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This work aimed to understand the mechanical behavior of siliceous and calcareous sand materials under uniaxial confined compression loading at high stress levels. For this purpose, a series of quasi-oedometric compression tests were conducted on sand materials, to examine the effects of grain size, nature, and moisture contents on the soil crushability and the compression behavior, using an upgraded thick pressure vessel device that can reach mean stress up to 500 MPa. All samples were prepared using an aspect ratio of 1:1 (diameter: height), placed inside a high strength steel vessel, and compressed at a uniform axial displacement rate of 5 µm/s. The vessel is instrumented with multiple strain gauges allowing for the characterization of the hydrostatic and deviatoric behavior of each test. The results of quasi-oedometric tests, conducted on these types of sand, up to a passive confinement of 500 MPa, show that particle breakage is enhanced by the presence of water. It was noticed that, for siliceous sand, smaller particles break more than larger particles, and that the calcareous grains manifest a rapid response to axial stress compared to siliceous sand. Testing various soil properties shows a great potential to better characterize the sensitivity to breakage of soils. Lastly, a post-mortem analysis of samples before and after testing, using the X-ray micro-tomography technique, was applied to study the mechanical damage of sand specimens.

This paper discusses the consistency of the theoretical basis behind the kinematic models of strong discontinuity methods for local fracture simulations using the Embedded Finite Element Method (E-FEM). A brief review is made on the elemental enhancement functions from the current E-FEM literature and how previous works managed to model mode I (normal) and mode II (parallel) fracture kinematics in multiple dimensions. Further analysis is made on how these approaches also introduce unintended mesh dependencies and basic kinematic inconsistencies of the fracture model with respect to its hosting element. Notable work from a few authors discussing some of these issues and their contributions to resolve them is reviewed as well. Based on this analysis, a new proposal of strong discontinuity enhancement functions is introduced to ensure a broader kinematic coherence within the element and to avoid the observed theoretical faults. This is done by making a more extensive use of the flexibility granted by the Hu-Washizu variational principle and by introducing new algebraic constraints that will ensure more correct fracture kinematics without compromising the acknowledged simplicity of the whole E-FEM framework. Element-level simulations are done to compare the outputs within a group of selected formulation approaches, including the novel proposal. Simulations show that the new element formulation grants a wider level of basic kinematic coherence between the local fracture outputs and element kinematics themselves, demonstrating an increase in robustness that might drive the usefulness and competitiveness of E-FEM techniques for fracture simulations to a higher level.

This paper analyses in detail the use of the Embedded Finite Element Method (E-FEM) to simulate local material heterogeneities. The work starts by a making a short review on the evolution of weak discontinuity models within the E-FEM framework to discuss how they account for the presence of multiple materials within a single element structure. A theoretical basis is introduced through some mathematical weak discontinuity definitions and the Hu-Washizu variational principle, for then establishing a set of requirements for retaining variational and kinematic consistency for any weak discontinuity enhancement proposal. From a general definition of a displacement enhancement field, two particular enhancement functions are derived by considering different consistency requirements: one which has been typically used in previous works and other which truly possesses variational consistency. A discussion is held on enhancement stability properties and the impact to global finite element solution processes. In the end, numerical simulations are made to assess the performance of each of these enhancements on the task of modelling a classical bi-material layered 3D tension problem. The final discussion evaluates both model performance and ease of implementation.

International audience; This paper provides both an analytical and a finite element model aiming at better predicting possible perforation of reinforced concrete slabs submitted to impacts. Both models account for free water saturation ratio and high triaxial stress induced into concrete by the impact. Finite element simulations are performed with Abaqus explicit code using a revised constitutive model for concrete; this coupled damage plasticity model (PRM) accounts for strain rate effects and the influence of saturation ratio on the triaxial behavior. Complementary original analytical predictions of ballistic limit and residual velocities are provided for both hard and soft impacts. These predictions depend on a recent deviatoric stress-based formulation of compressive strength of concrete. Numerical and analytical results are consistent with bending and punching experimental tests.

International audience; Shake table testing is a very efficient tool in earthquake engineering that may be used to enhance construction methods andthe overall safety of structures. Shake tables permit investigation of the dynamic behaviour of civil engineering structures.This paper details the development of a uniaxial shake table that was designed and built with the objective of limiting its costto a moderate amount, bellow 250 k€. The technical specifications of the shake table are presented as well as the evaluationof its performance and its validation. It is shown that it is possible to build an operational shake table capable of generatingreal seismic motion for the assessment of structural behaviour at a moderate cost.

International audience; This work examines the effect of porosity and free water on the behavior of concrete under high confinement. For this purpose, three types of concretes are designed with either low, medium or high porosities but featuring the same aggregate skeleton, yielding: an ordinary referenced concrete (OC), as already characterized in previous studies; a high performance concrete (HPC) with very low capillary porosity; and a low performance concrete (LPC) with the same composition as OC except for a much higher entrained air porosity. The results of triaxial tests, conducted on these three concretes up to a confinement of 600 MPa at both low and high saturation ratios, are presented. The conclusions from past studies on OC are extended herein for the cases of HPC and LPC: the free water quantity exerts a major influence on the concrete strength capacity as well as the volumetric stiffness for all three types of concretes. These results also show that the free water influence on concrete behavior depends on both the amount and nature of this porosity: the effect of modifying the entrained air porosity is much weaker than that of capillary porosity. To better quantify the effect of concrete saturation ratio, results are then completed by triaxial testing performed on OC with intermediate saturation ratios. Lastly, an empirical triaxial failure criterion of concrete, which takes into account the uniaxial strength, porosity and saturation ratio, is proposed; this new criterion accurately reproduces the results presented, as well as, a large database obtained from previous studies.

The modelling of thermal spalling of concrete structures is a challenging subject that requires relevant and reliable experimental data concerning temperature distribution and material degradations. For this purpose, a new device dedicated to the study of concrete thermal-induced spalling was designed and manufactured and is now under development. The experimental apparatus consists of a propane supplied burner surmounted by a furnace that can heat uniformly a concrete specimen surface according to ISO 834 standard fire curve. Temperature-time curves on the heated face and at different points of high performance concrete specimens that underwent spalling are reported and commented. The thermal conditions for which spalling and explosive spalling occurred are also examined and analyzed. An indicative thermal gradient value of about 14 °C/mm was found to be critical for the spalling occurrence. The analysis of the debris mass and dimensions distributions highlighted their usefulness in numerical modelling purpose. Lastly, the paper analyses how the undertaken tests results can ease the finite element modelling of spalling of concrete structures.

The objective of this study is to measure the interstitial pore pressure into saturated concrete under hundreds of megapascals of confinement. This study is carried out within a more general context aiming to understand the behavior of concrete structures under impact. It is well known that the water saturation in massive concrete structures evolves from quasi-dry state at the surface to reach a quasi-saturated state at the core. Since the response of these structures under impact is highly linked to the state of saturation into the material, it is suspected that the pore pressure plays a major effect. This paper presents a new testing technique developed to measure the concrete pore pressure at high confining pressure. This latter is generated by means of a high capacity GIGA press. The new concept consists in implementing a pressure sensor into a water collecting cap. This cap is designed specially to collect water from concrete subjected to mechanical confinement pressure. Experimental results show that concrete pore pressure can reach values of the order of the confining pressure.

This paper proposes a simple formulation for the impact analysis of a deformable projectile on reinforced concrete targets. This approach assumes the presence of soft strikers and rigid targets. Based on an energy balance, it aims to predict a perforation limit for different targets under soft impacts. The procedure employed is validated by means of tests performed on a rigid target (pendulum test) and reinforced concrete slabs. The article discusses various estimations of the crushing force of projectiles, as derived from experimental results and numerical approaches. The efficiency of simplified approaches is highlighted from an engineering point of view. The limit between soft and hard impacts is also analyzed according to a recent criterion. Moreover, the paper proposes validating the approach using experimental tests with both perforating and non-perforating impact tests on nine slabs.

Whenever concrete structures are subjected to impacts or blast loadings, high confining pressures generate a triaxial loading on the concrete at the material scale. This paper presents a new Radial Strain Sensor (RSS) dedicated to concrete specimens (7 cm in diameter) for use in triaxial compression tests under very high lateral confining pressure (about 600 MPa). The objective here is to better characterize the behavior of concrete under stresses potentially encountered during severe loadings, which would complicate the use of strain gauges, especially when the tested sample becomes saturated with water. Moreover, while volumetric behavior is an important parameter in concrete behavior modeling, strain gauge measurements constitute local observations and hence are not representative of global specimen behavior when localized damage appears (i.e., post-peak behavior). To achieve the stated objective, a new radial sensor that takes into account geometric and technical constraints imposed by the triaxial cell has been designed, tested and validated. This radial strain sensor (RSS) is capable of supporting very high confining pressures generated during the triaxial test (850 MPa). After introducing the geometric constraints leading to the initial sensor shape, the pressure effects, temperature and boundary conditions involved in strain analyses are all presented, in emphasizing that greater sensor sensitivity is indeed necessary. By means of a numerical approach, an RSS dimensional optimization protocol is proposed and discussed. Lastly, the calibration procedure is described and the sensor measurement validated thanks to a triaxial test conducted on concrete at a 600-MPa confining pressure.

cited By 41; International audience; Performance observation is a necessary part of the design and construction process in geotechnical engineering. For deep urban excavations, empirical and numerical methods are used to predict potential deformations and their impacts on surrounding structures. Two inverse analysis approaches are described and compared for an excavation project in downtown Chicago. The first approach is a parameter optimization approach based on genetic algorithm (GA). GA is a stochastic global search technique for optimizing an objective function with linear or non-linear constraints. The second approach, self-learning simulations (SelfSim), is an inverse analysis technique that combines finite element method, continuously evolving material models, and field measurements. The optimization based on genetic algorithm approach identifies material properties of an existing soil model, and SelfSim approach extracts the underlying soil behavior unconstrained by a specific assumption on soil constitutive behavior. The two inverse analysis approaches capture well lateral wall deflections and maximum surface settlements. The GA optimization approach tends to overpredict surface settlements at some distance from the excavation as it is constrained by a specific form of the material constitutive model (i.e. hardening soil model); while the surface settlements computed using SelfSim approach match the observed ones due to its ability to learn small strain non-linearity of soil implied in the measured settlements. © 2009 Elsevier Ltd. All rights reserved.

cited By 92; International audience; This study focuses on the identification of concrete behavior under severe triaxial loading in order to better evaluate the vulnerability of sensitive infrastructure to near-field detonations or ballistic impacts. For the purpose of reproducing high stress levels with well-controlled loading paths, static tests have been conducted on concrete samples using a triaxial press offering very high capacities (stress levels of around 1 GPa). It is a well-known fact that the concrete drying process is a slow phenomenon. Massive concrete structures, such as bridge piers, dams and nuclear reactors, could retain a quasi-saturated core throughout most of their lifetime, even though their facing dries very quickly. The objective of this article is to evaluate the effect of the saturation ratio on concrete behavior under high confinement; this article will present triaxial test results on concrete samples over a saturation ratio range extending from dried to quasi-saturated concretes. The subsequent analysis of results will show that the saturation ratio exerts a major influence on concrete behavior, particularly on both the concrete strength capacity and shape of the limit state curve for saturation ratios above 50%. This analysis also highlights that while the strength of dried concrete strongly increases with confining pressure, it remains constant over a given confining pressure range for either wet or saturated samples. © 2008 Elsevier Ltd. All rights reserved.

A series of pressuremeter and penetrometer (dynamic SPT) tests on a sandy soil of Unesco area in Beirut are presented. These results are compared by observing the variations of their parameters versus depth. Samples of this soil have been tested in laboratory. All these data are used for a numerical modelling of the pressuremeter test (software PLAXIS), by inverse analysis, for calculation of geotechnical constructions.

This paper deals with empirical formulae predicting the perforation of reinforced concrete barriers. These formulae are usually validated for hard impacts only. Consequently, on the one hand, a simple method is proposed in case of soft impacts. For various tests, experimental and model predictions of perforation show a good agreement. On the other hand, recent tests at the material scale show that the behaviour of concrete under high confinement does not depend on the unconfined compressive strength of concrete after 28 days, fc 28. Therefore analytical models based on fc 28 need to be considered with caution. Cet article se focalise sur les modeles analytiques de prevision de la perforation de voiles en beton arme sous impact. Ces modeles ne sont en general valables que pour des projectiles rigides. D'une part, une methode simple de prevision de la perforation est alors proposee dans le cas d'impacts mous. Les formules qui en decoulent permettent de prevoir la perforation de dalles impactees dans le cadr...

cited By 14; This study presents the probabilistic analysis of the inverse analysis of an excavation problem. Two techniques are used during two successive stages. First, a genetic algorithm inverse analysis is conducted to identify soil parameters from in situ measurements (i.e. first stage of the construction project). For a given tolerable error between the measurement and the response of the numerical model the genetic algorithm is able to generate a statistical set of soil parameters, which may then serve as input data to a stochastic finite element method. The second analysis allows predicting a confidence interval for the final behaviour of the geotechnical structure (i.e. second stage of the project). The tools employed in this study have already been presented in previous papers, but the originality herein consists of coupling them. To illustrate this method, a synthetic excavation problem with a very simple geometry is used. © 2010 Elsevier Ltd. All rights reserved.

Politano and Pouquet's law, a generalization of Kolmogorov's four-fifths law to incompressible MHD, makes it possible to measure the energy cascade rate in incompressible MHD turbulence by means of third-order moments. In hydrodynamics, accurate measurement of third-order moments requires large amounts of data because the probability distributions of velocity-differences are nearly symmetric and the third-order moments are relatively small. Measurements of the energy cascade rate in solar wind turbulence have recently been performed for the first time, but without careful consideration of the accuracy or statistical uncertainty of the required third-order moments. This paper investigates the statistical convergence of third-order moments as a function of the sample size N. It is shown that the accuracy of the third-moment depends on the number of correlation lengths spanned by the data set and a method of estimating the statistical uncertainty of the third-moment is developed. The technique is illustrated using both wind tunnel data and solar wind data., Submitted to: Nonlinear Processes in Geophysics

International audience; Cette étude concerne l'identification du comportement du béton sous sollicitations triaxiales sévères (chutes de blocs, explosions, impacts balistiques). Pour reproduire de forts niveaux de contrainte avec des trajets de chargement bien contrôlés, des essais statiques sont réalisés sur des échantillons de béton en utilisant une presse triaxiale de très grande capacité (niveaux de contrainte de l'ordre du GPa). En outre, il est connu depuis longtemps que le rapport E/C, entrant dans la composition du béton, est un des paramètres majeurs conditionnant la porosité et la résistance de la matrice cimentaire du béton durci. L'objectif de cet article est donc de quantifier l'effet de ce rapport sur le comportement du béton sous fort confinement. A partir de la composition d'un béton de référence « ordinaire » (E/C=0,6), deux autres bétons avec des rapports E/C respectivement égaux à 0,4 et 0,8 sont formulés. Cet article présente les résultats des essais triaxiaux réalisés sur ces trois bétons. Ces résultats montrent que, contrairement à ce qui est observé en compression simple, le béton se comporte sous fort confinement comme un empilement granulaire sans aucune influence de la résistance de la matrice cimentaire. ABSTRACT. This study concerns the identification of concrete behaviour under severe triaxial loadings (rock falls, explosions, ballistic impacts). In order to reproduce high stress levels with well controlled loading paths, static tests are carried out on concrete samples by means of a very high capacity triaxial press (stress levels of the order of the GigaPascal). In addition, it is known for a long time that the water/cement ratio (W/C), entering the concrete composition , is one of the major parameters conditioning the porosity and the strength of the cement matrix of the hardened concrete. The objective of this article is to quantify the effect of this ratio on the concrete behaviour under high confinement. From the composition of a reference "ordinary" concrete (W/C=0,6), two other concretes were formulated with W/C ratios equal to 0,4 and 0,8 respectively. This article presents experimental results concerning the effect of the water/cement ratio (W/C) of the concrete on its behaviour under high confinement. The test results presented in this article show that under high confinement, the concrete behaves as a granular stacking which the concrete constitutes without any influence of the cement matrix strength. MOTS-CLÉS : béton, rapport eau/ciment, essai triaxial, fort confinement.

We study the experimental dependence of the third-order velocity structure function on the Taylor based Reynolds number, obtained in different flow types over the range 72⩽Rλ⩽2260. As expected, when the Reynolds number is increasing, the third-order velocity structure functions (plotted in a compensated way) converge very slowly to a possible −4/5 plateau value according to the Kolmogorov 41 theory. Actually, each of these normalized third-order functions exhibits a maximum, at a scale close to the Taylor microscale λ. In this Brief Communication, we show that experimental data are in good agreement with the recent predictions of Qian and Lundgren. We also suggest that, from an experimental point of view, a log-similar plot suits very well to study carefully the behavior of the third-order velocity structure functions with the flow Reynolds number.

cited By 2; International audience; This paper focuses on the discrete modeling of triaxial behaviour of concrete. The originality of this work comes from two points. The first one concerns the predictive feature of the model developed for simulating the response of concrete specimens; the behaviour of mortar, rock, and their interaction being identified a priori or by means of experimental tests on the mortar and the rock. The second originality relates to the construction method of the discrete element assembly based on the 3D segmentation of tomographic images. Such a method allows modeling of concrete at the mesoscopic scale with an internal structure similar to the one of the concrete tested experimentally. The comparisons between numerical and experimental results show the model is capable to reproduce the triaxial behavior of concrete for confining pressure varying from 0 to 650 MPa.

This thesis focuses on the behavior of concrete under severe triaxial loading. The stress levels studied are, more accurately, those caused by a missile impact. The broader context of this study is to improve the understanding of infrastructures under dynamic loading type impact, which can generate mean stresses in the concrete of the order of GPa. Quasi-static tests are needed to characterize the behavior of a material in a homogeneous state of stress. The work presented in this thesis proposes the investigation of damage mechanisms of concrete during triaxial loading tests. For this, two innovative machines are used : a large capacity triaxial press and an X-ray computed tomography instrument. The process of conducting tests is to scan concrete samples in the undamaged state and after each loading cycle (load-unload) performed by mean of the triaxial press. The images from these tomographic examinations allow to assess the evolution of the mesostructure of concrete (scale where aggregates and macropores appear) for different loading paths and loading levels. Two samples were tested according to this process. The first one, at relatively low confinement (50 MPa), shows that the mechanisms leading to failure are very localized and mainly initiated at the microscopic scale. The second one, tested at a very high confinement (650 MPa), has a diffuse damage with also a localized failure mechanism. A mesoscopic discrete element model is also presented in this manuscript. The originality of this approach lies in the predictive model developed on one hand, the behavior of mortar, aggregates or their interaction is identified a priori or from tests on mortar and rock. On the other hand, the method of constructing the discrete element mesh is based on the 3D segmentation of tomographic images what allows a comparison between fracture facies of both experiment and numerical specimens. This model leads to a good prediction of the triaxial behavior of concrete and appears to be a promising tool to study damage mechanisms and the influence of parameters of concrete composition., Ce mémoire de thèse s'intéresse au comportement du béton sous chargement triaxial sévère, et plus précisément, aux mécanismes de dégradation associés. Le cadre plus général de cette étude est l'amélioration de la tenue des infrastructures soumises à des chargements dynamiques de type impact, pouvant générer des contraintes moyennes dans le béton de l'ordre du GPa. Des essais quasi-statiques sont nécessaires pour caractériser le comportement d'un matériau dans un état de contraintes homogènes. Les travaux présentés dans ce manuscrit proposent d'investiguer les mécanismes de dégradation du béton lors d'essais triaxiaux. Pour cela, deux machines innovantes sont utilisées : une presse triaxiale de grande capacité (Giga) et un tomographe à rayons X. Le processus de réalisation des essais consiste à scanner des échantillons de béton à l'état vierge et après chaque cycle de chargement (charge-décharge) effectué avec la presse triaxiale. Les images issues de ces examens tomographiques permettent d'évaluer la modification de la mésostructure du béton (échelle où les granulats et les macropores apparaissent) selon différents trajets et niveaux de chargement. Deux échantillons ont été testés triaxialement selon ce processus. Le premier, à relativement faible confinement (50 MPa), montre que les mécanismes menant à la rupture sont fortement localisés et principalement initiés à l'échelle microscopique. Le deuxième, testé à un très fort confinement (650 MPa), présente un endommagement diffus avec également un mécanisme de rupture localisé. Un modèle mésoscopique, utilisant la méthode aux éléments discrets, est également présenté dans ce manuscrit. L'originalité de cette approche réside d'une part dans le caractère prédictif du modèle développé, le comportement du mortier, des granulats ou de leur interaction étant identifié a priori ou à partir d'essais réalisés sur mortier et roche. D'autre part, la méthode de construction du maillage éléments discrets s'appuie sur la segmentation 3D d'images tomographiques ce qui permet une comparaison des faciès de ruptures expérimentaux et numériques. Ce modèle aboutit à une bonne prédiction du comportement triaxial du béton, et semble être un outil prometteur d'étude des mécanismes d'endommagement et de l'influence de paramètres de composition du béton.