Mitteldeutsche Flughafen was contracted to resurface the north runway and a number of taxiways and sub-contracted Kutter Construction Company, Germany’s biggest specialist in road milling, to handle the necessary planing work before paving could be carried out.
Leipzig/Halle Airport (LEJ) is a busy facility and is the hub facility for DHL Aviation in Europe. While LEJ handles a comparatively small number of passengers (around 2.61 million in 2019), it is Germany’s second busiest for cargo after Frankfurt, as well as being the fifth busiest cargo airport in Europe. It is also used extensively by military aircraft from NATO members and EU nations. The northern runway has been in operation since March 2000 with the concrete surface having deteriorated due to its handling around 400,000 aircraft over this time, including some of the largest cargo and passenger aeroplanes.
For the 3.8km-long, 60m-wide north runway and associated taxiways, around 300,000m2 of concrete surfaces, needed to be replaced. In order to preserve the cement treated base course (CTB), the owner stipulated that the removal of the concrete surface needed to be vibration-free. In addition, the new concrete covering would be installed directly onto the milled surface so the elevation needed to be kept within 10mm.
Kutter looked to its operators and 3D paving technology from Trimble to complete the project within the tight specifications.
The owner specified that the runway surface be milled with sufficient precision that it could be used as the base to support a new concrete covering. As the milling contractor, Kutter used a Trimble PCS900 3D Paving Control System linked to the milling machines to create the new surface, achieving an elevation accuracy of +/- 3mm over the entire area.
“Milling machines are the ideal solution for vibration-free removal of concrete coverings, and we achieved the precision needed using the 3D control system,” said Bernhard Fischer, head of Kutter’s Leipzig office.
Prior to 3D milling, the owner provided a digital model of the surface with defined locations of the upper edge of the new concrete pavement. By deducting the layer thickness, a dataset for the position of the milled surface was produced. This data was transferred to the Wirtgen milling machines, which would then have the necessary information to cut to the target height of the milled surface for every point.
In this instance, the 40cm-thick concrete overlay was reinforced with several layers of dowels and anchors. To minimise cutting tooth wear, the milling depth was geared to the position of the dowels and anchors.
Kutter accomplished the milling in several passes. “To hand over a completely concrete-free planum, we removed 42cm everywhere,” commented Matthias Dreer, site coordinator for the project, who coordinated deployment of the 15 large milling machines for Kutter, working over several weeks in a two-shift operation.
The final milling pass was critical for quality, because the milling machines needed to leave a textured surface at the contractually-agreed elevation. This pass milled the bottom 4-6cm of concrete and around 2cm of the underlying CTB using a standard milling drum. Using 3D-enabled milling, the CTB on the 80,000m2 of taxiways and 228,000m2 of runway presented a perfectly flat surface.
On the site, crews relied on the Trimble MT900 Machine Target with active tracking technology on the milling machines to provide a secure connection with a series of Trimble SPS930 Universal Total Stations placed strategically along the runway.
“Using the total stations, we were able to keep the milling process going smoothly, even if the site was very busy with traffic, numerous machines and trucks,” added Dreer.
A further advantage of this setup proved to be the stable communications between the total stations and the prism. With the Trimble system, each active prism device has an individual address. The total station is programmed to this precise address, which enables rapid relocation of the prism following a break in signal.
“This feature makes the ‘hot swap’ between total stations very easy. It’s an ideal setup for construction sites with several prism devices and other reflective objects,” said Norbert Mattivi, Trimble applications specialist. At Leipzig/Halle Airport, this feature proved useful when milling the runway.
Accordingly, the milling machine can be controlled with millimetre accuracy. For this, the Trimble prism uses LED lights that transmit on a particular frequency. A CCD camera in the total station recognises these actively-transmitted light waves via a filter.
“This signal is clearly defined and recognised even under adverse conditions. Dust, rain, snow or fog do not impair communications. And it worked even when the sunlight was striking the prism,” explained Mattivi.
Dreer confirmed, “Using active tracking, we were able to achieve excellent results at the airport, even under extreme wind conditions where a lot of dust was whipped up.”
During milling, a ground-based total station measured the spatial position of the prism on the cold milling machine and transmitted that data to the milling machine’s onboard computer. The onboard computer then compared the measured height with the target values.
The Trimble-enabled solution allows the machine controllers to react quickly in the event of deviations. This allows the milling machines to travel at speeds of up to 25m/min, with the system delivering high accuracy and boosting efficiency for milling large surface areas.
“Using the Trimble system, we are able to remove the runway covering so accurately that the concrete paver can install the new surface precisely on the milled surface. There is no longer any need for levelling layers. That saves our customers a lot of construction materials, and naturally costs, too,” explained Dreer. “With our large milling machines and the 2.2m milling width, we were able to realise over 7,000m2/machine per shift, on average.”
The team completed the milling project in seven weeks.
Kutter relied on a collaboration with proven system partners for the Leipzig/Halle Airport project. “We are given the target data by the clients, and then we always work with a local SITECH provider onsite,” said Dreer.
For the Leipzig/Halle Airport project, the survey team from ex-act from Zwickau worked with Kutter, in collaboration with SITECH, to handle total station calibration, installation of the components onsite and elevation monitoring behind the milling machines. SITECH consulted with the contractor and leased the right equipment for the project.
Due to preparation, the onsite management overseeing milling operations, the Trimble system and support from SITECH, Kutter was able to hand over a 300,000m2 surface at an elevation that never deviated by more than 3mm, well below the specifications from the client.