Study on mechanism of strength weakening of concrete subjected to microwaves in microwave-assisted concrete recycling

Abstract

This paper explores the multi-field distribution in concrete by coupling of the electromagnetics, heat transfer and solid mechanics in COMSOL Multiphysics software under microwave irradiation. The numerical model is validated by the temperature data captured from the experiment. The physicochemical reactions in mortar and aggregate during heating and evolution of surface cracks are further investigated by the tests. The results show that when the radiation effect is considered above 100 °C, the simulated temperature profile is in alignment with the experimental one. The hot spots formed by heating exist at surface edges and in aggregate area on the cross section. The maximum tensile stress and compressive stress appear at the boundary on the cross section. Although the stress in aggregate is greater than that in mortar, it maintains in the elastic deformation stage, whereas mortar will undergo the plastic strain with the strain over 0.04%, approximately rheological behavior. In the first 100 s of heating, the migration and evaporation of water and the dehydration of hydration products are the main factors of strength degradation of concrete, which is manifested by the generation and propagation of cracks and the decrease in density. The initial position of visible cracks has a lot to do with the path of moisture migration. The surface cracks have been coalesced over stress relief caused by the increase in open porosity after heating for 150 s. The pore pressure induced by moisture migration and thermal stress caused by temperature gradient initiate more internal cracks. This paper can provide reference and insight for effective strength weakening of concrete exposed to microwaves so as to make its weakened mechanical properties can work to recovery of high-quality aggregate after mechanical crushing.