Tunnelling and Mining Concrete

Modern tunneling methods in weak rock conditions demand concrete segments which are immediately load bearing as linings to the fully excavated tunnel section. Precast concrete units called tunnel segments perform this function.

Production Due to the large numbers required and heavyweight (up to several metric tons each), 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. It is then heated for about 5 hours in an autoclave at 50 – 60 °C to obtain the necessary strength for formwork removal.