Chloride-Induced Corrosion on a 35-Year-Old RCC Slab Bridge
A 35-year-old three-span RCC slab bridge on a state highway has a 7.5 m carriageway and 60 m total length. The soffit shows map cracking, rust staining and 10–15 mm deep spalling over about 15% of the area, with delamination near the expansion joints. A cover-meter survey gives 20–25 mm cover to reinforcement. Chloride content is 0.35% by weight of cement at 25 mm depth, carbonation depth is 15 mm, and half-cell potential readings average –350 mV CSE over 30% of the soffit. The bridge carries 20,000 PCU per day, including overloaded trucks, and the nearest alternative crossing is 30 km away. A load test is not immediately possible, but the structural drawings show 12 mm main bars at 150 mm centres and a 200 mm slab. The department must decide whether to repair the bridge, impose restrictions, or replace it within the next financial year.
1. Interpret the condition data and classify the corrosion activity.
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The data indicate active chloride-induced corrosion. Chloride content of 0.35% by weight of cement at 25 mm depth is close to or above the usual 0.4% threshold and is high enough to depassivate the steel, especially with only 20–25 mm cover. Carbonation depth of 15 mm has reached most of the cover, so carbonation-induced corrosion is also possible.
Half-cell potentials averaging –350 mV CSE over 30% of the soffit correspond to a high probability of corrosion activity, typically above 90%. Map cracking, rust staining, delamination and 10–15 mm spalling are visible consequences. The unknown is the loss of bar section; 12 mm bars at 150 mm centres can tolerate some loss, but the load rating must be checked.
The combination of chloride, carbonation and low cover means corrosion will continue unless the environment at the steel is changed.
2. Compare patch repair, full-depth repair with protection, and replacement. What would you recommend?
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Patch repair alone is risky. Cutting out spalled concrete and patching with ordinary mortar can create incipient anodes around the patch, because the surrounding steel remains corroded and the patch becomes a cathode–anode boundary.
A better strategy is full-depth repair of delaminated areas, removal of chloride-contaminated concrete behind the bars, and replacement with a low-permeability repair mortar. Then protect the whole soffit with a silane or migrating corrosion inhibitor, and consider cathodic protection if chloride contamination is widespread.
If the structural check shows the slab capacity is adequate for 20,000 PCU per day, repair and protection can extend life by 10–15 years. If the load rating is deficient, replace the deck or strengthen it with an overlay or external prestressing. Replacement within one year may be unavoidable if section loss is severe.
3. Recommend immediate actions, load restrictions and monitoring before the repair or replacement decision is finalised.
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Immediate actions:
- impose a load restriction, such as a 10 t gross vehicle limit, and enforce it with a weighbridge or physical barrier;
- carry out a detailed load rating using the measured section loss and original reinforcement details;
- map half-cell potentials and chloride profiles at 25, 50 and 75 mm depths to define repair areas;
- repair the expansion joints, because leaking joints are a major chloride source.
If the rating is marginal, provide temporary propping under the span or restrict traffic to one lane. Install corrosion monitoring and inspect after each monsoon. Plan replacement or major rehabilitation in the next financial year, and prepare a traffic management plan for the 30 km detour.