Multi-scale modeling of the microstructure and transport properties of contemporary concrete
by Hongyan Ma
Ph.D. Civil and Environmental Engineering
xxiv, 328 p. : ill. (some col.) ; 30 cm
In this thesis, frameworks for multi-scale modeling of the microstructure and transport properties of cement based materials have been developed....[ Read more ]
In this thesis, frameworks for multi-scale modeling of the microstructure and transport properties of cement based materials have been developed.
Experiments are carried out on cement based materials with various techniques, including thermo-gravimetric analysis (TGA), mercury intrusion porosimetry (MIP), nitrogen adsorption, back-scattered electron microscopy (BSE) image acquisition and processing, and transport property measurements. The experimental results provide database for identifying the input parameters for the simulation of the models as well as the references for validating the outputs of the developed models. Based on experimental results and proposed paste models, stoichiometry factors of the reactions of cement and mineral admixtures are determined, and the corresponding kinetics is investigated.
With the stoichiometry factors and kinetics, the microstructure of cement paste is modeled at two levels using a series of status-oriented computer models. The status of outer CSH layer at sub-micro-level is characterized by its small capillary porosity, while that of paste by w/c (or w/b) and cement degree of hydration (and degree of reactions in blended paste). Interfacial transition zone (ITZ) is characterized by a newly developed multi-aggregate approach. Mortar/concrete is modeled as a composite consisting of bulk paste, ITZ and aggregate particles.
Starting from the simulated microstructures, diffusion tortuosity (τD) is computed at each level, except nano-level, through random walker simulations. Diffusion tortuosity of LD CSH (Low Density Calcium Silicate Hydrates) at nano-level is determined separately as τDLDCSH=21.76. At sub-micro-level, tortuosity of outer CSH layer τDOCSHL is found to be a function of the small capillary porosity, and at micro-level, τDP and τDITZ are functions of large capillary porosity. In combination with the effective porosity, the computed diffusion tortuosity at each level can be transformed to formation factor, related to which electrical conductivity, chloride diffusion coefficients and permeability can be calculated through the definition of formation factor, the Nernst-Einstein equation and the Katz-Thompson equation, respectively. Most of the outputs of the models developed in this study have been compared to the experimental results and reasonable agreement has been observed.