Corrosion of Steel in Concrete Seen through Neutron and X-Ray Tomography

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Experimental study of inelastic deformation at the micro scale in cemented granular materials: some recent results

A novel thermomechanical constitutive model for cemented granular materials has been recently introduced. An essential ingredient of the model is the use of measurable and micro-mechanics based internal variables describing the evolution of dominant inelastic processes. In this paper, discuss about the model ability to reproduce material behaviour at specimens scale starting from a few physically meaningful parameters. These parameters link the macroscopic mechanical behaviour to the statistically averaged evolution of the micro structure. However, to fully justify this statement and given the bottom-up hierarchy in the model development, it is also important to check the model’s capability to capture the statistically averaged evolution of the micro structure embedded at its base. For that purpose we have used high resolution x-ray tomography to scan artificially cemented granular materials under a variety of loading conditions. X-Ray Tomography has been chosen for its capability to track inelastic processes in granular materials non destructively with a high spatial resolution (a few microns, in this study). The main objective of this work is to introduce the key features of the constitutive model and to report some recent results on the experimental quantification of the evolution of the microscopic internal variables in cemented granular materials

Fast vapour migration next to a depressurizing interface: A possible driving mechanism of explosive spalling revealed by neutron imaging

When exposed to high temperatures, concrete is prone to explosive spalling, resulting in the projection of concrete flakes and the reduction of the structural element cross-section. Elastic energy alone cannot justify the explosive nature of the process: accumulated thermal energy serves as a supplementary source, together with the water in the pores, which can generate kinetic energy through vaporization. The objective of this paper is to study the fast thermal and hygral transients occurring upon depressurization of an internal interface, emulating a crack developing in hot moist concrete. One possible mechanism that could justify this contribution is the flash vaporization of water. This paper presents a new experimental approach to directly measure flash vaporization through rapid-neutron imaging. Additionally, a thermo-hygral model assesses key parameters influencing fasttransient phenomena. Experimental observations reveal moisture loss in a 1-mm-thick layer, equating to a 90 degrees C temperature drop, justifying a significant amount of released energy

A two-stage study of steel corrosion and internal cracking revealed by multimodal tomography

Modeling of corrosion-induced cracking is limited by lacking knowledge on the behavior of corrosion products. In this work, the corrosion and cracking processes were experimentally investigated in 3D at two different stages. The processes were measured at micro-structural scale, applying nondestructive neutron and X-ray computed tomography in two scans at different stages in the corrosion process. A method to evaluate the average volumetric strain of the compressed corrosion layer was proposed and displacements in the concrete matrix were measured. Strain localization revealed cracks not directly visible in the images. Multimodal tomography demonstrated to be an effective method for investigating steel corrosion in reinforced concrete

Neutron imaging of hydraulic flow within structural concrete

La quanti?cation et l'analyse de la distribution spatiale des ?ux et la compétition entre diverses porosités est à ce jour un verrou scienti?que majeur ne permettant pas d'alimenter proprement des modèles de perméabilités si ce n'est par le biais de la perméabilité moyenne. Le but de cette étude est de développer une nouvelle méthodologie et de valider le dispositif experimental par imagerie neutronique à la ligne de faisceaux D50 à l'Institut Laue Langevin. Le test consiste à injecter de l'eau normale (H2O) sous haute pression dans un échantillon de béton coulé et saturé avec de l'eau lourde (D2O) a?n de suivre la progression d'un front d'eau dans le temps par di?érence d'atténuation des deux eaux. Un test préléminaire a été mené et les premiers résultats sont présentés

Fast 4D imaging of fluid flow in rock by high‐speed neutron tomography

High‐speed neutron tomographies (1‐min acquisition) have been acquired during water invasion into air‐filled samples of both intact and deformed (ex situ) Vosges sandstone. Three‐dimensional volume images have been processed to detect and track the evolution of the waterfront and to calculate full‐field measurement of its speed of advance. The flow process correlates well with known rock properties and is especially sensitive to the distribution of the altered properties associated with observed localized deformation, which is independently characterized by Digital Volume Correlation of X‐ray tomographies acquired before and after the mechanical test. The successful results presented herein open the possibility of in situ analysis of the local evolution of hydraulic properties of rocks due to mechanical deformation

Erbium emission in MOS light emitting devices: from energy transfer to direct impact excitation

The electroluminescence (EL) at 1.54 µm of metal-oxide-semiconductor (MOS) devices with Er3+ ions embedded in the silicon-rich silicon oxide (SRSO) layer has been investigated under different polarization conditions and compared with that of erbium doped SiO2 layers. EL time-resolved measurements allowed us to distinguish between two different excitation mechanisms responsible for the Er3+ emission under an alternate pulsed voltage signal (APV). Energy transfer from silicon nanoclusters (Si-ncs) to Er3+ is clearly observed at low-field APV excitation. We demonstrate that sequential electron and hole injection at the edges of the pulses creates excited states in Si-ncs which upon recombination transfer their energy to Er3+ ions. On the contrary, direct impact excitation of Er3+ by hot injected carriers starts at the Fowler-Nordheim injection threshold (above 5 MV cm−1) and dominates for high-field APV excitation

A micro-mechanical study of cemented granular materials

Cemented Granular Materials (CGMs) are ubiquitous in nature. Their behaviour is driven by phenomena occurring at the scale of individual grains and cement bonds. However, the effect of this scale on the engineering response has lacked a systematic description and is rarely acknowledged in the models portraying their behaviour. This dissertation contributes to the development of a methodological framework for CGMs, relating the behaviour of individual grains to their collective response at the macroscopic scale, through a combination of analytical, experimental and numerical approaches. Analytically, a novel constitutive model for CGMs is developed, which adopts measurable internal variables describing the evolution of key grain-scale processes. This model can successfully predict stress-strain responses as well as the onset and development of localisation patterns for a wide range of pressure regimes. Its constitutive parameters have a precise physical meaning and are directly quantifiable. Experimentally, a triaxial testing program is carried out on specimens of CGMs while acquiring x-ray tomographic images at a resolution sufficient to discern individual grains and cement bonds. A toolset is developed to characterise, for the first time, each grain and cement bridge in the specimen, their evolution, and to extract statistically representative measures of the grain-scale processes. Numerically, boundary value problems are solved using the finite element method to bridge experimental observations to analytical predictions, and vice versa, while accounting for the effects of strain localisation. The acknowledgement of grain-scale behaviour within this methodological framework allows for a realistic validation of the constitutive models not only at the scale of the specimen response and its localisation features, but also at the scale of individual grains. This is pivotal in several open engineering problems controlled by the localised evolution of the micro-structure