Soffit Delamination in a 1983 Parking Garage
A 1983 cast-in-place parking garage in a cold climate uses deicing salt. On the soffit of a two-way slab above a drive aisle, inspectors see map cracking and rust staining across a 6 m × 9 m bay. Chain-drag sounding finds 22% of the bay delaminated, concentrated along the bottom mat. Covermeter readings are 28–42 mm. Cores taken between delaminations show carbonation depth 6–9 mm, acid-soluble chloride 0.12% by mass of concrete at 25 mm and 0.05% at 40 mm. Half-cell potentials (CSE) range from −180 mV in sound areas to −420 mV in delaminated areas. Petrographic examination shows no freeze–thaw cracking and no alkali–silica gel. The owner wants to patch only where the hammer sounds hollow and reopen in 48 hours. Traffic must be maintained in the adjacent bay. No post-tensioning is present.
1. State the dominant deterioration mechanism and explain how the carbonation, chloride, half-cell, and petrographic data support it. What additional information would you request before selecting the repair type?
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The dominant mechanism is chloride-induced corrosion of the bottom mat. Carbonation is only 6–9 mm while cover is 28–42 mm, so the carbonation front has not reached the steel and cannot explain the delamination. Chloride at 25 mm is 0.12% by mass of concrete, above the usual 0.05–0.10% threshold region, and drops to 0.05% at 40 mm, consistent with a surface chloride profile. Half-cell potentials are much more negative in hollow areas, and rust staining plus delamination confirms active corrosion. Petrography rules out freeze–thaw and ASR. Before choosing partial-depth versus full-depth repair, request rebar diameter and actual depth to bottom steel, section-loss measurements, a full chloride profile to 50 mm, concrete compressive strength, and whether the slab is post-tensioned. Those determine whether 20 mm removal can get below the bar and whether remaining chloride will sustain corrosion.
2. The contractor proposes 20 mm deep partial-depth patches limited to hollow-sounding areas. Evaluate that proposal and describe a defensible repair boundary.
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Reject the proposal as written. With cover 28–42 mm and a typical 16 mm bar, 20 mm removal does not reach the bottom of the bar or the delamination plane, which usually follows the corroded bar. The patch would leave contaminated concrete and active corrosion at the perimeter, causing ring corrosion. Remove concrete around each corroded bar to expose its full circumference, and extend the cavity at least 25 mm beyond the hollow boundary into sound, uncontaminated concrete, with square saw-cut edges. Where chloride remains above threshold at the proposed depth, either increase removal or use a corrosion-control system. Confirm boundaries by re-sounding and chloride testing at the cavity base and sides, not by the original chain-drag alone.
3. What would you require before accepting the prepared cavities and allowing placement?
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Acceptance should be based on inspection, not time. Require all delaminated and unsound concrete removed; bar exposed around its circumference; no undercut or feathered edges; cavity edges square and at least 25 mm beyond the last hollow point; a roughened profile free of laitance, dust, and oil; and substrate saturated-surface-dry with no standing water if a cementitious material will be used. Rebar should be cleaned to bright metal without over-blasting, and section loss must be measured against the engineer's limit. Retest chloride at the cavity base and sides, confirm restored cover of at least 40 mm, and hold a joint inspection with photographs before placement.