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Due to its ecological interest and large availability, a renewed attention is paid to earth as building material. Indeed, raw earth consumes CO2 only during its processing and transportation, and it provides a natural hygrothermal comfort. However, its mechanical properties are highly linked to its composition, which causes an important variability of performances. That is why any soil has to be characterized before being used as a building material. The aim of this study is to propose a model able to predict the hydromechanical behavior of a reconstituted soil according to its composition. As earth is a heterogeneous material, the model is based on homogenization procedures. The sand is considered as spherical inclusions inside a clay matrix. The particularity of the model stands to consider both positive and negative effects of volume variation and mechanical properties of clay under hydric variations. The model parameters are determined according to an original experimental campaign, which is conducted on various mixes of a single type of clay (kaolinite) and of sand, and water. The experimental study provides some mechanical properties of the mixes versus water content and sand content to test the ability of the homogenization model to assess the main properties of this material.

International audience; The reduction of CO2 emissions has become an important parameter in the choice of construction materials. Earth guarantees a low environment impact due to the reduced need of energy for its processing and transportation and also provides natural hygrothermal comfort. To facilitate the design of earthen constructions, the development of models is necessary. This paper aims to provide a database of experimental results that could be easily used for developing models based on the competition between positive effect of capillary pressure on material cohesion and negative effect induced by matrix shrinkage restrained by aggregates. Mechanical and thermal tests were carried out on three reconstituted soils composed of pure kaolinite and three contents of fine sand (0, 30 and 60%), as well as different water content configurations (fabrication water content and after drying at 50% RH). The thermal properties analyzed included thermal conductivity and specific heat capacity. The mechanical properties studied encompassed the tensile (three-point bending test) and compressive strengths, Young's modulus and Poisson's ratio (using video correlation). Moreover, an original test was developed for measuring sample shrinkage during drying. Specifically, the results clearly showed that while shrinkage is divided by 5 for the maximal sand content, tensile strength is reduced by half. Therefore, these results allow to assess the balance between benefices and disadvantages of capillary pressure induced by clay drying on earth construction materials, which is of first interest to assess cracking risk in a structure.

We use an innovative combination of measurements to study reinforcement in a series of SBR elastomers filled with various amounts of submicrometric precipitated silica. While mechanical measurements give access to the overall response of the nanocomposite material, measurements of the chain segment average orientation induced upon uniaxial stretching give selective access to the response of the elastomer matrix only. Average segment orientation is measured by X-ray scattering. Reinforcement effects are analyzed in terms of the enhancement ratio of the mechanical modulus or induced segmental orientation in a reinforced sample over the corresponding quantity measured in the pure matrix. Cross-link densities are measured independently by NMR to account for possible impact of fillers on the cross-link density. It is demonstrated that in filled materials the orientational enhancement ratio does not decrease significantly as temperature increases, while the mechanical reinforcement ratio decreases as temperatur...

International audience; 3D concrete printing is a promising technology in the construction field. Un-availability of standards/specifications related to certification of 3D printed structures and lack of reinforcement in printed concrete are the major factors behind the under-utilization of such innovative technology. Addition of fibers as reinforcement in the printable mix could be considered as viable solution to improve mechanical properties of printable mix particularly tensile strength. In this regard, recently carried out studies have used mostly non-metallic fibers with low modulus of elasticity and tensile strength. In this study, effect of adding flexible amorphous metallic fibers on the print quality and mechanical properties (flexural and direct tensile strengths) of resulting mixes has been investigated. Fibrous and non-fibrous mixes were designed. To prepare fibrous mixes, metallic fibers were used at dosage of 20 kg/m3, 30 kg/m3 and 40 kg/m3. Further, to study the influence of fiber length on the print quality and strength, fibers of length 5 mm, 10 mm and 15 mm were used. Reference specimens for tensile and flexural strength tests were also cast in molds using all mixes used for printing. Results showed that addition of metallic fibers of length 10 mm and 15 mm exhibited detrimental effects on the print quality, however, strength values, particularly tensile strength of printed concrete were observed to be enhanced by the addition of these fibers. Although print quality of mix containing fibers of 5 mm length was least affected, no positive impact of these fibers on the mechanical strength of printed concrete was noticed. © 2020, RILEM.

The microstructure of polymer nanocomposites made with disordered silica filler (Zeosil(R) 1165MP) of industrial relevance and various coating agents is quantitatively analyzed using a combination of SAXS, TEM, and a recently developed structural model. The polymer matrix is formed by an end-functionalized styrene–butadiene statistical copolymer capable of covalent grafting on the silica nanoparticles. The effect of the coating agents with different alkyl chain length (C8, C12, and C18) on filler structure quantified in terms of aggregate formation, for different concentrations (up to 8 wt % with respect to silica), and the effect of a commonly added catalyzer, DPG, are studied using the structural model. As a result, we show that a strongly synergetic effect of both DPG and coating agent exists. Our findings open the road to a fundamental understanding and rational design of model and industrial nanocomposite formulation with optimized coating agents.

WOS:000310136500011; International audience; The orientation of shear bands is investigated analytically and numerically for a rigid plastic frictional material in simple shear. The model is based on co-axial flow rule, incompressible deformations and a friction factor which depends on the strain history. Since we are focussing on geological timescales, the influence of elasticity is neglected. Firstly, a linear stability analysis is performed confirming Rice's 1976 assertion [The localization of plastic deformation, in Proceedings of the 14th International Congress on Theoretical and Applied Mechanics, W. T. Koiter, ed., North Holland, Amsterdam, 1976, p. 207] that, in the hardening regime, bifurcation is possible at every stage. Orientation of shear bands against the less compressive principal axis lies anywhere between the Roscoe and Coulomb angles, namely between pi/4+psi/2 and pi/4+phi/2, where phi and psi are the mobilised angles of friction and dilatancy, respectively (in our study, we assume psi=0). The linear stability analysis leaves open the question of which orientation will actually emerge in a boundary value problem that consider all nonlinearities. This question is addressed in a finite element study of simple shear with periodic boundary conditions in the shear direction. Our simulations show temporary inclined shear bands in the hardening regime followed by a persistent horizontal shear band. A sensitivity study with respect to geometric and constitutive parameters indicates that the height of the sample controls the orientation of the inclined shear bands. Finally, we extend our analytical and numerical studies to Cosserat plasticity. It turns out that inclined shear bands are suppressed for large values of the internal length R (narrow bands). The case of a standard continuum is gradually recovered for small R (wide bands).

This paper discusses the reliability and the efficiency of a time homogenization method employed to reduce the computational time during cyclic loading for two common geotechnical tests and two elastoplastic models for clays. The method of homogenization is based upon splitting time into two separate scales. The first time scale relates to the period of cyclic loading and the second to the characteristic time of the fatigue phenomenon. The time homogenization method is applied to simulate an undrained triaxial test (homogeneous stress state) and a pressuremeter test (nonhomogeneous stress state) under one-way cyclic loading on normally consolidated clay. This method is coupled with two elastoplastic models dedicated to cyclic behavior of clay (a bounding surface plasticity model and a bubble model). Both linear and nonlinear elasticities are considered. The difficulty encountered when applying this method to models introducing nonlinear elasticity and kinematic hardening is pointed out. The performance of time homogenization related to the main parameters is numerically investigated by comparison with conventional finite element simulations.

This paper discusses the quality of the procedure employed in identifying soil parameters by inverse analysis. This procedure includes a FEM-simulation for which two constitutive modelsa linear elastic perfectly plastic MohrCoulomb model and a strain-hardening elasto-plastic modelare successively considered. Two kinds of optimization algorithms have been used: a deterministic simplex method and a stochastic genetic method. The soil data come from the results of two pressuremeter tests, complemented by triaxial and resonant column testing. First, the inverse analysis has been performed separately on each pressuremeter test. The genetic method presents the advantage of providing a collection of satisfactory solutions, among which a geotechnical engineer has to choose the optimal one based on his scientific background and/or additional analyses based on further experimental test results. This advantage is enhanced when all the constitutive parameters sensitive to the considered problem have to be identified without restrictions in the search space. Second, the experimental values of the two pressuremeter tests have been processed simultaneously, so that the inverse analysis becomes a multi-objective optimization problem. The genetic method allows the user to choose the most suitable parameter set according to the Pareto frontier and to guarantee the coherence between the tests. The sets of optimized parameters obtained from inverse analyses are then used to calculate the response of a spread footing, which is part of a predictive benchmark. The numerical results with respect to both the constitutive models and the inverse analysis procedure are discussed.

The dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of g...

Submitted to large sinusoidal strains, filled elastomers not only show a decrease in their storage modulus — the Payne effect, but also a nonlinear behavior — their response is not sinusoidal anymore and involves strain-stiffening. We show in this study that the two effects can be separated thanks to large amplitude oscillatory shear experiments. The stress signal of filled elastomers consisting of a dispersion of silica particles into a polymeric matrix was decomposed into an elastic and a viscous part and we could observe simultaneously the Payne effect and a strain-stiffening phenomenon. We showed that the strain-stiffening was correlated with the Payne effect but came from various intricated effects. It most probably also has its origins in the finite extensibility of the polymer chains confined between solid particles, where the strain is larger. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

We have been able to design model filled rubbers with exactly the same chemical structure but different filler arrangements. From these model systems, we show that the particle arrangement in the elastomeric matrix controls the strain softening at small strain amplitude known as the Payne effect, as well as the elastic modulus dependence on the temperature. More precisely, we observed that the Payne effect disappears and the elastic modulus only weakly depends on the temperature when the particles are well separated. On the contrary, samples with the same interfacial physical chemistry but with aggregated particles show large amplitudes of the Payne effect and their elastic modulus decreases significantly with the temperature. We discuss these effects in terms of glassy bridge formation between filler particles. The observed effects provide evidence that glassy bridges play a key role on the mechanical properties of filled rubbers.

International audience; Adding fillers in elastomers is known to increase the elastic modulus and the wear resistance of elastomers, but also to increase nonlinear dissipation, a phenomenon known as the Payne effect. Indeed, when submitted to deformations of the order of a few per cents or more, the elastic modulus can decrease down to values much smaller than the initial one. On the other hand, when submitted to large amplitude oscillatory shear at a frequency omega, frequency analysis shows that the contribution of higher harmonics 3 omega, 5 omega, .., to the response is quite small. This might appear somehow as a paradox since the nonlinear behavior of filled elastomers can be strongly marked. We discuss here in detail a possible physical origin of these various features. We do it by comparing experimental results performed on model elastomers to the prediction of a model proposed recently, based on the presence of glassy bridges linking neighboring particles. We show that the kinetics of rupture and rebirth of these glassy bridges can explain these effects.

WOS:000310136500008; International audience; At elevated stress levels, the deformation of granular assemblies has the tendency to localize in narrow shear bands. Recent research efforts within the granular material community are focusing on the morphology of deformation patterns within the bands. Understanding of the deformation mechanisms is a prerequisite for the quantification of energy dissipation, hardening and softening in the post-localization range. Despite significant experimental research efforts, a clear understanding of the flow properties and constitutive behaviours within the shear band is still elusive. This could mainly be attributed to the fact that shear bands are thin objects, making a detailed characterization of the particles' behaviour and their interactions within shear bands difficult. As a numerical method, the Discrete Element Method (DEM) has demonstrated its ability to address this problem, owing to its intrinsic characteristic of tracing particle behaviour. This paper presents the results of application of DEM to model two-dimensional, densely packed, cohesionless, polydisperse granular assemblies under simple shear. Meso-scale kinematical information such as displacement field is evaluated. Within the shear band, the deformation field displays a vortex-like structure, and such vortex structures are accompanied by strong pore space production within the shear band. Analysis of the history of force chains inside the localized shear zone reveals that the formation of vortex structures is related to the kinematical evolution of a cycle encompassing buildup, buckling and collapse of the force chains.

The reinforcement of elastomers by inorganic fillers, a concept of very high technological importance, is commonly understood to result from the presence of a mechanical network of partially aggregated filler particles. The non-linear mechanical properties, in particular the decrease of the modulus at high strain (Payne effect), are further interpreted to be a consequence of the breakdown of this filler network. There are, however, many open questions concerning the actual nature of the interparticle connections, where a modified polymer layer forming "glassy bridges" constitutes one possibility. In this work, we address this issue with a suitable silica-filled model elastomer, where we characterize the silica dispersion by SANS in combination with reverse Monte-Carlo modeling, and the mobility modification of the polymer by low-field proton NMR spectroscopy. In our samples, we identify a glassy layer as well as a region of intermediate mobility (possibly modified Rouse modes). Based on the structural information from SANS, we are able to quantify the amount of interparticle connections, and correlate it with the magnitude of the Payne effect taken from shear rheology. This works only if we assume that these connections comprise both, the glassy layer as well as the region of intermediate mobility. The amount of glassy immobilized polymer only does not suffice to explain the mechanical properties.

International audience; Numerical modeling of the behavior of geotechnical structures subjected to cyclic loading requires the consideration of two main aspects: the constitutive relationship and numerical strategy. The constitutive model has to be representative of the clay behavior, whereas the numerical strategy should be efficient in order to reduce the computation time. To simulate undrained triaxial cyclic tests on clay, a method of time homogenization is applied to a bounding surface plasticity model. This method of homogenization is based upon splitting time into two separate scales. The first scale relates to the period of cyclic loading and the second to the characteristic time of the material. Simulations of undrained triaxial cyclic tests on normally consolidated clay under one-way cyclic loading are carried out. The performance of time homogenization is numerically validated. The sensitivity of the time increment is also investigated

International audience; The identification of constitutive parameters by inverse analysis of limited experimental information is investigated. In order to ensure the quality of measurements, all the data come from a thick wall cylinder test performed on a saturated sand sample. A numerical FEM-simulation considering a linear elastic perfectly plastic model (Mohr-Coulomb) is carried out in order to determine the pressure – cavity deformation relationship. Two kinds of optimization algorithms, a deterministic and a stochastic one, are presented and implemented in the inverse procedure. The performance of theses algorithms taking for identifying three parameters (Young’s Modulus, friction angle and dilatancy angle) is compared. The two kinds of algorithms provide a quite similar response for the mathematical optimum, but some parameters sets did not have an acceptable physical meaning. Contrary to the deterministic algorithm, the genetic method provides an assembly of “satisfactory” individuals. Some of the parameters sets have acceptable physical meaning and the expert has to choose the best one considering his scientific and technical background.

A series of Macromolecular Design by Interchange of Xanthates (MADIX)-derived amphiphilic block copolymers exhibiting lower critical solution temperature behaviour was synthesized and evaluated as silicone oil-in-water emulsion co-stabilizers. It was demonstrated that the cloud point of the block copolymer is mainly controlled by the chemical composition of the N-isopropylacrylamide-based hydrophilic block. The reversible stabilization and destabilization phenomena of the silicone emulsions occurred at a temperature equal to the cloud point of the block copolymer taken alone.

The thesis work is part of the numerical modeling of the flow of cementitious materials in the fresh state coupled with an identification procedure of the parameters. It deals in particular with the step of the development of the identification by inverse analysis. First,the literature review reveals the existence of rheometric tools dedicated to cementitious suspensions; The passage from the macroscopic quantities to the local ones is made either by the use of conventional geometries or by means of calibration methods. Nevertheless, these tools do not make it possible to find the expected single rheological signature for a given suspension. In addition, there are few studies reporting strategies for identifying constitutive parameters in the case of fresh cement-based materials and they are limited to local data. Then, a strategy consisting in identifying the parameters of a supposed law, directly on the basis of the simulated macroscopic measurements (torques, rotational speeds imposed on the shearing tool) was developed and validated in 2D, discussing in particular the efficiency Of the optimization algorithms tested (simplex method and genetic algorithms), according to the degree of knowledge that the user has of the material. Finally, the method has been applied in 3D on model fluids, assuming that they are homogeneous. The method appears effective in the case of pseudo-plastic fluid, in particular by combining both optimization algorithms used. But there remain obstacles to overcome in the case of visco-plastic fluids, probably related to the experimental tools rather than to the procedure of identification itself.; Le travail de thèse s'inscrit dans la modélisation numérique de l'écoulement des matériaux cimentaires à l'état frais couplée à un outil d'identification des paramètres. Il traite en particulier l'étape de mise en place de l'identification par analyse inverse. D'abord, une analyse de la littérature fait ressortir l'existence d'outils rhéométriques dédiés aux suspensions cimentaires ; le passage des grandeurs macroscopiques à celles locales est faite, soit par le biais de l'utilisation de géométries conventionnelles, soit au moyen de méthodes de calibration. Néanmoins, ces outils ne permettent pas de trouver une signature rhéologique unique pour une même suspension. De plus, les stratégies d'identification des paramètres relatifs aux matériaux cimentaires frais sont peu nombreuses et limitées aux données locales. Ensuite, une stratégie qui consiste à identifier les paramètres d'une loi supposée, directement à partir des mesures macroscopiques simulées (couples, vitesses de rotation imposées au mobile de cisaillement) a été développée et validée en 2D, en discutant notamment de l'efficacité des algorithmes d'optimisation testés (méthode du simplexe et algorithmes génétiques), en fonction du degré de connaissances que l'utilisateur a du matériau. Enfin, la méthode a été appliquée en 3D sur des fluides modèles supposés homogènes. Elle apparaît efficace en fluide pseudo-plastique, notamment par combinaison des algorithmes d'optimisation. Mais il reste des obstacles à lever en fluide visco-plastique, vraisemblablement liés aux outils expérimentaux plutôt qu'à la procédure d'identification elle-même.