Microstructural effects in the simulation of creep of concrete

The influence of the microstructure on the visco-elastic properties of concrete is investigated using finite element simulations at the meso-scale. We first derive a constitutive law for the creep of the cement paste which accounts for both recoverable and permanent deformations, as well as the influence of temperature and internal relative humidity. The model is calibrated on a set of experiments at the cement paste scale, and then validated after upscaling to the concrete scale. The model is then applied to study the influence of the microstructure on the macroscopic creep of concrete. We show that materials with finer particles exhibit less creep, and that the anisotropy of creep can be explained with the shape and orientation of the aggregates. Furthermore, the acceleration of the stress relaxation in the presence of damage is explained by the micro-mechanical interaction between the aggregates, the cement paste, and the micro-cracks

Computing Creep-Damage Interactions in Irradiated Concrete

Among various degradation mechanisms possibly affecting the long-term operation of nuclear power plants, the effects of induced expansion and internal degradation occurring in concrete exposed to high-flux neutron radiation require additional research. Notably, using short-term test-reactor data to assess the long-term structural significance of light-water reactor concrete biological shields necessitates properly capturing the concurrent time-dependent effects, e.g., creep and damage caused by radiation-induced volumetric degradation. As this poses significant numerical challenges, a creep-damage algorithm was developed to account simultaneously for the progress of damage and viscoelastic processes in the concrete microstructure. The algorithm uses a time-adaptive scheme in which the instants at which damage occurs are explicitly searched for. This provides a nonlocal continuum damage procedure with very low sensitivity to the time or loading step. The proposed method is then used to simulate creep and restraint effects on radiation-induced degradation in concrete.This research is sponsored by the United State DOE Light Water Reactor Sustainability Program

Microstructural effects in the simulation of creep of concrete

The influence of the microstructure on the visco-elastic properties of concrete is investigated using finite element simulations at the meso-scale. We first derive a constitutive law for the creep of the cement paste which accounts for both recoverable and permanent deformations, as well as the influence of temperature and internal relative humidity. The model is calibrated on a set of experiments at the cement paste scale, and then validated after upscaling to the concrete scale. The model is then applied to study the influence of the microstructure on the macroscopic creep of concrete. We show that materials with finer particles exhibit less creep, and that the anisotropy of creep can be explained with the shape and orientation of the aggregates. Furthermore, the acceleration of the stress relaxation in the presence of damage is explained by the micro-mechanical interaction between the aggregates, the cement paste, and the micro-cracks

Finite elements in space and time for the analysis of generalised visco-elastic materials

SUMMARY: Numerical analysis of linear visco-elastic materials requires robust and stable methods to integrate partial differential equations in both space and time. In this paper, symmetric space-time finite element operators are derived for the first time for elementary linear elastic spring and linear viscous dashpot. These can thereafter be assembled in parallel and in series to simulate an arbitrarily complex linear visco-elastic behaviour. The flexibility of the proposed method allows the formulation of the behaviour, which closely reflects physical processes. An efficient algorithm is proposed to use the generated elementary matrices in a way that is comparable with finite difference schemes, in terms of both processor and memory costs. This unconditionally stable and convergent procedure is equally valid for space domains in which geometry or material properties evolve with time. © 2013 John Wiley & Sons, Ltd

Influence of visco-elasticity on the stress development induced by alkali-silica reaction

The alkali-silica reaction causes long-termdegradation in the microstructure of affected concrete as well as macroscopic expansion. In this paper, a micro-mechanical model based upon an explicit representation of the microstructure has been used to simulate the role of creep on the expansion and damage induced by the reaction. The model accounts for the coupling between damage propagation and stress relaxation in the cement paste. This study indicates that the influence of the visco-elastic nature of the material on the overall expansion is within the experimental scatter. However, it is shown that creep can explain the comparatively low amount of damage in the cement paste as observed experimentally. Creep also increases the amount of damage in the aggregates when the rate of reaction increases. Overall, the results presented in this paper indicate that accelerated experiments may not be representative of the degradation in the field at equivalent degrees of reaction

Computing Creep-Damage Interactions in Irradiated Concrete

Among various degradation mechanisms possibly affecting the long-term operation of nuclear power plants, the effects of induced expansion and internal degradation occurring in concrete exposed to high-flux neutron radiation require additional research. Notably, using short-term test-reactor data to assess the long-term structural significance of light-water reactor concrete biological shields necessitates properly capturing the concurrent time-dependent effects, e.g., creep and damage caused by radiation-induced volumetric degradation. As this poses significant numerical challenges, a creep-damage algorithm was developed to account simultaneously for the progress of damage and viscoelastic processes in the concrete microstructure. The algorithm uses a time-adaptive scheme in which the instants at which damage occurs are explicitly searched for. This provides a nonlocal continuum damage procedure with very low sensitivity to the time or loading step. The proposed method is then used to simulate creep and restraint effects on radiation-induced degradation in concrete

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 (μ-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. © 2013 Elsevier Ltd. All rights reserved