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Opened during the summer, the Adjaye Associates-designed Idea Store on Whitechapel Road in east London realises the high-quality interior finish potential of precast concrete.

A modern redefinition of the neighbourhood library, Idea Stores are state-of-the art facilities with educational learning areas in partnership with Tower Hamlets College. The idea is being introduced to increase usage of library facilities by providing a modern, high-quality environment akin to a retail operation. This building accommodates traditional learning spaces, library stacks, after-school study zones, business facilities and formal and informal reading areas, along with retail areas and a café.

The hung facade's panels of green and blue glass take the external market awning atmosphere into the building through broad stripes of coloured light that fall onto the monolithic walls and ceilings of the exposed, deliberately visual concrete structure.

With fl exibility a key client requirement, engineer Arup set the building out on a rectangular grid designed to work as a sway frame - stability provided by the frame itself, avoiding the need for any internal shear walls or bracing that could have constrained the spaces in use. The intentional monolithic look was achieved by a combination of cast in situ beams and columns, and precast ribbed soffits slabs, a solution whose thermal capacity enhances environmental performance as well as making for a more efficient construction sequence.

Early meetings were held between structural and mechanical engineer Arup, architect Adjaye Associates, precaster Hanson Floors and main contractor Verry Construction to discuss buildability issues, including those involved with the precast planks for the project. Hanson supplied 3,000m² of bespoke precast concrete flooring units and 18 concrete staircases for the Idea Store.

Discussions included questions about the maximum size and weight of the slabs that could be placed on a tight city-centre site using the proposed tower crane. Once all these issues had been agreed, the in situ frame could be designed although, with substructure work already under way, some redesign was needed by Arup because of the store's close proximity to the soon-to-be-built Crossrail project.

Adjaye wanted an exposed-concrete finish for the building and Arup proposed a hybrid precast and in situ solution. Hanson was given the task of designing, manufacturing and supplying units to create an exposed concrete ribbed ceiling to the open expanse of floors on the three upper storeys of what promises to become a landmark building in the area. Initially, the concept called for hundreds of separate beams and short concrete-deck elements to act compositely using a structural concrete topping. However, Hanson was able to redesign the units so that, in most cases, multiple ribs and decking were cast in one piece. This greatly aided the handling process during transport and installation and significantly reduced the number of construction joints. Hanson also designed and produced the bespoke staircase elements that are left exposed to match the rest of the building.

As the concrete frame was to be fully visible from outside and inside the building, the colours of the in situ frame and the precast floor planks had to be matched closely - a task made more difficult by the fact that the mix used for precasting was not suitable for site work. Following trials, Verry Construction built a full-size mock-up of the floor panel, supported by four columns and beams.

Connecting in situ concrete to ribbed precast planks created the challenge of sealing the junction between the two while achieving a visually acceptable finish. Several possible solutions were considered and it was finally decided to provide individual shutters for each rib. These were machine cut and then fixed in place before the precast planks were located. They had to be slightly smaller than the rib and be moveable to allow for tolerances. Once the precast slabs had been placed, mastic was applied to ensure a tight seal.

With the shutters sealed, the reinforcement was fixed, tying in the precast slabs to the in situ frame.

A further implication of the design was that for the frame to be fully monolithic, the 500m 2 floor with its 300 individual rib shutters had to be poured in one go. To ensure everything went smoothly, a vigorous inspection system was instigated.

Main contractor Verry Construction managed the construction of the in situ structural frame with Modebest, a special formwork process developed to ensure that the in situ and precast elements formed a seamless match.

The five floors of the Ideas Store were completed within 24 weeks and the standard of concrete finish is exemplary.

Gerry Feenan works for Hanson Building Projects

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