Durability of reinforced concrete bridges in marine environments

Achieving and predicting long-term durability for reinforced concrete structures particularly in marine environments remains a matter of much interest. Recent research findings based on observations from actual bridge performance and specially conducted long-term experiments for a wide variety of concretes indicate that chloride concentrations are involved only indirectly in initiating reinforcement corrosion and in serious active corrosion. For high quality concretes the critical aspect for long-term durability is the loss of concrete alkalinity, i.e. the acid neutralising capacity of the concrete, principally imparted by its cement content. Alkalinity is reduced, normally very slowly, by the rate of dissolution of calcium hydroxide. However, it is increased by chlorides. It is also increased by a greater internal surface area available for dissolution, such as in poor quality, porous, concretes. Only when the concrete pH drops sufficiently, to a level set by chemical thermodynamic conditions, will large-scale reinforcement corrosion be possible. Importantly, cracking of concrete cover through to the reinforcing bars is likely to cause accelerated localised loss of alkalinity, and thus severe very localised corrosion. Several examples of this are given, drawn from practical reinforced concrete structures. These observations have serious implications for serviceability inspections and for predicting remaining structural life.