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Concrete focus: Supporting the vase

5 February 2018 | By James Kenny

The 9m structural concrete columns had to reach strengths of 95N/mm to support 50 storeys

One Blackfriars, known as “The Vase”, required a high-strength concrete mix for its substructure, supplied by materials firm Tarmac. James Kenny reports.

The 50-storey One Blackfriars tower, rising to 170m high, will be one of the tallest residential buildings in Europe when it completes this summer. As well as 274 luxury apartments, the development includes a hotel, spa facilities, retail units and a new public plaza.

Berkeley Group subsidiary St George’s asymmetrical tower, which looks out over the Thames in central London, is now in fit-out stage. Finished in a double-envelope facade of transparent glass and cladding, it will have 57,000 tonnes of concrete in its structure.

Concrete provider Tarmac was brought on board for the significant challenge of creating a high-strength mix for the substructure – working with main contractor Brookfield Multiplex, engineer WSP, concrete frame contractor Byrne Bros and Laing O’Rourke ground engineering subsidiary Expanded.

“It was vital that a low-heat solution was designed to address the potential risk of thermal cracking, and it obviously needed to be strong enough to support the weight of the structure above,” says Tarmac business manager Steve Hyde.

To supply the enormous amount of concrete needed, Tarmac dedicated two twin-batching plants at King’s Cross and Silvertown to the job. It also installed a new Microsilica dispensing system at its factory in King’s Cross.

This system was created specifically for One Blackfriars, with Tarmac working closely with Multiplex at development stage, testing the concrete to ensure the designed strength was reached. Full-size mockups of the structures were tested to ensure the right mix of concrete was found.

“The early engagement, laboratory trials and mockups meant we could be sure we brought to site the right product,” says Hyde. 

The project broke ground in late 2013 with the excavation of an 18m-deep basement. The substructure work was carried out by Expanded and, due to the project’s complexity, techniques included a secant pile wall, temporary mini piled wall, rotary piles as well as large-diameter bentonite piles.

The raft slab had to be constructed in one sitting, with some 35 trucks making more than 400 deliveries in a 24-hour period. With 3,200 cu m of concrete, it was Expanded’s largest single pour. 

This filled the 18m deep excavation, creating a slab up to 4.5m thick. This had to carry the weight of the concrete superstructure, distributing the weight across 36 large-diameter bearing piles. “A Topflow concrete mix was selected as it is capable of reaching strengths of 50N/mm2,” explains Hyde.

The next major task was to create 15 9m-tall ground level structural columns, which had to reach strengths of 95N/mm2 to support the 50 storeys above ground. After trials that focused on strength, colour and finish, a Topflow mix using 10mm limestone with a limestone powder was selected.

Hyde says: “Each column was poured using a tremie pipe process. The tremie pipe was lowered inside the heavily congested 40mm reinforcement bar framework of each column and slowly lifted up with each pour.” A new column was poured every three days, requiring 22 cu m for each one.

The geometry of the building was a significant challenge, says Aret Garip, technical director at engineer WSP, which used 3D modelling software to study numerous framing configurations.

“We specified high-strength concrete including stiffness testing criteria to ensure the building’s performance matched the engineering design.”

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