CONCRETE program

The CONCRETE consortium's scientific programme, launched in September 2020 for a period of 4 years, aims to remove the scientific barriers to controlling the ageing of concrete structures, particularly in the case of structures affected by internal swelling reactions classically known as "concrete pathologies".

The programme brings together a consortium of 4 major French players in the field of research into the ageing of concrete and reinforced concrete structures: the Gustave Eiffel University (UGE), the Mechanics and Durability of Constructions Laboratory (LMDC) in Toulouse, the Mechanics and Acoustics Laboratory (LMA) in Aix Marseille and the Institute for Radioprotection and Nuclear Safety (IRSN).

Characteristics

Completion date: 2020 - 2024
Partners: Materials and Durability of Constructions Laboratory (LMDC) - Mechanics and Acoustics Laboratory (LMA) - Gustave-Eiffel University

Context

As part of the lifespan extension of the French nuclear power plants currently in operation, assessing the performance of civil engineering structures over time and making relevant predictions are challenges with major safety, economic and environmental implications.

One way of assessing the long-term performance of a concrete structure is to evaluate the durability of the material, often using laboratory tests. Indeed, a defect in durability on this scale will necessarily lead to a defect in durability on a larger scale, such as that of the structure. The first challenge that will guide the research in the CONCRETE programme is therefore to develop a detailed understanding of the local mechanisms responsible for the ageing of reinforced concrete structures.

The performance of structures at a given point in time is most often quantified by measuring limit state functions. These functions describe the position of the operating point of a structure at a given time. To be evaluated, limit state functions require variables describing resistance and stress. These variables can very rarely be obtained by measurement, given the scale of the objects studied. It is therefore necessary to use reliable and robust calculation models. This is the second barrier that the CONCRETE programme will seek to overcome. The consortium aims to develop methods and tools for assessing and predicting the performance of structures based on models that take account of the scale of the structure. The aim is to develop a multi-scale approach.

The final area being studied by the CONCRETE programme concerns methods for diagnosing and monitoring concrete structures. Their development requires knowledge of the phenomena that are likely to occur throughout the life of a structure. However, this is rarely the case and the methods have to be continually adapted according to the level of knowledge available. In the case of existing structures, the application of certain techniques is conditioned by the nature of the structure, which was often not designed to be inspected. In addition, in view of nuclear safety requirements, destructive methods are generally not feasible. In the case of structures under design, for which the mechanisms of degradation are still poorly understood (storage and disposal of nuclear waste, for example), dedicated diagnostic and inspection methods need to be developed. The CONCRETE programme therefore aims to improve the quality and relevance of the data provided by on-site diagnostic and monitoring techniques.

Progra​m overview and areas of research

The CONCRETE programme is organised around 4 components, one for each scientific issue to be resolved and a final component focusing on the study of the ageing of nuclear civil engineering structures:

The aim of this section is to study the pathologies of reinforced concrete structures. The aim is to improve knowledge of concrete degradation mechanisms as a function of its environment. The research focuses on :

  • the influence of the "transition halo" (or ITZ - interfacial transition zone), the interface zone between cement and aggregate, on healthy or pathologically-affected materials
  • the mechanical characteristics and cracking behavior of concrete at the interface between concrete and steel
  • the impact of late heating of concrete on its mechanical properties and durability
  • the specific features of storage structures in a clay-limestone environment, similar to the CIGEO project site[1].
  • The various lines of research focus on the physical, chemical and mechanical parameters acting on concrete and likely to generate cracking.

[1] CIGEO: le center industriel de stockage géologique is the French project for a deep repository for radioactive waste. It is designed to store the long-lived, highly radioactive waste produced by all current nuclear facilities, until their decommissioning, and by the processing of spent fuel used in nuclear power plants.

 

  • The second part of the CONCRETE program aims to develop strategies for predicting the performance of concrete structures, based on a multi-scale approach.

    The models developed will have to take into account the constraints specific to each scale (microscopic, mesoscopic and macroscopic) that can promote concrete swelling and cracking pathologies. This may be due to the internal pressure of the material through the formation of ettringite[2] on the microscopic scale, or to interactions between the different phases of the concrete, as studied in the first section, on the mesoscopic scale.

    Ultimately, the aim is to develop macroscopic models that take into account the physical-chemical phenomenology revealed at lower scales, and describe the mechanical consequences.

    [2] Ettringite: ettringite is a mineral species composed of calcium sulfate and hydrated aluminum. Ettringite can generate internal swellings that can lead to the appearance of disorders in structures.

  • The third part looks for correlations between changes in various physical, chemical and mechanical parameters and the state of development of concrete pathologies. In particular, it should propose a qualitative and/or quantitative indicator to reveal the presence of internal or surface swelling pathologies in the structure.

    The project also aims to explore different techniques for the implementation of non-destructive evaluation methods and tools, notably based on non-linear acoustic techniques that can be used on an industrial scale, and the use of optical fibers.

  • In the case of massive concrete structures, pathologies develop heterogeneously throughout the volume. This spatial variability can be explained by various mechanisms such as thermal history, chemical potential, hydric and thermal gradients due to storage conditions and mechanical stresses. The first part of this section aims to establish a link between these conditions and the amplitude of concrete swelling on the one hand, and its appearance and development kinetics on the other.

    Drawing on data from the ODOBA[3] project, this section also aims to validate the modeling strategies and diagnostic techniques developed in sections 2 and 3.

    [3] ODOBA: the ODOBA project (Observatoire de la durabilité des ouvrages en béton armé) was launched by IRSN in 2016 for a duration of at least 10 years. Its aim is to study concrete pathologies and their consequences on the scale of nuclear structures (e.g. reactor containments).