Size effect of quasibrittle materials and simulation of concrete decontamination using microwave heating.

摘要

The existence of fracture process zone in a structure leads to a size effect on the nominal strength of the structure. This means that the structural response determined for small sizes must be properly extrapolated to large sizes used in practice, in order to ensure safety of structures. The size effect is studied in the first part of this thesis in Chapters 2 and 3 by means of a new eigenvalue method, which makes it possible to obtain the size of the cohesive crack model without solving the entire load-deflection curve. The size effect for very large structures is studied in Chapter 4 by using a new K-version of the smeared tip method. The size effect of particles associated with the alkali-silica reaction in concrete is discussed in Chapter 5.; When a detailed information on the deformations and stress distributions in a structure is required, one can use the finite element analysis, in which the use of a proper constitutive relation is essential. With this aim, the microplane model, which has been developed in Northwestern University, is adopted. In Chapter 6, the viscoelastic creep is incorporated into the kinematically constrained microplane constitutive model. For this purpose, a technique in which the partial moduli of a Maxwell chain representation are obtained unambiguously is developed. In Chapter 7, the microplane model is applied to a porous rock such as Salem Limestone. In this application, the shear enhanced compaction in triaxial states is introduced to improve the microplane model.; In Chapter 8, the radionuclide decontamination of concrete using microwave heating is studied. The purpose of the decontamination is to remove radionuclide-contaminated surface layers from the concrete structures in nuclear facilities. The rapid temperature increase caused by microwave heating produces thermal stress, which then causes mechanical spalling. The analysis includes both the heat and moisture transfer, and the mechanical deformation of the concrete. It is shown that the proposed microwave decontamination technique is feasible for concrete structures and that the spalling is caused by biaxial compression failure of concrete rather than an explosion due to internal pore pressure generated by microwave power.