During the reconstruction of runway 10/28, BOMAG tandem rollers with ASPHALT MANAGER provided the necessary rapid compaction of the stone mastic asphalt.
Zurich Airport is considered one of the best in the world. It regularly receives awards for achieving the highest quality standards. In 2019, the airport recorded more than 275,000 aircraft movements. The operator, Flughafen Zürich AG, has assured safe, attractive and punctual air traffic for many decades through a variety of structural measures. The existing facilities have to be optimized and maintained on an ongoing basis.
Runway 10/28, which runs in a west-east direction, was already commissioned when the airport opened in 1948. In the 1980s, the heavily used central strip had to be renewed for the first time. In 2022, the runway's entire length of 2,500 m was once again due for refurbishment. As before, only the concrete of the central strip, with a width of around 22 m, was replaced by a three-layer surface. At the same time, the outdated halogen technology was replaced by a new intelligent LED lighting system.
Around 19,000 m3 of concrete, 4,500 m3 of cement stabilization and 4,000 m3 of gravel had to be removed here and then replaced by a three-layer asphalt surface. At its peak, a removal rate of over 250 m3/h was required. And just as much material had to be delivered, installed and perfectly compacted in the same night. Because by 7 a.m. at the latest, the first aircraft would be taxiing to take off from Zurich on runway 28. Flight operations were not to be disrupted.
Delays were unacceptable. There was a threat of high penalties if the refurbished section was not optimally compacted and load-bearing and the runway could not be released in time. But here, we could count on our experience and the tandem rollers from BOMAG.
ARGE Midnightforce was contracted to carry out the central strip reconstruction of runway 10/28. The work was scheduled to be carried out in 62 night stages between April 2022 and July 2022 outside the airport’s operating hours. An ambitious plan – but the weather played along.
The services required for the civil engineering and lighting systems were provided by companies of the Midnightforce consortium: Walo Bertschinger AG, Implenia Schweiz AG, Eberhard Bau AG, Marti AG, Bauunternehmung and Letech AG. Up to 100 specialists and around 90 construction machines, including 8 pivot-steered tandem rollers from BOMAG, were in action every night. Under the leadership of Walo Bertschinger AG, those responsible built on the experience they had already gained in 2014 with the reconstruction of runway 14/32.
Work began at around 11:30 pm. First, noise barriers were erected. Then two large excavators began demolishing the old concrete slabs. At about 1 a.m., another 8 cm of subgrade had to be milled off. Compared to the old concrete pavement, the new asphalt structure, consisting of a 28 cm base course, 6 cm binder course and 4 cm top layer course, was 8 cm thicker. Afterwards, the pipes, cables and connections for the runway lighting had to be laid. Only then could the asphalt paving and subsequent compaction begin with the BOMAG tandem rollers, which were optimally equipped for the job. And so it went every night.
At Zurich Airport, warm mix asphalt was used for both the base and binder course. The asphalt mix was pre-mixed at temperatures between 120 °C and 140 °C and initially stored in a specially built paving hall on the airside. The base course was then laid at Zurich Airport using a crawler.
Using warm mix asphalt takes the time pressure out of construction work: Thanks to the approximately 20 °C to 30 °C cooler paving temperature compared to standard asphalt, warm mix asphalt has a faster load-bearing capacity. This means the stretch can be released for traffic much sooner. Standard mixes would not cool down quickly enough with the thick paving – in this case up to 28 cm in one layer.
Moreover, the use of warm mix asphalt offers other decisive advantages: lower energy consumption and thus reduced CO2 emissions during production, as well as less exposure for the paving crew to fumes from the bitumen. Working conditions on construction sites are significantly improved by the use of warm mix asphalt and the lower exposure levels.
The aspect of working conditions on construction sites and the lower exposures due to the use of arm mix asphalt is all the more remarkable because a new, stricter maximum allowable concentration of only 1.5 mg/m³ will apply in Germany as early as 2025.
A special paving scheme was implemented in Zurich, as is usual for many projects of this kind. The paving was staggered: Initially, only the base course (28 cm) and binder course (10 cm) were laid. Both layers were paved with warm mix asphalt. After about 10-15 night stages, it was then the turn of the top layer. The cold planers first milled off another 4 cm of the binder course. The milled surface then had to be cleaned with sweepers. Next, bitumen emulsion was sprayed on with a ramp sprayer.
The paving of the top layer could often not be started before 2 a.m., but then things progressed. The paving team managed at least 120 m, sometimes even 300 m per night. The mix used for the top layer at Zurich Airport was stone mastic asphalt SMA 11 without chippings. By the end of the project, around 65,000 m2 of airport surface had been renewed.
On one such night, at around 2:15 a.m., a total of 8 heavy, pivot-steered BOMAG tandem rollers arrived on the site. The BOMAG machines (BW 174 AP-4 AM HYBRID and BW 174 AP-4 AM) were equipped with the ASPHALT MANAGER automatic compaction system. ASPHALT MANAGER enables particularly fast and powerful compaction.
And that was necessary because the paving material used here for the top layer, SMA 11 (stone mastic asphalt with 11-grain size), can be described as averse to compaction. A high compaction energy needs to be applied.
Thanks to the infinitely variable compaction system, rollers with ASPHALT MANAGER can work more powerfully and gently than conventional systems. Due to the integrated measurement of the compaction parameter EVIB, the driver is also informed about compaction progress in real time. The result is maximum efficiency and quality, no pass too many or too few.
During the first two to three passes, the asphalt compaction of the top layer on runway 10/28 was initially carried out with large amplitude. The effective direction of the compaction power was initially applied vertically to the surface. Shortly before the target value of 320 MN/m2 was reached, ASPHALT MANAGER automatically and continuously adjusted the effective direction of the compaction energy applied from vertical to horizontal until the target value was achieved over the entire area. Further compaction was then carried out purely statically, with the top layer ironed. Thanks to ASPHALT MANAGER, the roller drivers at Walo Bertschinger AG and Implenia Schweiz AG only had to select one setting according to layer and thickness – the BOMAG rollers with ASPHALT MANAGER then did the rest almost entirely on their own. The right force was always applied in the right direction: as powerfully as possible and as gently as necessary. This made it possible to achieve the fastest possible compaction progress on the difficult-to-compact stone mastic asphalt.
A longitudinal evenness of sW [؉] < 1.4 was required for the top layer. The planning specifications were achieved without any problems in Zurich. The stone mastic asphalt (SMA) needed to be compacted to an average of at least 98%. This goal was also achieved in good time with the BOMAG rollers, because every morning at 6 a.m., it was clear to everyone involved: “We have to be off the surface in 30 minutes.”
From 7 a.m. onwards, the first jets took off: to Portugal, Greece, and Kosovo.