Eric Lemarchand

A multiscale micromechanics-hydration model for the early-age elastic properties of cement-based materials

Abstract The E-modulus of early age cement-based materials, and more importantly, its evolution in time, is one of the most critical material-to-structural design parameters affecting the likelihood of early-age concrete cracking. This paper addresses the problem by means of a multistep micromechanics approach that starts at the nanolevel of the C–S–H matrix, where two types of C–S–H develop in the course of hydration. For the purpose of homogenization, the volume fractions of the different phases are required, which are determined by means of an advanced kinetics model of the four main hydration reactions of ordinary portland cement (OPC). The proposed model predicts with high accuracy the aging elasticity of cement-based materials, with a minimum intrinsic material properties (same for all cement-based materials), and 11 mix-design specific model parameters that can be easily obtained from the cement and concrete suppliers. By way of application, it is shown that the model provides a quantitative means to determine (1) the solid percolation threshold from micromechanics theory, (2) the effect of inclusions on the elastic stiffening curve, and (3) the development of the Poissons ratio at early ages. The model also suggests the existence of a critical water-to-cement ratio below which the solid phase percolates at the onset of hydration. The development of Poissons ratio at early ages is found to be characterized by a water-dominated material response as long as the water phase is continuous, and then by a solid-dominated material response beyond the solid percolation threshold. These model-based results are consistent with experimental values for cement paste, mortar, and concrete found in the open literature.

Elements of chemomechanics of calcium leaching of cement-based materials at different scales

Abstract Calcium leaching of cementitious materials has been identified as a severe long-term chemical degradation scenario of concrete structures. While the coupled diffusion–dissolution processes are well understood now, separating the effects of the incongruent dissolution of calcium (bound in different forms in the solid phases of the porous material) on the mechanical performance of cement-based materials remains a challenging task, requiring a break-down of the problem in its elementary components situated at different scales. This is the focus of this paper which reviews recent developments in chemomechanics of calcium leaching induced aging of cement-based materials: (1) a combined experimental–theoretical investigation of the strength domain of unleached and homogeneously leached cement-based materials, by means of a non-linear homogenization approach, which captures the frictional enhancement of a cement-based matrix reinforced by rigid inclusion; (2) a micro-to-macroscopic chemomechanics theory based on volume averaging of the local energy dissipation, which distinguishes the effect of the two governing leaching processes in cement-based materials: Portlandite dissolution and leaching of calcium from the calcium–silicate–hydrate structure. Combined with (3) an advanced predictive model of the two-phase dissolution process, prediction and anticipation of the mechanical integrity of concrete structures subjected to severe chemical degradation becomes possible.

Macroscopic and Micromechanical Approaches to the Modelling of the Osmotic Swelling in Clays

The osmotic swelling in clays has been extensively studied at the physico-chemical scale. The present paper addresses the question of the modelling of this phenomenon from the mechanical point of view. First, the classical macroscopic thermodynamic framework for saturated porous continua is extended in order to take into account the solid-salt interaction through the concept of macroscopic activity coefficient of the salt. The micromechanical approach then incorporates this interaction through the concept of swelling pressure which is used for describing the internal forces in the fluid phase at the microscopic scale. The results of a physico-chemical theory for the solid-salt interaction, such as the e.d.l. theory, can be introduced in both approaches. Each of them leads to the identification of a deviation, of chemical origin, to Terzaghis effective stress principle. Besides, the micromechanical approach allows us to clearly differentiate the mechanical and the chemical parts of clay materials elasticity.

Micromechanics investigation of expansive reactions in chemoelastic concrete

Expansive reactions damage porous materials through the formation of reaction products of a volume in excess of the available space left by the reactants and the natural porosity of the material. This leads to pressurizing the pore space accessible to the reaction products, which differs when the chemical reaction is through-solution or topochemical or both in nature. This paper investigates expansive reactions from a micromechanical point of view, which allows bridging the scale from the local chemo-mechanical mechanisms to the macroscopically observable stress-free expansion. In particular, the study of the effect of morphology of the pore space, in which the chemical expansion occurs locally, on the macroscopically observable expansion is the main focus of this paper. The first part revisits the through-solution and the topochemical reaction mechanism within the framework of micro–macro-homogenization theories, and the effect of the microscopic geometry of pores and microcracks in the solid matrix on the macroscopic chemical expansion is examined. The second part deals with the transition from a topochemical to a through-solution-like mechanism that occurs in a solid matrix with inclusions (cracks, pores) of different morphology.

A Micromechanics Model for Solute Diffusion Coefficient in Unsaturated Granular Materials

A new micromechanics analysis of solute diffusion in unsaturated granular materials is put forward. This permits predicting the percolation effect. To do so, the pore water is divided up into four phases that account for the different spatial distributions and diffusion properties: interconnected capillary water, isolated capillary water, wetting layer and water film. A parameter denoted as connectivity ratio is introduced to account for the connectivity of the capillary pore water. Our model agrees well with experimental results on unsaturated sands and glass beads from literature. It also permits interpreting the physical meaning of the saturation exponent of Archie’s law: The latter is found to be correlated with the connectivity ratio of the unsaturated granular materials.

Micromechanical Modeling of Transport Properties of Cement-Based Composites: Role of Interfacial Transition Zone and Air Voids

The transport properties of cement-based composites, including solute diffusivity, electrical conductivity and water permeability, are regarded as durability indicators of cement-based composites. These transport properties are closely related to the microstructure, or rather to the pore structure of materials. Among all the microstructural aspects, the interfacial transition zone (ITZ) between the cement paste matrix and aggregates, and the air voids are believed to play an essential role in the transport properties. However, their impacts on the transport properties are difficult to be quantified. This paper develops a closed-form four-phase micromechanical model accounting for the local properties of ITZ and the saturation states of air voids. The effects of ITZ and air voids on the transport properties of cement-based composites are addressed quantitatively in the model. The Katz–Thompson equation is reinterpreted by the model in particular. It is shown that the local properties of ITZ and volume fraction of aggregates act mutually on the overall transport properties, the influence of air voids depends significantly on the water saturation, and a critical saturation degree is found to be 1/3.

Poroelastic Properties of a Nanoporous Granular Material with Interface Effects

AbstractMany research activities have contributed to extend the homogenization schemes and variational bounds to account for surface stresses, in the case of matrix-inclusion composite materials. The nanostructure of clay-based and cement-based materials rather exhibits a disordered granular-like morphology which is usually well described by using the self-consistent scheme. Within this context, this paper proposes an extension of Kroener’s self-consistent scheme incorporating the physics of surface stress. The poromechanical coupling is also considered through the concept of disjoining pressure. Closed-form solutions for the homogenized elastic and poroelastic moduli that are derived and simplified expressions of these moduli are reported for asymptotic cases.

Analyse micromécanique du couplage perméabilité-contrainte d'une argilite fracturée

ABSTRACT A micromechanics-based analysis is developed throughout this paper in order to study stress loading/permeability couplings in cracked porous media. Fracture closure phenomena under macrocopic compressive stresses are addressed for a fracture modelled as an arrangement of parallel cracks. This first multiscale step emphasizes the key role of the cracks connectedness on the macroscopic permeability evolution with respect to the applied confining pressure.

An energy-based equivalent inclusion for the determination of nanocomposite behavior

Abstract Achieving remarkable mechanical properties for very low levels of reinforcement, nanocomposites are nowadays a new class of materials with high potential. Thanks to the stiffening aspect of the interface region between the matrix and the inclusions, nanocomposites often exhibit improved mechanical and physical properties compared to the conventional composites reinforced with micron-sized particles. This paper presents a strategy based on the introduction of an equivalent inclusion phase that allows managing the ellipsoidal morphology of inhomogeneous inclusion nanophase in the determination of estimates for the homogenized elastic moduli. Application to clay-polymer nanocomposites is presented in the framework of the dilute scheme. The latter gives theoretical estimates in good agreement with experimental results reported in the literature.

A Micromechanics Approach to the Mechanically-Induced Dissolution in Porous Media

When subjected to a mechanical loading, the solid phase of a saturated porous medium undergoes a dissolution due to strain-stress concentration effects along the fluid-solid interface. Through a micromechanical analysis, the mechanical affinity is shown to be the driving force of the local dissolution. For cracked porous media, the elastic free energy is a dominant component of this driving force. This allows to predict dissolution-induced creep in such materials.