Matthias ASCHABER

Development and Application of a three-phase-model for concrete
2017

Abstract

This thesis deals with the development and application of a three-phase-model for concrete on the basis of the theory of porous materials. Starting from the microscopic balance equations the macroscopic balance equations of a multi-phase-system are derived. Due to the existing pore structure, caused by hydration, concrete is a porous material and moisture within the pores can be transported and stored. The content and the distribution of moisture influences the mechanical behaviour of concrete, for example the progress of drying shrinkage strains. By means of a three-phase-model for concrete the coupled hygric, thermal and mechanical behaviour of concrete can be numerical simulated. From the moment of concrete setting the progress of the distribution of moisture and temperature within a concrete member can be calculated with respect to different initial and boundary conditions. The dependence of the reaction rate of hydration from the temperature and the moisture is considered. The latter result in the evolution and the spatial distribution of stiffness and strength. One important item of this thesis is the realistic calculation of drying shrinkage strains. They are determined with the calculated moisture distribution by means of the concept of effective stress. For the long-time behaviour of concrete not only drying shrinkage strains are of note but also concrete creep, which is determined with the microprestress-solidification theory of Bažant [14].

For the developed three-phase-model of concrete a lot of material parameters are necessary. To determine some of them and to validate the three-phase-model tests, which has been carried out at the university of Innsbruck at the unit of Strength of Materials and Structural Analysis in line with a research project, are computed numerical. To validate the creep model publicized test results are used. Bridge structures strengthened by concrete overlays are a field of interest for an application of the three-phase-model. Particular the mechanical behaviour of the concrete overlay is investigated, because strains of the concrete overlay are restrained due to the existing concrete structure, which results in restraint stress. By means of the developed three-phase-model of concrete two numerical computations of concrete structures strengthened by concrete overlays are presented. One numerical computation deals with the retrofit of a real bridge structure with adding a concrete overlay. Thereby the environmental conditions as temperature, humidity and wind speed have been recorded and are considered as boundary conditions for the numerical computation.