An experimental program is oriented to the mechanical behaviour of the Interfacial Transition Zone (ITZ) exposed to leaching, for a single mix. Leaching kinetics of the degraded depth, measured through grey level on cross sections, reveals a maximum gap of 10 % between ITZ and bulk cement paste. ITZ, with a main thickness of 15 mu m suffers a significant decalcification, which is reflected on the mechanical properties of the bond. Grey level, Calcium/Silicon (Ca/Si) molar ratio and Young's modulus in the degraded zone present similar evolution. Effect of ITZ causing cracking and then debonding is noticed, through chemical analysis, image observation and mechanical tests.

The influence of the normal mode cohesive parameters of both cement paste and cement-steel interface on the overall mechanical behavior of concrete samples is investigated through multiple numerical simulations. Damage behavior is modeled by cohesive zones localized on the boundaries of the elements of a planar Delaunay mesh allowing for a progressive separation between adjacent elements.Using these numerical results, practical relations are obtained for estimating normal mode cohesive parameters of both a cement paste and a cement-steel interface at the cement-aggregate interface scale. The cohesive parameters are computed by using the breaking force value as well as an average crack speed which, in opposition to cohesive parameters, are more easily measurable during experimental tests. Complementary studies are presented for quantifying estimates accuracy as well as their sensitivity to measurement errors.

This work consists in highlighting the influence of granulates rubber on the shear strength of sand.It deals with the study of sand-rubber mixtures and in particular saturation and mechanical properties.In order to study the influence of granulate rubber content on the mechanical properties of sand, direct shear tests were performed.The shear results show that the strength of the sand in the dry or saturated state increases with increasing rubber content.A rubber granulate content of 20% is the optimum value for improving the mechanical behavior of sand in dry and saturated conditions.This work also compares the shear results obtained for a dry and saturated sample under various normal stresses (100, 200, and 400 kPa).Increasing the normal stress improves the shear strength of pure sand in both dry and saturated states.A reduction in the maximum shear strength is observed in the saturated condition compared to the dry condition for the same relative density (D r = 55%).

The development of tools, using a micromechanical approach, predicting the macroscopic behaviour of heterogeneous materials such as concrete, requires the knowledge of their microstructures (geometrical properties of phases), the behaviour of phases and the interaction laws between phases. This study is focused on a numerical modelling of a local shear test on a cement paste-aggregate composite using a cohesive zone model with the objective to identify the behaviour of the cement paste-aggregate interface. The computations use a 3D finite element modelling of the composite, using a cohesive law at the interface between the two phases. The cohesive model mimics the behaviour of the well-known interfacial transition zone. This work presents a methodology for the identification of cohesive law parameters at different stages of hydration and for different confining stresses using experimental results.

This paper presents results of experimental tests on soil specimens reinforced by stone columns. The experimental investigations have been conducted by setting up a laboratory scaled model. The main objective is to evaluate the effects of various reinforcing materials and stone columns configurations on the axial deformation during a vertical loading test of cylindrical samples of unreinforced and reinforced soil. The findings have been compared to existing experimental results proposed in literature. The specimens of soil consisted in an analogic material of poly-dispersed glass beads (GB) with a grain size smaller than 50 mu m. Two reinforcing materials have been considered as ballasts: crushed sand (CS) and coarser granularity GB (0.7-1.0 mm). The results show that the material type and spacing between columns in a triangular or square configuration can greatly affect the reinforcement efficiency. The proposed results are applicable to predict the improvement of granular soils of medium density when subjected to the vibrocompaction technique.

Concrete rupture is mainly influenced by the mechanisms occurring at the scale of the Interfacial Transition Zone (ITZ) between cement matrix and aggregates. However, experimental quantitative assessment of concrete cohesive behavior at that scale (centimeter scale) remains an open issue among the scientific community. In this paper, we present a three-point bending test allowing for a measure of the mechanical response of centimeter concrete samples. CEM I Portland cement paste sample, as well as composites cement-silica and cement-steel samples are tested. Weibull parameters for estimating the probability of crack initiation in concrete at the centimeter scale are computed based on the experimental results. A device for recording crack position on the surface of a sample is introduced. Its use in future studies might bring information on the crack dynamic propagation in concrete at the centimeter scale. (c) 2022 The Authors. Published by Elsevier B.V.

Tensegrity systems are a class of reticulated space structures composed of compressed bars maintained in equilibrium by a network of tensioned cables. Their stiffness depends both on elements’ mechanical properties and their internal self-stress state. Taking advantage of their structural properties, we respond to the challenge of accessibility for everybody to the sea with a new concept of modular lightweight and deployable platforms. Variable configurations are developed to fit ecologically into the marine environment thanks to the transparency of double layer tensegrity structures. Moreover, allowing practical assembly and disassembly is considered in the design to respect the coastal law. Through a numerical study, we demonstrate in this paper the capability of this solution under various representative load cases and support conditions. After the structural and design optimization of elements constrained by weight and stiffness, we detail the design of the nodes, which are the key components ensuring geometry and foldability of the structure. Finally, on-site setting and interfacing with ground supports is experimented in marine conditions to proof the feasibility of this concept.

This article presents an experimental study of the chemical and mechanical properties of the cement paste / aggregate interphase. Interfacial Transition Zone (ITZ) represent the contact zone of the cement paste with the aggregate with a thickness of about 20 - 50 μm. This zone has different mechanical and chemical properties compared to the bulk paste. Cement paste / limestone aggregate composite samples were performed to allow the study of the chemical and mechanical properties of ITZ. The cement paste is made with Portland cement with a water/cement mass ratio of 0.4. Calcium concentration profiles in the ITZ show a high concentration of calcium due to the presence of portlandite and ettringite. The nano-indentation tests indicate weakness of ITZ compared to bulk cement paste.

Delayed Ettringite Formation (DEF), is one of the different Internal Sulfate Attack (ISA), is a slow chemical reaction that can be responsible for the degradation of cementitious materials, through the swelling of the material followed by crack diffusion in the structure. The objective of this study is the experimental characterization at the local scale (10 x 10 x 30 mm(3) samples) of the concrete reached by DEF. The composite samples, consist of a 15 mm thick cement paste placed in contact with a siliceous aggregate. The shape of these samples makes it possible to study the mechanical behavior of the interface between the cement paste and the aggregate. This zone is identified as a privileged zone of the development of DEF. Given the slowness of this pathology, the experimental setting in the laboratory is accelerated by choosing conditions favoring the appearance of DEF. The tests are exploited at the local scale by measuring expansions by image correlation of the degraded samples and by scanning electron microscope observations. The results showed a higher expansion at the cement paste-aggregate interface compared to the cement paste. A tensile test performed at local scale allowed to characterize the impact of DEF on mechanical properties of the cement paste-aggregate interface. A drop in strength was observed with heterogeneous formation and localization of ettringite inside the interface. (C) 2021 Elsevier Ltd. All rights reserved.

The behavior of a natural soil is known to change substantially in presence of water under unsaturated conditions, due to additional capillary forces. Water can be absorbed by hygroscopic soil particles (such as clay), or remains at the surface of solid grains (sand, silt) and forms either a discontinuous (pendular regime) or a continuous phase (funicular regime), depending on the water content of the soil. Capillary bridges exist solely between pairs of grains at small water contents, giving rise to simple capillary force expressions and straightforward subsequent modeling. For larger water contents, these generic capillary bridges progressively merge into more complex coalesced bridges involving several grains (i.e. at least three) and whose description remains little known. In the present study, a numerical approach based on surface energy minimization is proposed to compute capillary forces for assemblies of two or three grains. The methodology is first validated for a standard capillary bridge between two grains by comparison both with previous experiments and with other alternative theoretical and numerical approaches. The method is next extended to a triplet of grains within a wide range of water content (or equivalently reduced water volume) during imbibition, to switch from uncoalesced to coalesced bridges. Eventually, the influence of contact angle, surface tension and gravity on the capillary force, the volume of coalescence and the morphology of the bridge as well is investigated. The present study paves the way for the implementation of capillary effects in micromechanical models relying on mesostructures composed of a few grains.

The main objective of this study is to compare the mechanical behavior of two sands (Hostun or Dune sands) mixed with crushed rubber obtained from used tires. However, it is essential to ensure that his geotechnical application do not result in long-term negative impacts on the environment. The chemical properties of these two sands are given by energy dispersive analysis X-ray fluorescence spectrometry. The mineral composition of these two sands is performed by X-ray diffractometry. The morphological characteristics of the sand grains are given by the analysis of the images of the two sands given by the scanning electron microscope. This study is based on 120 direct shear tests performed on sand-rubber aggregate mixtures. The results show that the rubber content of the aggregates has a significant effect on the shear strength of sand-rubber mixtures in both cases of sand. In fact, the shear strength of the sand-rubber mixture increases with increasing crushed rubber up to 20% for different normal stresses. The analysis of the test results also shows the effect of the angular shape of the sand grains on the interparticle friction. The contribution of the structure effect in the mobilized friction is analyzed by comparing the shear test results of Hostun and dune sand mixtures.

Une approche par elements finis cohesifs-volumetriques, tenant compte des proprietes mecaniques de la zone de transition interfaciale (ITZ), est utilisee pour etudier le comportement mecanique du beton lors d'un essai de compression uniaxiale. Dans ce travail, un modele geometrique bidimensionnel a l'echelle mesoscopique du beton numerique a ete considere. Les echantillons obtenus sont mailles a l'aide du logiciel GMSH avec une methode de Delaunay. Les resultats des simulations a l’aide du Modele de Zones Cohesives Frottantes (MZCF) permettent, a travers une etude de criblage de type Hadamard, d’apporter des elements de reponses concernant les facteurs les plus influents et leur contribution a la resistance maximale du beton en compression uniaxiale.

Cement paste/aggregate bond influences durability of concrete subjected to leaching, following the existence of a particular, more vulnerable zone in cement paste, adjacent to aggregates - the interfacial transition zone (ITZ). In order to assess the mechanical behaviour of concrete at local scale of cement paste/aggregate bond, tensile tests were carried out on cement paste and cement paste/aggregate composite samples. The relationship between chemical degradation and mechanical properties (Young's modulus and tensile strength) was expressed through the notion of chemical degradation rate. Other aspects discussed, such as the leaching kinetics, chemical dissolution and cracking were used to highlight the origin of the loss of mechanical properties of the cement paste and of the cement paste/aggregate bond. Following an important dissolution within ITZ, a total loss of adhesion between cement paste and aggregate following leaching occurs gradually. Concerning the cement paste, it undergoes a smaller decrease of Young's modulus and tensile strength compared to the composites, being also less affected by cracking. Overall, this study highlights the mechanism by which the mechanical behaviours of cement paste and cement paste/aggregate bond are affected by leaching.

Ce travail s’interesse a une caracterisation a l’echelle locale de l’interphase ciment-granulat sain et degrade thermiquement, puis, a terme, atteint de la pathologie de Reaction Sulfatique Interne (RSI). Cette caracterisation est realisee a partir de plusieurs essais mecaniques associes a des mesures d’expansion et a des observations au microscope electronique a balayage (MEB). Les echantillons de forme parallelepipedique, dits composites, sont constitues d’une pâte de ciment liee a un granulat siliceux. La forme de ces echantillons permet de rendre plus accessible l’etude du comportement mecanique de l’interphase. Deux sens de coulage ont ete effectues (horizontal et vertical). Dans une enceinte climatique permettant de reguler l’humidite relative proche de 100%, un cycle thermique a ete applique aux echantillons au jeune âge avant qu’ils soient conserves dans de l’eau demineralisee a 38°C. Les resultats montrent une expansion du composite plus importante que celle de la pâte de ciment apres 28 jours de conservation. Suite au traitement thermique applique, la resistance a la traction du composite est plus affectee que celle du ciment ou le sens de coulage lors de la preparation presente une incidence sur les resultats. Par ailleurs, des observations au MEB montrent que le sens de coulage et le traitement thermique modifient la microstructure de l’interphase ciment/granulat.

Les structures de tensegrite sont des structures reticulees composees de barres en compression en equilibre dans un reseau de câbles en traction. Leur rigidite et stabilite dependent a la fois des proprietes mecaniques des elements et des efforts internes introduits durant la mise en service. Outre son aspect artistique, la tensegrite peut egalement etre a la base d’applications concretes notamment a travers une solution de plateforme pour l’accessibilite a la mer, caracterisee par sa configuration geometrique variable, modulaire, legere et de faible impact environnemental. Ce document presente la suite des etudes menees sur cette solution de plateforme, notamment aux travers des aspects d’optimisation structurelle des modules isoles et des assemblages de plusieurs modules. Enfin, on mettra en lumiere les tests de mise en service realises sur site littoral.

The study of the durability of concrete subjected teaching requires knowledge of the mechanisms of chemical degradation of cement paste and of cement paste/aggregate bond. The Interfacial Transition Zone (ITZ), the portion of cement paste in contact with aggregates, is a vulnerable zone in the context of chemical degradation due to its high degree of solubility and diffusivity. In order to evaluate the leaching kinetics as well as the chemical dissolution of bulk cement paste and of the ITZ, parallelepiped cement paste samples and cement paste/aggregate composites were produced and submitted to accelerated leaching. The measurement of the degraded depth was carried out by image analysis of the cross sections of the cement paste and of the interface. The degradation kinetics of the bulk cement paste is proportional to the square root of time. On the other hand, that of the ITZ initially follows the same trend, but slightly accelerates after a critical threshold. Chemical analyses were performed by Energy Dispersive Spectroscopy (EDS) to measure the Ca/Si molar ratios of ITZ and cement paste for a single degradation time. The greater drop of the Ca/Si ratio at the ITZ reveals a significantly greater dissolution of the portlandite than in the bulk cement paste.