For reliable modelling of microwave heating of concrete its complex permittivity has to be known precisely within the full range of working temperatures. Dielectric characterization of dry concrete cured with different water-to-cement (w/c) ratios and concrete samples from nuclear power plant constructions was performed during heating and cooling cycles from room temperature to 700 degrees C and back. On average, higher permittivity values are found for concretes cured with smaller w/c ratio (more dens and less porous) as compared to concretes cured with higher w/c ratio (lighter and more porous). Samples from nuclear power plant reveals a permittivity close to the concrete prepared with lowest w/c ratio. Permittivity change along increasing temperature correlates with moisture loss and thermal decomposition reactions. These reactions are irreversible that lead to a permittivity divergence in heating and cooling scenarios. The variations of concrete permittivity because of w/c ratio, water transport and decomposition reactions are discussed. (C) 2016 Elsevier Ltd. All rights reserved.

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The use of high power microwaves to perform explosive spalling of surface concrete is a promising technique with applications in the area of concrete facilities decommissioning. The mechanism that creates explosive spalling is an interactive process of the thermal stress from high temperature gradients and the pore pressure generated from the water vaporization. In order to better predict the total stress distribution, the temperature has to be calculated by including the effect of water vaporization and water transport through a porous medium. In this paper, a one dimensional model solving the heat and diffusion equation for liquid and vapor phase with COMSOL finite element software is presented. The modelling of the drying process is based on the Spatial Reaction Engineering Approach (SREA developed by X.D. Chen). This paper discusses the influence of the relative energy activation parameter and effective diffusion coefficients on the temperature, water content and pore pressure in the case of fast microwave heating of concrete. This model is then used for a 3-D geometry with a sealed insulated concrete block and a conical antenna to compute the thermal stress, pore pressure and total stress.

AbstractMicrowave heating is becoming of growing importance in material sciences in the last two decades. Due to the volumetric heating effect, microwave heating offers an energy efficient way of processing materials in several ways. The modeling and simulation of material processing with microwaves with respect to the material structure is a challenging task. The corresponding mathematical modeling results in a coupled, multiscale problem in space and time consisting of Maxwell equations, the heat equation and a temperature‐dependent description of the dielectric properties of them material under investigation. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The use of high power microwave energy for ablation of contaminated concrete is a promising technique to speed up the dismantling of nuclear power plants. A coupled simulation using COMSOL Multiphysics® finite element software is performed by solving the electromagnetic wave equation at 2.45 GHz for a standard wave guide and a concrete block. The temperature field is obtained with the heat equation and the microwave power dissipation as a source term. The displacements and stress fields are obtained by solving a thermo elastic model.