Asphalt compaction: a race against time

BOMAG asphalt rollers proved their outstanding performance capabilities during the full reconstruction of the 16/34 runway at Zurich International Airport. There were barely six hours available during the overnight cessation of flights to completely remove sections up to 70 metres long and 23 metres wide, to lay pipes for the runway lighting, to install a new asphalt surface and to hand over sections "swept clean" and ready for work to the airport company. A race against time - night after night.

Ueli Stalder, construction engineer from the Swiss Walo Group, hardly remembers the warm summer days of 2008 as he was predominantly working nights over the summer period. Ueli Stalder was managing one of the most spectacular construction sites in Switzerland.

Precise: Perfect teamwork under cramped conditions.

23:30, engines are warming up.

The aim was to completely remove a 23 metre wide, 3,700 metre long central concrete strip during night-time work, taking it in sections of up to 70 metres at a time. At the same time the runway lighting including power supply was renewed, new asphalt laid and the renovated sections handed back each morning to Unique, the airport's operating company, on time and in a condition to start work immediately. On time meant 5.30 as airport operations could not be interrupted. "Sometimes the plane that was scheduled to land at 6.00 was already in the air when we started to remove the concrete surface at 23.30." Any overrun would incur contract penalties: 25,000 Swiss Franks for each quarter hour started.

The clock began ticking after each start. Every night on weekdays the machine convoy, including ten pivot steered BOMAG asphalt rollers - types BW 174 and BW 184 equipped with BOMAG ASPHALT MANAGER - started up. And while part of the crew was still working on cordoning off the site, excavators with special grab buckets were already removing a concrete layer up to 35 centimetres thick. On some nights up to 1,000 tonnes were shifted in 1.5 hours.

Ready to go: BOMAG asphalt rollers with engines running, ready to start work.

1:00, 4.5 hours to go.

The concrete had only just been removed, the subbase meticulously cleaned and the empty pipes laid for the beacon's power supply when the contractor began laying the asphalt. "Specifications were very high," remembers Ueli Stalder. The reason lay not only with his experienced and well-drilled team but in his confidence in the technology, which made it possible to reduce the original three layers of asphalt base layer to one layer (25 cm), all of which was achieved while maintaining the specified compaction of 98 percent. To achieve this, tonnes of the low temperature asphalt that had been in temporary storage at the airport were trucked in. The material was spread by dozer and compacted in about eight passes, followed by a 10 centimetre asphalt binder layer.

In addition to the exceptional compaction performance from the BOMAG rollers, the BOMAG ASPHALT MANAGER automatic measuring and control system played an indispensable role in meeting the strict quality and placing specifications in spite of the time constraints. The system works with directed oscillation which progressively adjusts the applied direction of force between vertical and horizontal. The dynamic stiffness value E_VIB is calculated in terms of MN/m² as a reference and comparative value. The acceleration transducer mounted on the front drum detects the interaction between asphalt and oscillating mass with increasing compaction. This interaction is then regulated by a rapid control circuit. The compaction force automatically adjusts to the stiffness of the material to be compacted. A central display summarises all of this in a simple colour schematic and keeps the roller driver, Michele Greco, and his colleagues continually aware of the results. This not only gives high compaction performance but also valuable time and cost benefits.

Ueli Stalder: "BOMAG ASPHALT MANAGER has completely won us over under these tough conditions." And by the way, Michele Greco is also a long-term employee at Walo. "Thanks to the early training we all received from Probst Maveg, it was easy for us to work with the roller and the BOMAG ASPHALT MANAGER even at night and under such time pressure; we were able to exploit the full potential of the machine." With growing experience, the crew around Ueli Stalder was able to increase the section lengths from an initial 30 metres to over 70 metres after just a few days - due to the lack of downtime and the high steering precision of the BOMAG rollers. At peak times they were able to place up to 1,500 tonnes of asphalt in three hours.

Intelligent: Even the strictest specifications were met on time thanks to the BOMAG ASPHALT MANAGER.

4:15, 1.25 hours still to go.

At 4.15 the finishing bell sounded for the asphalt specialists from Walo and Implenia. This was the latest time to start installation of the so-called centre-line lamps (the beacons) in order to guarantee punctual hand-over of the section. In addition to this, the airport company required that the asphalt surface temperature should not exceed 80 degrees Celsius. To save time, cleaning and runway marking were carried out at the same time. By 5.30 there had to be no sign of the night work; this was when construction safety experts from Zurich Airport carried out the quality inspection, not a minute earlier or later.

The final placement of the approx. four centimetre thick chip mastic cover layer would have been impossible under the time constraints, so this was carried out after complete placement of the first two asphalt layers. For this, the newly laid asphalt binder layer was milled by four centimetres. Subsequently the surface was cleaned and binder was applied. "Even here we had to work with great precision, in spite of the time pressure," says Stalder. After all, even the layer specifications were exceptional: "The layers had to be double the strength of those on motorway projects, specifically 28 kN per square metre." This was achieved.

"It's only possible," says Ueli Stalder, "When everything is planned in minute detail and everyone knows what they're doing." Quality requirements and time constraints don't allow errors. This is why Implenia and Walo (who worked alongside a total of four other companies in the "Midnight Run" joint working project) wanted to play it safe; the preparations alone for this construction site took months. Consequently, the co-ordination of multiple, closely interlinked work phases was tried out on a test site before the first day of actual construction (as specified by the client).

5:28, 2 minutes still to go.

Thanks to the precision units from BOMAG, Ueli Stalder was able to watch the approval process as well as the landing of the first plane. It was not just that the crew had once again finished on time, Stalder also knew already, thanks to the BOMAG ASPHALT MANAGER, what the strict checks by the construction safety department at the airport would confirm: compaction target, quality and evenness achieved.

Six months and 56 million Swiss Franks later, over 52,000 cubic metres of concrete had been removed, over 81,000 tonnes of asphalt had been placed, 100,000 metres of cable conduits were laid and 13,000 metres of trenches for industrial lines were excavated. In the end, Ueli Stalder and Michele Greco were certain they had found the right partner in BOMAG, particularly for extreme challenges such as this.

About BOMAG

BOMAG is a global market leader in compaction technology. The company, situated in Boppard, Germany, belongs to the French FAYAT Group and produces compaction machines, stabilisers/recyclers, milling machines and pavers. Six branches within Germany, 11 subsidiaries worldwide and more than 500 dealers in over 120 countries guarantee global distribution and service.

Contact:

BOMAG GmbH
Hellerwald
D-56154 Boppard
E: info@bomag.com
www.bomag.com