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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.

The discrete element model proposed in this paper addresses the macroscopic behavior of concrete taking into account the presence of free water in pores, thanks to a new interaction law between spherical discrete elements (DE). When concrete structures are subjected to a severe loading, e.g., an impact, material exhibits high triaxial compressive stresses which are highly influenced by the saturation ratio. In this new constitutive model, cracking and compaction are modeled at the interaction level between DEs and free water effects are taken into account by introducing a dependency between the water saturation ratio and the inelastic deformation due to the pore closure. The present numerical model has been implemented in the Yet Another Dynamic Engine code in order to deal with extreme loading situations leading to stress states characterized by a high mean stress level.

This article presents the behaviour under fire of a specific ultra-high performance concrete (UHPC) called BCV. Several compositions with synthetic additions such, as polypropylene (PP) fibres and powder or acrylic (PAN) fibres, are mixed to produce these specimens. As a first step, a blowtorch test on prismatic specimens shows that small PP fibres are more efficient in resisting fire. Next, scanning electron microscopic observations after heating reveal significant behavioural differences between PP fibres and PAN fibres during their vaporisation process. Lastly, a mercury intrusion porosimetry investigation following a heating cycle at 350°C indicates that concrete porosity is not a sufficient parameter for determining whether or not a given material composition is resistant to fire spalling. A critical factor dependent on pore size distribution, is proposed herein; its threshold value, i.e. under which the material exhibits a very high probability of being prone to fire spalling, is also estimated.

This paper presents an experimental study of concrete at meso-scale (aggregates, macro-pores and mortar matrix) in order to get a better understanding of the local failure mechanisms known to drive the macroscopic mechanical behaviour of the material. The main originality comes from conducting in-situ X-ray mechanical tests on micro-concrete samples of realistic composition (including cement, sand, aggregates and water), under uniaxial compression and, for the first time, under triaxial compression at 5 MPa, 10 MPa and 15 MPa confining pressures. A timeseries analysis of the set of 3D images coming from each test allows for the measurement of the 3D kinematic fields (displacement and strain fields) throughout the experiments. The different failure patterns observed for each loading path are discussed, along with a quantification of the 3D fracturing processes at the scale of the largest heterogeneities (aggregates and macro-pores). With an increasing level of confinement, the transition from brittle to ductile response is observed, as well as an increase of the strength of the material. The pronounced impact of the meso-scale heterogeneities of concrete on their local failure mechanisms is highlighted. It is shown that strain localisation mainly originates between aggregates and mortar matrix, with the shape and location of the largest aggregates and macro-pores essentially driving the propagation of the cracking network.

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 available models for estimating the creep strains of concrete generally assume that concrete is a homogeneous material. Since concrete is a composite, such models are incapable of taking into a...

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 work makes the link between the experimental technique of x-ray tomography and the reliable representation of the heterogeneities of a cementitious material at the meso-scale (aggregates, mortar matrix and macro-porosity). An image analysis procedure and its validation are presented. Images coming from x-rays scans are treated to finally obtain a trinary 3D image, where each phase of the material's meso-structure is separated. The separation of the aggregates from the mortar matrix is the technical challenge, due to the physics of x-ray interaction with matter, addressed on this work. Validation of the proposed method is made by comparing the grain size distribution curves computed from images of the same set of aggregates coming from x-rays and neutron scans. The reliably obtained morphology can be used as a basis for direct physical simulations, as well as data to improve morphological models of any type.

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.

This article focuses on identifying concrete behavior under high triaxial loading. The study is carried out within a more general context of understanding the behavior of concrete under impact. The effect of coarse aggregate shape and composition on concrete under high triaxial compression is examined by means of a very high-capacity triaxial press. The shape effect is further investigated using rolled aggregates, crushed aggregates and glass balls. The influence of aggregate composition is determined on concretes mixed with siliceous aggregates, glass aggregates and limestone aggregates. These analyses specifically indicate that coarse aggregate shape exerts no influence on the concrete response at high confinement. Moreover, concrete behaves like a non-cohesive granular stacking that seems to be governed by the compacted cement matrix provided a sufficiently high aggregate strength. Otherwise, a lower aggregate strength serves to weaken this granular stacking. At an intermediate level of confinement, the shear strength of concrete is mainly controlled by aggregates strength; however, irregularly-shaped coarse aggregates slightly increase the overall strength of concrete.

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 42; International audience; This study focuses on identifying concrete behavior under severe triaxial loadings (near field detonation or ballistic impacts). In order to reproduce high stress levels with well-controlled loading paths, static tests have been carried out on concrete samples by mean of a very high-capacity triaxial press (stress levels on the order of 1GPa). It is a longstanding fact that the water/cement ratio (W/C), upon entering the concrete composition, is a major parameter affecting the porosity and strength of the cement matrix of hardened concrete. The objective of this article is to quantify the effect of this ratio on concrete behavior under conditions of high confinement. From the composition of a reference 'ordinary' concrete (i.e. W/C=0.6), two other concretes have been produced with W/C ratios equal to 0.4 and 0.8, respectively. This article presents experimental results and their analysis regarding the effect of water/cement ratio (W/C) on concrete behavior under high confinement. It shows that when placed under high confinement, concrete behaves like a granular stacking composed of concrete without any influence from the level of cement matrix strength. Copyright © 2009 John Wiley & Sons, Ltd.

cited By 50; International audience; When impact loaded, concrete is submitted to high triaxial stresses. The experimental response of concrete under quasi-static triaxial compression is studied using a triaxial press capable of applying a mean pressure greater than 1GPa on cylindrical samples measuring 7 cm in diameter and 14 cm high. A numerical analysis of these previous experiments is performed herein at a mesoscopic scale. Concrete is modelled as a biphasic material consisting of a mortar (cement paste and fine aggregates) and roughly spherical aggregates (with a diameter exceeding 2 mm) whose characteristics are applied on a regular cubic finite element mesh. A damage-plasticity model is then used to model the behaviour of mortar. An identification of model parameters on mortar samples and the subsequent comparison between numerical and experimental tests will be presented for hydrostatic and triaxial compression. Copyright © 2009 John Wiley & Sons, Ltd.

This study concerns the identification of constitutive models from geotechnical measurements by inverse analysis. Soil parameters are identified from measured horizontal displacements of sheet pile walls and from a measured pressuremeter curve. An optimization method based on a genetic algorithm (GA) and a principal component analysis (PCA), developed and tested on synthetic data in a previous paper, is applied. These applications show that the conclusions deduced from synthetic problems can be extrapolated to real problems. The GA is a robust optimization method that is able to deal with the non-uniqueness of the solution in identifying a set of solutions for a given uncertainty on the measurements. This set is then characterized by a PCA that gives a first-order approximation of the solution as an ellipsoid. When the solution set is not too curved in the research space, this ellipsoid characterizes the soil properties considering the measured data and the tolerate margins for the response of the numerical model. Besides, optimizations from different measurements provide solution sets with a common area in the research space. This intersection gives a more relevant and accurate identification of parameters. Finally, we show that these identified parameters permit to reproduce geotechnical measurements not used in the identification process. Copyright © 2009 John Wiley & Sons, Ltd.

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.

cited By 102; This paper is dedicated to the identification of constitutive parameters of the Mohr-Coulomb constitutive model from in situ geotechnical measurements. A pressuremeter curve and the horizontal displacements of a sheet pile wall retaining an excavation are successively used as measurements. Two kinds of optimization algorithms are used to minimize the error function, the first one based on a gradient method and the second one based on a genetic algorithm. The efficiency of each algorithm related to the error function topology is discussed. Finally, it is shown that the use of a genetic algorithm to identify the soil parameters seems particularly suitable when the topology of the error function is complex. Copyright © 2007 John Wiley & Sons, Ltd.

This article focuses on the static and dynamic behavior of two concretes under very high pressure loaded at different strain rates. Concretes differ by the type of coarse aggregates (limestone or siliceous). Specimens are confined in an elastic steel ring, insuring a quasi-uniaxial strain state of loading during axial compression, the so-called quasi-oedometric compression (QOC) tests. The effect of the loading path is first investigated by comparing their response under static QOC tests to triaxial compression results. Compaction of concrete made with strong siliceous aggregates is much higher in QOC test than in triaxial compression test, contrary to concrete made with soft limestone aggregates. Both concretes are then subjected to Split Hopkinson Pressure Bar testing. At high strain rate volumetric stiffness and deviatoric strength of both concretes are increased. Moreover, the increase of concrete strength with strain rate is more pronounced for concrete made with limestone aggregates than with siliceous aggregates.

This article focuses on the influence of the cement matrix porosity on the triaxial behaviour of concrete in the presence of high confining pressures. It offers new experimental results on two concretes, respectively a high performance concrete (HPC) and a low performance concrete (LPC). These results complement recent studies on ordinary concrete (OC), in using the same large-capacity triaxial press. This paper serves to better characterise the behaviour of these 3 concretes, whose porosities are very different in terms of volume and size; moreover, it emphasises that the contribution of cement matrix strength (mainly linked to its porosity) decreases at high confining pressures and that concrete behaviour is governed by the granular skeleton, even when the capillary porosity is low or the entrapped air porosity is high. To reach this level of residual behaviour, higher confining pressures are needed for HPC, even if the amount of porosity compacted is smaller.

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.

cited By 32; International audience; The velocity increments statistic in various turbulent flows is analysed through the hypothesis that different scales are linked by a multiplicative process, of which multiplier is infinitely divisible. This generalisation of the Kolmogorov-Obukhov theory is compatible with the finite Reynolds number value of real flows, thus ensuring safe extrapolation to the infinite Reynolds limit. It exhibits a β estimator universally depending on the Reynolds number of the flow, with the same law either for Direct Numerical Simulations or experiments, both for transverse and longitudinal increments. As an application of this result, the inverse dependence Rλ = f(β) is used to define an unbiased Rλ value for a Large Eddy Simulation from the resolved scales velocity statistics. However, the exact shape of the multiplicative process, though independent of the Reynolds number for a given experimental setup, is found to depend significantly on this setup and on the nature of the increment, longitudinal or transverse. The asymmetry of longitudinal velocity increments probability density functions exhibits similarly a dependence with the experimental setup, but also systematically depends on the Reynolds number.

cited By 0; International audience

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...

This paper is intended to assess the mesostructural damage mechanisms of concrete under hydrostatic and triaxial loadings. Such a study is possible thanks to two state-of-the-art laboratory instruments: a high-pressure triaxial press, and an X-ray computed tomography instrument. The laboratory protocol consists of scanning the concrete prior to the initial loading and after each cycle. An analysis of the resulting images indicates that under high hydrostatic loading, significant damage is visible in cement paste at the mesoscopic scale. For two triaxial tests conducted at 50 and 650 MPa of confining pressure, results reveal major differences in both damage and failure mechanisms. At the lower pressure, shear loading creates a localised failure mechanism characterised by sliding on an inclined plane, whereas at the higher pressure, the strain and damage mode are much more homogeneous with a failure localisation after unloading. Cet article a pour objectif d’estimer les mecanismes d’endommagement du beton p...

cited By 28; International audience; When massive concrete structures (high-rise buildings, tunnels, dams, nuclear power plants, bridges, protection structures, ..) are subjected to extreme loadings (aircraft shocks, rock falls, near-field detonations, ballistic impacts, ..), the material undergoes triaxial compression loading at a high confinement. In order to reproduce high stress levels with well-controlled loading paths, static triaxial tests are carried out on concrete samples by mean of a very high-capacity triaxial press. It is a well-known fact that the cement paste volume and the coarse aggregate size are two important parameters of concrete formulation. This article focuses on identifying the effect of coarse aggregate size and cement paste volume on concrete behavior under high triaxial compression. This article shows that at low confinement, the concrete strength slightly increases as the coarse aggregate size increases. At high confinement, the coarse aggregate size has a slight influence on concrete deviatoric behavior and a significant influence on concrete strain limit-state. The higher the coarse aggregate size, the lower is the mean stress level corresponding to concrete strain limit-state. Furthermore, this article highlights that at low confinement, the concrete strength significantly increases with an increase in cement paste volume. Increasing confinement tends to reduce cement paste volume effect on concrete strength. At high confinement, contrary to what has been observed in unconfined compression, the cement paste volume has little effects on concrete deviatoric behavior. Otherwise, decreasing cement paste volume increases concrete deformation capacity. At very high confinement levels and at very high deviatoric stress levels, the axial tangent stiffness of concrete increases as the coarse aggregate size or the cement paste volume is reduced. © 2011 Elsevier Ltd. All rights reserved.

cited By 0; International audience; This study aims to characterize mechanical behaviour of light concrete under very high stress levels of the order of the Giga Pascal. The studied light concrete, called S800, has low density (800 kg/m 3) and high measured porosity accessible to water (47%). This paper presents the development of a radial strain measurement device and first tests results obtained on light concrete, using a large capacity triaxial press. These tests results show that the concrete volumetric strain reaches more than 150% at maximum mean stress of 700 MPa. Triaxial tests with confinements ranging from 50 MPa to 400 MPa, carried out on the studied concrete, highlight the evolution of light concrete behaviour with confinement. The increase of confinement strongly increases both the ductility and the loading capacity of light concrete. © 2010 Taylor & Francis Group, London.

International audience; This article presents the first part of a study concerning the behaviour of ultra-high performance fibre concrete (UHPFC) under fire. Four kinds of synthetic fibres were added: polypropylene (PP), acrylic (PMMA), polyvinyl acetate (PVA) and polyethylene (PE). After a mechanical characterization of the material, blowtorch tests were performed on it. The expected efficiency of PP and PE fibres was observed. Microscopic observations enabled us to put forward a hypothesis regarding the efficiency of fibres in concrete. © 2010 Taylor & Francis Group, London.

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.

cited By 5; International audience; Prestressed concrete is commonly used in a wide range of applications, yet on rare occasions spalling problems have been reported for concrete structures prestressed in two directions. The tensile behaviour of concrete may be affected by prestressing; consequently a number of experiments have been performed to reproduce this load combination on laboratory samples. Tests were carried out in a compression-tension triaxial cell suitable for the independent application of a lateral confining pressure, as well as a direct axial tension load. Results reveal a measurable but limited reduction in concrete tensile strength with a confining pressure of a similar magnitude as the prestress (order of magnitude of 10 MPa). Further experiments were conducted at higher confining pressures, in both confined tension and in triaxial extension, in order to generate an overview of limit state behaviour for the studied high performance concrete. © RILEM 2009.

cited By 57; International audience; This study is intended to characterize the evolution in triaxial behavior of a standard concrete subjected to confining pressures varying from 0 to 600 MPa. Hydrostatic and triaxial tests, with several unloading-reloading cycles, are carried out on concrete samples using a high-capacity triaxial press. These tests serve to identify the evolution of the elastic unloading characteristics of concrete, depending on both confining pressure and axial strain. A number of optical observations are also provided to allow visualizing the evolution in concrete damage mode in the middle of the sample. Experimental results indicate a sizable change in concrete behavior with confining pressure. At low pressure values, Young's modulus decreases and Poisson's ratio rises sharply with axial strain. The concrete exhibits brittle behavior with failure caused by a localized damage mechanism. In contrast, at high confining pressures, the concrete becomes a ductile material, and the evolution in its unloading characteristics is negligible. Failure is thus associated with diffuse material damage. The concrete behaves like a granular material controlled by plasticity, meaning that the damage phenomenon observed at low confinement is completely inhibited. © 2009 RILEM.

cited By 26; This study focuses on identifying concrete behavior under severe loading (near field detonation or ballistic impacts). In order to reproduce high stress levels with well-controlled loading paths, static tests are carried out on concrete samples by mean of a very high-capacity triaxial press. Experimental results indicate a sizeable change in concrete behavior with confining pressure. At low pressure values, the concrete exhibits brittle behavior with failure caused by a localized damage mechanism. In contrast, at high confining pressures, the concrete becomes a ductile material, and failure is associated with diffuse material damage. These tests also show an evolution of the elastic characteristics of concrete. A numerical modeling of these previous experiments is performed at a mesoscopic scale. It provides a reproduction of the main characteristics of concrete behavior under high confinement, both qualitatively and quantitatively. © 2010 Taylor & Francis Group, LLC.

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

The causes of the nonlinear behavior of concrete until failure are numerous and complex, particularly for nonmonotonic and rapid loadings. A model is presented coupling damage and plasticity including several effects: Development and closure of cracks, damping, compaction, and strain rate effects. The idea is to describe, with the same tools, a wide variety of problems, the model is of explicit form, and what makes possible its implementation into explicit numerical scheme well adapted to the treatment of fast dynamic problems. In this context, the finite element "Abaqus explicit" code is used, and the model has been successfully applied during the past few years to model a large range of complex reinforced concrete structures subjected to severe loadings. In this paper, the main model concepts are presented, and some examples of numerical simulations are given and compared with experimental data. The applications proposed are related to quasi-static loading as well as to rapid loading (impact); In particular, one of them is within the framework of an experiment linked to the design of a reinforced concrete rock-shed gallery located in the French Alps. The results show the relevance of the modelling used, which makes some real numerical experiments very useful for complex structures and/or extreme loadings.

This article presents the production of strain measurements on porous concrete samples for use in triaxial compression and extension tests with a very high lateral confining pressure. When a massive concrete structure is subjected to severe loadings (e.g. rock falls, near-field detonations, and ballistic impacts), the material undergoes triaxial loading at a high confining pressure. To reproduce high levels of stress with well-controlled loading paths, static tests are carried out on concrete using a high-capacity triaxial press, called GIGA. This press allows the testing of concrete specimens (7 cm in diameter and 14 or 15.5 cm long) for levels of confining pressure ranging up to 850 MPa and axial stresses of up to 2.35 GPa. The porous characteristic of the material together with the high confining pressure require both developing a material protection device and building strain gauge-based instrumentation of unprecedented design for such confining pressures. In addition, the effect of pressure and other sources of error on strain and stress measurements are identified herein thanks to tests performed on model materials. This study shows that the effect of pressure on strain gauge measurements is negligible, whereas this same effect proves significant in the axial displacement measurement by means of a linear variable differential transformer (LVDT) sensor and must be taken into account therefore during the data processing phase. This article will present the initial results of triaxial compression tests conducted at high confining pressure on both dry and saturated concrete samples instrumented with gauges. It will also provide results of a triaxial extension test conducted at high confinement on dry concrete: a unique step in characterizing the triaxial behaviour of concrete. Moreover, it will be demonstrated that simultaneous axial strain measurements using gauges and the LVDT sensor serve to evaluate strain homogeneity of the sample tested at high confinement.

International audience

International audience