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 NZS 3101 Concrete Structures Standard (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.