r - The Hong Kong Polytechnic University

eserve. Each SAL value is that figure associated with the most corroded section of the bars exposed in a small
open-up of typically 300mm by 150mm. Determination of representative values for these parameters for a
tenant-occupied building is however extremely difficult. Not only sampling and testing are greatly affected by
the presence of tenants and furniture, these parameters themselves are very complex variable, which vary
between buildings, between elements and even within element. Strength reserve determination is even more
complex bearing in mind of the large number of elements, many loading combinations and re-distribution of
loadings.
The rate of deterioration of an existing building and hence its residual service life are related to a multitude of
factors, from the original design standards, the construction workmanship to subsequent level of maintenance as
well as other internal and external environmental effects. The factors will act interactively to give complicated
multiplying effects.
All these call for a simplified approach. Strength reserve is established generally being mindful that proactive
inspection and intervention focused on the vulnerable local elements may be a better option to keep the building
at an acceptable reliability level. Prediction of building residual life which is related to structural safety,
demands a higher reliability in comparison with repair cycle projection. Mean values of SAL and corrosion
rate are not used in the assessment as a significant probability of target reliability level may not be achieved.
Instead, 90 percentile values of the parameters are adopted as a prudent approach.
METHODOLGY
Through-life maintenance cost
Spalling repair for HKHA aged buildings has long been attributing a considerable proportion of the overall
building maintenance expenses. The average yearly repair cost for different concrete elements or the same
element type but at different locations varies greatly, depending very much on the present condition and its rate
of deterioration. Reinforcement corrosion rates so far measured in 7 HKHA older estates for elements under
different usages show great variances. The toilet slabs which are subject to cyclic wet and dry exposure due to
showering water were found to be undergoing much acute corrosion whilst readings taken at constantly dry
living room slabs and walls are generally mild. As shown in Table 1, the overall mean values for toilet, balcony,
living room slabs and walls are respectively 0.0454, 0.0201, 0.0023 and 0.0022 mm depth/year. On estate basis,
the mean and 90 percentile values for all four types of elements are also shown in Table 1. The values used in
the assessments, i.e. mean values for slabs and 90 percentile values for walls are then gauged against the values
of similar exposure conditions specified in EN206 (ECS 2000). Table 2 shows the corresponding corrosion rates
for various exposure classes from CONTECVET Report 2001 (CONTECVET 2001).
Table 1 Corrosion rates measured in 7 old estates
Corrosion rates [mm/year]
Toilet slab Balcony slab Living room slab Wall
Mean 0.0207 – 0.0725 0.0086 – 0.0298 0.0012 – 0.0044 0.0006 – 0.0053
90 Percentile 0.0510 – 0.2570 0.0119 – 0.0760 0.0019 – 0.0138 0.0018 – 0.0150
Overall mean 0.0454 0.0201 0.0023 0.0022
Table 2 Corrosion figures in CONTECVET Report 2001
CONTECVET Report 2001
Exposure Classes
(Chloride initiated corrosion)
Icorr [μA/cm 2 ]
D1 Moderate humidity 0.1 - 0.2
D2 Wet - rarely - dry 0.1 - 0.5
D3 Cyclic wet - dry 0.5 - 5
S1 Airborne sea water 0.5 - 5
S2 Submerged 0.1 - 1.0
S3 Tidal zone 1 - 10
Note 1: CONTECVET 2001 (Section C5) also suggests applying a pitting factor (α) of 5 to 10 to arrive at the
depth of pitting penetration.
A comparison plot of measured corrosion rates against those in (CONTECVET 2001) is given in Figure 2. It is
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