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Simulating the microstructure of cement-based construction materials

Simulating the microstructure of cement-based construction materials

Led by:  P. Wriggers, S. Löhnert
Team:  N. Dabagh
Year:  2008
Is Finished:  yes

In this thesis three-dimensional computational homogenization of hardened cement paste (HCP) including micro-structural damage due to frost is introduced. Based on a computer-tomography at a resolution of 1µm a finite-element model of HCP is developed with different elastic and inelastic constitutive equations for the three parts unhydrated residual clinker, pores, and hydration products. The introduced finite-element model is restricted to mild loadings, since cracks can not be described by the constitutive equations. The reliability of the selected micro-structural constitutive equations is validated using a multi-scale model, where at each integration point of a finite-element mesh the micro-structure of HCP is evaluated. In order to keep the overall computation time within reasonable bounds a client-server based system is used which distributes the time consuming evaluation of the micro-structure within a TCP/IP based network automatically and thus allows a parallel computation of the micro-structural response. Subsequently, computational homogenization of HCP yields probability densities of the effective elastic properties. The effective inelastic constitutive equation of HCP is selected to be of Perzyna-type including isotropic damage and contains parameters which can not be obtained using experimental testings or computational homogenization. Therefore, these parameters are identified numerically by solving an optimization problem through a combination of the stochastic genetic algorithm and the deterministic Levenberg-Marquardt method. During a freezing process the water filled pores of HCP increase in volume which yields inelastic material behavior. Due to the resolution of the computer-tomography scans the porosity can not be resolved exactly, such that a computation of the freezing process yields only qualitative results. The corresponding simulations are performed for different moistures and temperatures which yield a correlation between moisture, temperature, and the inelastic material behavior. Then, thermo-mechanically coupling is introduced and an effective constitutive equation for HCP is developed in which the temperature-moisture-damage correlation obtained from micro-structural observations and the probability densities of the effective elastic properties are used.