India Coastal Exposure Map — IS 456 Cl. 8.2
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About IS 456 exposure classes
Salt-laden coastal air carries chlorides that penetrate concrete cover, depassivate steel, and trigger corrosion. IS 456 Cl. 8.2 defines five exposure classes with progressively stricter mix + cover requirements. Classifications scale with distance from coast: 0-5 km (Very Severe), 5-30 km (Severe), > 30 km inland (Moderate / Mild).
Mix + cover requirements (IS 456 Table 5 + Table 16)
- Mild: cement ≥ 300 kg/m³, w/c ≤ 0.55, M20+, cover 20mm.
- Moderate: 300 kg, w/c ≤ 0.50, M25+, cover 30mm.
- Severe: 320 kg, w/c ≤ 0.45, M30+, cover 45mm. PPC/PSC preferred.
- Very Severe: 340 kg, w/c ≤ 0.45, M35+, cover 50mm. PSC mandatory + surface coating.
- Extreme: 360 kg, w/c ≤ 0.40, M40+, cover 75mm. SRC + microsilica + cathodic protection.
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Frequently asked questions
What exposure class applies to Mumbai?
Mumbai (within 5 km of Arabian Sea): Very Severe. Cement ≥ 340 kg/m³, w/c ≤ 0.45, M35+ minimum grade, cover 50 mm. PSC (IS 1489) preferred over OPC for chloride resistance. Outer cladding / surface concrete benefits from silane / siloxane water repellent treatment.
Does the entire coast need Very Severe spec?
No — only the 0-5 km strip from coast. Beyond 5 km, exposure drops to Severe. Beyond 30 km, Moderate. Mumbai south island = Very Severe; Mumbai outer suburbs (Thane, Vasai-Virar) = Severe; Pune (135 km inland) = Moderate. Apply IS 456 by site-specific distance to coast.
Can OPC be used in coastal projects?
Per IS 456 Cl. 8.2, OPC is allowed for Mild/Moderate. For Severe and stricter, prefer PPC (IS 1489 Part 1, with fly ash) or PSC (IS 455, with slag) — both have lower permeability and better chloride resistance. For Extreme exposure, use Sulphate-Resistant Cement (SRC, IS 12330).
Coastal exposure classification is based on distance from coast. Local microclimate (winds, salt-spray patterns, urban heat island) can extend or contract effective exposure zones. For projects near coast or critical infrastructure, conduct site-specific chloride monitoring + rapid chloride permeability test (ASTM C1202).