CEMENTITIOUS MATERIALS/AGGREGATES.......................................................................................

Concrete applications using GGBSDenis Higgins of the Cementitious SlagMakers Association outlines some recentexamples using high levels of ground­granulated blast-furnace slag (GGBS).

Hanson Cement has recently been involved withfour projects where the cementitious contentof the concrete has comprised 70%, or more, of

GGBS. These have ranged from a pedestrian footbridge,to the Tyne Tunnel, to a storage facility for nuclear­waste products, to the highest building in Europe.

The company assisted with the design andconstruction of the Bourg pedestrian bridge inAylesbury town centre. The single-span cable-staybridge spanning the gap from the bus station to theFriars Square shopping centre was part of a majordevelopment to promote sustainable transport links inand around Aylesbury. The engineer Jacobs wanted toenhance the visual appearance of the bridge and alsothe sustainability of the construction materials. A70%GGBS mix was selected to give the mast its aestheticallypleasing lighter colour and to reduce the greenhousegas emissions associated with the concrete.

Afurther technical requirement for the concretewas that it had to be self-compacting, to assist in theconstruction of the 35m-high reinforced concrete mast.Test panels of the proposed concrete were prepared forcolour matching and calculations were done to ensurethat the heat evolution in the nearly 3m-thick plinth andbase was adequately low to avoid cracking. Hanson wasinvolved at various points of the design and specificationprocess and was frequently on-site for trial panels andtechnical control tests.

TyneTunnelThe new Tyne Tunnel is due to open in February 2011and is the third vehicle tunnel in the UK to be builtusing the immersed-tube technique. The 1.5km-Iongtunnel runs parallel to the existing Tyne Tunnel andwill create a dual carriageway between East Howdon innorth Tyneside and Jarrow in south Tyneside.

GGBS has been extensively used in the tunnel'sconstruction, in slurry cut-offwalls, in he precasttunnel segments and in the diaphragm walls. The fourimmersed tubes were precast concrete sections 90mlong x 8m high x 15m wide and required a total of14,400m3 of70% GGBS concrete, which was supplied byBardon Concrete.

Using its database of previous temperature studies,Hanson was able to provide Bardon Concrete with thedata and information needed to progress the mix designfor these tunnel sections, where the concern was tocontrol the early-age temperature rise and avoid thermalcracking.

A70% GGBS mix was also used in diaphragm wallsthat were up to 35m deep and in the north approachbase and roof slabs. The concrete for these was suppliedby Tarmac and as Dave Dobson, Tarmac's technicalmanager, says, "The area surrounding the diaphragmwalls had been used for shipbuilding and the concreteis up against some nasty chemicals in places." Theexposure classes determined by the ground conditionsled to the use of 70% GGBS in order to protect the

concrete from adverse chemical reactions. To speedup formwork striking times for the roof slab, HansonCement lent Tarmac its temperature-matched curingapparatus and trained Tarmac's staff to use it. Thisapparatus cures test cubes at the same temperature asthat achieved in the concrete structure and provided anaccurate method of determining the actual strength inthe roof slab.

The test cubes were tested at Tarmac's laboratory.Downloading the temperature data and preparingmaturity graphs enabled the concreting subcontractor,Dunne, to decide when it was safe to remove thesupporting props.

SellafieldAspecially produced GGBS is used by Sellafield Ltdin grouts for the encapsulation ofwaste from nuclearpower stations; these have a high GGBS:cement ratio, upto 9:1. As well as the provisionof grouts, Hanson reviewed andcommented on the 30-pagehandbook that Sellafield hasproduced to provide best­practice guidance for theproduction and use of concreteacross the whole Sellafieldsite. One current project is theconstruction of a 23m-high,football pitch-sized, additional'encapsulated product store' forintermediate-level waste.

The 32,000m3 of concreteneeded for the constructionof this store is being suppliedfrom an on-site ready-mixedplant situated inside theSellafield complex. Theconcretes contain up to 70%GGBS and were developed bySellafield, in conjunction withHanson Concrete and LaingO'Rourke, specifically for thischallenging application.

The ShardIn April, Hanson Cement supplied thousands of tonnesof GGBS to London Concrete for 5000m3 of 75%GGBS concrete in the base slab for the London BridgeTower. Nicknamed 'The Shard of Glass', this tower isdesigned by the Italian architect Renzo Piano and isbeing constructed over a working underground station.Standing at 310m tall, it is destined to be the tallestbuilding in Europe. At the bottom is the redevelopmentof London Bridge station's concourse and there will bepublic areas on floors 1-3. Above these will be 27 floorsof offices and then a luxury hotel on floors 34-52 andfinally ten floors of flats with panoramic views overLondon.

This is one of several jobs where Hanson has workedwith structural designer WSP Group and concretecontractor Byrne Bros. With the designer aiming to setnew sustainability standards for tall buildings, the use ofhigh-percentage GGBS in the base slab not only reducedthe propensity for early-age thermal movement andcracking but also provided a sustainable solution to acomplex engineering problem.•

A 70% GGBS mix suppliedby Hanson was used ascement replacement onBourg Bridge in Aylesbury.

www.concrete.org.uk JUNE 2010 concrete 55