Study On Multi-field Coupling Effect And Strength Weakening Mechanism In Concrete Under Microwave Irradiation

Demand for aggregate is growing with the rapid construction of public infrastructure worldwide.Microwaves have increasing potential in the field of concrete aggregate recovery because of the characteristics of selective and integral heating.Based on theoretical analysis,numerical simulation and experiment,this paper analyzed the relationship between the multi-field coupling effect and the separation of mortar and aggregate under microwave irradiation,and then explored the underlying mechanism of concrete strength weakening and the effect of microwave pretreatment on aggregate recovery,as follows:(1)Given theory and simulation analysis,the distribution of electromagnetic,heat and stress fields in mortar and aggregate under uniaxial microwave irradiation was studied,and then the separation criterion of mortar and aggregate was established.The results show that the electric field intensity and temperature in mortar are higher than those in aggregate.The electric field at the interface keeps continuous,while the temperature and stress are abrupt change.In the plane in which the wave propagates,there are two neutral axes located in mortar and aggregate,respectively.The tensile stress on the surface gradually increases as the neutral axis in mortar moves towards the interface.As the tensile stress exceeds the critical strength of mortar,microcracks appear at the mortar surface and gradually expand to the interface until the mortar fractures and sheds away from the aggregate surface.(2)COMSOL Multiphysics software was employed to study the multi-field distribution in the mortar and aggregate under 3D microwave irradiation and the effect of location on the separation of mortar and aggregate.The simulation results show that the separation of mortar and aggregate is caused by the normal stress perpendicular to the interface induced by the high temperature gradient due to thermal mismatch.The microcracks first appear at the edge of the interface.With the increase of time,the propagation and connection of the cracks will occur,leading to the increase of interfacial separation rate.There is no direct relationship between separation efficiency and energy utilization rate.For the sample placed at the appropriate location in the cavity,the aggregate damage rate is basically 0,while the interfacial separation rate reaches 95%.(3)The mechanism of concrete strength weakening under microwave heating is explored over numerical simulation and experiment.The results show that the radiation effect caused by surface temperature should be considered as the temperature is above100 ℃.The maximum tensile stress and compressive stress occur at the interface.Although the stress of aggregate is greater than that of mortar,it still maintains in the elastic deformation stage,whereas mortar will undergo the plastic stage,approximately rheological state,as the strain exceeds 0.04%.The differences in temperature and stress-strain due to different phases are smaller than those due to position.In the first 100 s of heating,the migration and evaporation of water and the degradation of hydration products are the main factors to weaken the strength of concrete which is manifested by the generation and expansion of cracks and the decrease of density.The position of the cracks is potentially related to the transport channel formed by bleeding during sample preparation.After heating for 150 s,with the decrease of initial moisture,the surface cracks have been completely coalesced but developed inward.In addition,the surface cracks show a certain closing effect as the water migrating to the surface is gradually evaporated.The pore pressure triggered by moisture migration and thermal stress induced by temperature gradient generate more internal cracks.Microwave pretreatment promotes concrete breakage and aggregate liberation by increasing mortar porosity and weakening interfacial transition zone strength.In addition,high power microwave heating can promote the efficiency of aggregate recovery.This study can provide reference and insight for multi-field distribution in concrete exposed to microwaves and effective strength weakening,which is helpful for the recovery of high-quality concrete aggregate after mechanical crushing,and lays a foundation for the commercial application of microwave-assisted concrete recycling technique.