One Coleman Street, London
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The Project
Located in the heart of the City of London, this prestigious
office development has set a new benchmark for the use of recycled
secondary aggregates and secondary cementitious materials.
Developed by Stanhope, the nine-storey, 18,000m2
office demonstrates sustainability as an integral part of the
construction process. The engineer, Arup, chose to use china clay
stent as a natural coarse aggregate to replace the use of primary
quarried Croft granite. In addition, the project used a secondary
cementitious material - fly ash. Although the use of secondary
cementitious materials such as fly ash is well established, what
makes One Coleman Street different is that rather than the
commonly employed level of 30% fly ash by mass of cement, Arup used
a higher content of up to 40% as standard.
The use of china stent and fly ash plus the use of reinforcement
manufactured from 100% scrap metal increased the recycled/secondary
value of the concrete to as much as 77% depending on the amount of
reinforcement in an element.
Stent solution
Stent is the term used to describe the waste granite rock
material that has been separated from kaolin (china clay) by
high-pressure water jets. For every tonne of china clay,
approximately 4.5 tonnes of stent is produced along with other
waste, which is usually tipped onto ever-growing surface spoil
heaps.
Conventional wisdom has been that stent was not of a
sufficiently high quality to enable it to be used as a concrete
aggregate. Evidence to refute this comes from the long history of
stent's use in ready-mix concrete in Cornwall and Devon. One
Coleman Street is the first major use of stent outside the
South-West.
The stent that was supplied by Bardon Aggregates from the
Littlejohn Quarry near St Austell was selected in order to meet the
requirements of BS EN 12620 and PD 6682-1 for concrete aggregate.
Despite sourcing the stent from a quarry 250 miles away there were
no additional lorry journeys. Instead, a total of five 1,200 tonne
train loads travelled from the quarry site to the rail head at
Bardon's concrete plant in Bow, London. The use of the stent meant
that 6,000 fewer tonnes of china clay waste were deposited onto the
spoil heaps and equally, 6,000 fewer tonnes of primary aggregates
were quarried.
Another environmentally sensitive decision was to use 40% fly
ash by mass of cement in the structure's C32/40 pile caps and 35%
in the C28/35 and C32/40 superstructure elements. In the watertight
concrete ground slab, the fly ash content was restricted to 30%
simply to meet the specification requirements of the admixture
producer.
Project team liaison
Because of the innovative nature of the concrete, discussions
between the parties involved commenced well in advance of
construction to allow time to obtain test data, develop mix designs
and perform trials. Full test data for the aggregate's physical
properties, petrographic characteristics and alkali-silica
reactivity were obtained to ensure that no risks were taken and
that the material could fully meet the project procurement
requirements. It was realised at the specification stage that the
stent aggregate concrete would cost a little more per cubic metre
than its conventional equivalent. This was largely due to the
transportation and testing costs. However, these extra costs were
partially offset by the fact that stent is exempt from the UK
aggregates levy and if used more often for future projects there
would be less need for extensive testing.
Project team
Client: Stanhope
Architect: Swanke Hayden Connell Architects;
David Walker Architects
Engineer: Arup
Construction Manager: Bovis Lend Lease
Main contractor: John Doyle