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 C435 Farringdon Station and C410 Bond Street and Tottenham Court Road Stations projects were major concrete tunnel projects designed under key high-value structures in London.
Crossrail's shotcrete lining (SCL) design at C435 Farringdon Station consists of a permanent primary shotcrete lining and, for the most part, a secondary in-situ lining for both the main platform and concourse tunnels.
The SCL design was used for the platform tunnels, concourses, crosspasses, and tunnels and ventilation shafts.
The Challenge
Of particular importance at Farringdon station was the need for very high initial strength concrete to ensure full ground support in the mixed formations of the Lambeth Group, which can often be sandy and sometimes flow uncontrolled if there is a lot of groundwater present.
Above the Crossrail tunnels, at Farringdon station, are key London Underground and Overground rail tracks. It was therefore vital to adopt measures such as dewatering and borehole drilling, together with a reliable, high-performance shotcrete solution to ensure rapid ground stabilization and reduce settlement.
The shotcrete challenge was to develop a wet, pumpable, flow-retaining mix that could be supplied from an on-site or off-site batching plant to meet the "modified J2" early strength requirement from 10 minutes of age and the physical properties of a permanent, durable, fiber-reinforced shotcrete lining.
Quick Facts
- A pumpable, flow retaining wet mix sprayed concrete designed by Normet
- Supplying over 50% of the sprayed concrete solutions of Europe's largest Metro project
- Contractor's downtime and wastage reduced thanks to efficient logistic control
- The mix could be delivered from an on-site or off-site batching plant
The Solution
Normet worked full time with the BFK JV team to develop and refine the concrete mixes even after the successful trial period and ultimately defined two mixes based on open time: a 4 to 6 hour mix for in-situ batched concrete, and an 8 to 10 hour mix needed 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 used to extend the open time of the mixes.
To improve early-age strength performance compared with C410 requirements, BFK JV selected higher dosages of TamShot 80AF set accelerator, typically 7-8% by weight of cement, to achieve modified J2 requirements (as illustrated) after 30-minute age screening. The higher-performing TamShot 100AF was used for sections where a higher risk of soil problems could arise.
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.
