During the construction of a particle accelerator at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, a building complex was erected on a 20-hectare site with heights ranging from 21 metres above to 17 metres below ground level. The central element is an underground accelerator tunnel approximately 1,100 metres long and with a variable width of 8.3 to 10.3 metres. The tunnel height varies between 6 and 7.5 metres, depending on the floor slab and ceiling thickness.

A particular challenge was to develop a technology for a 6-metre-thick tunnel ceiling that would allow a free span of 26 metres – while another building was being constructed directly above it. In addition to the enormous static requirements, it was necessary to ensure maximum precision, extreme robustness, a joint-free design and freedom from cracks. These requirements went far beyond the previous state of the art and necessitated innovative solutions.

To achieve these goals, a special concrete concept was developed that enables structures with exceptionally large wall thicknesses (6 metres) to be built using solid reinforced concrete construction. The complex structural design and the requirement for a jointless finish called for a novel, self-compacting concrete mix and particularly high-density reinforcement in the components. The innovative concrete mix enabled the controlled setting of large, crack-free components. In addition, specially developed formwork and supporting scaffolding were used to withstand the extreme forces. The reinforcement density is around 700 kg/m³, which is significantly higher than the usual values of 100–150 kg/m³. In addition, care was taken to minimise hydration heat during the curing process.

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