Piercing London’s Skyline with a Shard of Glass

Author: Grace Goss-Durant

Through expertise and careful planning, London Concrete successfully overcame the significant challenges associated with supplying concrete to construct the tallest building in western Europe. A recent award is testament to their success.

Standing at 310 meters tall in the London borough of Southwark, the Shard of Glass dominates the London skyline. Designed as a multiuse building by the architect Renzo Piano, who first conceived the idea during a meeting in a Berlin restaurant and hastily sketched it on the back of a menu, the Shard houses offices, restaurants, the 5* Shangri-La hotel, residential apartments, and a viewing gallery affording an unrivalled panorama of the city. The building points towards the sky in a manner reminiscent of a church spire and is clad in clear blue glass in order to reflect light and mirror the changing weather. 72 meters higher than its closest London competitor, One Canada Square at Canary Wharf, on completion the Shard became the tallest building in western Europe.

Concrete is a fundamental component of the Shard, which combines composite steel frames for the bottom levels and spire with a three-story reinforced concrete basement, concrete structural core, and in-situ concrete frame for the upper levels. The purpose of this hybrid structure is to improve the dynamic performance of the tower in response to wind loads, with the weight gaining properties of steel in the lower floors complimented by the inherent stiffness of the concrete, limiting horizontal deflections and making any additional dampening devices unnecessary. An ambitious building in scale and scope then, the construction of the Shard required tailored solutions, careful planning, and close collaboration from its concrete supplier - London Concrete.

 

Overcoming core challenges
Construction began in February 2009, with the first concrete pour taking place in October of that year. In order to conform to the tight program schedule, the substructure was constructed using top down techniques to allow the core slipform to be launched in advance of the main structural works. This enabled the installation of lifts to occur in tandem with core construction and resulted in a critical program saving of 20 weeks. This technique necessitated the installation of 24 meter long plunge columns, steel members embedded in concrete piles to serve as structural columns, in order to support the weight of the core prior to the construction of the basement.

The slipform core was under continuous construction, and therefore required consistent supply of concrete 24 hours a day. A small site situated within an extremely complex urban environment, coupled with severe fines for delays to construction, demanded intricate logistics planning from London Concrete to navigate the congested London streets and ensure this requirement was met. The average daily increase in slipform height of three meters to meet the ten-month program was achieved using a high early strength mix designed to pump up to 72 floors in a single operation, meaning that no intermediate level pump was used, and adjusted to account for temperature and height.

 

Intricate logistics for concrete delivery
Careful planning required that the basement raft be completed at precisely the same time as the core slipform reached level 26 so that the overall building works were able to continue without any interruption. The construction of the basement was therefore critical to the overall program. Originally intended to be supplied in ten pours of 560m3, the timely completion of the basement was ultimately achieved within a tight 12 week schedule which culminated in the installation of the basement slab in a single pour of 5,600m3.

Supplied by two concrete plants, with another on standby, an average of 175m2 of concrete per hour was delivered by 44 truck mixers at peak times over the course of the ambitious 36 hour basement slab program. The scale of this operation not only posed challenges to London Concrete in terms of logistics but also required the development of an appropriate mix. A pour of this size generates significant heat of hydration. London Concrete therefore worked closely with Byrne Bros, the concrete frame contractor, to design a slow-setting mix with 75 percent ground granulated blast furnace slag (GGBFS) that would keep heat gain within acceptable limits to prevent cracking.

Raft temperatures were closely monitored using thermocouple sensors, which revealed that the concrete temperature achieved was within two degrees centigrade of the design temperature. Successfully meeting the various challenges, London Concrete completed the largest pour ever undertaken in the UK within 32 hours, beating the schedule on both time and volume.



 

 

A collaborative success
As a result of careful planning and implementation construction proceeded rapidly, and in September 2011 the Shard’s concrete core topped out 19 days early on the 72nd floor. Concrete pumps then began to pour the floors from level 41 upwards, with a new floor being poured each week on average, and the steel-framed spire which comprises the remaining levels to the 87th floor was installed. On 30th March 2012 the Shard became the tallest externally completed building in western Europe.

This project's success was capped when the Shard was named Best Large Project in the Engineering News-Record (ENR) Global Best Projects Awards. These awards recognize excellence in construction, with a panel of industry experts choosing winners on the basis of them being outstanding examples of the challenges, risks, and rewards of designing and building internationally, placing special emphasis on the global diversity of project teams and how well they worked together. 

The safe and successful execution of the concrete works on this project was a tremendous achievement for Byrne Bros, London Concrete, and their supply chain partners, with the now completed landmark standing as testament to their efforts in the heart of England’s capital.