Replacement of ceiling in Sonnenberg tunnel, Cityring Lucerne

Initial Situation

As part of the complete renewal of the Cityring Lucerne, several challenging construction projects had to be carried out. Traffic through the city was severely affected by the construction measures. It was a major concern to shorten the closure periods. The intermediate ceiling in the Sonnenberg tunnel had to be constructed in just one weekend closure. This included formwork, reinforcement, concreting, and stripping. The time window from concrete production to reaching a compressive strength of 40 N/mm² was a mere 10 hours. Nevertheless, the concrete had to have an open time of 3 hours for pumping.

Solution and Assessment

The ambitious solution concept was implemented with an adapted variant of Concretum® Q-FLASH 2/20. The work schedule was very detailed and tailored to the concreting work. Excellent planning and very good cooperation between the concrete producer, concrete pump, and contractor were the key to success. The concrete could be pumped and placed without any loss of consistency for 3 hours. With the beginning of the setting process, a rapid strength development started. The formwork could therefore be lowered as planned, and the tunnel could be reopened to traffic on Monday morning.

Explanation of Key Terms and Concepts

• Cityring Lucerne: A ring road around the city of Lucerne.
• Sonnenberg tunnel: A specific tunnel within the Cityring Lucerne.
• Intermediate ceiling: The ceiling between two floors or levels within a structure.
• Concretum® Q-FLASH 2/20: A specific type of high-performance concrete designed for rapid strength gain.
• Compressive strength: The maximum amount of compressive stress that a material can withstand before failure.
• Open time: The period during which fresh concrete can be worked with before it starts to set.
• Formwork: The temporary structure used to support concrete while it cures.
• Stripping: The process of removing the formwork from the hardened concrete.

Summary of the Challenge and Solution:

The construction team faced the challenge of completing a complex concrete structure in a very short time frame. They were able to achieve this by using a specially formulated high-performance concrete that developed strength rapidly. This allowed them to remove the formwork quickly and reopen the tunnel to traffic on schedule. The success of the project was attributed to careful planning, coordination between different teams, and the use of innovative materials.