Due to the large numbers required and heavy weight, tunnel segments are almost always produced close to the tunnel portal in specially installed precasting facilities. They have to meet high accuracy specifications. Heavy steel formwork is therefore the norm. Because striking takes place after only 5 – 6 hours and the concrete must already have a compressive strength of >15 N/mm², accelerated strength development is essential. There are several methods for this. In the autoclave (heat backflow) process, the concrete is heated to 28 – 30 °C during mixing (with hot water or steam), placed in the form and finished.
Tunnel segment production combines the challenge of realization of a specified high early strength and fulfilment of highest requirements regarding durability. Strength development is usually secured by utilization of heat or steam curing which can be contradictory to durability if the concrete core temperature is too high. The concrete performance regarding early strength and durability can be enhanced with the SikaRapid® technology.
With the application of SikaRapid® the hardening process of the concrete was optimized, with the result that approximately 150 minutes of heating could be eliminated. At the same time the early and final strength requirements were attained. Moreover the durability of the tunnel segments was improved as the concrete peak temperature was limited to less than 60°C.