A framework for use of durability indexes in performance-based design and specifications for reinforced concrete structures

Durability of reinforced concrete remains a pervasive concern. At present, as-built concrete quality and hence durability is inadequate in many cases. This relates partly to use of prescriptive specifications that do not appropriately address actual quality concerns. Performance-based specifications may offer greater advantages in improving concrete durability, but to be viable require suitable quality parameters to be defined and measured. In South Africa, a 'Durability Index' (DI) approach has been developed to address these concerns. 'Durability indexes' are quantifiable parameters which characterise concrete quality and are sensitive to material, processing, and environmental factors. The approach is based on measurement of transport-related properties of the cover layer of laboratory and in-situ concrete, thus reflecting the dual aspects of material potential and construction quality. Rational durability design and performance-based durability specifications are being developed and in some cases applied in actual construction. The paper presents a framework within which the DI approach is used to craft performance-based specifications, based on service life models that utilise the relevant DI values. Steps to establish appropriate DI test values for a given structure are described, and a procedure for implementing their use as a quality control measure is recommended. The approach is integrated, allowing for continual improvement and modification as additional data become available.