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APARTMENT DESIGN IS IDEALLY SUITED TO PRECAST CONSTRUCTION

CONCRETE QUARTERLY/ SUMMER 06 - HANOVER QUAY

It's hard to imagine a city where house prices are higher than in London, but this is the case in Dublin, a consequence of economic boom, shortage of old housing stock and the lack of a skilled construction workforce. To satisfy demand, apartment blocks are now beginning to rise on brownfield sites such as the former docklands on the banks of the River Liffey. To solve the labourshortage problem many of them are built with factory-made, precast concrete components. The cellular and repetitive nature of apartment block design is ideally suited to precast construction.

Sound reduction at party walls is easy to achieve and the solidity of precast concrete recalls traditional concepts of enclosure.

One of the largest docklands developments, designed by O'Mahony Pike Architects (OMP), is Hanover Quay. The site, once a gasworks, stretches from the Liffey quayside to the banks of the Grand Canal dock. It is a mixed-use development, 13 blocks of four to eight storeys accommodating 292 one- to three-bedroom apartments (including affordable apartments) with basement car parks, a crèche, retail units, a pub and restaurant. The blocks looking directly on to the river contain two lower oors of office/ retail units and upper oors of apartments with large balconies, some of which are 'winter gardens' with glazed sliding screens.

The apartment blocks are a composite structure of loadbearing precast concrete external wall panels, precast concrete internal wall panels and precast concrete oor slabs. On the top oors the external precast panels have been extended to create a parapet. 'This must be the sixth or seventh job where we have used precast concrete panels, ' explains the architect. 'In Dublin, where labour is scarce and time is at a premium, it's the way to go.'

However, this is the first time that OMP has used precast concrete insulated sandwich panels, produced by Techrete, a manufacturer based in Ireland and the UK, with a head office at Howth, just outside Dublin. The Techrete insulated sandwich panels comprise an outer leaf, a layer of insulation and a backing leaf of plain grey concrete. They are cast as one. The insulation, installed under controlled factory conditions, is well protected by the concrete. Panels have a range of options: the outer leaf of precast concrete is available with a wide choice of stone-like finishes or applied facings such as stone, brick or terracotta. Panels can be integrated into the building fabric as part of a load-bearing structure to support either precast or cast-in situ oor slabs.

A combination of Techrete's single-layer precast-concrete panels and, on the gable walls, precast-concrete insulated sandwich panels, were used at Hanover Quay. The sandwich panels have a 150mm load-bearing structural inner leaf connected to a 75mm outer leaf with stainless steel wind/shear tie connectors, with a cavity former, 75mm of mineral-fibre insulation and a vapour barrier between. The outer leaf is separated from the inner leaf by the connectors; it is free to expand and contract and is not intended to support any load. (See Working Detail, pages 54-55. ) Both single-leaf and sandwich panels were produced by Techrete with an identical mix. The main panels are a light buff reconstructed stone with an acid-etched finish. Stair core and balcony edge panels are of dark charcoal grey reconstructed stone with an acid-etched finish. The panels incorporate window openings and circular apertures for ventilation extract from the apartments. Damp-proof coursings around window openings were pre-fixed to the panel at the Techrete factory.

The outer leaf of each sandwich panel extends at the top above the floor junction to co-ordinate the live joints at the external face with the finished floor level. The outer leaf is watertight, with double seals at joints. There is a vapour control layer behind the insulation. Calculations established that the dew point could not occur within the cavity. Each panel has a horizontal dummy joint aligning with the window head to reduce the scale of the panelled facade.

Sandwich panels offer many advantages. They provide a strong, durable, energy-efficient, fire-resistant cladding system.

And all aspects of the production process are carried out in a factory, ensuring the highest quality possible. Hanover Quay shows how a fully integrated structure and skin sandwich panel system, where load-bearing wall panels provide both structural support and external finish, speed up construction and minimise on-site labour costs.

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