Supplying the contractor with over 60% of the sprayed concrete solutions for Europe’s largest Metro project
Contractor
Bam Ferrovial Keir JV
Processes
Sprayed ConcreteLocation
United Kingdom
The Project
The BFK JV projects C435 Farringdon Station and C410 Bond Street and Tottenham Court Road Stations were significant major sprayed concrete tunnelling projects beneath London’s key high-value structures.
The Crossrail Sprayed Concrete Lining (SCL) design at C435 Farringdon Station comprises of a permanent sprayed concrete primary and for the most part a cast insitu secondary lining for both the main platform and concourse tunnels.
The SCL design was used for platform tunnels, concourses, cross passages and ventilation tunnels and shafts
The Challenge
At Farringdon Station, of particular importance was the need for very high early-strength concrete to ensure full ground support in the mixed Lambeth Group formations, which can often be sandy and sometimes can flow uncontrollably if there is a lot of groundwater present.
Key London Underground and Overground railways exist above the Farringdon Station Crossrail tunnels. Therefore toolbox measures such as dewatering and probing were vital, coupled with a high-performance, reliable sprayed concrete solution to ensure fast ground stabilisation and reduced settlement.
The sprayed concrete challenge involved developing a pumpable, flow-retaining wet mix sprayed concrete that could be delivered from either an on-site or off-site batching plant to meet the early strength requirement of “J2 modified” from 10-minute age and the physical properties of a permanent, durable, fibre reinforced sprayed concrete lining.
Quick Facts
- Supplying over 60% of the sprayed concrete solutions on Europe’s largest Metro project
- A pumpable, flow retaining wet mix sprayed concrete designed by Normet
- The mix could be delivered from an on-site or off-site batching plant
- Contractor’s downtime and wastage reduced thanks to efficient logistic control
The Solution
Normet worked full time with the BFK JV team to develop and fine-tune concrete mixes even after the successful trial period and eventually defined two mixes based on open time: a 4 to 6 hr mix for site-batched concrete, and an 8 to 10 hr mix required to facilitate off-site batching and delivery through the busy London area.
Both mixes used Normet’s TamCem 60 high-performance plasticizer and TamCem HCA hydration control admixture, the latter being used to extend the open time of the mixes.
To provide the enhanced early age strength performance, compared to the requirements at C410, BFK JV selected higher dosages of TamShot 80AF set accelerator, typically between 7 and 8% by weight of cement to achieve the J2 (as illustrated) modified requirements after spraying to 30 minutes age. The higher-performing TamShot 100AF was used for sections where a higher risk of ground problems could arise.
Graph showing compressive strength vs concrete
Project Outcome
Normet successfully supplied BFK JV with over 60% of the sprayed concrete solutions on the largest Metro project in Europe.
The logistic control of the mix and supply arrangements enabled the contractor to reduce downtimes and wastage to a minimum and achieve extremely high-performance permanent sprayed concrete for both primary and secondary lining shells.
