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It's in the mix

Producing consistent fair-faced exposed concrete only 500m from the Arabian Gulf is a thankless task. So says Buro Happold project manager Gary Cutler, who is ultimately responsible for the supervision of the £20M APICORP headquarters building in Al Khobar, near Dhahran. 'The C40 concrete specified has a water/cement ratio of only 0.35, which means having to use both superplasticisers and retarders,' he adds.

'These are right at their limits of dosage and very sensitive to small changes in temperature - even 5degreesC makes a big difference. This means big variations in dosage and a marked colour difference between pours.'

Increasing water/cement ratio to 0.38 almost eliminates the problem, Cutler reports, and 0.40 is even better. 'I think 0.35 is just too high a specification for this climate,' he concludes.

In fact, building the spectacular design drawn up by London-based architect DEGW would be a challenge in almost any climate. A distinctive vaulted roof supported on a perimeter colonnade shelters three storeys of virtually independent offices. Roof, colonnade and offices sit on a complex insitu concrete plinth. Hot air from the offices exits via central atria, rises into the roof cavity and returns down hollow, elliptical perimeter columns.

'Originally the double-skinned roof vaults were to be made up of an insitu curved outer shell supported by precast sections sitting on insitu spine beams between the columns, with a precast lower curved soffit,' Cutler says.

'But during a pre-tender value engineering exercise the contractor, the Saudi Binladin Group, came up with a radically different alternative.'

This involved the use of internal steel portal frames spanning between modified spine beams to support the roof shell. Soffit shells would be ferrocement, produced on site by specialist subcontractor Retsei.

Reinforcement is placed over a single curved former at ground level, and a mortar mix hand trowelled to encapsulate it. Cutler says the results have been 'excellent.'

These changes, however, came at a price, not least a complicated joint detail between the modified spine beams and the elliptical perimeter columns. Specially-made steel forms had to be used, with the associated risk of flash setting and skinned-over boreholes if the steel got too hot before the pour.

'They had to be wrapped in hessian and constantly sprayed with water to keep temperatures down,' Cutler reports. 'And all concrete had to be made with chilled water and ice.

'It's not impossible to produce good quality fair-faced concrete here - just very hard.'

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