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Earth concrete, constituted mainly of raw earth, is a promising ecofriendly material for future constructions. However, it is composed of fine particles like clay which make it very sensitive to drying. The volume change induced by desiccation shrinkage can be responsible of cracking which can have a detrimental effect on the durability and mechanical properties. Thus, in order to design reliable earth concrete structures, considering a coupling between drying shrinkage and damage is important. The effect of curing conditions on the mechanical properties of earth concrete with different percentages of flax fibers has been studied. The evolution of damage during drying has been monitored continuously using the ultrasound and the acoustic emission (AE) techniques. The mechanical properties of earth concrete decreases for specimens stored at low relative humidity. In addition, the AE results associated with shrinkage and mass loss measurement made it possible to describe qualitatively the drying process.

International audience; The evaluation of the spatial variability of concrete properties is an important issue for a better diagnosis of reinforced concrete structures. The combination between destructive techniques and nondestructive techniques (NDT) is a common practice to establish relationships between concrete properties and NDT measurements. Concrete properties can then be estimated at each test location using the corresponding NDT values on the basis of the calibration and inversion of these relationships. However, NDT measurements include many sources of uncertainties that can lead to biased or even inaccurate estimation. Thus, the improvement of the reliability of this estimation requires to specify and control the principal influencing factors on these uncertainties. The main objective of this paper is to propose a calibration methodology of conversion models and to study the reliability of concrete properties assessment considering the effect of the number of measurements, the uncertainty of NDT measurements and the combination of NDT techniques. Three conversion models linking the ultrasonic pulse velocity, the electrical resistivity, and the dielectric permittivity to two physical concrete properties, the porosity and saturation degree, are considered. The results show that the inversion of the proposed analytical models enables an accurate evaluation of concrete properties. In addition, it has been shown that there is a minimal number of measurements needed for an efficient non-destructive evaluation of concrete properties considering their variabilities.

International audience

The mechanical behavior of earth concrete for sustainable construction has been studied in this paper. The effect of flax fibers on the fracture behavior has been monitored using digital image correlation (DIC) and acoustic emission (AE) techniques to better understand the fracture process and damage mechanism. Three-point bending tests have been conducted on prismatic specimens with different percentages (0%, 0.3% and 0.6%) and lengths (12 mm, 24 mm, 50 mm) of flax fibers. The AE activity and fracture energy increased with the percentage and length of fibers. Moreover, the AE signal parameters varied with the loading rate and different damage mechanisms have been distinguished. Crack opening resistance also increased due to the bridging effect of fibers along developed cracks. Those results show the benefits of adding flax fibers to enhance the ductility of earth concrete.

This study aims to optimize the composition of new ecological concrete constituted of upgraded excavated soil. The novelty of this study is the fact that earth concrete is poured with classical vibration as ordinary concrete. Several soil concrete mixtures, composed of different proportions of clayey soil, sandy soil and small amounts of cement, lime and hemp fibers have been tested. The mineralogical composition of clayey soil was studied by X-ray diffraction (XRD) analysis The chemical composition was established by Environmental Scanning Electron Microscopy (ESEM) coupled with the X-Ray Energy Dispersive Spectrometry (EDS). Casting of concrete mixtures has been realized by vibration, as ordinary concrete, to obtain the required workability on construction sites. Compressive tests have been carried out on samples at different curing times. Ultrasonic non-destructive technique has been used to monitor the hardening of soil concrete. As this concrete presents important volumetric changes that can cause the infiltration of water and impact their durability, an experimental investigation on autogenous and drying shrinkage is reported. Water porosity and water absorption tests have been also carried out to evaluate the transfer property of this porous material. The results show that the compressive strength of soil concrete is greater than 1 MPa, which is sufficient for filling application (wall of a frame building) and increases to 5 MPa after 6 months. Compressive strength, density, and workability of the tested concrete mixtures decrease with increasing clayey soil and hemp fibers. Desiccation shrinkage increases with the volume fraction of clayey soil and hemp fibers.

Earth concrete or concrete based on soil materials presents a high percentage of fine particles (clay) which makes it susceptible to plastic shrinkage cracking in its fresh state that affects its durability. In this study, natural flax fibers with different mass percentages (0%, 0.3% and 0.6%) and lengths (12 mm, 24 mm, 50 mm) have been added to earth concrete in order to study their effect on plastic shrinkage and cracking at early age. The effect of flax fibers on the mechanical properties of hardened earth concrete has been first investigated. Free horizontal plastic shrinkage has also been measured in parallel with the evaporation rate, the capillary pressure and the temperature of earth concrete at early age. Furthermore, restrained plastic shrinkage tests have been conducted according to ASTM C1579-13 standard. The Digital Image Correlation technique have been used to monitor the strain distribution and crack width during the first 24 h after casting. The results show a reduction of the total free plastic shrinkage at early age with the addition of flax fibers. In addition, flax fibers reduce the strain localization of restrained shrinkage and the plastic shrinkage cracking.

This paper deals with the detection and monitoring of damage of a three-dimensional carbon/carbon composite (3D C/C) under tensile loading. In fact, due to the manufacturing process, the existing cracking network of 3D C/C at the initial state may be quite extended which may affect the mechanical behavior and in some cases lead to the premature rupture under loading. The objective of this work is to investigate the validity of the acoustic emission as an advanced health monitoring system to provide warning of the distress and characterize the damage state during loading. Load-unload-reload tensile tests have been conducted and monitored in parallel with the Digital Image Correlation (DIC) method and the acoustic emission (AE) technique. The AE technique allows to monitor the evolution of damage inside the volume of the material and the growth in situ. This is very interesting since the other non-destructive techniques usually used allow only a posteriori evaluation of damage. The influences of stress states on tensile properties and the dependency with the AE activity have been analysed. Different stages of damage have been identified based on the acoustic emission activity and AE signals parameters. In addition, the AE technique provides accurate location of damage based on the density of AE events location maps.

International audience; One of the main interests of the Acoustic Emission (AE) technique is its capability to detect and localise damage in structures. In this paper, the accuracy of AE sources localisation in concrete is studied. Several methods related to the choice of the onset detection of AE signals and to the location algorithm are evaluated and compared. The results show an important influence of the onset detection method on the location accuracy of AE events. The choice of the minimisation algorithm depends on the desired accuracy and the computation time. A statistical analysis based on the distribution of AE events and energy is presented for the detection of damages and the monitoring of the crack propagation. The comparison of the presented methodology demonstrates comparative results with Digital Image Correlation (DIC).

International audience

International audience; Cracking in reinforced concrete structures is of major concern since it may affect their durability and structural integrity. Cracking is mainly influenced by stress distribution along the interface between steel and concrete which is responsible of transferring load from steel bar to the surrounding matrix. Thus, quantitative evaluation of steel-concrete bond effect on fracture properties is important. An experimental investigation on Reinforced Concrete (RC) ties is reported in this paper. Pull-out and tensile tests have been conducted on cubic specimens with high adhesion reinforcement bars with different diameters in parallel with a continuous monitoring using the Acoustic Emission (AE) and the Digital Image Correlation (DIC) techniques. Two failure modes have been observed according to the steel bar section: concrete cover splitting and pull out with damage concentrated near the concrete rebar interface. The results show a good correlation between the loading force and the AE activity with AE signal of different characteristics indicating different active fracture mechanisms. The order of appearance of cracks, their width and their spacing have been also measured in correlation with the loading level for short ties with different lengths and bar diameters.

The diagnosis of reinforced concrete is essential to detect the degradation and thus maintain the structural performance of civil engineering structures. This paper aims to establish a mathematical relationship between the ultrasonic pulse velocity UPV (considered as an observable variable) and two concrete properties indicators (compressive strength fc and water content W) within a probabilistic framework. Synthetic simulations are proposed to derive a conversion model between the statistical properties of the output and the input parameters for a reinforced concrete structure by taking into account spatial variability of concrete.

International audience; Cracking in Reinforced Concrete (RC) structures may impact their durability and their structural integrity. Cracking is mainly influenced by stress distribution along the interface between steel and concrete. Thus, quantitative evaluation of steel-concrete bond, which is responsible of transferring load from steel bar to the surrounding matrix, and its effect on fracture properties is of major important. An experimental investigation on RC ties is reported in this paper. Tensile tests have been conducted on cubic specimens with different high adhesion reinforcement diameters. Those tests have been monitored continuously using the Acoustic Emission (AE) technique for a better evaluation of the damage process. The results show a good correlation between the load and the AE activity. AE signals with different characteristics have been observed during the different fracture stages. Parameters such as duration and energy increased with the loading level and can be used as indicators to detect the macrocracking of concrete. A parametric analysis is performed between average frequency and RA value. Ib-value of AE hits has been also investigated and decreases to 0.12 prior to the first macro crack.

International audience; The aim of this paper is to study the reinforced concrete short tie-rods behavior using the adhesion-slip curve shape between steel and concrete adopted by the European Concrete Committee. We are interested here in short tie-rods without main cracks for which we calculate and measure the maximum mobilization state of steel-concrete adhesion, and beyond the decrease mode of this bond. For this, tests of short tie-rods, with different high adhesion rebar diameters have been carried out. To characterize the first phase of the adhesion-slip behavior law (τ−g), pull out tests have been carried out with the same concrete, the same reinforcement and the same cross-section such as the tie-rod tests, with a proposed method to estimate the adhesion peak and the corresponding slip. For this adhesion peak value, slightly underestimated by the conventional curve of the European Concrete Committee, a new expression is suggested. A numerical model with theoretical relations of the behavior of such tie-rods is proposed. The comparison of this model with the obtained test curves of the short tie-rods shows a suitable approach. Also, we deduce that the steel (coated with concrete) fictitious module slope is even higher than the percentage of reinforcement is low. These results may help to understand the tie-rods behavior generally, in the phase of cracks stabilization, during which the tie-rod is composed of short tie-rods without main cracks.

Le béton de terre est composé essentiellement de particules fines comme l'argile qui le rend très sensible au retrait plastique. De plus, la faible résistance de ce béton au jeune âge augmente le risque de fissuration. Dans ce travail, l'effet des fibres de lin sur la fissuration des bétons de terre au jeune âge a été étudié selon la norme ASTM C1579-13. Le pourcentage et la longueur des fibres ont été variées de 0,6 à 1,2 et de 12, 24 à 50 mm respectivement. Le béton testé, est constitué d'un sol artificiel composé de sable du béton recyclé et d'argile avec 8% de ciment et 3% de chaux. Les champs de déplacement dans la zone de fissuration ont été mesurés à l’aide de la technique de traitement d'images. Le temps de fissuration et l’évolution de l’ouverture de la fissure ont été ainsi évalués. Les résultats montrent l'efficacité des fibres de lin dans la réduction de la largeur de fissuration dû au retrait plastique., Academic Journal of Civil Engineering, Vol 36 No 1 (2018): Special Issue - RUGC 2018 St Etienne

L’objectif de ce travail est de valoriser des sédiments non traités dans la formulation d’un béton allégé. Pour cela, un béton mousse innovant a été développé en remplaçant le sable fin par des sédiments. Plusieurs essais expérimentaux ont été réalisés pour, dans un premier temps, caractériser les sédiments et dans un second temps déterminer les propriétés du béton de mousse à l’état frais et à l’état durci. Le pourcentage en masse de ciment et le pourcentage volumique de mousse ont été variés de 12% à 18% et de 0, 30 à 50% respectivement. Des essais de compression ont été effectués sur des éprouvettes à 7 et 28 jours. La technique non destructive des ultrasons a été aussi utilisée pour suivre le durcissement du béton allégé. Les résultats ont montré que les propriétés mécaniques comme la résistance à la compression, le module de Young et la vitesse des ondes ultrasonores augmentent avec l’ajout du ciment et diminuent avec le % de mousse avec des valeurs acceptables pour des travaux de remplissage., Academic Journal of Civil Engineering, Vol 36 No 1 (2018): Special Issue - RUGC 2018 St Etienne

International audience; In civil engineering, the control of creep has a major role in concrete structure design. Indeed, the viscoelasticity of concrete leads to delayed deformations which could be at the origin of cracking in structures. So, these delayed strains have an impact on their safety margin and life time, especially when they are subjected to high loads. For example, underground concrete structures and other infrastructures undergo, during the construction phase, load due to soil pressure, which leads to important creep deformations.

The purpose of this paper is to analyse the development and the evolution of the fracture process zone during fracture and damage in quasi-brittle materials. A model taking into account the material details at the mesoscale is used to describe the failure process at the scale of the heterogeneities. This model is used to compute histograms of the relative distances between damaged points. These numerical results are compared with experimental data, where the damage evolution is monitored using acoustic emissions. Histograms of the relative distances between damage events in the numerical calculations and acoustic events in the experiments exhibit good agreement. It is shown that the mesoscale model provides relevant information from the point of view of both global responses and the local failure process.

International audience; In order to build sustainable structures, the study of mechanical behavior must integrate with local phenomena, e.g. fracture propagation and localization zone. Fracture in concrete usually develops in the form of localized zone of microcracks which then coalesce into macrocrack of significant crack openings. In this paper, fracture process in geometrically scaled concrete beams under bending test is analyzed. Acoustic emission (AE) and digital image correlation (DIC) techniques are simultaneously applied to identify fracture parameters such as crack openings and size of fracture zone. The AE technique is useful to identify the location of fracture growth due to microcracks and macrocrack, however, DIC is useful to measure crack openings at various locations of crack. The location of crack tip is also estimated from both techniques. It is observed that the two techniques in coupled position proved effective in identifying the fracture process zone and cracking mechanisms of concrete.

International audience; Many researchers have proposed hypotheses concerning the physical mechanisms that govern creep and among them the development of microcracks is well recognized. For high load levels, microcracking may initiates at the moment of load application and begins to grow to form a time-dependent crack path. An experimental investigation is proposed here in order to provide interesting insight into the coupling between creep and damage with specimens loaded in flexure. The acoustic emission (AE) technique is used as a tool to provide information on the pertinence of the physical hypothesis that microcracks appear during creep. An original test is performed to accelerate the creep phenomenon by submitting concrete beams to desiccation after a basic creep period. The results show a good proportionality between the creep deformation and the AE activity and thus the efficiency of acoustic measurements for the estimation of the state of damage. In addition, an unsupervised pattern recognition analysis is used as a tool for the classification of the monitored AE signatures. The cluster analysis shows two clusters during basic creep and three clusters during desiccation creep indicating different damage mechanisms.

International audience; The computational power allows nowadays the development of mesoscopic models of concrete, based on finite element or lattices approaches, which represent the contribution of inclusions to the behavior of concrete. However, the smallest heterogeneities are often removed to these simulations for decreasing the computation time. In this paper, the effect of aggregate classes on the fracture behavior of a plain concrete is studied. Different simulations are performed from a mesoscopic model based on a diffuse meshing technique and Fichant's damage model, in which the smallest aggregates are successively removed from the granular skeleton to the benefit of a homogenized continuous mortar. The effects of these simplifications are then evaluated by comparing the fracture behaviors obtained to the one of the reference concrete. The results show the relevance of modeling all classes of aggregates in order to obtain an accurate description of the failure behavior of concrete.

WOS:000403042900102; International audience; Two functions are regularly tested on containment walls in order to anticipate a possible accident. The first is mechanical to resist a possible internal over-pressure and the second is to prevent leakage. The AAPR reference 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 and cracks. This 4-year-project is segmented into two parts. The first consists in developing and selecting the most relevant NDEs 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 structure is proposed by ONERA and the second is a mockup of a containment wall on a 1/3 scale made by EDF within the VeRCoRs project. Communication is focused on the part of the project that concerns the damage and crack process characterization by means of NDT. The tests are done in 3 or 4 points bending in order to study the cracks' generation, their propagation, as well as their opening and closing. The main ultrasonic techniques developed concern linear or non-linear acoustic: acoustic emission [1], Locadiff [2], energy diffusion, surface wave's velocity and attenuation, DAET [3]. The recorded data contribute to providing the mapping of the investigated parameters, either in volume, in surface or globally. Digital image correlation is an important additional asset to validate the coherence of the data. The spatial normalization of the data in the specimen space 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.

International audience; In this paper, the fracture process zone (FPZ) is investigated on unnotched and notched beams with different notch depths. Three-point bending tests have been realized on plain concrete under crack mouth opening displacement (CMOD) control. Crack growth is monitored by applying the acoustic emission (AE) technique. The comparison with a numerical model is also realized by using a mesoscopic approach. Such an approach is of particular interest in the analysis of interactions between the cementitious matrix and aggregates. Several AE parameters are examined during the entire loading process, and show that the relative notch depth influences the AE characteristics, the process of crack propagation, and the brittleness of concrete. The numerical load-CMOD curves show that the mesoscopic modelling reproduces well the notch effect and concrete failure. In order to improve our understanding of the FPZ, the width and length of the FPZ are followed based on the AE source locations maps in parallel with the numerical damage fields. An important energy dissipation is observed at the crack initiation in unnotched beams.

International audience; In this paper, the fracture process zone (FPZ) is investigated on unnotched and notched beams with different notch depths. Three-point bending tests have been realized on plain concrete under crack mouth opening displacement (CMOD) control. Crack growth is monitored by applying the acoustic emission (AE) technique. The comparison with a numerical model is also realized by using a mesoscopic approach. Such an approach is of particular interest in the analysis of interactions between the cementitious matrix and aggregates. Several AE parameters are examined during the entire loading process, and show that the relative notch depth influences the AE characteristics, the process of crack propagation, and the brittleness of concrete. The numerical load-CMOD curves show that the mesoscopic modelling reproduces well the notch effect and concrete failure. In order to improve our understanding of the FPZ, the width and length of the FPZ are followed based on the AE source locations maps in parallel with the numerical damage fields. An important energy dissipation is observed at the crack initiation in unnotched beams.

A new approach to simulate interface damage in brittle matrix composites Alexia Este 1, Bernard Toson 1, Jacqueline Saliba 2, Jean-Christophe Mindeguia 2, Stephane Morel 2, Eric Martin 3 1 CEA-CESTA, F-33114 Le Barp, France 2 Universite de Bordeaux, UMR 5295, Institut de Mecanique et d’Ingenierie - Bordeaux (I2M), Dept. Genie Civil et Environnemental (GCE), F-33000 Bordeaux, France 3 Laboratoire des Composites Thermo-Structuraux, CNRS UMR 5801, Universite Bordeaux, F-33600 Pessac, France alexia.este@cea.fr Keywords: damage, crack deflection, fiber/matrix interface, brittle matrix composites. Abstract: Under mechanical loading, brittle matrix composites develop various damage processes which include cracking of matrix and fiber/matrix interface [1]. Cohesive zone models are widely used to model interface damage [2]. The aim of this paper is to propose an alternate approach based on a damage model associated to a diffuse meshing method. In this study, the deflection of a matrix crack at the fiber/matrix interface in a single fiber composite is simulated using CZM and the results are compared with those obtained with the new approach. The damage model requires the relation between stress and strain vectors on planes of various directions which are superposed to obtain the macroscopic strain-stress relation [3]. The damage model used was developed and implemented by S. Fichant and C. La Borderie [4,5] in the finite element modelling software Cast3M [6]. With this model, a diffuse meshing method [7] is used which consists in projecting material properties on the shape functions of a finite element mesh. At each Gauss point, the properties of the fiber, matrix or interface are assigned based on the integration point position. This meshing method allows the user to override the geometry of heterogeneities and therefore to use a regular mesh. The simulation of a tensile test (in the fiber direction) on a brittle single fiber composite is performed by using the damage model and shows a good agreement with the reference one obtained from CZM. On this basis, it is shown that the damage model is able to reproduce complex phenomenon as periodic matrix crack deflections at the fiber/matrix interface. References: 1. Evans, Zok, Davis. 1991. The role of interfaces in fiber-reinforced brittle matrix composites. Composites Science and Technology, Vol. 42, Issues 1-3: 3-24. 2. Alfano, Crisfield. 2001. Finite element interface models for the delamination analysis of laminated composites: mechanical and computational issues. International Journal for Numerical Methods in Engineering. Vol. 50, Issue 7: 1701-1736. 3. Bazant, Ozbolt. 1990. Nonlocal Microplane Model for Fracture, Damage and Size Effect in Structures. Journal of Engineering Mechanics, Vol. 116, No. 11: 2485-2505. 4. Fichant, Pijaudier-Cabot, La Borderie. 1997. Continuum damage modelling: approximation of crack induced anisotropy. Mechanics Research Communications, Vol. 24, No. 2: 109-114. 5. Fichant, La Borderie, Pijaudier-Cabot. 1999. Isotropic and anisotropic descriptions of damage in concrete structures. Mechanics of Cohesive-Frictional Materials, Vol. 4: 339-359. 6. CEA, Cast3M, http://www-cast3m.cea.fr/. 7. Daoud, Maurel, La Borderie. 2013. 2D mesoscopic modelling of bar-concrete bond. Engineering Structures, Vol. 49: 696-706.

International audience; The use of shrinkage-reducing admixtures (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. Various forms of SRA are commercially available and they may act through different mechanisms. Some SRA mainly acts on drying and weight loss leading to shrinkage. In this paper, the influence of a liquid SRA on plastic shrinkage, long-term shrinkage, mechanical characteristics as well as concrete pore structure were investigated. Samples of concrete were prepared with two water-to-cement ratios (w/c) to design ordinary and high strength concrete. The effect of the shrinkage-reducing admixture was studied by adding 1% to the total mass of binder, while keeping the other parameters constant. At early age, the results indicate that the SRA lead to the same plastic shrinkage for w/c = 0.65 while it reduced the plastic shrinkage by 25% for w/c = 0.43. Drying shrinkage was assessed from 1 day on hardened concrete. The SRA reduced the 7 day drying shrinkage for w/c = 0.65 and w/c = 0.43 concrete mixtures by up to 56%–31%, respectively, and the 70 day drying shrinkage by up to 33%–25% when the specimens were cured for 24 h then stored at relative humidity of 50%. At equal water-to-cement ratios, the SRA is seen to be more efficient in reducing drying shrinkage at early ages. These findings suggest that the SRA is most effective when internal relative humidity is relatively high or when higher porosity exists in the material. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 300 to 1000 nm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels where capillary stresses are the main cause of shrinkage. Thus, this phenomenon may contribute in the reduction of drying shrinkage that occurs when this liquid SRA is used in concrete.

International audience; The fracture process zone (FPZ) was investigated on unnotched and notched beams with different notch depths. Three point bending tests were realized on plain concrete under crack mouth opening displacement (CMOD) control. Crack growth was monitored by applying the acoustic emission (AE) technique. In order to improve our understanding of the FPZ, the width and length of the FPZ were followed based on the AE source locations maps and several AE parameters were studied during the entire loading process. The bvalue analysis, defined as the log-linear slope of the frequency-magnitude distribution of acoustic emissions, was also carried out to describe quantitatively the influence of the relative notch depth on the fracture process. Theresults show that the number of AE hits increased with the decrease of the relative notch depth and an important AE energy dissipation was observed at the crack initiation in unnotched beams. In addition, the relative notch depth influenced the AE characteristics, the process of crack propagation, and the brittleness of concrete.

International audience; Les bétons présentent une rupture quasifragile du fait de la présence d'une zone d'élaboration de la fissure (FPZ), principalement microfissurée, se développant en avant du front de fissure. La puissance de calcul actuelle a permis le développement de modèles mésoscopiques ayant pour objectif de représenter la participation de la géométrie dans le comportement du matériau hétérogène. Ces modèles s'appuyant sur des approches de type élément fini ou lattices, ont montré leur intérêt dans la description détaillée de la rupture ainsi que pour celle des phénomènes couplés. Cependant pour des raisons souvent liés à la difficulté de mailler différentes tailles d'hétérogénéités, la plupart des modèles se contente de représenter les plus grosses hétérogénéités. Cette étude montre l'influence de l'exhaustivité de la représentation du squelette granulaire du béton sur la réponse du modèle mésoscopique. Suite à différentes simulations où nous mêlons progressivement les plus petits granulats à la pâte de ciment dans un mortier dont les caractéristiques sont homogénéisées numériquement, nous soulignerons l'importance de l'exhaustivité des classes d'inclusions lors d'une modélisation.

International audience; Le suivi d'endommagement du béton par émission acoustique (EA) est parmi les seules techniques qui permettent de détecter et de localiser les micros-endommagements qui surviennent lorsque les capacités de résistance locales du matériau sont dépassées. Cette localisation s'effectue à partir de l'analyse des temps de vol des ondes élastiques reçues par un ensemble de capteurs disposés sur la surface du matériau. L'utilisation de différentes hypothèses pour effectuer cette localisation conduit à une importante incertitude sur la position des micros-endommagements. Nous proposons ici de corriger deux de ces problèmes. Enfin, un traitement statistique des cartes de micros-fissurations basé sur la densité spatiale d'endommagement est proposé dans le but de reconstruire le profil de fissuration du béton.

International audience; Le comportement à long terme des structures en béton est conditionné par la viscoélasticité de la pâte de ciment et l'évolution des fissures. La prédiction des déformations différées est donc primordiale pour l'étude de l'aptitude à la fonctionnalité pérenne des structures. Une investigation expérimentale est proposée ici pour étudier l'effet des granulats sur l'évolution des propriétés de rupture du béton sous fluage. Pour cela, des essais de fluage propre ont été menés sur des poutres en béton et en mortier avec de fort niveau de chargement. L'influence du fluage sur la résistance résiduelle et l'énergie de fissuration a été étudiée en soumettant à la rupture les poutres chargées préalablement en fluage de flexion. En parallèle, la technique d'émission acoustique (EA) a été exploitée afin de suivre l'évolution de l'endommagement. Afin d'avoir plus d'informations sur l'effet de l'hétérogénéité du béton, des essais de fluage de dessiccation ont été aussi réalisés avec un suivi continue par la technique d'EA. Les résultats montrent que les granulats jouent alors un rôle important dans le développement des microfissurations et augmentent ainsi le risque de rupture sous fluage.

International audience; Concrete structures are susceptible to degradation due to delayed strains that have an impact on their safety margin and lifetime, especially when they are subjected to a high loading level. Few studies have investigated the effects of creep loading history on failure mechanisms, including its influence on strength, stiffness, and fracture energy. Most of these studies have tried to quantify the effect of creep on the behavior of concrete mainly by measuring the residual strength (Carpinteri, Valente, Zhou, Ferrara, & Melchiorri, 1997; Cook & Haque, 1974; Denarié, Cécot, & Huet, 2006; Liniers, 1987; Omar, Loukili, Pijaudier-cabot, & Le Pape, 2009; Shah & Chandra, 1970). Deformation and fracture of concrete are associated with very complicated progressive failure, and it is usually accepted that the failure process under a sustained load is associated with the development and growth of micro-cracking (Barpi & Valente, 2005; Bazant & Li, 1997; Rossi, Godart, Robert, Gervais, & Bruhat, 1994; Rossi, Tailhan, Le Maou, Gaillet, & Martin, 2012). Thus, understanding the behavior of concrete requires the detailed assessment of creep and the part of damage. Micro-cracking and damage localization in concrete beams cannot be investigated by the classical mechanical measurements. So, as a part of the investigation, the acoustic emission (AE) technique can be applied.

International audience; The service-life of concrete structures depends on the delayed strains that appear due to creep phenomenon. Few are the studies that treated the effect of the dimensions of concrete specimens on the amplitude and the kinetics of creep and the results show many contradictions. Thus, to design reliable civil engineering structures, the knowledge of the behaviour of concrete under a sustained load including size effect is necessary and performing calculations are needed. In this paper, the physical mechanisms behind the size effect on creep rate are evaluated at the mesoscopic scale. The material volume is modeled, by a Digital Concrete model which takes into account the microstructure heterogeneities and the “real” aggregate size of concrete. Calculations are performed in 2D by considering a viscoelastic damage behaviour law for the matrix and an elastic behavior for aggregates. The numerical results show that size effect is well reproduced by the meso-scale approach. The stresses under a sustained load are induced by strain incompatibilities between the components at the mesoscale. Accordingly, the evolution of the microcracked zone with the size of the bending specimens can be related to the creep rate.

International audience; In order to build sustainable structures, the study of mechanical behavior must integrate with local phenomena, e.g. fracture and strain localization. The fracture usually develops in the form of main crack, with branches, secondary cracks and the microcracking zone ahead. Various experimental methods are already employed to detect the fracture process. In this study, digital image correlation is used to measure displacement field in the cracking area. Using displacement field data, crack opening at various locations of crack is measured. The location of crack tip is estimated using crack opening data. Acoustic Emission technique is applied simultaneously with digital image correlation to identify the size of microcracking zone ahead of the propagating crack. It is observed that the two techniques in coupled position proved effective in identifying the fracture process zone and cracking mechanisms of concrete.

International audience; Despite the large number of studies, the mechanisms involved during creep of concrete are still not well known and particularly tensile creep which is a major issue especially in the case of special concrete structures. The objective of this study is to suggest an assessment of the damage mechanisms occurring during the different phases of creep. To achieve this, an experimental investigation is proposed on mortar and concrete beams loaded with flexural creep at high stress levels. Damage evolution under desiccation creep tests is assessed by using the acoustic emission technique. The results show a good proportionality between creep displacement and the acoustic emission activity and thus the efficiency of acoustic measurements for the estimation of the damage evolution. In addition, the results show the importance of aggregates in the fracture behaviour under desiccation creep.

International audience; In its service-life concrete is loaded and delayed strains appear due to creep phenomenon. Some theories suggest that micro-cracks nucleate and grow when concrete is submitted to a high sustained loading, thereby contributing to the weakening of concrete. Thus, it is important to understand the interaction between the viscoelastic deformation and damage in order to design reliable civil engineering structures. Several creep-damage theoretical models have been proposed in the literature. However, most of these models are based on empirical relations applied at the macroscopic scale. Coupling between creep and damage is mostly realized by adding some parameters to take into account the microstruc-ture effects. In the authors' opinion, the microstructure effects can be modeled by taking into account the effective interactions between the concrete matrix and the inclusions. In this paper, a viscoelastic model is combined with an isotropic damage model. The material volume is modeled by a Digital Concrete Model which takes into account the "real" aggregate size distribution of concrete. The results show that stresses are induced by strain incompatibilities between the matrix and aggregates at mesoscale under creep and lead to cracking.

In order to design reliable concrete structures, prediction of long term behaviour of concrete is important. In fact, creep deformation can cause mechanical deterioration and cracking, stress redistribution, loss in presteressed members and rarely ruin the structure. The aim of this research is to have a better understing of the interaction between creep and crack growth in concrete. An experimental investigation on the fracture properties of concrete beams submitted to creep bending tests with high levels of sustained load is reported. The influence of creep on residual capacity and fracture energy of concrete is studied. In parallel, the acoustic emission technique (AE) was used to monitor crack development. The results give wealth information on damage evolution and show a decrease in the width of the fracture process zone (FPZ) characterizing a more brittle behaviour for beams subjected to creep. The AE shows that this may be due to the development of microcracking detected under creep Based on those experimental results, a mesoscopic numerical study was proposed by coupling a damage model based on the microplan theory and a viscoelastic creep model defined by several Kelvin-voigt chains. The numerical results on concrete specimens in tension and in bending confirm the development of microcracks during creep at the mortar-aggregate interface.; La prédiction des déformations différées est d'une très grande importance pour l'étude de la durabilité des structures en béton. En effet, elles peuvent être à l'origine de la fissuration, de pertes de précontrainte, d'une redistribution des contraintes et même, plus rarement, de la ruine de l'ouvrage. L'objectif de cette étude est de mieux comprendre l'interaction entre les déformations de fluage et l'évolution de la fissuration dans le béton. Une investigation expérimentale sur l'évolution des propriétés de rupture de poutres en béton soumises à des hauts niveaux de fluage en flexion a été menée. L'influence du fluage sur la résistance résiduelle et l'énergie de fissuration du béton a été étudiée. En parallèle, la technique d'émission acoustique (EA) a été exploitée afin de suivre l'évolution de l'endommagement. Les résultats montrent une diminution de la zone de microfissuration caractérisant un comportement à la rupture plus fragile pour les poutres ayant subi le fluage. L'EA indique que ceci peut être dû au développement des micro-fissures observées pendant le fluage. L'ensemble de ces résultats conduit à proposer une approche de modélisation à l'échelle mésoscopique couplant un modèle d'endommagement basé sur la théorie des microplans et un modèle de fluage viscoélastique de type Kelvin-Voigt afin de mieux comprendre le processus de fissuration sous fluage. Les calculs du fluage d'échantillons de béton en traction directe et de poutres en flexion trois points ont confirmé la formation de micro-fissures pendant le fluage aux interfaces entre le mortier et les granulats.

In its service-life concrete is loaded and delayed strains appear due to creep phenomenon. Some theories suggest that micro-cracks nucleate and grow up when concrete is submitted to a high sustained loading, which contribute to make the concrete weaker. Thus, it is important to understand the interaction between the viscoelastic deformation and damage in order to design reliable civil engineering structures. Several creep-damage theoretical models have been proposed in the literature. However most of these models are based on empirical relations applied at the macroscopic scale. Coupling between creep and damage is mostly realised by adding some parameters to take into account the microstructure effects. In the author opinion, the microstructure effects can be modelled by taking into account the effective interactions between the concrete matrix and the inclusions. In this paper, a viscoelastic model is combined with an isotropic damage model. The material volume is modelled by a Digital Concrete model which takes into account the "real" aggregate size distribution of concrete. The results show that stresses are induced by strain incompatibilities between the matrix and aggregates at mesoscale under creep and lead to cracking.

International audience; The use of CODA wave interferometry (CWI) is an emerging topic in civil engineering for the non destructive characterisation of concrete. The idea is to take benefit from the heterogeneity of concrete with regard to ultrasonic wave propagation. Indeed when wavelength are of the same size as that of the aggregates, ultrasonic waves are multiply diffracted and carry information about very small perturbations of the propagating medium that is it possible to measure with inter-correlation with a reference signal. This technique is used here to follow modifications of concrete micro-structure during the beginning of creep tests. A three points bending specimen is instrumented with ultrasonic transducers located in its central area and velocity modifications and decorrelation of the signals are monitored as a function of the load applied and time. We show that CWI is potentially complementary to measurement of the global displacement of the beam by carrying more localised very sensitive information.; L'utilisation de l'interférométrie de la CODA (CWI) est un sujet émergent dans le génie civil pour la caractérisation non destructive du béton. Il s'agit de tirer profit de l'hétérogénéité de ce matériau lors de la propagation des ondes ultrasonores. Ainsi, lorsque les longueurs d'onde des signaux sont du même ordre de grandeur que la taille des granulats, les ondes multiplement diffusées portent une information sur de petits changement du milieu qu'il est possible de quantifier par inter-corrélation avec un signal de référence. Cette technique est utilisée ici pour suivre les modifications de la microstructure du béton au début d'un essai de fluage. Une poutre en flexion trois points est instrumentée avec des traducteurs ultrasonores dans la zone centrale et les modifications de vitesse et la décorrélation des signaux au cours du temps en fonction de différents chargements sont étudiées. Nous montrons que la CWI est potentiellement complémentaire à la mesure du déplacement global de la poutre en apportant une information plus localisée très sensible.

Creep and damage in concrete govern the long-term deformability of concrete. Thus, it is important to understand the interaction between creep and damage in order to design reliable civil engineering structures subjected to high level loading during a long time. Many investigations have been performed on the influence of concrete mixture, the effect of the bond between the matrix and the aggregates, temperature, aging and the size effect on the cracking mechanism and fracture parameters of concrete. But there is a lack of results on the influence of the creep loading history. In the present paper, an experimental investigation on the fracture properties of concrete beams submitted to three point bending tests with high levels of sustained load that deals with creep is reported. The results aim first to investigate the ranges of variation of the time response due to creep damage coupled effects under constant load and secondly to evaluate the residual capacity after creep. For this purpose a series of tests were carried out on geometrically similar specimens of size 100x200x800mm with notch to depth ratio of 0.2 in all the test specimens. The exchange of moisture was prevented and beams were subjected to a constant load of 70% and 90% of the maximum capacity. Three point bending test were realized on specimen at the age of 28 days to determine the characteristics of concrete and the maximum load so we could load the specimens in creep. Threepoint bend creep tests were performed on frames placed in a climate controlled chamber [1]. Then after four months of loading, the beams subjected to creep were removed from the creep frames and then immediately subjected to three-point bending test loading up to failure with a constant loading rate as per RILEM-FMC 50 recommendations. The residual capacity on the notched beams and the evolution of the characteristics of concrete due to the basic creep was considered. The results show that sustained loading had a strengthening effect on concrete, probably because of the consolidation of the hardened cement paste. The influence of creep on fracture energy, fracture toughness, and characteristic length of concrete is also studied. The fracture energy and the characteristic length of concrete increases slightly when creep occurs prior to failure and the size of the fracture process zone increases too. The load-CMOD relationship is linear in the ascending portion and gradually drops off after the peak value in the descending portion. The length of the tail end portion of the softening curve increases with beams subjected to creep. Relatively more ductile fracture behavior was observed with beams subjected to creep. The contribution of non-destructive and instrumental investigation methods is currently exploited to check and measure the evolution of some negative structural phenomena, such as micro-and macro-cracking, finally resulting in a creep-like behaviour. Among these methods, the non-destructive technique based on acoustic Emission proves to be very effective, especially to check and measure micro-cracking that takes place inside a structure under mechanical loading. Thus as a part of the investigation quantitative acoustic emission techniques were applied to investigate microcracking and damage localization in concrete beams. The AE signals were captured with the AE WIN software and further analyzed with Noesis software analysis of acoustic emission data. AE waveforms were generated as elastic waves in concrete due to crack nucleation. And a multichannel data acquisition system was used to record the AE waveforms. During the three point bending tests, quantitative acoustic emission (AE) techniques were used to monitor crack growth and to deduce micro fracture mechanics in concrete beams before and after creep. Several specimens are experimented in order to match each cluster with corresponding damage mechanism of the material under loading. At the same time acoustic emission was used to investigate characteristic of the fracture process zone (length width and macro crack propagation) [4] and to measure energy associated with fracture of concrete test specimens [5]. It is further suggested that AE energy release can be related to actual crack formation energy and the study of the nature of waves release may explain different mechanism as friction and other internal energy dissipation or toughening mechanisms. In addition more experiments were conducted using acoustic emission with the three point bending test and AE with specimen under creep so that the analysis could be capable of detecting events during creep. Then acoustic emission is analyzed and illustrated the correlation between basic displacement and total number of acoustic events or in other terms the damage occurring inside the material under creep [2, 3].

The use of shrinkage reducing admixture (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. In this paper, the influence of SRA on plastic shrinkage, long term shrinkage, mechanical characteristics as well as concrete pore structure were investigated. Samples of concrete were prepared with two water-binder ratios (w/b) to design ordinary and high strength concrete. The effect of the shrinkage-reducing admixture was studied by adding 1 % to the total mass of binder, while keeping the other parameters constant. The results indicate that SRA reduced plastic shrinkage, and reduce the 7 day drying shrinkage for w/c=0.65 and w/c=0.43 concrete mixtures by up to 56% to 31% respectively and the 70 day drying shrinkage by up to 33% to 25% when the specimens were cured at relative humidity of 50%. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 0.13 to 1µm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels which can explain the reduction in drying shrinkage that occurs when SRA are used in concrete., Les ouvrages du génie civil sont victimes de l'apparition de microfissures dans le béton au jeune âge due aux phénomènes de retraits de la pâte de ciment. L'objectif de ce travail est d'étudier l'effet d'un agent compensateur de retrait, sous forme d'adjuvant, sur les déformations libres du béton. Ainsi trois axes ont été suivis : le retrait plastique, le retrait du béton durci, ainsi que l'analyse des propriétés mécaniques. Les essais sont réalisés sur deux formulations ayant deux rapports eau sur ciment (E/C) différents : 0,65 et 0,43. L'effet de l'adjuvant a été étudié en l'ajoutant au béton à hauteur de 1% en masse du liant tout en conservant les autres paramètres de composition constants. L'amplitude du retrait de dessiccation est diminuée de 57% après 7 jours de durcissement pour le béton ayant un E/C=0,65 et 31% pour le béton avec E/C=0,43 à la même échéance. A l'âge de trois mois, cette différence diminue à 33% et 25% pour les deux bétons respectivement. En effet, une légère augmentation de la porosité totale du béton a été mesurée, ainsi qu'une diminution du pourcentage de pores les plus larges de diamètre compris entre 0,13 et 1µm. Ces pores étant les premiers à perdre l'eau interne, cette modification du réseau poreux peut expliquer la diminution du retrait du béton durci.

International audience; This paper is focused on the part of the project ANR ENDE that concerns the damage and crack characterization by means of NDT. The tests are done in 3 or 4 points bending in order to study the cracks' generation, their propagation, as well as their opening and closing. The main ultrasonic techniques developed concern linear or non-linear acoustic: acoustic emission [SBA 2015], Locadiff [LAR 2015], energy diffusion, surface wave's velocity and attenuation, DAET [3]. The recorded data contribute to providing the mapping of the investigated parameters, either in volume, in surface or globally. Digital image correlation is an important additional asset to validate the coherence of the NDT results. RÉSUMÉ. Il est nécessaire d'utiliser les méthodes non-destructives pour une bonne évaluation des structures en béton et notamment les ouvrages sensibles comme les centrales nucléaires. Dans le cadre du projet ANR ENDE, plusieurs développements ont été effectués en méthodes non destructives. Cet article présente les résultats d'une large campagne expérimentale qui a pour objectif de qualifier des méthodes END nouvelles pour la détection de l'endommagement diffus et la détection et la caractérisation des fissures (position, longueur, etc.).

International audience; Le bois est un matériau naturel de plus en plus utilisé dans la construction. Afin d'améliorer les outils de conception des structures en bois, la connaissance des mécanismes de fissuration est un élément primordial à appréhender. L'objectif de ce travail s'oriente vers l'étude des propriétés mécaniques et des mécanismes d'endommagement du bois massif épicéa et du bois lamellé collé avec des lamelles de pin. Une investigation expérimentale préliminaire est proposée en réalisant un essai de fissuration en traction (Mode I) provoquée par enfoncement d'un coin (Wedge Splitting Test). Le suivi de l'endommagement durant ces essais est effectué à l'aide de la technique d'émission acoustique (EA). La localisation cumulée des évènements acoustiques et les paramètres des signaux (énergie, amplitude, etc.) durant les différentes phases de l'essai de traction ont été étudiés. Les résultats montrent une bonne corrélation entre les courbes force-CMOD et l'activité acoustique. La largeur de la zone de microfissuration a été aussi estimée à partir des cartes de localisation.

International audience; Cracking in reinforced concrete structure is of major concern since it may impact their durability and their structural integrity. Cracking is mainly influenced by the stress distribution along the interface between steel and concrete which is responsible of transferring load from steel bar to the surrounding matrix. Thus, quantitative evaluation of the influence of steel-concrete bond on the fracture properties is important. An experimental investigation on pull-out and tensile tests on reinforced concrete ties is reported in this study. Pull-out and tensile tests are conducted on cubic specimens with different reinforcement diameters of high adhesion in parallel with a continuous monitoring using the Acoustic Emission (AE) technique and the Digital Image Correlation (DIC). The AE technique allows real time monitoring of the fracturing stages and can provide useful information about the location of defects and damage characteristics. Two failure modes are observed according to the steel bar section: concrete cover splitting and pull out with damage concentrated near the concrete rebar interface. The results show a good correlation between the loading force and the AE activity with AE signal of different characteristics indicating different active fracture mechanisms. The order of apparition of cracks, their width and their spacing was also measured in correlation with the loading level for short ties with different lengths and bar diameters. In addition, the location of AE events is consistent with the actual crack development.

International audience; Concrete is a heterogeneous material that develops delayed strains under a constant load. These deformations are related to micromechanisms which occur in its microstructure. To model this behaviour a new numerical multi-scales method is suggested. Based on a method for the calculation of the effective response for a non-aging material, an extension to aging materials has been developed. This method operates directly in space-time and avoids the difficulties of Laplace Carson transform inversion. The delayed strains are calculated in a representative elementary volume thanks to classical finite elements method which allows determining the components of the creep compliance tensor. First of all, calculations have been performed for an uni-axial loading to model the creep strain at different scales in order to give the visco-elastic property of calcium silicate hydrates. Then, the evolution of the microstructure with age has been considered to model the creep at early-ages of concrete.