Cutting-edge Wirtgen technology in successful operation on Interstate 81

Cold recycling – leading technology in structural road rehabilitation

One of the major north-south routes in the eastern United States of America runs across the state of Virginia with two lanes in each direction – Interstate I-81. Having been built in the late 1960s, I-81 had been in service for well over forty years. And time had not passed without leaving its mark on the heavily frequented highway. Continuously increasing traffic volumes and in particular the loads imposed by heavy-vehicle traffic had taken a heavy toll on the road pavement and caused damage to varying degrees. As a result, the pavement surface was covered with innumerable alligator cracks, wheel ruts, deformations and patches where minor repairs had been carried out. On several sections of Interstate 81, however, more and far greater evil was hiding a little bit deeper: the right-hand truck lane, which suffered the most from the high volumes of heavy-vehicle traffic, showed significant structural damage and an inadequate load-bearing capacity in its lower pavement layers. In previous years, Virginia Department of Transportation (VDOT), the building authority in charge, had been able to keep traffic up and running by carrying out extensive repair and maintenance operations at regular intervals but limited to the surface layer. In the spring of 2011, however, a 6-km long stretch of I-81 had clearly reached the end of its service life, and suitable measures that would go really deep needed to be taken as quickly as possible in the interests of traffic safety. Conclusion: a structural rehabilitation of the aged section of Interstate I-81 required more than just milling off the existing pavement and placing new base course and surface course layers.

Instead, the asphalt layers of the right-hand truck lane needed to be removed at full depth and the subgrade needed to be stabilized. The asphalt material reclaimed during this operation was to be recycled and then reused in a cost-efficient and environmentally-friendly process. As it was mostly used by passenger cars and was therefore subject to lower loads, the left-hand lane was to be rehabilitated by cold recycling to a depth of 12 cm. In this highly challenging construction project, rehabilitating the complete pavement structure involved the use of as many as three different recycling methods. All the more reason for VDOT to rely on the tried-and-tested high-performance machines from the Wirtgen Group and use powerful cold milling machines and cold recyclers, a mobile cold recycling mixing plant from Wirtgen and an entire fleet of Hamm rollers to breathe new life into the age-worn road.

Rehabilitation of the right-hand lane

In a first step, the schedule of operations specified the renewal of the right-hand truck lane over a length of 6 km. The compact dimensions of the Wirtgen Group machines used in the process caused only minimal disruptions to traffic: both the trucks and passenger cars were guided past the job site on the adjacent, left-hand lane, thus maintaining a continuous flow of traffic. In the first step of the construction project, W 210 and W 2100 large milling machines from Wirtgen reported for duty. The two workhorses removed the entire asphalt material down to a depth of 25 cm in a single pass. A big help in this truly demanding job: LEVEL PRO – the levelling system specifically designed by Wirtgen for use in cold milling machines precisely maintained the selected milling depth. At the same time, the powerful conveyor systems of the two cold milling machines kept the waiting trucks busy. The trucks transported the reclaimed granulated material to the mobile mixing plant, which had been set up in the vicinity of the construction site, for recycling and subsequent reuse. In addition, traffic conditions required the entire milling operation to be carried out during night hours – it was completed “just in time” thanks to the machines’ excellent lighting systems and backlit control panels. A Wirtgen WR 2400 cold recycler with a working width of 2.40 m was at the ready for the soil stabilizing operation to be carried out next, which would restore the subgrade to the required bearing capacity. Prior to stabilizing, a binding agent spreader spread 5% of a lime-cement mixture in the milled cut. The WR 2400 then travelled over the milled cut and binding agent. In the process, the recycler’s milling and mixing rotor mixed the existing, damaged subgrade material and pre-spread binding agent down to a depth of 30 cm, adding specified amounts of water to produce a homogeneous construction material.

At the same time, the recycler pushed a tanker truck which provided water for the mixing process via a hose connection. The WR 2400 cold recycler thus produced a perfect foundation with excellent load-bearing properties that merely required compacting by single-drum compactors from Hamm and levelling by a grader. While the WR 2400 was completing the soil stabilizing operation, the KMA 220 cold recycling mixing plant was busy recycling the reclaimed asphalt material in an “inplant” process for full reuse. The binding agents added to the recycling process included 1% of cement and 2.2% of innovative foamed bitumen. Preliminary investigations using the Wirtgen WLB 10 S laboratory plant enabled the foamed bitumen quality and, in combination with the Wirtgen WLM 30 laboratory mixer, the optimal mix proportion to be defined even prior to the start of construction. The powerful twin-shaft continuous mixer of the KMA 220 transformed the source material and the two additives into a homogeneous cold mix of high quality that was directly discharged into a waiting truck via a loading conveyor. Its mobile design enabled the mixing plant to be set up in the immediate vicinity of the construction site, which not only reduced the distance to be covered by the transport trucks, fuel consumption rates and CO2 emissions but, in the final analysis, also resulted in a significant reduction of the overall construction costs. The material recycled and upgraded by the KMA 220 mixing plant was transported back to the construction site by trucks. In the next step, asphalt pavers placed it on the stabilized subgrade as a 20-cm thick layer that was then effectively compacted by tandem rollers from Hamm. The base course built from the recycled cold mix impressed with optimal resistance to deformation, improved flexibility and excellent durability. The last operational step in the rehabilitation of the right-hand lane consisted in a 5-cm thick asphalt layer being placed on top of the base course prior to the lane being reopened for use by traffic.

Rehabilitation of the left-hand lane

Following completion of the right-hand truck lane, the second project phase was next on the agenda: rehabilitation of the left-hand traffic lane. As it had mostly been used by passenger cars in the past, it was not as severely damaged, meaning that it also required less extensive rehabilitation methods. Similar to the rehabilitation of the right-hand lane, the traffic was guided past the construction site on the adjacent lane, this time on the previously recycled right-hand lane. In a first step, the schedule of operations specified milling off the 5-cm thick surface course at a gradient of 2% over the entire length of the construction site. The W 2100 large milling machine tackled this part of the job, completing it with great ease in cooperation with the tried-and-tested LEVEL PRO levelling system. After pre-spreading 1% of pure cement, the Wirtgen 3800 CR cold recycler equipped with a 3.8-m wide integrated paving screed gained centre stage for the “in-situ” processing of the traffic lane. It is a major advantage of the “in-situ” recycling process that neither the source material nor the recycled construction material mix require any transport effort. Preceding tanker trucks delivered bitumen and water to the 3800 CR via hose connections. The cold recycler milled off and granulated the damaged asphalt layer down to a depth of 12 cm in a single pass – while the milling and mixing rotor mixed in foamed bitumen, cement and water at the same time. Foamed bitumen was produced by the 3800 CR by injecting water and compressed air into hot bitumen. The precise mix proportion was again determined in advance by the WLB 10 S laboratory plant and WLM 30 laboratory mixer. Following recycling, the 3800 CR paved and pre-compacted the re cycled mix by means of the integrated Vögele paving screed, thus producing a bituminous bound base course of superior quality even at this early stage. Final compaction was, as usual, a routine job for the tandem rollers and pneumatic-tyred rollers from Hamm. Rehabilitation of the left-hand traffic lane – and therefore the entire construction project – was completed by paving an asphalt course on top of the recycled base.

Wirtgen technology makes the difference

Ground-breaking cold recycling technologies from Wirtgen were the key to success in this showcase project: traffic on Interstate I-81 fl owed smoothly again after a short construction period – on a roadway of optimal structural integrity – on a roadway of excellent durability – on a roadway recycled in an economical, resource-efficient and environmentally friendly process.