r - The Hong Kong Polytechnic University

Balcony Slab
Repair Cost ( $ / slab )
Toilet Slab
(year)
(year)
– mode of repair (2)
Residual service life prediction
Figure 4 Illustrative diagram of repair cycles
If concrete repair is not so excessive as to be less than cost-effective, prediction of the building residual service
life can be made based on wall assessment. Using the corrosion rates measured in walls of these estates, the
likely residual period of time before the steel corrosion reach a state that inadequate reserve is available can then
be predicted at 90% probability. As discussed above, it is a prudent approach to adopt the values at 90%
probability for both the SAL and corrosion rate for strength assessment. The residual service life calculation
model is outlined as follows:
• Plot the Cumulative Frequency Curve of the latest measured SAL for vertical elements.
• Plot the steel corrosion rate of vertical elements based on representative corrosion rate measurements and
determine the 90 percentile value.
• Project the Cumulative Frequency Curves of SAL at different ages based on the 90 percentile steel
corrosion rate.
• Determine the strength reserve of the vertical elements, in terms of allowable SAL, based on loadings and
capacity of the elements.
• Predict the residual life of the estate at 90% probability based on the above.
AN ILLUSTRATIVE EXAMPLE
To illustrate the above prediction models, the details of the assessment carried out in one of the aged estates,
Estate E, was given below. This estate comprises buildings with age approaching 40 years. In this exercise, the
corrosion rate adopted is the mean or 90 percentile value of corresponding measurements taken in the group of
estates of similar conditions.
Based on the measured corrosion rates, the repair cycle for various categories of slab elements for Estate E is
established and integrated over the coming years, say 50 years, to arrive at a total life time cost.
As for the prediction of the residual service life of Estate E, the cumulative frequency plots of the SAL
measured at walls of the Estate at various residual life were shown in “Figure 5” below. Based on the allowable
strength reserve, it is estimated that Estate E can enjoy a residual life of more than 59 years under proper
building maintenance. It can be argued that with intermediate in-depth inspections and focused interventions,
further extension of the residual service life is achievable.
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