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BHS-Sonthofen twin-shaft batch mixer used in Germany

Published by , Editorial Assistant
World Cement,

Concrete manufacturer Godel-Beton, based in Stuttgart, Germany, has used BHS-Sonthofen twin-shaft mixing technology for decades – and is currently using the technology in the Stuttgart 21 project. The architectural centerpiece of the new Stuttgart underground station is starting to take shape, and three of the impressive pillars that will provide stability and daylight in the concourse have been completed. Two twin-shaft batch mixers with an output of 2.25 m3 of compacted concrete per batch are used to produce the highly complex concrete needed for the construction.

The most impressive part of the railway station concourse is the ceiling, which is supported by large pillars. The flower-shaped concrete pillars are designed to look as if they are flowing down from the ceiling. Openings in the pillars allow light to flow into the underground concourse. These features – which will give the building its striking aesthetic – put great demands on the engineers, construction staff, the material, and the technology. A special work group deliberated the technical possibilities for the pillars ad nauseam – from the unique formwork to the functional requirements for the materials, right through to its look. The first of the 28 planned pillars was built at the end of 2018. Two more followed in spring 2019.

This project included many requirements for the concrete used. The concrete was produced by Godel-Beton, which has been involved in the Stuttgart 21 project since 2013 and supplies its concrete to a number of different sub-sectors. A mobile mixing system with a twin-shaft batch mixer of type DKX was installed on the construction site of the underground station in order to produce the concrete needed. The mixing system, which was previously used to mix concrete for the Scheibengipfel tunnel in Reutlingen, produces 35 m3 of concrete needed for the base of each pillar. A second mixing system is located in nearby Fellbach for the chalice-shaped top, which requires several times the amount of concrete needed for the base (500 – 800 m3).

High requirements for concrete

The concrete mixture was specially developed for the pillars by Marko Aldag, a civil engineer at Godel-Beton, together with his team. “The requirements for the concrete were very exacting and sometimes even contradictory,” said Aldag. The fact that the supports have to achieve their necessary compressive strength is only one of the challenging aspects that the expert had to contend with. The concrete used is white exposed concrete. Its light colour – a basic requirement from the architect – is the result of using white cement as well as very bright rocks. The challenge here was that white cement releases a lot of hydration heat, which creates tension in massive structures, such as these pillars. Adding cement with granulated blastfurnace slag reduces this effect. And to ensure that the pillars met stringent fire protection specifications, the formula also includes a significant amount of PP fibres. These must be completely encapsulated within the cement glue during the mixing process. An unfortunate side effect is that the fibres increase the need for water, affect the consistency, and harden the material, which makes the concrete difficult to process. Additives ensure that the mixture remains pourable and stable. “Many months of development went into finding the right formula. The concrete and casting process were tested in advance in a model pillar and continuously adapted relative to the outcome,” said Aldag. Distributing the many additives throughout the concrete requires a reliable and effective mixer that ensures consistently high mixing homogeneity and short mixing cycles. Godel-Beton has always relied on the twin-shaft batch mixers from BHS-Sonthofen that was once again the partner of choice for this important project. “Some of the experts involved thought that the formula would require a mixing time of at least 4 to 5 min.,” said Aldag. “we were able to show that our mixer can do it in 90 sec.”

Reliability, spare parts, and service

Godel-Beton has twin-shaft batch mixers of all sizes working all over Germany. At BHS, important wear parts fit any mixer, which means that the size of the mixer does not matter. Therefore, Godel-Beton is able to keep important wear parts in stock, without requiring a large parts warehouse. If a part is not available onsite, the BHS service department steps in. “At Godel-Beton, our mixers have been operating daily for many years. A while ago, one of the machines was impacted by gearbox damage,” said Gerd Schuler, a sales representative at BHS. “But thanks to our large in-house parts warehouse, the new gearbox was onsite in just four hours.”

The concrete is subjected to several controls to ensure the quality of the material. Starting at the Fellbach plant, Godel-Beton employees check the raw materials used during production. A second check takes place on the construction site, and the installation company checks the material a third and final time at the pump. Only the white colouring cannot be checked, even after the formwork has been removed. Sulfides in the granulated blastfurnace slag initially give the pillars a blue tint. The pillars will gradually turn white once they are exposed to atmospheric oxygen. Until then, nothing stands in the way on constructing the remaining pillars for the Stuttgart underground station.

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European cement news Cement news 2018