Reinforced Concrete is a highly durable construction material. However, it has a well-recognised vulnerability to aggressive chemical agents which, if they penetrate the cover in sufficient concentration, can depasivate the steel reinforcing, leading to corrosion followed by the cracking and ultimate failure of the concrete.

Under New Zealand's maritime climate it is chloride ions from sea salts which pose the most significant risk.

The New Zealand Building Code (NZBC) mandates a durability performance for structural building components based upon a 50-year design life, unless a lesser life is nominated for temporary buildings. NZBC B2/AS1, an Acceptable Solution, specifically lists compliance with the Concrete Structures Standard NZS 3101: Part 1 section 5 (subject to certain modifications) as a means of meeting this durability requirement. The standard divides the country into four geographic zones based on increasing severity of exposure to marine chlorides - Inland (A2), Coastal Perimeter (B1), Coastal Frontage (B2), and Tidal/Splash (C). For each exposure classification, a minimum OPC concrete quality is specified by compressive strength and then a minimum cover is given to obtain the desired durability.

There are several good sources of information which can assist engineers in developing a specification for concrete in a marine environment.

Concrete Construction article on 'Designing Concrete for Exposure to Seawater'

'concrete' article on 'Designing for the marine environment.'