A Phase Field Microcracking Description Coupled with Viscoelastic Concrete Behavior
Résumé
Concrete is an important constitutive material of nuclear power plants and waste storage structures whose main
role is to ensure high level of performance regarding the required middle to long-term containment function. As
such, creep and microcracking are known significant factors affecting the mechanical properties and the long term
behavior of concrete, and may call into question the safety of nuclear facilities. The accurate modelling of these
phenomena including their coupled effects have then to receive a special attention. We propose in this study to
apply the relatively recent method making use of a phase field approach to reproduce the initiation and propagation
of microcracking in concrete. This method rests on a diffuse description of the microcrack surfaces by one scalar
variable whose values result from a specific balance equation. As a main contribution, it is extended here and
coupled to a linear viscoelastic behavior to mimic the creep (and possibly shrinkage) of the material. A classical
generalized Maxwell model is retained for modelling this viscoelastic behavior. At first, an energy-based formulation
is developed to express the equations governing both viscoelastic mechanical and phase field problems. In this first
attempt, only cracking due to extensions is considered. Different simple fracture criteria based on positive stresses
or strains with and without threshold are applied and tested. The resulting approach is next implemented in the FE
code Cast3M so as to solve both systems of equations. Applications to a homogeneous material subject to loading
with various durations and intensity are then analyzed and discussed with regard to the different crack driving state
functions. In a second stage, 3D simulations of heterogeneous concrete samples made up of elastic aggregates
dispersed in a mortar matrix concentrating the viscoelastic behavior are performed. These numerical specimens
are generated with polyhedral aggregates having various size and shapes, and which are randomly distributed in a
box. The effects of the aggregate shape on the microcracking development and the macroscopic behavior of
samples subject to creep loading are investigated. To this aim, different specimens are created with elongated and
flattened polyhedral aggregates with given aspect ratio, and the obtained results are analyzed and confronted in
particular in terms of microcrack patterns.