The Shard represents technical milestone for concrete
In the year since the Shard topped out, Renzo Piano’s glass-clad church spire has created a new landmark for London, and one which due to its extreme height, dominates the skyline south of the river.
The tallest building in the European Union, the Shard is more than 300 m high and has 95 storeys of office, residential and mixed-use space, of which 72 storeys are habitable. On a clear day, views from the top of the Shard can span 60 miles.
From the moment of the Shard’s conception – that iconic shape reportedly roughed out on the back of a napkin in a Berlin restaurant – it has been clear that delivering on site would pose considerable technical challenges.
Tight site, tight schedule
The Shard replaced Southwark Towers, a more conventional office block less than a third the size of its successor. Located in an area which has been built up for more than 300 years, the site was constrained, located between one of London’s busiest transport hubs and Guy’s hospital – and the delivery schedule famously tight.
Through its subsidiary London Concrete, Aggregate Industries was part of the Shard’s development from the beginning – quite literally in at the foundations.
“The pace of work was such that by the time the foundations were completed, the building was already 21 storeys high”
New ground was broken logistically, with 700 truckloads of concrete delivered with military efficiency within a tight 36-hour window.
Within that window, amounts of concrete that might ordinarily be poured in a day were poured every hour, creating a raft approximately 50 m x 60 m and up to 3 m deep in places.
The 5,500 cu m single pour set a new record for the UK’s largest continuous concrete pour, beating the previous record – 4,800 cu m delivered at Wembley Stadium – by nearly 15 per cent.
Crack and shrink factors
In addition to the logistical challenges, a pour of this scale raised technical issues which had to be considered. At this scale, cracking and shrinking caused by heat build-up during the curing process was a real possibility.
Mix design was crucial, with temperature differentials avoided through use of ground granulated blastfurnace slag (GGBS) as an alternative to Portland Cement.
As this mix doesn’t attain full strength until 56 days, the ready-mix was further modified to make sure that enough strength was available at 14 days for work to proceed, and to ensure that the ready-mix flowed smoothly around the tightly packed reinforcement bars at the base of the Shard.
The pace of work was such that by the time the foundations were completed, the building was already 21 storeys high. As a last stage, self-compacting ready-mixed concrete was pumped bottom up into shuttering, creating walls which linked the foundation slab to the building core.
Miles Watkins is director of sustainability at Aggregate Industries