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Cathodic protection Ministry of state

Concrete conservation - Cathodic protection is saving the historic facade of a London building, as Christina Taylor reports.

The huge former Ministry of Pensions offi ce in East Acton, London, is being converted into 491 new luxury fl ats by Berkeley Homes (West London) Ltd. This complete identity change is cautiously described by structural consultant Cameron Taylor Bedford associate director Mike Broderick as 'the biggest current change of use project in Europe, as far as we know'.

Externally, the distinctive cast stone façade dating from 1922 will be retained. But inadequate original construction details and the effects of carbonation, water infiltration and thermal action have led to some corrosion of the supporting steelwork and cracking in the façade.

Cameron Taylor Bedford and cathodic protection specialist consultancy Electro-Tech CP have been brought in to develop a package of remedial works on the external structure.

But although it is the concrete façade which gives the building its character, this is where most of the problems lie.

The stone effect, siteproduced, 108mm thick exterior blocks are separated from the inner, 57mm concrete block wall by a 57mm cavity. Concrete encased steel beams and channels carry the masonry loads at each level.

Differential thermal movement has taken its toll on the external blockwork over the years, as has the absence of an expansion joint between the steel elements.

However, the most severe cracking has been caused by steel corrosion due to carbonation of the relatively thin layer of concrete encasing the edge channels along the top of the windows.

All 166 external steel columns sit in the cavity with the external flange of the I-section just 6-12mm from the external block work. A mortar fill between the block work and the flange has provided a conduit for capillary water flow through the semiporous external blocks making contact with the steel.

The resulting expansive corrosion has pressured the concrete façade, producing vertical cracking along the column line.

Where this occurs on the internal courtyard facades, over-cladding has been specified.

'Planning permission issues made over-cladding a non starter for the front facade, ' explains Broderick.

Instead, a cathodic protection (CP) system is being installed to arrest corrosion insitu (see box).

'CP is a good solution for residential buildings because not many owners want the maintenance associated with a steel frame building to be an issue, ' says Electro-Tech CP director Peter Gibbs.

'With correct maintenance of component parts, this CP system will protect the structure for 60 years or more, ' he adds.

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