Véronique Baroghel-Bouny, Laboratoire Central des Ponts et Chaussées (Research Laboratory of Bridges and Roads Administration), France
Nowadays, worldwide efforts are being made to develop durability design approaches in order to ensure a longer life for reinforced concrete (RC) structures at the lowest cost. With the increasing use of complex concrete mixtures incorporating hydraulic and pozzolanic materials, a performance based approach seems particularly relevant for durability issues.
Such an approach(1) has been developed in France within the framework of an AFGC* Working Group. It is based on key material properties called durability indicators (DI), on the specification of appropriate performance based criteria, and on the use of predictive models. The purpose of this approach is to design concrete mixtures capable of protecting RC structures against a given degradation, such as reinforcement corrosion or alkali-silica reactivity (ASR), for a target lifetime in given environmental conditions.
Durability Indicators
The DIs include universal indicators, such as water porosity and chloride diffusion coefficient, which are relevant to many degradation processes, as well as indicators specific to a degradation process such as ASR or freeze-thaw damage. Each DI has a test method that is well defined and produces consistent results. Each DI is classified into five levels of potential durability ranging from very low to very high depending on the laboratory test results. With this classification system, various concrete mixes can be ranked, selected to meet specified criteria, or optimized to satisfy several criteria. When several DIs are used, the durability of a concrete mix can be based on an overall weighted rating.
Performance Based Criteria
Performance criteria for the DIs have been developed for different target service lives and several exposure conditions. These criteria have been based on experimental data obtained on a broad range of concretes and verified using several analytical or numerical models. As the target service life increases and the environment becomes more aggressive, more DIs are specified and the criteria are more stringent. In practice, the suggested criteria can be adapted for specific project conditions depending on local environment, concrete cover, or economics. The criteria are also likely to evolve as further experience is obtained.
Service Life Prediction
With the purpose of predicting the service life of RC structures, several predictive models have been selected for each degradation process.(1) These models, in which the DIs are introduced as input data, have different levels of sophistication and thus address different issues. The most sophisticated models are based on well-identified physical and chemical mechanisms including moisture transport and take into account the microstructural changes induced by the degradation process. But simple engineering models are also proposed. This multi-level modeling approach can be applied at the design stage and during the monitoring of existing structures.
Concluding Remarks
This new approach offers greater freedom to engineers and designers. It takes advantage of all the technical and economical benefits of new concepts of mix design and high-technology materials such as high performance concretes, for which an extended service life can be expected for the structures. This has been confirmed by field performance data already available. Moreover, this approach is being used as a basis for revisions of current French and European documents for test procedures, cement and concrete standards, and design codes.
Further Information
For more information, the author may be contacted at [email protected].
Reference
Reference
- Baroghel-Bouny, V., et al., “Concrete Design for Structures with Predefined Service Life – Durability Control with Respect to Reinforcement Corrosion and Alkali-Silica Reaction. State-of-the-Art and Guide for the Implementation of a Performance-Type and Predictive Approach Based upon Durability Indicators,” (in French), Documents Scientifiques et Techniques de l’Association Française de Génie Civil, AFGC, Paris, July 2004, 252 p.
*French Association of Civil Engineering (http://www.afgc.asso.fr).