Thermal Stresses in the Microwave Heating of Concrete

The significance of compressive thermal stresses generated during microwave heating on concrete surface delamination is investigated analytically in this paper. Microwave energy acts as an internal heating source that causes concrete to heat up. The amount of microwave energy dissipated in concrete depends on microwave frequency, microwave power and the electromagnetic properties of the concrete. Electromagnetic properties of concrete change as a function of concrete water content, microwave frequency and temperature. The variation in microwave energy dissipation of concrete can be modeled using Lambert’s law which assumes an exponential decay in energy dissipation with distance. The exponent of energy dissipation function depends on the electromagnetic properties of concrete. The decay in microwave energy dissipation gives rise to temperature gradient in the concrete which results in compressive stresses being generated in the heated zone. Another factor that is postulated to affect concrete delamination by microwave heating is the generation of pore pressure within the concrete. However most of the previous studies have overlooked the importance of thermal stresses. In this paper a finite element modeling is used to examine the temperature distribution and thermal stresses generated within the concrete. Results show high compressive stresses being generated especially at high frequencies and in concrete with high water content. The results also reveal that the water content of concrete plays a very important role in the microwave heating process.