r - The Hong Kong Polytechnic University

observed that the measured values closely aligned with the figures quoted in (CONTECVET 2001) of similar
exposure classes.
Figure 3 Corrosion rates measured in HKHA estates
Corrosion rate has much effect on the repair frequency, and the repair method used relates closely to the extent
of deterioration. Slab categorization is therefore necessary for these buildings for life-cycle repair cost analysis.
A virtual repair cycle taking into account of the above has been modeled as outlined below, for through-life
repair cost estimation for slabs in an aged estate:
• Determine the corrosion rates and present SAL for toilet, balcony and living slabs based on the corrosion
current measurements and opening-up.
• Categorize the slabs into 4 grades according to the SAL so measured. Grade A is the best with SAL less
than 8% and Grade D be the worst with SAL exceeding 30%, and estimate the relative proportion of slabs
falling within slab grade.
• Formulate repair solution for each grade of concrete deterioration and determine the relevant repair market
unit rates.
• Calculate the time taken by corrosion of each slab type (toilet, balcony and living room) propagating
through each specified SAL range.
• Determine the different starting points in the typical virtual repair cycle because of different present SAL
of each slab grade. It is assumed that the slab after complete re-hauling repair, a new repair cycle then
restarts again from zero SAL.
• Calculate the repair costs of each slab category along period of each deterioration grade and then sum up
all for over a specified time period, say 50 years, to obtain the estimated total slab repair cost for a typical
flat in the period.
The above concept is illustrated in “Figure 4”. Each slab may go through cycles of repairs of 4 repair modes (1)
no repair, (2) patch repairs, (3) extensive patch repairs and (4) large area spalling repair with bar replacement.
The cycles would depend on the choice of repair strategy.
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