Benoit Bary

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.

Analytical and 3D numerical analysis of the thermoviscoelastic behavior of concrete-like materials including interfaces

Abstract We investigate in this paper analytically and numerically by means of 3D simulations the viscoelastic behavior of concrete and mortar subjected to creep loading and moderate temperatures at mesoscale. These heterogeneous materials are assumed to be composed of thermoelastic aggregates distributed in a linear thermoviscoelastic matrix; moreover, the Interfacial Transition Zones (ITZ) between aggregates and matrix, whose behavior is also considered as linear thermoviscoelastic, are explicitly introduced. The numerical specimens consist in unstructured periodic meshes containing polyhedral aggregates with various size and shapes randomly distributed in a box. Zero-thickness interface finite elements are introduced between aggregates and matrix to model the ITZ. Macroscopic response and averaged stresses and strains in the matrix and aggregate phases are compared to analytical estimations obtained with classical mean-field approximation schemes applied in the Laplace–Carson space, in which the ITZ are introduced via imperfect interfaces modeled with the Linear Spring Model (LSM). The effects of ITZ thickness, aggregate shape and uniform temperature increase are then studied to evaluate their respective influence on the local and macroscopic creep behavior of mortar and concrete. Globally, it is found that the model response is in relatively good agreement with numerical simulations results, and that as expected while the ITZ do not affect significantly the concrete behavior, they have a non-negligible impact on the mortar one.

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.

Thermoviscoelastic Analysis of Concrete Creep at Mesoscale

Concrete is a heterogeneous material made up at mesoscale of linear elastic aggregates distributed in a mortar matrix whose behavior is time and temperature dependent. The Interfacial Transition Zone (ITZ) between aggregates and matrix also influences the overall behavior. We investigate here analytically and numerically by means of 3D simulations the creep behavior of concrete and mortar subjected to moderate temperatures at mesoscale. The numerical specimens consist in unstructured periodic meshes of polyhedral aggregates with various size and shapes randomly distributed in a box. Specific interface finite elements are introduced between aggregates and matrix to model the ITZ. Both matrix and ITZ are considered as linear thermoviscoelastic materials. Averaged stresses and strains in the matrix and aggregate phases are compared to analytical estimations obtained with classical mean-field approximation schemes applied in the Laplace-Carson space, where the ITZ are introduced via imperfect interfaces modelled with the Linear Spring Model (LSM). The effects of ITZ thickness, aggregate shape and temperature are then studied to evaluate their respective influence on mortar and concrete creep behavior.

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.