Micro-reinforcement of 2,000 m³ concrete with 4 tons of Sika Crackstop Fiber

The “Hamburger Deckel“ is a noise protection project of the Hanseatic City for which the A7 federal motorway north of the Elbe Tunnel is widened by two traffic lanes, and which provides comprehensive noise protection from tunnels and walls in the boroughs of Altona and Eimsbüttel. This has also a positive effect on the development of the city because of the extensive parklands and allotment gardens that will be created on the loadbearing tunnel covering and that will enable city quarters to merge. For the production of the prestressed concrete beams for the covering in front of the Altona tunnel entrance, Sika Crackstop micro-polymer-fiber from Sika Deutschland GmbH was used for meeting the requirements for fire protection.

The Hamburger Deckel consists of a series of three individual tunnels, of which the Altona Tunnel with 2,230 m is the longest. Other than was the case for the already completed tunnels (Schnelsen and Stellingen), no temporary bridges could be used here. The crossings that were erected following dismantling of the bridges will therefore already become part of the completed tunnel. The prestressed concrete beams required for this task were manufactured by Bettels Betonfertigteile GmbH in Emden, Germany, placed in temporary storage and transported to the construction site on heavy-duty trucks. No easy task for a total of 114 beams with a length of 25 m each, a width of 1.20 m and a total weight of 45 tons.

 Water-soluble paper bags facilitate application

In the search for a fire-protection fiber, Olaf Stracke of Bettels Betonfertigteile GmbH had also turned to Sika. A mandatory specification had called for approval in accordance with certification as per ZTV-ING. But the actual problem was somewhere else: “We did not want to invest in a batching plant, but most fibers can be batched only loosely” explains Stracke. Oliver Herzog, Regional Sales Manager of Sika, had the solution: the Sika Crackstop Fiber 6 mm micro-polymer fiber, which had just very recently obtained the required certification, can be directly placed in the compulsory mixer in the water-soluble paper bag. The bag dissolves without residue, which greatly facilitates application. To determine the correct mix ratio, Bettels had carried out several tests in its in-house laboratory to observe how the fiber behaves in the concrete. The tests also examined how the bags dissolve.

A total of 114 prestressed concrete beams, each 25 m long, in intermediate storage

Sika Crackstop Fiber 6 mm is a micro-polymer fiber for fire protection with concrete. It provides the fresh concrete with improved cohesion and stability and increases the hardened concrete’s resistance to impact and shock. The specified ratio of 4 tons fiber for 2 kg per cubic meter was used for every batch. The completed concrete was subsequently cast with pumps from the mixing tower into steel formwork 25 m long, and compacted. Two days of hardening was followed by demolding. The beams were subsequently prestressed and the prestressing steel installed. Eight prestressed concrete beams per week were produced in this manner, always one to two per day. The 28 beams were subsequently left to mature in the plant. “We have stored a total of 114 beams in the yard – not everyone has so much space,” as Olaf Stracke emphasizes the impressive dimensions.

Verification of fiber in the concrete

To ensure that the concrete contains sufficient fibers, both MFPA Leipzig GmbH and the joint venture (ARGE) that participated in the project, regularly took samples. “For five months, twice a week, we provided valid verification. It all went like clockwork,” said Stracke pleased.

With heavy-duty trucks to the construction site

The 114 beams were transported 250 km to Hamburg by a total of five heavy-duty trucks. For erection, special-purpose cranes lifted the 45-ton elements and deposited them on the existing lateral and middle walls. The bond was then established by the commissioned erection firm with engineers and staff of the companies participating in the joint venture. “In 2023 we will produce 108 beams once again for the second construction section,” noted Stracke. “Not every precast plant has such comprehensive production and storage possibilities as we have. But for us, too, the magnitudes, the sizes and the timing were a challenge that we mastered – and we are proud of that.”

Reducing shrinkage cracks, diminishing spalling in case of fire

Sika Crackstop Fiber will also be used to ensure higher durability in the 108 beams to be produced in 2023. Apart from the benefits already mentioned, the fiber reduces plastic shrinkage cracks and improves behavior in case of fire, by diminishing spalling. The prestressed concrete beams used in the covering of the Altona Tunnel meets all of the specified requirements, thanks to the synthetic micro-reinforcement with Sika Crackstop Fiber 6 mm. This is achieved with a marvelously simple application using water-soluble paper bags, which dissolve without residue in the compulsory mixer.