Julien Sanahuja

A Burger Model for the Effective Behavior of a Microcracked Viscoelastic Solid

This article aims at the determination of the effective behavior of a microcracked linear viscoelastic solid. Due to the nonlinearity of the strain concentration in the cracks, the latter cannot be derived directly from a combination of the correspondence theorem with the Eshelby-based homogenization schemes. The proposed alternative approach is based on the linear relationship between the macroscopic strain and the local displacement discontinuity across the crack. An approximation of the effective behavior in the framework of a Burger model is derived analytically.

Micromechanical Explanation of Elasticity and Strength of Gypsum: From Elongated Anisotropic Crystals to Isotropic Porous Polycrystals

Gypsum is made up of interlocked and elongated crystals. The random nature of its morphology suggests to resort to homogenization of random media to investigate its mechanical properties from the scale of the single crystals upwards. Unfortunately, the usual homogenization schemes fail to quantitatively predict the influence of the porosity on the effective Youngs modulus of gypsum. This is clearly due to the inability of such approaches to take into account the elongated nature of the crystals. A modification of the classical self-consistent scheme is proposed. It is validated against elastic characteristics computed by finite element analyses, and also against experiments on real dried gypsum samples (with empty pores). Finally, a strength model based on brittle failure is presented. The whole strength domain in the space of macroscopic principal stresses is derived. The comparison to experimental data in both simple tension and simple compression is remarkably good.

A critical comparison of several numerical methods for computing effective properties of highly heterogeneous materials

Modelling transport and long-term creep in concrete materials is a difficult problem when the complexity of the microstructure is taken into account, because it is hard to predict instantaneous elastic responses. In this work, several numerical methods are compared to assess their properties and suitability to model concrete-like microstructures with large phase properties contrast. The methods are classical finite elements, a novel extended finite element method (@m-xfem), an unconstrained heuristic meshing technique (amie), and a locally homogenising preprocessor in combination with various solvers (benhur). The benchmark itself consists of a number of simple and complex microstructures, which are tested with a range of phase contrasts designed to cover the needs of creep and transport modelling in concrete. The calculations are performed assuming linear elasticity and thermal conduction. The methods are compared in term of precision, ease of implementation and appropriateness to the problem type. We find that xfem is the most suitable when the mesh if coarse, and methods based on Cartesian grids are best when a very fine mesh can be used. Finite element methods are good compromises with high flexibility.

Creep of a C-S-H gel: a micromechanical approach

Both clays and calcium silicate hydrates(the main hydration products of Portland cements) exhibit a microstructure made up of lamellar particles. The microscopic mechanism responsible for the macroscopic creep of such materials is often described as the relative sliding of the sheets. This paper proposes a micromechanical approach to estimate the macroscopic creep behavior rising from this microscopic mechanism. The asymptotic evolution of creep at both short- and long-term is especially investigated. More precisely, a non-vanishing initial elastic strain is retrieved. At long-term, a threshold on porosity appears. At lower porosities, the creep evolution admits an asymptotic strain. At higher porosities, it admits an asymptotic strain rate.

Concrete desorption isotherms and permeability determination: effects of the sample geometry

The isotherm sorption curve is a first-order parameter used in the Finite Element modelling of concrete moisture transport, shrinkage and creep behaviour. An original experimental campaign was developed by EDF R&D in order to characterise the first desorption isotherm at room temperature of a laboratory concrete. Long-term drying tests were carried out on three sample geometries: radial and axial one-dimensional drying on thin discs and multi-dimensional drying on representative elementary volumes (REV), in order to evaluate the possibility of accelerating the tests. Porosity, densities and mass loss curves are measured and the first-desorption isotherms obtained for the three different configurations are compared. Several analyses of these results are proposed including the assessment of a criterion for the determination of the moisture content final balance (estimation of the asymptotic mass loss) and the back analysis of equivalent permeability. The tests results show the significant time gain using thin (2.5 mm) concrete half samples drying in radial direction compared to the REV samples.

Upscaling concrete properties: a rational approach to account for the material complexity and variability

Life management of electric hydro- or nuclear power plants requires estimating long-term concrete properties on concrete facilities for obvious safety and serviceability reasons. Decades-old structures are foreseen to be operational for several more decades. Operational time-scale is thus far more extended than laboratory test duration. Additionally, tests on rather old concrete can hardly be done again due to the difficulties of finding genuinely representative cement or aggregates and, as far as dams are sometimes concerned, to the large size of the coarse aggregates. In order to estimate long-term mechanical properties upscaling techniques offer an interesting alternative. Two approaches are described in the sequel: 0D analytical and semi-analytical simulation based on homogenisation techniques and 3D numerical simulation.

Numerical benchmark campaign of COST Action TU1404 – microstructural modelling

This paper presents the results of the numerical benchmark campaign on modelling of hydration and microstructure development of cementitious materials. This numerical benchmark was performed in the scope of COST Action TU1404 “Towards the next generation of standards for service life of cement-based materials and structures”. Seven modelling groups took part in the campaign applying different models for prediction of mechanical properties (elastic moduli or compressive strength) in cement pastes and mortars. The simulations were based on published experimental data. The experimental data (both input and results used for validation) were open to the participants. The purpose of the benchmark campaign was to identify the needs of different models in terms of input experimental data, verify predictive potential of the models and finally to provide reference cases for new models in the future. The results of the benchmark show that a relatively high scatter in the predictions can arise between different models, in particular at early ages (e.g. elastic Young’s modulus predicted at 1 d in the range 6-20 GPa), while it reduces at later age, providing relatively good agreement with experimental data. Even though the input data was based on a single experimental dataset, the large differences between the results of the different models were found to be caused by distinct assumed properties for the individual phases at the microstructural level, mainly because of the scatter in the nanoindentation-derived properties of the C-S-H phase.

Effective ageing linear viscoelastic properties of composites with phase precipitation: comparisons between numerical and analytical homogenization approaches

Dissolution and precipitation processes are present in key phenomena affecting the behavior of cement-based materials. Additionally, cement–based materials exhibit viscoelastic behavior. Recently, analytical homogenization tools have been developed to upscale the effective properties of composites in an ageing linear viscoelastic framework [1,2]. Taking advantage of these tools, an extension of Bazant’s original solidification theory [3] was proposed in a 3D tensorial context [4]. In this paper, we propose to benchmark these analytical approaches by comparing with numerical homogenization to estimate the behavior of ageing composites in different scenarios. To this end, 3D numerical samples are generated by randomly distributing inclusions of various sizes and shapes in a box [5,6]. We compare the response of solidification in two main morphologies: spherical and convex polyhedral inclusions. The results provided here go towards a better description of the dissolution/precipitation processes, which is an important feature in the characterization of cement-based materials ageing behavior.