Multiscale experimental and numerical study of creep and microcracking in cementitious materials

The safety of double wall concrete containment buildings in the French nuclear fleet primarily depends on the integrity of prestressed concrete. The delayed strains induced by creep and shrinkage are the cause of a loss of prestress that may increase the risk of potential leaks in accidental conditions. In order to predict in a reliable manner the long term performance of nuclear civil engineering structures, concrete constitutive models require a basis on realistic deformation mechanisms at the micro scale. Creep of concrete depends on multiple coupled factors among which microcracking is of major significance. This work aimed to gain a better understanding on the mechanisms of creep/microcracking coupling with changes in water content, and to develop a numerical creep/damage model at the mesoscale for the prediction of long-term delayed strains. This study was based on an extended interplay between experiments and modeling. The campaign of macroscopic compressive creep tests on mortar and cement paste aimed at decoupling the influence of several factors on the creep rate: material heterogeneity (influence of inclusions), water content, stress state and drying. The obtained data were used for a better understanding of the physical mechanisms, calibration of the creep numerical model and as a benchmark for numerical simulations of creep/damage/drying interactions. The mesoscopic damage was studied with in situ micro-flexural tests on mortar with X-Ray tomography. The test was analysed with a mechanically regularized Digital Volume Correlation technique adapted for a heterogeneous material to study the damage processes at lower scales. The calibration of finite element damage models was carried out for the cementitious matrix and matrix-aggregate interfaces based on in situ micro-flexural tests. The simulations were performed on image-based meshes of real samples and with measured kinematic boundary conditions via Digital Volume Correlation. The calibrated creep and damage models were applied on realistic and artificial microstructures of mortar to simulate basic creep, and evaluate damage effects on delayed strains. Then, the drying model was introduced for drying creep/damage simulations. (author)