Expansion of the Riedgrabenweg railway underpass, Zürich

Initial situation

The railway underpass in Riedgrabenweg in Zürich – Oerlikon had to be extended. Rail traffic had to be maintained throughout the entire construction period with as little disruption as possible. The project envisaged prefabrication of the bridge slab in two parts as precast elements, which were then pushed together on two weekends so that only one track had to be closed at a time during this period. Only the header beams, a type of frame wall for the bridge slabs and supports, were to be concreted on site and bonded to the bridge slab monolithically.

Solution and assessment

The rapid-hardening concrete Concretum^® Q-FLASH 2/20 was selected. A concrete with Concretum^® C-STRONG was also discussed during the project planning phase. The ten hours in which this concrete develops a strength of 20 N/mm², would just have been enough for the construction program to be complied with. However, with rapid-hardening concrete applications the drying often plays a critical role as well, so that any sealing can be applied as rapidly as possible. In this case, the seal had to be applied no later than 7 hours after completion of the concreting work. This is only possible with Concretum^® Q-FLASH 2/20. While the fact that a strength of 20 N/mm2 is reached after 2 hours was not solely critical for this project selection, this quality gave the contractor a welcome degree of flexibility in a very short construction program.