Maintenance of the underground line requires teams of workers to enter the tunnels during weekends and nights. Any concrete used during the maintenance works must achieve the required strengths incredibly quickly, so that the underground line may be operated the following morning. Efficiency is therefore critical to enable the maintenance team to carry out the necessary activities and hand over the line for operation within very small windows of time.
Rapid-setting concrete mixes are used to enable key construction activities to be carried out quickly, but they need to meet the required strength in < 24 hours from the time of placement before the line becomes operational again. Cubes are collected when the concrete is delivered, and taken to a lab to be stored and crushed periodically. Labs can remain open during antisocial hours, but communication between the lab and the workers in the tunnels is particularly challenging. In addition, the cube samples themselves cure more slowly in the lab than the in-situ material (with ambient temperatures of 30°C in the tunnels, curing the cubes at 20°C results in underestimates in strength) building in unnecessary delays.
Closely supported by the Converge team, the TFL maintenance team installed a network of repeaters from above ground down into the tunnel. Temperature sensors were installed into the concrete and strength estimates were obtained via the Converge platform. Key strength milestones were set up as text and email alerts so that the team could be informed as soon as the concrete reached strength. The in-situ strength estimates are not only more accurate than cubes, but there is no delay in obtaining that information. Instead of requesting cubes to be crushed every other hour by a test-house, the Converge platform calculated the strength every 10 minutes.
The Converge system consistently improved the speed at which the maintenance team could work, allowing them to hand over the line by the agreed time. During the first deployment, works began at midday on Sunday, with handover by 6:00am on the Monday morning. The concrete needed to be at 15MPa before the line can be operated, and the cubes had still not reached 11MPa by handover time. Had Converge not been used, this would have been a disastrous result. Fortunately, due to the use of Converge’s in-situ strength monitoring system, the maintenance team were able to hand over the line with 20 minutes to spare.