The 250m high core was slip-formed and started at basement level B2, nine metres above the lowest excavation

level. This was achieved by sinking large steel box columns into support piles directly beneath the core, and

constructing a grillage of reinforced concrete beams at B2 level, connected to the piles with welded couplers. This

takes the load as the core is constructed skywards. The basement below was excavated and the large raft foundation

poured. The six metre high reinforced concrete core walls from level B3, the lowest level, to B2 were constructed

using self-compacting concrete pumped from the bottom of the form, so that a tight fit is achieved between the walls

and the grillage of beams.

The concrete for the slip-formed core is grade C50/60 concrete and 30 MPa cube strength was achieved after 24

hours. There are 36 different concrete mix types to allow for variations in weather, time of year and height of pumping

etc. The slip-form rig also held one of the cranes for the construction so that the crane moved up the building with the

core. This has added an extra 180 tonnes to the weight of the rig which had an area of 26m x 22m. The core itself

covers an area of 22m x 19m. The slip-forming started in January 2010 and 72 storeys of slip-forming were complete

at the end of 2010. The maximum speed that the slip-forming achieved was 3.6m in 24 hours.

The core reached 21 storeys high by the time 700 truckloads of concrete were poured into the basement to form the

three metre deep raft foundation, upon which the tower sits. Carried out over 36 hours, the 5,500m3 single concrete

pour is one of the largest ever undertaken in the UK. At the peak of the pour, trucks arrived on site every two minutes.

Three concrete pumps were installed to pump up to 150m3 an hour to ensure a fast truck turnaround time. The

concrete was poured in layers 750mm deep, this helped to reduce heat build-up and regulated the concrete

placement. Reducing heat was important in order to limit the potential for high levels of shrinkage and cracking.

C35/45 concrete contains a cement blend using 70% ground granulated blast-furnace slag (GGBS) to limit early heat

gain. This high level of cement replacement has the potential disadvantage of low early strength gain, but this was

overcome by developing the concrete mix so that it achieved sufficient strength gain to meet initial structural

requirements within 14 days, with the full strength being achieved at 56 days.

Levels 40 to 72 were constructed in post-tensioned concrete on high strength (C65/80) concrete columns. Pumping

concrete up 250m was another of the challenges this project's concrete subcontractor had to overcome.

Project team