S. Helland · Design for service life: implementation of fib Model Code 2010 rules in the operational code ISO 16204
Level of reliability – consequences of failure
fib MC SLD, fib MC-2010, EN 1990 and ISO 2394 all suggest a three-level differentiation of the consequences upon passing an LS: a) risk to life low, economic, social and environmental consequences small or negligible b) risk to life medium, economic, social and environmental consequences considerable c) risk to life high, economic, social and environmental consequences very great Based on the relevant consequence class, combined with a consideration of the cost of safety measures, a relevant level of reliability for not passing the LS during the design service life should be selected. Within the limitations normally found in national building legislation, the reliability level used in the design should be agreed with the owner of the structure. fib and ISO suggest a probability of failure pf = 10–1 for depassivation of reinforcement (by carbonation or ingress of chlorides) in cases where the presence of oxygen and moisture makes corrosion possible. If collapse is the LS considered, pf = 10–4 to 10–6 may, as for traditional structural design, be the relevant level if the possible consequences are in classes b) and c).
Deterioration ((corrosion)
4
Collapse of structure pf ≈ 10 -4- 10 -6
20
15
Spalling
10
Formation of cracks Depassivation pf ≈ 10 -1
5
0 0
5
10
15
20
25
30
35
Time
Fig. 2. Various limit states and related reliability levels shown for corrosion of reinforcement
100
C 50% cumulative failure (%)
tion), LS depassivation is the choice of convenience for most engineers.
75
B 30%
50
A 2%
25
10%
0
5
End of service life
0
50
100
150
years
Based on the above, a main element in the fib and ISO documents is therefore an amended quantitative definition to the qualitative one we find in traditional standards such as the ones in ISO 2394 or EN 1990: Traditional qualitative definition: The design service life is the assumed period for which a structure or part of it is to be used for its intended purpose with anticipated maintenance but without major repair being necessary. Quantitative amendment by fib and ISO: The design service life is defined by: – A definition of the relevant LS – A number of years – A level of reliability for not passing the LS during this period Fig. 2 indicates how various limit states may be associated with corresponding levels of reliabilities for not passing the LS within the design service life in the case where corrosion of reinforcement is the critical case. In principle, the verification of the design has to demonstrate that the structure will satisfy all combinations of LS and pf. For practical design, however, we do not have time-dependent models with international consensus to predict the corrosion phase after depassivation. The calculation therefore often has to be based on the time up to depassivation. The corresponding pf must then be sufficiently low to ensure that this LS results in equal or stricter requirements for material and depth of cover than the other combinations.
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Structural Concrete 14 (2013), No. 1
Fig. 3. Time until depassivation of surface reinforcement (example derived from [16]). The Norwegian Standardization body applied a 10 % acceptance for depassivation as a criterion when determining its durability provisions, whereas countries A, B and C applied 2, 30 and 50 % respectively.
When considering the effect of corrosion of the reinforcement after its depassivation, splitting stresses in the cover zone from the reinforcement due to the effects of other mechanical actions/loads should also be considered. Wherever there are bond stresses in the reinforcement there are also “bursting stresses” in the concrete of the same nature as those from the expanding corrosion product, ultimately leading to the same type of cracking and spalling of the cover. This is another argument for avoiding the minefields of using cracking and spalling as the LS for service life deign. If we are pursuing the example of depassivation due to carbonation, all the characteristics that determine when the individual reinforcing bars will depassivate in a structure will have a statistical spread. This includes the actual depth of cover, the microclimatic conditions, the humidity of the concrete, its curing, etc. As a result, the initiation period will also exhibit a statistical spread. Fig. 3, derived from Bamforth [16], indicates the accumulative time for depassivation of the surface rebars in a structure subjected to carbonation. To assess the actual service life of this structure, the depassivation LS has to be