Moisture and Chloride Transport in Concrete

Doctoral thesis, 2014

Material properties pertaining to the physical phenomena of mass transport in concrete are important for the durability of concrete structures, and they can ultimately be derived from material heterogeneities found at different subscales. On the mesoscale level, these heterogeneities are mainly characterized by the presence of embedded aggregates in the cement paste, together with cracks. This thesis concerns modelling of moisture and chloride transport in concrete, where the mesoscale heterogeneities are explicitly accounted for by means of geometrical and constitutive descriptions along with computational homogenization. The objective of this work was to numerically study how the heterogeneous and random mesoscale composition of concrete influence the homogeneous transport properties on the macroscale. This objective was ultimately met by the development of two- and three-dimensional Statistical Volume Elements (SVEs) of mesoscale concrete, which are geometrical representations of the material morphology. The developed SVEs contained the mesoscale constituents cement paste, aggregates and the highly porous interface material - the so-called Interfacial Transition Zone (ITZ) - which has important implications on the macroscale behaviour of concrete, both in terms of mass transport and deformational characteristics. The SVEs were employed numerically using the finite element method (FEM) to simulate mass transport - for both stationary and transient conditions - in order to compute macroscale diffusivities. Computations were carried out for various volume fractions of aggregates in the SVEs and for different diffusivities in the ITZ. For stationary conditions, the decrease in macroscale diffusivity attributed to increased volume fraction of aggregates was numerically determined. By use of the ITZ, the magnitude of this decrease in macroscale diffusivity could readily be controlled and altered. The influence of cracks on the macroscale diffusivity was also modelled and numerically evaluated. The numerical simulations showed that the macroscale diffusivity became anisotropic due to cracks and that the macroscale diffusivity rapidly increased in magnitude once cracks had begun to develop in the SVE. This work forms a base for further modelling of the influence of mesoscale composition and cracking on the diffusivity of concrete, which has important implications for the durability of concrete structures.