Study On Mechanism Of Microwave Assisted Recycling Coarse Aggregate Based On Finite Element And Discrete Element Method

Recycling coarse aggregate from waste concrete can alleviate the shortage of natural coarse aggregate and reduce the environmental pollution caused by construction waste.Microwave irradiation can produce cracks in concrete.Because of its high effi ciency,instant and clean characteristics,it can not only improve the quality of recycled coarse aggregate,but also reduce energy consumption.It is considered to be a promising method.Microwave irradiation of concrete is a multi field coupling problem.A one-way coupling method of finite element and discrete element was proposed to simulate the evolution of electric field,temperature,stress and crack distribution in concrete more accurately,and its effectiveness and accuracy were verified by experiments.With this method,microwave-assisted crushing concrete and microwave-assisted selection of coarse aggregate were studied;the mesoscopic mechanism of microwave-assisted selection of coarse aggregate was revealed.The main contents and results are as follows:(1)The electric field and heat production power density distribution of concrete under microwave irradiation were simulated by finite element method based on Maxwell equation and Helmholtz equation.The evolution of temperature and crack distribution in concrete were simulated by discrete element method based on Fourier heat transfer theory and parallel bond constitutive model.A one-way coupling subroutine of the finite element and discrete element was written to realize the whole process simulation of microwave-assisted recovery of coarse aggregate with fish language.(2)The concrete strength and crack shape were taken as the evaluation index of microwave-assisted crushing concrete,the effects of coarse aggregate content and microwave power on microwave-assisted crushing concrete effect were systematically studied based on the numerical simulation of microwave irradiating multi aggregate concrete.The results show that,the depth of concrete heated by microwave was limited,the cracks could only appear near the two corners of the concrete near the microwave emission port.Most cracks did not develop along the interface between mortar and coarse aggregate.The effect of microwave-assisted crushing concrete decreased with the increase of coarse aggregate content and increased with the increase of irradiation power.Microwave irradiation could reduce the strength of concrete,but the irradiated concrete could not be completely broken,and the effect of microwave-assisted breaking concrete was very limited.(3)The selection ratio and crack shape were taken as the evaluation indexes of microwave-assisted selecting of coarse aggregate,the effects of coarse aggregate particle size,mortar thickness,the shape of coarse aggregate and microwave power on the effect of microwave-assisted selecting of coarse aggregate were systematically studied based on the numerical simulation of microwave irradiating single aggregate concrete.The results show that,the selecting process can be divided into three stages:the beginning stage,the main selecting stage and the finial stage.The selecting process mainly occurs in the main selecting stage,and the end time of the main selecting stage was the best irradiation time.The effect of microwave-assisted selection of coarse aggregate decreased with the increase of coarse aggregate particle size,increased first and then decreased with the increase of mortar thickness,and increased with the increase of microwave power.In addition,the smoother the shape of coarse aggregate,the better the selecting effect.The selection ratio of coarse aggregate under microwave irradiation reached 50～70%,and the coarse aggregate in concrete was hardly damaged under microwave irradiation.Microwave-assisted selecting of coarse aggregate was effective.In actual production,high-power microwave should be used and appropriate irradiation time should be selected for auxiliary selecting.(4)The single aggregate concretes were taken as the research object,the changes of electric field,temperature and stress distribution in the sample with irradiation time were analyzed.The evolution law of temperature gradient and stress distribution in the concrete were studied,and the mesoscopic mechanism of microwave-assisted selecting coarse aggregate was revealed.Under microwave irradiation,the coarse aggregate was tensioned in both circumferential and radial directions,while the mortar was compressed in circumferential direction and tensioned in radial direction,and the bonding performance on the interface between mortar and coarse aggregate was the weakest,so the interface cracked under the action of radial tensile stress.