This thesis is part of the research carried out at the Institute of Radioprotection and Nuclear Safety (IRSN) concerning the aging of concrete containment building (extension of the operating life of nuclear reactors from 40 to 60 years) In partnership with the LMDC (Materials and Durability of Construction Laboratory) of Toulouse and the LMGC (Mechanics and Civil Engineering Laboratory) of Montpelier, the aim of this thesis is to propose a characterization method of transition zone between cement paste and aggregate, in order to better understand the development of internal swelling reactions in the concrete of existing structures. These pathologies can be attributed to two types of chemical reactions: the Internal Sulfate Reaction (ISR) and the Alkali-Aggregate Reaction (AAR). These reactions cause, when they develop, the setting in traction of the concrete leading to the creation of cracks in the structures affected by these pathologies. At the microstructural scale, one area receives particular attention for the development of these endogenous pathologies; the Interfacial Transition Zone (ITZ). In fact, the wall effect caused by aggregate presence in a paste at the fresh state generates an excess of local water, leading to porosity increase locally. Therefore, the cement paste structure is modified over a few tens micrometers, accompanied by the development of larger crystals and an oriented growth of the Portlandite. These localized physicochemical characteristics make the ITZ a more permeable area in concrete, which is involved both in the strength and mechanical adhesion and in the durability of the cementitious material. In addition, ITZ seems to be a more porous area allowing the formation of AAR gel and / or delayed ettringite during these pathologies. There is a real scientific interest to characterize accurately ITZ in the case of endogenous degradations such as AAR and ISR in order to predict the evolution of these pathologies within the concrete and their effect on their property (mechanical, permeability and durability). To answer these problems, this experimental program will focus on research into fine techniques of physical and chemical characterization (Scanning electron microscopy, EDS, X-ray tomography, electron microprobe, micro-indentation, XRD, etc.) and mechanics depending on the pathologies evolutions. This work will be structured around several points: