A LIFE SPAN OF AT LEAST 100 YEARS - The Westerschelde Tunnel: Approaching Limits - page 69

Fig. 6.9
Basic cross section of a
bored tunnel for road
traffic indicating the
various salt
environments for the
sustainability

Zone subjected to
thawing salts
(chloride penetration)

Splash zone

Tidal zone

Underwater zone

The penetration of chloride in the concrete of the tunnel in which the critical
content at the position of the reinforcement is exceeded, can be portrayed
in a calculation model.The calculation result then provides a definite answer
about the compliance or non-compliance with the requirement of a 100 year
life span.

Design proposal of segments and optimizations

The first design proposed by the contractor did not withstand the life span
calculations and that is why the design of the segments was adjusted.

Composition of concrete
Although the client's preference was for a composition of concrete with
blast-furnace cement, the contractor eventually proposed applying Portland
cement.
This was mainly inspired by the desire to be able to demould the segments
quickly so that a higher rate of production was possible. Due to the fact that
Portland cement has a higher early strength, the application of this cement type
was desired. This desire was honoured. In order to be able to achieve a suf-
ficiently dense concrete however, fly ash was added.This gives the concrete
a finer pore structure through which penetration by (hazardous) substances
is limited.
The eventual composition of the concrete was thus:
- Portland cement (CEM-I) 310 kg/m 3

80 kg/m 3 - fly ash

120 kg/m 3

- water

750 kg/m 3

- sand 0/2

520 kg/m 3

- gravel 2/8

590 kg/m 3 - gravel 8/16

Quality of concrete
The required concrete strength of B55 was amply achieved with the above-
mentioned composition and its associated additional measures: on average
one could count on a pressure strength of B65-B70.

Concrete cover
The first design proposal by the contractor provided for a concrete cover of
50 mm on the inner side and outer side of the segments. However, on the
side surfaces the contractor wanted a smaller cover. By keeping the con-
crete cover small here, the jacking forces of the tunnel boring machines
would quickly be transferred onto the reinforcement. The smaller the cover,
the smaller the chance of damages as a result of this mechanism, but the