Balancing Contractor’s needs with the Clients’s requirements beneath the heart of London
The BFK JV projects, including C435 Farringdon Station, are major sprayed concrete tunnelling projects beneath London’s high-value structures.
The Crossrail Sprayed Concrete Lining (SCL) design at C410 is comprised of a permanent sprayed concrete primary and secondary lining for Tottenham Court Road and Bond Street underground stations.
The SCL design was created for all platform tunnels, concourses, and cross passages, including ventilation tunnels and shafts. In addition to the C410, the project included two crossover caverns and a shaft at Fisher Street.
The key challenge on Crossrail C410 was balancing the Contractor’s need for an economical, logistically smart solution, with the Client’s requirements of safe, early-age strength development coupled with long-term durability and mechanical performance.
The mission was to develop a pumpable, flow-retaining wet-mix sprayed concrete that could be delivered from either an on-site or off-site batching plant. The mix had to meet the early strength requirement of “J2 modified” and the physical properties of a permanent, durable, fibre-reinforced sprayed concrete lining.
Regulating the layer mixes for the successful application of spray-applied waterproofing membranes and the fire protection sprayed concrete finishing layers were also addressed during the project.
Normet worked full-time with the BFK JV team to develop and fine-tune concrete mixes even after the successful trial period. The team 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 use Normet’s TamCem 60 high-performance plasticizer and TamCem HCA hydration control admixture, the latter extending the open time of the mixes.
The early age strengths required for the project were initially based on the addition of a 6% TamShot 80AF AFA set accelerator at the nozzle, and later this increased slightly as the project demanded higher earlier strengths from an age of 10 mins.
Later in the project, The BAM JV team employed Normet spraying equipment to improve the quality and the homogeneity of the sprayed concrete lining at Fisher Street. Normet’s latest pulsation free pumps and robots, such as the Spraymec 8100, NorStreamer and Minimec proved it was feasible to reduce accelerator dosage as the spraying performance was optimized.