Etude expérimentale et numérique des interactions entre le fluage en traction et l'endommagement du béton

L'évaluation du risque de fissuration du béton et des ouvertures de fissures constitue une problématique majeure lors de la conception de certains ouvrages de génie civil (enceintes de confinement de bâtiments réacteurs de centrales nucléaires, ouvrages de stockage de déchets radioactifs, etc.). Dans ce but, le comportement mécanique instantané et différé du béton doit être parfaitement maitrisé. Compte tenu du rôle majeur joué par la traction dans les processus d'endommagement du matériau béton, ce travail de thèse se fixe comme objectif d'approfondir les connaissances sur sa réponse en traction et plus particulièrement en fluage par traction. Le travail porte sur trois grands axes : la mise au point de bancs de fluage en traction directe et en traction par flexion, la réalisation d'une campagne d'essais de fluage propre sous différents taux de chargement pour trois types de sollicitations (traction, flexion et compression) et enfin une analyse basée sur un modèle couplant retrait, fluage et endommagement. Les résultats expérimentaux indiquent que le comportement différé est différent selon la nature de la sollicitation appliquée. Ils mettent de plus en avant un couplage fort entre les déformations de retrait et celles de fluage. L'effet de la consolidation, de l'endommagement et de leur couplage permet alors de mieux appréhender les résultats obtenus. Les résultats expérimentaux sont ensuite confrontés aux prévisions d'un modèle couplant retrait, fluage et endommagement afin de tester sa capacité et éventuellement ses limites à reproduire les phénomènes observés.The assessment of cracking risks is of a great interest in civil engineering field, particularly when considering structures such as nuclear power plants or nuclear wastes repository in deep geological formation. Accordingly, mechanical behaviors of concrete (instantaneous and delayed behaviors) have to be clearly understood. In the present work, the mechanical behaviour in tension has been studied given the key role played by tension in the damage process of concrete. For this purpose, tensile and flexural creep devices have been developed. Then, basic creep tests under different types of load (compression, tension and bending) and under different load levels have been carried out. Finally, experimental results have been compared with those resulting from a numerical modeling. Experimental results show that the delayed behaviour of concrete depends on the type of load. Moreover, they highlight a strong interaction between shrinkage and creep strain. Consolidation effect, damage effect and their coupling lead to a satisfactory reproduction of obtained results. A numerical model which takes all involved phenomena (shrinkage, creep and damage) and their coupling into account, allows a good restitution of the experimental results to be obtained

Non Destructive Evaluation of Containment Walls in Nuclear Power Plants

Two functions are regularly tested on the containment walls in order to anticipate a possible accident. The first is mechanical to resist at a possible internal over-pressure and the second is to prevent leakage. The reference accident LLOCA (Large Loss of Coolant Accident) is the rupture of a pipe in the primary circuit of a nuclear plant. In this case, the pressure and temperature can reach 5 bar and 180°C in 20 seconds. The national project ‘Non-destructive testing of the containment structures of nuclear plants’ aims at studying the non-destructive techniques capable to evaluate the concrete properties and its damaging or progression of cracks. This 4-year-project is segmented into two parts. The first consists in developing and selecting the most relevant NDEs (Non Destructive Evaluations) in the laboratory to reach these goals. These evaluations are developed in conditions representing the real conditions of the stresses generated during ten-yearly visits of the plants or those related to an accident. The second part consists in applying the selected techniques to two containment structures under pressure. The first (technique) is proposed by the ONERA (National Office for Aerospace Studies and Research of France) and the second is a mock-up of a containment wall on a 1/3 scale made by EDF (Electricity of France) within the VeRCoRs program. Communication bears on the part of the project that concerns the damaging and cracking follow-up. The tests are done in bending on 3 or 4 points in order to study the cracks’ generation, their propagation, as well as their opening and closing. The mostly ultrasonic techniques developed concern linear or non-linear acoustic: acoustic emission [1], LOCADIFF (Locating with diffuse ultrasound) [2], energy diffusion, surface waves velocity and attenuation, DAET (Dynamic Acousto-Elasticity Testing) [3]. The data contribute to providing the mapping of the parameters searched for, either in volume, in surface or globally. Image correlation is an important additional asset to validate the coherence of the data. The spatial normalization of the data allows proposing algorithms on the combination of the experimental data. The tests results are presented and they show the capacity and the limits of the evaluation of the volume, surface or global data. A data fusion procedure is associated with these results

COMPARATIVE STUDY OF COMPRESSIVE AND TENSILE BASIC CREEP BEHAVIOR OF CONCRETE

International audienceDue to its poor strain capacity and a low tensile strength, concrete is brittle and highly sensitive to cracking detrimental to application sustainability. Despite this well-established knowledge, the irony today is that investigations on concrete are usually limited to the compression behavior. The creep behavior that is a major concern for concrete structures is no exception to this observation. Only one reason can explain this aberration: the difficulty to perform a tensile test on cement-based materials, particularly their fixture to the loading device. This paper describes the experimental setup developed to achieve direct tensile and bending creeps. The precautions taken to obtain relevant data are described. For comparison, tensile, flexural and compressive basic creep test were conducted in parallel. Although the approach is still controversial, the basic creep strain was determined by subtracting the shrinkage strain and instantaneous strain from the total strain. Results available for specimens subjected to 50% of the strength in tension or in compression are presented. The final discussion compares the basic creep under the different types of loading

Tensile, compressive and flexural basic creep of concrete at different stress levels

International audienceConcrete is brittle and highly sensitive to cracking, which is detrimental to the sustainability of its applications. Although it is well known that cracks occur mainly in tension, research on the mechanical behavior of concrete is usually limited to compression and investigations of creep behavior, a major concern for concrete structures, are no exception in this respect. This paper is intended to help remedy the situation. First, the new experimental set-ups developed to achieve tensile and bending creep are presented. The precautions taken to obtain relevant experimentation are also described. Results for specimens subjected to sustained stresses of 30, 40 and 50% of the tensile or compressive strength are then presented. The final discussion compares basic creep under the different types of loading for the three stress levels

Étude expérimentale et numérique des interactions entre le fluage en traction et l'endommagement du béton

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Basic creep of concrete under compression, tension and bending

International audienceInvestigations on concrete creep are often limited to the compression behavior due to the difficulty of performing tensile tests on cement-based materials. This paper describes the experimental setup developed to achieve direct tensile and bending concrete creep. The precautions taken to obtain relevant data are described. For comparison, tensile, flexural and compressive basic creep test were conducted in parallel. Although the approach is still controversial, the basic creep strain was determined by subtracting the shrinkage strain and instantaneous strain from the total strain. Results available for specimens subjected to 50% of the strength in tension or in compression are presented. The final discussion compares the basic creep under the different types of loading

Numerical modeling of basic creep of concrete under different types of load

International audienc

Numerical modeling of basic creep of concrete under different types of load

International audienc

Characterization of damage in concrete beams under bending with Acoustic Emission Technique (AET)

International audienc

Damages Assessment of Cement-Paste and Cobblestone Interfacial Transition Zone by Acoustic Emission

In cementitious materials, bonding at the contact surface between cement-paste and coarse aggregates plays an important role and determines the durability of the material. Recent studies have evaluated the impact strength on this contact surface by experiments on different surface roughness aggregates and concluded that the contact area between the two components was the weakest zone in concrete. This paper presents experiment test on cement-paste and cobblestone contact surface to determine the types of damage modes using Acoustic Emission Technique. The result is that the shear stress occupies only about 10% of the total stress experienced on the contact surface