Saving time onsite has been one of the key elements of this project, with the building's substructure and superstructure both being worked on simultaneously. However, the raft foundation work had to be added with a massive pour of 5,460m³ to create a raft of approximately 50m x 60m and up to 3m deep in places. Logistical problems, as well as potential cracking and shrinkage caused by heat build up during the curing process of such a large pour, had to be addressed. Another key element was using a high quality lightweight concrete that could be pumped to a very high distance. Lytag was therefore chosen for this challenge.
London Concrete was able to call upon four concrete plants to service the pour. Although this was more than necessary, it meant that should there be a failure, then the work would not be affected.
The job was also scheduled for a weekend to avoid traffic and concrete mixer trucks from the four plants gathered at a muster point prior to having their loads checked before their final journey to the site.
The mix design for the ready-mixed concrete was also crucial. Temperature differentials were avoided as the mix utilised ground granulated blastfurnace slag (GGBS), as an ordinary Portland Cement replacement. As this mix doesn’t attain full strength until 56 days, then it was modified to make sure enough strength was available at 14 days for work to proceed. As the ready-mixed concrete needed to flow around the tightly packed reinforcement bars at the base of The Shard, plasticisers, retarders, and other additives were added to the mix.
The next stages of construction have seen the use of self compacting ready-mixed concrete pumped bottom up into shuttering, creating walls connecting the foundation slab to the bottom of the building core.
Construction of the Shard also required a lightweight concrete and Lytag® was utilised for the low density, increased fire protection and increased spans that the aggregate enables. An impressive 15,000m³ of Lytag lightweight concrete was supplied with pumping height reaching the 40th storey.
Renzo Piano Building Workshop
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