A LIFE SPAN OF AT LEAST 100 YEARS - The Westerschelde Tunnel: Approaching Limits - page 67

Fig. 6.6
Calculation of the
failure rate over all the
years provides the life
span spread

Design on the basis of a reference period

R(t)

R, S

Spread of R(t)

S(t)

Spread of S(t)

Time

Average life span

Design
life span

P acc

Spread of life span

Life span design

Deterioration mechanisms

Within the state of the technology at that time, it was not entirely possible to
draw up a life span design for all forms of deterioration. No life span calcu-
lations could be made for example, for frost/thawing salt attack and alkali-
silica reaction. In order to solve this problem, the choice was made to take
on the strategy of the avoidance of these forms of deterioration. For this,
requirements were set for the water/cement ratio, the composition of the
binding agent and the sensitivity of the aggregate for alkali-silica reaction.
The assessment by experts in respect of the expected life span were reverted
to for the rubber seals. This assessment came about on the basis of data
gathered about the composition of the rubber, the glueing together, the
available results of 'accelerated ageing tests' and experience.
The life span calculations were made for the following limit states:
- corrosion arising; corrosion can occur if there is too high a content of
chloride present in the concrete at the reinforcement, or when the con-
crete is carbonatated.The non-exceeding of this limit state within a period
of 100 years was normative for the design of the Westerschelde Tunnel.
- depression of the cover as a result of corrosion of the reinforcement,
which could lead to serious leakages;
- failure of the structure.

The following deals with the first abovementioned limit state (arising of
corrosion).

Causes of corrosion

Corrosion produces rust that forms a deposit on the reinforcement steel.The
volume of rust is greater than the volume of the original steel. This causes
stresses in the concrete which, in the long run, the material cannot withstand.
The concrete breaks and eventually this can lead to serious leakages.
Normally concrete forms a perfect protection of the reinforcement against
corrosion, whereby the basic environment avoids rust-forming.This protection
can however be countered in two ways: through penetration by chlorides -
from sea water or road salt - or through carbonatation in which carbon di-
oxide (CO 2 ) in the air fuses with hardened cement paste in the concrete and
the alkaline environment is lost. In any case, a condition for corrosion of
concrete is, that there is an adequate supply of oxygen in the concrete and