Chloride-Induced Corrosion in a Coastal Office Building
An eight-storey reinforced concrete office building in Colombo 04, completed in 1999, is 1.2 km from the sea. A 2024 condition survey found 0.3–0.8 mm vertical cracks and rust staining on 12 ground-floor columns on the seaward face. The columns are 450 × 450 mm with 8-T25 bars and carry four floors. Cover meter readings are 18–25 mm; half-cell potentials are more negative than −350 mV versus copper–copper sulfate over 40% of the tested surface; carbonation depth is 8–12 mm; chloride content is 0.25% by mass of cement at 20 mm depth. The building is occupied and the owner wants a 20-year extension without disrupting tenants. Original drawings show 30 MPa concrete and 40 mm cover, with no seismic detailing. You must advise on structural safety and repair.
1. Is corrosion active, and which mechanism dominates? Justify using the data.
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The data point to active chloride-induced corrosion, not carbonation-dominated corrosion. Chloride content of 0.25% by mass of cement at 20 mm depth is above the usual 0.2% threshold for acid-soluble chloride and is at rebar depth. Carbonation of 8–12 mm has not reached most of the 18–25 mm cover. Half-cell potentials more negative than −350 mV versus CSE over 40% of the surface indicate a high probability of active corrosion, although moisture and cover affect readings. The 0.3–0.8 mm cracks and rust staining are visible evidence of distress. Cover is below the specified 40 mm, so the chloride front can reach steel quickly, and the seaward face at 1.2 km from the sea is consistent with airborne chloride deposition.
Conclusion: corrosion is active and chloride-driven. Patch repairs alone risk incipient-anode corrosion around repaired areas.
2. What further investigation is needed before deciding between repair and strengthening?
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Before deciding, establish residual capacity and the extent of contamination. Breakouts on at least three columns should measure actual cover, bar diameter and section loss, and expose the bar/concrete interface. Take chloride profiles at 10, 20, 30 and 40 mm depth on several columns, plus carbonation depth and petrography. Use half-cell and resistivity maps to delineate anodic zones.
Check for vertical cracking, delamination, column shortening and beam deflection. Then assess column axial and bending capacity using measured section loss and the original loads; if capacity is uncertain, a load test may be justified. Review the foundation and beam-column joints, because the building has no seismic detailing and corrosion may have reduced confinement. The assessment should identify critical columns, whether temporary propping is needed before breakout, and whether repair alone can restore a 20-year life.
3. Recommend a durable repair and strengthening strategy for a 20-year life with tenants in place.
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Remove contaminated concrete behind the bars by hydrodemolition or controlled breakout rather than feather-edged patching. Clean bars to bare metal and replace any bar with more than 10% loss. Treat with a corrosion inhibitor and apply a shrinkage-compensated, low-permeability repair mortar. Because chlorides remain in adjacent concrete, patch repair can accelerate corrosion at the perimeter; consider electrochemical chloride extraction or cathodic protection for the most contaminated columns, especially on the seaward face.
For columns with significant section loss, strengthen with FRP wrapping or steel jacketing, which also provides confinement. Improve durability by increasing cover with a protective render or coating, sealing cracks, and improving drainage. Sequence the work column-by-column with temporary propping and tenant exclusion zones. Verify with half-cell readings and monitoring after repair; a 20-year life requires maintenance, not a one-off patch.