Two weeks ago, over a gruelling 36-hour operation, 700 truckloads of concrete were deposited at the London Bridge site of the Renzo Piano-designed Shard tower. The 5,500m3 single concrete pour ranks among the largest ever undertaken on a building in the capital and marks the first major milestone in the construction of what will become Europe's tallest building. This culmination of the building's groundworks package has created the huge raft foundation that will support the tower (see graphic below story). However, for regular commuters and passers-by it begs a question: what has been holding up the 21-storey core which over the last few months has been pushing its way into the skyline? The answer lies in the novel approach that has been used to get this building up as fast as possible.
So why the hurry? Well, a switch from construction management to a fixed-price contract had delayed the start of the Shard on site. Some way was needed of getting the programme back on track. But constructing the building in the first place was always going to be tricky. Not only does the site have to operate among the thousands of commuters that come into London Bridge station every day, but it also has to have minimal impact on its surroundings (see box, left). The site sits right up against the corner of the station which is supported on old brick railway arches, the tunnels of the Jubilee Line run underground close by and Guy's hospital is next door. Plus, a large Victorian water mains runs below the road next to the site. It was calculated that any structure within 100m of the site's perimeter could be at risk from any movement caused by excavating the ground and building the 306m tower.
Into the mix the concrete pour
At around 5,500m3, the concrete pour to create the raft foundation to support the Shard was among the largest undertaken on a UK building, if not Europe. As such, a conventional concrete mix would not do. When concrete cures it generates heat and the sheer volume of the pour, which measures about 50m by 60m and is up to 3m deep in places, means that the high levels of heat produced could result in shrinkage and cracking.
According to Don Houston, senior project manager with concrete contractor Byrne Bros, the overriding specification from WSP was to limit cracking. "What we needed to avoid was a large temperature differential between the centre and the top surface of the raft, so we had to look very carefully at the mix design."
Houston is coy about the exact formulae because of the time and cost it has taken coming up with it. However, he will reveal that it uses ground granulated blast furnace slag to replace 75% of the cement, which helps limit the amount of heat generated.
The downside of using a cement replacement is low early strength gain – 56 days compared with 14 for a mix using Ordinary Portland cement – so the mix was developed to make sure it would achieve sufficient strength gain over the first 14 days to meet the structural requirements, with the full strength coming later.
The concrete also needed to flow easily around the densely packed reinforcement bars at the base of the slab. Additives – plasticisers, retarders and others – were included to give good flow characteristics, delay setting times and prevent "bleed", which is a common result of using high levels of cement replacement.
The concrete was poured in layers 750mm deep. This helped limit heat build-up and also regulated the placement so that succeeding layers could be poured before early layers had set.
A computer programme was created to predict the temperature of the core, and thermocouples were fixed to the reinforcement cage in the raft to enable the temperature to be monitored. The mix recipe could then be altered at the batching plant if needed.
To limit heat build-up in the confined space while the pour was under way, fans were used to draw air through the basement and ventilate it out through the mole hole where the excavation occured.
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