via a spray bar directly into the milling and mixing chamber,? explains Singh.

The total quantity of foamed bitumen required is determined by microprocessors, as is the quantity of water. This may sound complicated, but it isn?t: the operator simply enters the desired quantities and the requisite parameters on a control panel in the cab. The intelligent workhorse from Wirtgen then handles everything else itself and doses the materials with utmost precision, depending on the pre-configured volume, the working depth and the feed rate.

Variable recycling width
The width of the milling and mixing rotor cannot be altered. However, the addition of water and binding agents can be varied across the width by deactivating individual nozzles in the spray bar. The 9 m wide surface of the NH-5, for example, was recycled in four lanes. To this end, the WR 2400 processed the first lane at its full working width of 2.40 m and the next two lanes at a width of 2.30 m (full working width + 10 cm overlap). On the final lane, water and foamed bitumen were only applied across a width of 2.10 m.

Compaction crucial to success
The tremendously important compaction work directly behind the cold recycler was handled on the Chennai job site by a Hamm 3520 P vibrating pad-foot compactor. The aim of this compaction process is to reduce the cavities filled with air and water, thus increase the density and load-bearing capacity of the layers. The 20T Hamm compactor was chosen because it boasts high compaction and penetration.

311 compactor- Compaction technology for India, by India
The subsequent compaction phase was carried out with a 311 compactor from Hamm. ?We?re particularly proud to be working with this compactor,? says Sanjay Bajaj, Sales Manager North, Wirtgen India. ?This compactor was developed by Wirtgen India and has been in series production at our factory in Pune since 2012.? The 311 was able to demonstrate its capabilities on the job site. And it did so admirably: all the experts were impressed by the ease of operation and the high level of compaction.

Recycling, compaction, job done
In the final pass, the moist surface was sealed with tandem and rubber-tyred rollers. ?Then we just had to clear everything away. The freshly recycled section that now serves as a new base course was subsequently re-opened to traffic  in places just a few hours after being recycled. It does not get any quicker than that,? says S Elangovan, Project Manager, L and T IDPL.

Final asphalt surface course
Last but not the least, a fleet of Wirtgen Group machines moved onto the job site following completion of the recycling work: to produce the 4 to 5 cm surface course above the recycled base course, GVR, a subcontractor of L and T IDPL, used a Super 1800-2 from V?gele together with a fleet of Hamm rollers.

Perfect service support
A team from Wirtgen India headed by service engineer Siva Kumar provided support for the job site during the entire project. He assisted and advised and helped with configuration and maintenance of the recycler and rollers. So, it is no wonder that all machines worked perfectly during the entire course of the project.

Trainers from the service team of Wirtgen India had already taken care of training the machine operators in advance. At the headquarters in Pune, the machine operators were familiarised in depth with the special characteristics of cold recycling and the operation and maintenance of the machines from Wirtgen, Hamm and Streumaster. This ensured the ideal prerequisites for a successful project.

Cold recycling: Ideal construction method for India
The quality of the new road surface, the speed of the construction work and the huge savings in fuel and construction materials impressed everybody involved in the cold recycling project. ?With minimum costs, we have created a highly durable base course that will serve for a long time,? says Elangovan. He also recognises the potential of this road construction method for many other roads on the subcontinent. ?With cold recycling, we can upgrade our infrastructure to a high technical level in an environmentally compatible manner while making a sustainable contribution to the development of India,? he concludes.

?With cold recycling, we can upgrade our infrastructure to a high technical level in an environmentally compatible manner while making a sustainable contribution to the development of India.?