Development of a global, predictive and performance approach of reinforced concrete structure durability based on durability indicators. Overview and future prospects. Microstructure characterization of concretes, study of their hydric and transport properties, assessment of free deformations and prediction of buildings lifetime

This document synthesizes the objectives, the adopted approach, as well as the main scientific results and the products (test methods, for example), obtained during the researches carried out or supervised by the author within various frameworks, primarily over the period 1995-2005, at LCPC as Head of the Section 'Microstructure and Durability of Concretes'. This document presents in particular a performance-based, global and predictive approach of the durability of (reinforced) concrete structures, based on the concept of durability indicators, and combining lab tests and numerical simulations. This type of approach was developed for the protection against rebar corrosion of reinforced concrete and against the degradations generated by alkali-silica reaction, within the framework of the Working Group of the French Association of Civil Engineering (AFGC) 'Concrete design for a given service life of structures - Durability indicators'. The objectives of this new approach and the selected durability indicators - simple but particularly relevant parameters with respect to the implied physicochemical mechanisms in a given environment - as well as the bases of this selection, are first of all presented. Significant examples, resulting in particular from the Research Project 'Transfers in concretes and durability of structures', from the Research Project 'Durability of reinforced concrete and of its components: management and performance-based approach', as well as from studies carried out within the framework of the Topic 'Durability' of the French National Project BHP 2000, are then presented. These examples address the understanding of the mechanisms (moisture transport, carbonation, chloride penetration, freezing, non-restrained deformations,.) or the development of tools for the characterization of concrete microstructure and of parameters related to the durability with respect to reinforcement corrosion. Among the examples given, are reported the quantification of the moisture properties, the description of the couplings between hydration and external drying and of their consequences, the evaluation of the physicochemical interactions between chloride ions and the cement matrix, and the description of the determining influence of concrete specimens pretreatment on the test results. Examples are also given relatively to the modelling of isothermal moisture transfers, of carbonation and of chlorides penetration. Works relating to the modelling of moisture transfers in particular brought major conclusions on the relative importance of the various moisture transport modes during the drying of weakly permeable materials. These works moreover were used as a basis for the development, not only of models of carbonation and transport of (chloride) ions in non-saturated conditions, but also of indirect methods of assessment of the transport properties (permeability to liquid water). The test methods (or possibly of calculation) developed for the assessment of the durability indicators are also discussed. A methodology, for the implementation of the performance-based approach suggested, is in addition introduced. It relies in particular on: - a system of classes defined for each indicator, allowing to evaluate the 'potential' durability of (reinforced) concretes, specifications relating to the durability indicators, which are a function of the environment type and of the service life required for the structure, in order to select or to qualify concrete mixtures for this structure. The approach suggested for the prediction of the service life of a new structure at the design phase, or for the diagnosis and the evaluation of the residual service life of existing structures (possibly degraded), is then described. This is a 'multilevel' approach, which is articulated around three essential tools: - predictive models of service life, - the durability indicators previously mentioned (inputs of the models), - monitoring parameters (outputs of the models). The characteristics required for the models are recalled. A panel of models of carbonation and of chlorides penetration, which have various levels of sophistication (from the simple engineer model, only based for example on the strength, to the 'sophisticated' numerical model, which takes into account several physical mechanisms and chemical reactions as well as their interactions), and thus intended to solve various types of problems, was selected among those developed recently or under development (at LCPC in particular). Methods of measurement of the monitoring parameters, applicable in laboratory on samples or on cores from structures, are in addition proposed. Examples of validation of the approach, and in particular of the performance criteria suggested, are moreover given, with regard to the protection against reinforcement corrosion. The use of the selected models, as well as the application of the experimental methods suggested, are also illustrated by several examples. Beyond the immediate benefit from these works (advances in the understanding of the phenomena, new approach of durability, 'tool-kit',...), proposals are furthermore given at the end of this document, on the one hand to continue the theoretical and experimental researches in the field of the durability of materials and building and civil engineering structures, and on the other hand to disseminate the works completed and to transfer to the practice the already achieved results. A new research project, which lies within the framework of the Research Projects of the LPC network, is in particular presented. (author)