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working details: Coffered precast concrete slab floor

The first floor of the two-storey office building consists of a series of coffered precast concrete slabs supported on precast columns. Precast was chosen for its energy efficiency - the concrete mass provides thermal energy storage - and to keep the overall height of the building as low as possible. A suspended ceiling is not needed as services are contained in the suspended floor above the slab. The slabs incorporate pipework carrying chilled water for office cooling.

The precast slabs are 14.9m long; they span across the main office space, stopping short of the circulation space which runs along the east wall. To accommodate the curved form of the building, the slabs taper from a width of 3.1m at the east wall to 2.9m at the west wall. At the east all the ends of the slabs are exposed and the staircase landing are bolted to them. At the west wall the coffers are coved to align the slab ends with horizontal window heads.

Each slab was cast in a timber-framed glass fibre mould together with the reinforcement and pipework for chilled water. To reduce weight and cost they were cast as 150mm-thick shells, transported to site, propped on columns and scaffolds and filled with cast in-situ concrete.

The slabs and the precast columns which support them were cast with a mix of white cement and Spanish Dolomite coarse and fine aggregates.

The surfaces were delicately grit-blasted - as the slabs were cast soffit-side down, acid etching would have been dangerous. The casting of 30 slabs in two moulds, with each one taking a week to cast and cure, required a special procurement process. Histon Concrete received instructions before the main contract went out to tender.

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