Borewell water with 640 mg/l chloride for a hospital raft
Your RMC plant supplies an RCC raft for a hospital annexe. The only assured water source is a borewell; municipal supply is intermittent. A NABL laboratory reports on the borewell water: pH 7.8, chloride 640 mg/l, sulphate 310 mg/l, total dissolved solids 4,200 mg/l, organic matter 150 mg/l. The plant manager wants to use it 'because the pH is fine and the sulphate is under limit', and proposes adding extra cement to compensate. The raft has 50 mm cover over reinforcement and will be cast in a hot, humid coastal district. A temporary aluminium access ladder has been bolted to the top reinforcement and shows white corrosion product within a week, while the steel below it is still bright. The client's engineer has asked for a written decision before batching starts.
1. Evaluate each water parameter against IS 456 limits and decide whether the water may be used for the raft.
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Compare with the IS 456 Table 3 limits for mixing water:
| Parameter | Result | Limit | Verdict |
|---|---|---|---|
| pH | 7.8 | not less than 6 | OK |
| Organic matter | 150 mg/l | 200 mg/l | OK |
| Sulphate (SO₄) | 310 mg/l | 400 mg/l | OK |
| Chloride (Cl) | 640 mg/l | 500 mg/l for RCC (2,000 mg/l for plain) | Exceeds |
| Total dissolved solids | 4,200 mg/l | 3,000 mg/l | Exceeds |
Decision: reject for the raft. The water fails on chloride, which matters because the raft contains embedded steel, and on total solids, which fails even the plain-concrete limit. The plant manager is applying only two of five criteria. Extra cement does not remove chloride; it only dilutes it slightly while raising heat of hydration and cost. The coastal, humid environment makes the chloride risk worse, because oxygen and moisture at the cover zone sustain the corrosion cell.
2. Explain chemically and electrochemically why 640 mg/l chloride is harmful, and why the aluminium ladder matters.
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In sound concrete the pore solution is highly alkaline (pH about 12.5–13.5) and maintains a passive iron-oxide film on the steel. Chloride depassivates that film locally, so pitting corrosion starts even where the pH is high. The reactions are:
- Anode: Fe → Fe²⁺ + 2e⁻
- Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻
Chloride raises the pore solution's conductivity (so the corrosion current flows more easily), migrates to the anode to maintain charge neutrality, and forms soluble Friedel's salt and iron chloride complexes that remove corrosion product and keep the pit active. The result is localised pitting, which is far more damaging to a 16 mm bar than uniform loss.
The aluminium ladder creates a galvanic couple: aluminium is anodic to steel in this environment, so it corrodes first, but the couple also changes the local potential and can accelerate attack at the steel contact, and aluminium corrosion product contaminates the cover concrete.
3. State the actions you would take on water, concrete specification and site hardware.
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Water: stop using the borewell water for reinforced concrete. Bring in potable water or treat the borewell water by reverse osmosis or blending with low-chloride water, then retest until chloride is below 500 mg/l and TDS below 3,000 mg/l. If the contractor disputes this, run comparative cube tests (borewell water versus distilled water) and require the 28-day strength not to be reduced by more than 10 per cent - but note that a strength test does not clear the chloride limit, because corrosion is a durability issue.
Concrete: keep total acid-soluble chloride in the concrete below 0.4 kg/m³ for RCC (0.2 kg/m³ if any prestressing is introduced). Increase cover only with the designer's approval; do not treat extra cement as mitigation. Specify a suitable cement type and a dense, well-cured mix.
Hardware: remove the aluminium ladder and use timber or FRP access; never leave dissimilar metals in contact with reinforcement. Record the decision and the retest results in the pour card.