A LIFE SPAN OF AT LEAST 100 YEARS - The Westerschelde Tunnel: Approaching Limits - page 63

Introduction
A requirement set for the design of the Westerschelde Tunnel, is that the
tunnel must have a life span of at least 100 years. Of course this does not
mean that the tunnel would collapse after 100 years, but it does mean that
the chances of a certain limit state being exceeded after 100 years, would
be acceptably small. In the Netherlands, the requirement of a life span of
at least 100 years was first set for the Second Heinenoord Tunnel, the first
traffic tunnel bored in the soil of the Netherlands. However, it was not
explicitly determined whether that tunnel complies with this requirement.
In the case of the Westerschelde Tunnel, the requirement was proposed
again, and by making use of the European research programme DuraCrete,
it could actually be proven that the design of the tunnel complies with this
limit state.
Apart from the life span of the tunnel, calculations were also carried out in a
similar fashion for the access ramps with a concrete quality of B-35 with
blast furnace cement (CEM-III), and required concrete cover of 45 mm
(50 mm has been applied). For the outside of steel segments with an
average corrosion rate of 0.08 mm per year, a corrosion allowance of 23 mm
is calculated.
Practices in the past
Naturally there have always been requirements set in respect of the life span
of structures, at least for those sections of structures which are not simply
replaceable.
There are detailed provisions in the Regulations for Concrete Structures and
in the Regulations for Concrete Technology for concrete structures, which
must ensure that a structure actually lasts for a sufficient length of time.
These provisions are implementation rules and concern matters such as the
least amount of concrete cover required on the reinforcement, the maximum
value of the water/cement ratio and the type of cement. The concrete struc-
tures that comply with these provisions are assumed to last for a sufficient
length of time. In practice it has been proven, that some maintenance
and repairs are necessary to maintain concrete structures.The disadvantage
of the life span requirements that follow on from the mentioned concrete
regulations is that no relationship is placed between the envisaged life span
and the maintenance that will be necessary.
For the design of important concrete structures in the Netherlands, often extra
requirements are set on the life span. For the Haringvliet locks this meant
an increased concrete cover, the use of blast-furnace cement concrete, a
water/cement ratio of 0.45 at most, and the requirement to apply preten-
sioned concrete as much as possible. For storm-surge barriers built later,
such as the Oosterschelde and the Maeslant storm-surge barriers, the explicit
requirement of the life span was set at 200 and 100 years respectively.
However, it was rather difficult to prove that these life span requirements had
been complied with.The most important reason for this was the absence of a
generally acceptable method of determination. For the Oosterschelde storm-
surge barrier, deterioration models were set up for example, from which the
expectation was assumed that the concrete cover would have to be
replaced after just more than 80 years.This section could therefore not com-
ply with the requirement set. For the Maeslant storm-surge barrier, enlarged