Time to initiation of reinforcement corrosion in marine environments: a review of older concrete structures

The time before visible evidence appears on the external part of a reinforced concrete structure is often used an indicator of the rate at which the reinforcement is corroding. The conventional wisdom is that this is a function of the rate at which chlorides from the marine environment (or from de-icing salts) reach the reinforcement. Current good practice requires high quality concretes with thick and low permeability cover to ensure long initiation times. This has been known for many years. Many modern structures show evidence of corrosion initiation already within 10-20 years. On the other hand, there is recorded evidence that many older reinforced concrete structures exposed to aggressive marine environments have survived for long periods of time with little or no evidence of reinforcement corrosion. In many cases this is despite thin covers and relatively poor quality concretes. This paper gives a summary of many structures recorded in the literature, both for cases with and without early corrosion evidence. It is proposed that understanding the differences in behaviour is the key to understanding longer-term durability of reinforced concrete structures in marine environments. The available evidence to date indicates that concretes with high pH buffering, such as imparted by calcareous aggregates, have considerably greater durability against reinforcement corrosion. Three Australian examples are given.