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Precast quality

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Michael Hammett looks at two schemes which combine speed, consistency and brickwork of exceptional character

The new City of London headquarters for international finance company Merrill Lynch certainly called for prestige, and the very large size of the building and the complexity of its historic location in Newgate Street EC1, close by St Paul's Cathedral, presented architect Swanke Hayden Connell with major challenges.

The client's desire for a building of permanence, gravity and solidity has been interpreted in the form of a steel-framed structure clad with Portland stone, glass curtain walling and the refined appearance of gauged brickwork.

Gauged brickwork 2000 style

Since its inception in the seventeenth century, gauged brickwork of cut-and-rubbed units jointed with precisely regulated, thin, limeslurry joints has been regarded as the highest form of the bricklayer's craft, calling for great skill and patience from the craftsman and superior brick materials from the manufacturer.

Its expense has confined use over large areas to prestigious buildings where cost was not a primary consideration.

There are three serious impediments to the use of gauged brickwork today: the rarity of bricklayers with the singular skills required; the very limited availability of the special bricks used; and the slow rate of building that the technique entails. Therefore, the only practical solution was prefabrication in the form of loadbearing reinforced concrete structural elements with brickwork facing.

The unusual nature of the brickwork facing with joints of only 3mm called for special development and Techrete (UK), a company with expertise in the manufacture of precast concrete cladding, took approximately eight months to evolve the production method.

The red perforated wirecut bricks were of through colour and fine texture similar to traditional 'rubbers'. They were made to special dimensions and cut by the cladding manufacturer to an accurate and consistent size.

They were also reduced in width by cutting lengthwise across the perforations to form keys for the concrete backing.

The cladding units were cast in timber moulds in which the bricks were placed face down and butt-jointed without mortar. Steel bar reinforcement was placed on the bricks in the mould and structural concrete backing then cast.

When set, the unit was turned over and the mould removed.

The 3mm joints were then cut into the surface using a diamond circular saw, following a guide rail aligned on the original butt joints, and then they were filled with a specially formulated white grouting material. When the grout had set and hardened, the whole brickwork surface was ground flat with a buffing machine fitted with abrasive pads.

The units are of various sizes and shapes; some are storey height, others are panels below windows. Lengths of flat panels vary up to 12m and curved sections up to 15m. The largest unit weighs 18 tonnes. The units were transported to site, craned into position and bolted into place.

Curves and vertical courses

Sidell Gibson Architects presented brickwork of more conventional appearance at Aldercastle, an office development by Argent in Noble Street EC2. This brickwork is also facing to precast concrete cladding.

For broad areas of walling the cladding panels are faced with yellow-multi stock bricks in normal stretcher bond. But there are some eye-catching features.

Staircase towers are enclosed with curved brickwork of 3m diameter. Strong curvature such as this would require special-shaped radial bricks for a smooth result in conventional brick bonding. However, standard bricks have been used laid on-end. The resultant narrow face of the brick and greater number of vertical mortar joints when brick is used in this way allows greater deviation from straightness and produces a very satisfactory curved form.

There are two choices for laying bricks onend; they can be stack bonded in soldier courses or, as here, lap bonded in courses running vertically. In hand-set work both forms require great care in laying. Normal techniques using line and level to maintain accurate alignment are not appropriate and consequently the work is more time consuming. However, prefabrication of brick-faced concrete cladding panels presents no such distinction between these forms and conventional, horizontal, bonded brickwork.

Vertical-bonded coursing was also used to form feature bands, 1227mm (5 clad beams over major openings adjacent to the rear service yard.

The cladding panels were manufactured offsite by Trent Concrete. The stock bricks were sorted for size as the limits of size specified in the standard for clay bricks, BS 3921, were considered too lenient for the dimensional constraints of the precast concrete cladding system. To limit overall panel thickness, each brick was cut to reduce its width to 60mm.

Cutting a perforated brick automatically produces a keyed surface, but these are stock bricks with frogs and so a dovetail key was cut in the back of each.

Panels were formed in timber moulds in which half-round battens were fixed to regulate coursing alignment. Thin polyethylene sheet liner was used to aid demoulding. A debonding solution was applied to the faces of the bricks before they were laid face down in the mould with polystyrene foam spacing strips between them. Reinforcement and concrete were then cast onto the bricks.

After demoulding, the faced surface was power-washed to remove any staining by leaked concrete. The spacing strips were removed and the joints pointed with mortar.

Michael Hammett is senior architect at BDA

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