Trials organized within a week from contact to find effective rock support for wet conditions
The contractor had been experiencing issues with inadequate early age strength development of their sprayed concrete during construction of tunnel cross passages under the River Thames.
The biggest issue was the application of sprayed concrete onto extremely wet chalk excavated surfaces with active water ingress through many fissures.
The contractor was not achieving was to meet the early strength requirement of “J2 modified” set by the Crossrail project which with its original supplier and approach.
Normet was urgently called in by Hochtief-Murphy JV to attend the site and assist with the early age strength issues. A solution had to be found to allow safe and effective rock support in the wet conditions. Normet had to mobilise quickly to help the contractor get back on schedule.
- Trials within a week to find a solution for the contractor’s urgent problem
- Valuable insight on improving early age strength development optimization
- Cemex DSM solution helped the contractor to get back on schedule
- Auditing procedures on-site to ensure an efficient process
Trials were organised within the same week contact was made. Normet supplied the contractor with two types of alkali-free set accelerator for trial. Normet provided the standard TamShot 80AF, supplied to the majority of SCL contracts at Crossrail, and the more powerful accelerator TamShot 100AF.
In discussions with the Hochtief-Murphy JV team, the additional outcome to ensure improved performance was to switch to the Cemex dry silo mortar solution (DSM), as the Normet admixtures and cement type in the silo are also optimized for early age strength development, good opening times and ease of pumping.
Normet supported the contractor by auditing their procedures on-site to make sure all steps of the process from batching through to spraying were performed efficiently. Normet provided recommendations to further optimise early strength development, such as improved logistic options and using hot water and air from the compressor.
A Normet Minimec, a small tunnel diameter spraying robot, was utilized for sprayed concrete applications.
Graph showing compressive strength vs concrete age
Following successful trials with both set accelerators and the Cemex DSM solution, Hochtief-Murphy JV achieved very high early age strength development as shown in the chart. The construction of cross passages was conducted safely and effectively in the extremely wet ground conditions.