Composites cimentaires à module d'élasticité contrôlé : conception, caractérisation et modélisation micromécanique

Cette thèse, réalisée dans le cadre d'une convention CIFRE (société Ménard), s'inscrit dans la problématique du renforcement de sols par inclusion de renforts (Colonnes à Module Contrôlé ou CMC) selon un procédé qui consiste à réaliser ces colonnes de renforcement par injection d'un matériau cimentaire via une tarière creuse. Le matériau cimentaire utilisé doit respecter un cahier des charges spécifique concernant des propriétés à l'état frais (affaissement) et à l'état durci (résistance en compression et module d'élasticité). Le premier objectif de cette thèse était de formuler des matériaux cimentaires destinés au procédé CMC, présentant des propriétés respectant le cahier des charges tout en étant optimisées par rapport à l'application visée. Une attention particulière a été portée sur la rigidité du matériau, ainsi que sur l'amélioration de son comportement mécanique lorsqu'il était renforcé par des fibres. Des matériaux à base de granulats caoutchouc ou d'argile expansée ont ainsi été développés et caractérisés. Le deuxième objectif de la thèse était d'étudier la pertinence des codes règlementaires étant donné que les formules fournies actuellement par ces codes n'étaient pas utilisables dans le domaine de résistance mécanique des matériaux cimentaires développés dans cette thèse. Si certaines lois peuvent être extrapolées sans problème, d'autres ne s'avèrent pas suffisamment précises ou sécuritaires pour que leur extrapolation soit possible. Le troisième objectif de la thèse était de prédire les propriétés élastiques des matériaux développés grâce à un modèle micromécanique adapté aux besoins spécifiques de ce type d'application (CMC). Le modèle développé permet de remplacer avantageusement les formules règlementaires empiriques défaillantes pour la prédiction des propriétés élastiques (notamment le module d'élasticité).This PhD was realized thanks to a CIFRE partnership between the company Menard and the university of Toulouse. The purpose of this PhD was to develop and study new cement-based materials destined for the CMC (controlled modulus columns) technique. This technique belongs to a wider family of ground improvement processes called rigid (or semi-rigid) inclusions, which are soil-stiffening techniques. Those cement-based materials shall respect the company specifications regarding fresh and hardened state properties. The first goal was to design mixtures compositions which properties at fresh and hardened state respect the company specifications and are optimized for the application which they are destined for. A specific attention was given to the rigidity of the material, and its brittleness when it is reinforced by fibers. Mortars incorporating expanded clay aggregates, rubber aggregates and metallic fibers were developed. The second goal was to study the efficiency of regulatory building codes formulas with the developed mortars, since most of the developed mortars do not meet with the application scopes of the building codes. A few formulas turned out to be as efficient with the developed mortars as with regular structural concrete, while others turned out to be imprecise and unfavourable to security. The third goal was to predict the elastic properties of the linear elastic properties of the developed mortars thanks to a micromechanical model adapted to the specific needs of those specific materials. This model shall replace the empirical formulas advantageously for predicting the modulus of elasticity of the developed cement-based composites

CRMP5 Regulates Generation and Survival of Newborn Neurons in Olfactory and Hippocampal Neurogenic Areas of the Adult Mouse Brain

The Collapsin Response Mediator Proteins (CRMPs) are highly expressed in the developing brain, and in adult brain areas that retain neurogenesis, ie: the olfactory bulb (OB) and the dentate gyrus (DG). During brain development, CRMPs are essentially involved in signaling of axon guidance and neurite outgrowth, but their functions in the adult brain remain largely unknown. CRMP5 has been initially identified as the target of auto-antibodies involved in paraneoplasic neurological diseases and further implicated in a neurite outgrowth inhibition mediated by tubulin binding. Interestingly, CRMP5 is also highly expressed in adult brain neurogenic areas where its functions have not yet been elucidated. Here we observed in both neurogenic areas of the adult mouse brain that CRMP5 was present in proliferating and post-mitotic neuroblasts, while they migrate and differentiate into mature neurons. In CRMP5−/− mice, the lack of CRMP5 resulted in a significant increase of proliferation and neurogenesis, but also in an excess of apoptotic death of granule cells in the OB and DG. These findings provide the first evidence that CRMP5 is involved in the generation and survival of newly generated neurons in areas of the adult brain with a high level of activity-dependent neuronal plasticity

Experimental Investigation on the Compressive Stress-Sensing Ability of Steel Fiber-Reinforced Cement-Based Composites under Varying Temperature Conditions

This study investigates the piezoresistive (self-sensing) properties of short stainless-steel fiber-reinforced mortar under varying temperature conditions. Different reinforced mortars were produced by varying fiber and aggregate content. First, Electrical Impedance Spectroscopy (EIS) measurements were used to characterize the electrical properties of the mortar specimens. EIS measurements were performed at temperatures of 24 °C, 35 °C, and 50 °C. Second, to investigate the self-sensing capacity of the different composites, the fractional changes of electrical impedance at 1 kHz were monitored under two conditions: temperature variation alone (cooling down from 35 °C or 50 °C to room temperature), and temperature variation combined with cyclic compressive loading (up to 5 MPa). The results of the former were used to compensate for the effect of temperature variations in the latter. Both temperature and mechanical loading produced meaningful variations in the electrical impedance and piezoresistivity of the investigated composites. Conclusions are drawn with respect to the stress and temperature sensitivity of the composites. The real and imaginary parts of the electrical impedance of the mortar produced with the highest fiber volume fraction (0.01%) and higher aggregate content (volume fraction of 60%) were distinctly sensitive to temperature and stress, which suggests the possibility of using the same composite as a stress and temperature sensor

Assessment of manufacturing process efficiency in the dispersion of carbon fibers in smart concrete by measuring ac impedance

International audienceSmart concrete is a construction material designed to perform structural and self-sensing roles simultaneously. self-sensing behavior stems from the inclusion of conductive fibers within the cementitious matrix, reducing its electrical impedance and enabling piezoresistive behavior. good dispersion of fibers is essential to ensure good mechanical and sensing properties. this study assesses the efficiency of different mixing sequences and energies in dispersing fibers within the mortar. the efficiency is assessed with 0.1% and 0.5% carbon fiber volume, by measuring the ac impedance: those volume fractions correspond to two fiber percolation states: around the percolation threshold (0.1%), and in the saturation zone (0.5%). the study showed that the mixing sequence has a significant effect on the final impedance of the material, and that increasing mixing energy is relatively ineffective in enhancing the fiber dispersion. differences between different mixing sequences were evident with 0.1% carbon fibers, while, to a certain extent, saturating the system with conductive fibers reduced the gaps between different mixing sequences

About the self-sensing behavior of smart concrete and its interaction with the carbon fiber percolation status, sand connectivity status and grain size distribution

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Carbon-fibred mortar: Effect of sand content and grain size distribution on electrical impedance

International audienceSelf-obtained by including electrically conductive fibers in cement-based materials. These fibers may allow to reduce electrical resistivity and develop a piezoresitive behaviour. Smart Concrete could therefore be simultaneously both a structural and a sensing material, which eliminates the need for external instrumentation in Structural Health Monitoring. By increasing the fiber volume fraction within the cement matrix, the electrical resistivity (or impedance) of the material is reduced once percolation threshold is reached. Above this percolation threshold, the fiber content is high enough to allow conductive particles to be in contact or very close to each other, thus creating a continuous conductive network within the insulative matrix. in case of fibred mortar: a high sand content may prevent the network of conductive fibers from percolating. This phenomenon is referred in order to allowi fibers to maintain their efficiency in reducing the electrical resistivity of composites. However, little attention has been given to the impact of the size of sand grains on the electrical percolation. This work intends to study the effect of the grain size distribution and volume fraction of sand within mortars containing various fiber volume fractions. The results confirm. t phenomena: when the volume fraction of sand is close to its maximum packing density, the addition of fibres was not as effective in reducing the electrical impedance of mortar samples. In addition, grain size distribution proved an influence on impedance of mortar: fine sand showed higher impedance compared to standard sand, especially in case of high sand volume fraction. This could be related to the smaller maximum packing density in case of fine sand, where distance between particles would be in average reduced. This effect, combined with the higher number of insulative particles, could probably disrupt the continuity of the conductive network of fibres within mortar

About electrical resistivity variation during drying and improvement of the sensing behavior of carbon fiber-reinforced smart concrete

International audienceThe addition of conductive fibers to a cementitious material reduces its electrical impedance, for monitoring purposes, based on the relationship between external stresses (mechanical or thermal) and changes in electrical properties. In this context, variations in the electrical properties due to drying prevent the application of this technique, unless such variations are negligible or precisely predictable. The aim of this paper, therefore, is to study the variation of the complex electrical impedance during drying, for measurement frequencies between 4 Hz and 1 MHz. Shrinkage, weight loss and electrical impedance spectrum were measured on three mortars with carbon fiber volume fractions (FVF) of 0, 0.1 and 0.5%. The results show that in the absence of conductive fibers, the real and imaginary parts of the impedance increase due to water loss. On the other hand, in the presence of a percolated carbon fiber network, the real impedance of the material decreases at low frequencies and increases at high frequencies during drying; for intermediate frequency ranges, quasi-constant values can be observed. In addition, as the material dries, the capacitive behavior of the material wanes, and the imaginary impedance values tend towards 0. The electrical behavior then approaches ideal resistive behavior with a real impedance value independent of the measurement frequency. Blind frequencies have been identified for fibrous mortars around 1 kHz and 40 kHz for 0.1% and 0.5% FVF, respectively. For monitoring purposes, coupling the presence of fibers with an appropriate measurement frequency would minimize the effects related to the variation of impedance as a function of time – i.e., allowing more precise monitoring of mechanical loads with time