Tunnel Segment Concrete

Tunnel Segment Concrete

Modern tunneling methods in unstable rock use 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 heavy weight (up to several tones each), tunnel segments are almost always produced near 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 5hours in an autoclave at 50–60°C to obtain the necessary demoulding strength.

Composition

Aggregate

·     Normally 0–32 mm in the grading range according to EN 480-1

Cement

·     Cement content 325 or 350 kg/m³

·     CEM I 42.5 or 52.5

Placing

·     The fresh concrete mix tends to stiffen rapidly due to the high temperature, making correct compaction and finishing of the surface difficult.

·     Due to the rapid industrialized process, a plastic fresh concrete consistence can be used. The desired initial strength can only be obtained by a low water/cement ratio, which should therefore always be<0.48.