Jointless concrete roof with curved geometry

World First: Jointless Roof in Merseburg

The all-plastic, 100 % wood-free facing alkus is the only formwork facing that can be welded to form large forming areas. The roof of this bus terminal in Merseburg, Germany, was poured

without a single seam or joint, in Aalborg White cement.

Data & Facts

  • Project 

    Bus Terminal, Merseburg, Germany

  • Architects 

    Weiß/Schellenberg, Merseburg

  • Contractor 

    Kramer GmbH + Co KG Merseburg 

  • MEVA Systems 

    all-plastic facing alkus

  • Formwork Engineering 

    Steffen Klaus GmbH

Use of alkus-panels when concreting the roof.

760 m² Roof without Joints

Merseburg is a small town near Halle in eastern Germany, where a new bus terminal is being built. Its most conspicuous feature is a flat roof, standing on steel columns 5.88 m high, that will protect a traffic and passenger Island (86 m long and 13 m wide) from rain and snow. The architects designed the roof in concrete and shaped like an M for Merseburg. Glass and steel elements complete the structure. The concrete roof is designed in architectural concrete, to remain exposed as poured, with a seam and jointless, white concrete finish. No touching-up or remedial work is allowed.

The Tender
Tenders for architectural concrete ofte follow the exceptionally strict Standards of the professional association DBV (German Concrete Building Association). Its top category describes what concrete finish applies to buildings with special architectural designs and representative functions. The tender described a facing sheet that would be welded together without any joints or seams that might imprint on the concrete. The roof area spans 760 m². The tender documents outline exactly with what demands the facing must comply and the architects got to great lengths to specify exactly what end result is required. A few examples:
Discolourations are inacceptable, as are any prior treatment of the facing surface.

  • Discolourations are inacceptable, as are any prior treatment of the facing surface.
  • Rust and dirt marks and stains of any kind are not acceptable.
  • The surface was to be absolutely even, so that a depth gauge of < 10 mm on 4 m would be tolerated. Also, these standards were specified for the facing sheet:
    - no drill holes
    - no nail or screw holes
    - no shrinking or swelling
    - no scratches
    - no cement haze

Plywood excluded
Any wooden facing – irrespective of type, grade, coating – was excluded on technical grounds. Only the all-plastic facing alkus can be welded easily and fulfills the high requirements without any restrictions. The facing sheet can be fully welded on all sides, can be stripped after the pour and – important for economic reasons – will be re-used as a facing sheet afterwards without loss of functional value. Such a constellation of requirements on a high technical level proved to be a world first in concrete contracting.

Detailing and precision work
MEVA engineers developed a unique technical procedure with the 100 % wood-free, all-plastic facing alkus to achieve the desired pour results. First, the imprint of the roof was optimised geometrically to correlate with standard alkus sheet measurements. A main sheet measuring 3.60 x 1.35 m, complemented by a 1.35 x 1.35 m compensation, proved viable. The sheets were cut to precision by MEVA in Haiterbach and transported to the site, where they were placed on the support grid and aligned in rows with a continuous 2 mm gap between each sheet.

Fixed from below
Each sheet was fixed to a screw bracket attached to the cross beam. Inserting the screw from behind prevented any surface damages or bulging on the facing which would have made welding the sheets together difficult. A mobile 6 m tent was set up to enable welding work to be done in any weather. A specially designed apparatus was created to handle the welding gun easily and smoothly along 900 m of running joints.

Openings for columns
Openings for the steel columns had to be prepared with calibres. They were fitted in with alkus sheets that were cut to shape on site. After the joints were meticulously welded and sealed, they were smoothed down with milling cutters and straightened with special tools.

Rotation cleaner, no release agent
In order to achieve a homogenous, evenly coloured finish, the entire forming surface including the edge sheet was smoothed with a rotation cleaner and special pads. Test pours showed that the best results were achieved when using zero release agent. Based on this, site management decided to do without release agent altogether.

Forming the roof’s edges
The formwork for pouring the 36 cm edge of the roof was pre-assembled by joining 2-metre alkus sheets to form 8 to 10 m long strips. These were then set up against wooden brackets and screwed to them from behind. This delicate work was completed in a mere 10 days, during which time rebar work was well underway on the first half of the roof area. The joint between the formwork on the edge strip and roof surface was sealed using a 10 mm chamfer strip pressed into a bed of silicone sealant.

Flooding the formwork
After rebar work was completed, the entire forming area was flooded to remove any residues of dust, flying rust and other dirt. As if by chance, the flooding also provided a thermal cushion that prevented the forming surface and joints from cracking at temperatures below 0°C – nothing unusual during the winter months. This procedure would not have been possible with any other forming material. The actual concrete pour was done in a single cycle after the water had been siphoned off and the forming surface cleaned with a high-pressure washer. In addition the surface was kept clean by vacuuming it constantly.

Re-using the alkus sheets
After the concrete works were completed, the all-plastic facing sheet was cut into into small transportable sheets. The screw holes were repaired and smoothed down before the sheets were fitted into standard formwork panels without any limitation whatsoever. This re-use of the forming sheet made the use
of the alkus sheet economically feasible.

10 years of experience using the all-plastic facing alkus
Over the last decade, MEVA has built up a wealth of know-how, experience and specialist expertise when it comes to employing the all-plastic, 100 % woodfree facing alkus. This has been applied with great success in a large number of technically demanding, high-profile building projects. The bus terminal in Merseburg is a prime example of where a smooth, completely joint-free and seamless concrete finish was achieved in Aalborg White Cement – a world first.

Base made of alkus solid plastic panels.
Siliconising a joint in the alkus all-plastic facing.
Finished roof construction in curved form.
Finished concrete roof in curved form.