Durability

A durable concrete keeps doing its job for its design life, often 50 to 100 years, without major repair. Most real deterioration is driven by water carrying aggressive agents into concrete. Durability therefore rests on low permeability, adequate cover, the right materials for the exposure and good curing.

The main mechanisms are corrosion of reinforcement, triggered either by carbonation, which lowers the alkalinity that protects steel, or by chlorides from sea water, de-icing salts or contaminated materials; sulphate attack, which forms expansive products in the paste; alkali-aggregate reaction between reactive silica in some aggregates and alkalis in cement; freeze-thaw damage; acid attack in sewers and industrial floors; and abrasion. Each has its own defence: low water-cement ratio and good cover against corrosion; sulphate-resisting or blended cements against sulphates; non-reactive aggregate or low-alkali binder with fly ash or slag against alkali-aggregate reaction; air entrainment against frost.

Codes link exposure classes to maximum water-cement ratio, minimum cement content, minimum grade and nominal cover. Performance tests such as rapid chloride permeability, water permeability and sorptivity check that the concrete delivered actually resists penetration. Inspections of older structures use cover meters, half-cell potential, carbonation depth with phenolphthalein and chloride profiles.

Design life thinking helps. A structure in a marine splash zone may need lower water-binder ratio, more cover, blended cement and sometimes coatings or corrosion inhibitors, while an interior column in a dry building needs much less. Spending a little more on the right materials at the start usually saves far larger repair bills, traffic closures and safety risks later in the life of a bridge, jetty or parking structure.