Civil Engineering Reference
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greater the 'conflict' with the requirement of a 100 year life span. At the pos-
ition of the rubber seal and the 'dowel recess' connection, the cover on the
reinforcement however, was only 20 mm. On the basis of the life span cal-
culation it was necessary to adjust the design of the segments with regard to
the cover. The concrete cover on the inner side and outer side remained
50 mm, but on the side surfaces the cover was increased to 50 mm, with the
exception of the places for the rubber seal and the dowels and recesses.
Here a new calculation was made, in the first instance with 30 mm and when
this also appeared to be inadequate, even a concrete cover of 35 mm.
The enlargement of the concrete cover on the side surfaces of the segments
and at the positions of the genuinely critical points of the concrete dowels
and recesses, brought the design close to the required life span, but the
chances were still greater than 1% that the reinforcement could corrode within
the period of 100 years:
Carbonatation (requirement
1.8)
-
no problem for all covers; 8.6
Fig. 6.10
The reliability index
versus the exposure
time in the event of
penetration by chloride
up to a level whereby
corrosion comes about
3.60
3.00
2.40
1.80
1.50
1.20
0.60
0
20
30
40
50
60
70
80
90
10 0
Time of exposure in years
Chlorides from sea water or thawing salt (requirement
1.8)
-
cover 50 mm; 3.5
-
cover 30 mm; 0.7
-
cover 35 mm; 1. 5
(in which a dependability (beta) of 1.8 is equal to a chance of 1/100 that the
limit state is exceeded).
Additional measures
After 100 years the adjustments would result in a reliability index of
1.5.
The design requirement was
1.8, which was not entirely complied with.
It was therefore necessary to take additional measures. In principle various
avenues were open for this, which included the application of a stainless
steel reinforcement at those sections of the segment where concrete cover
was inadequate; the fitting of a cathodic protection; or the taking of after-
treatment measures which include the hydrofobing of the concrete.
The most far-reaching measure is the application of a cathodic protection.
Due to the relatively high price tag of this solution, it was not an option for
the contractor. The use of stainless steel with which corrosion is postponed
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