回收废旧混凝土中的粗骨料，可以缓解天然粗骨料短缺并减轻建筑垃圾对环境的污染。微波辐照可以在混凝土内部产生裂纹，不仅能提高再生粗骨料质量而且能降低能耗，由于其具有高效、即时和清洁等特点，被认为是一种非常具有应用前景的方法。

       微波辐照混凝土是一个多场耦合问题，为了更加准确地模拟混凝土内电场、温度、应力以及裂纹分布演化过程，提出了一种有限元离散元单向耦合的模拟方法并通过试验验证了其有效性和准确性。利用该方法，对微波辅助破碎多骨料混凝土以及微波辅助剔选单骨料混凝土展开了数值模拟研究；从细观角度揭示了微波辅助剔选粗骨料机理。主要研究内容和结果如下：

（1）基于麦克斯韦方程、亥姆霍茨方程，采用有限单元法模拟微波辐照下混凝土内的电场以及功率密度分布；基于傅里叶热传递理论和平行粘结本构模型，采用离散元法模拟微波辐照下混凝土内的温度分布及裂纹分布演化规律；采用Fish语言，编写有限元离散元单向耦合子程序，实现了微波辅助回收粗骨料全过程模拟。

      （2）基于微波辐照多骨料混凝土数值模拟试验，以混凝土强度和裂纹形态作为微波辅助破碎混凝土效果评价指标，系统地研究了粗骨料含量和微波功率对微波辅助破碎混凝土效果的影响。结果表明：微波加热混凝土深度有限，导致裂纹只能出现在混凝土中靠近微波发射口的两角点附近。大多数裂纹没有沿着砂浆和粗骨料交界面开展。微波辅助破碎混凝土的效果随粗骨料含量增加而下降，随微波功率增大而提升。微波辐照可以降低混凝土强度，但辐照后的混凝土未能被完全破碎，微波辅助破碎混凝土效果十分有限。

      （3）基于微波辐照单骨料混凝土数值模拟试验，以剔选率和裂纹形态作为微波辅助剔选粗骨料效果评价指标，系统地研究了粗骨料粒径、粗骨料形状、砂浆厚度以及微波功率对辅助剔选效果的影响。研究结果表明：剔选过程可以分为3个阶段：剔选开始阶段，主要剔选阶段和剔选末段。剔选过程主要发生在主要剔选阶段，主要剔选阶段结束的时间是最佳辐照时间。微波辅助剔选粗骨料的效果随粗骨料粒径增大而下降，随砂浆厚度增加先提升后下降，随微波功率增大而提升。此外，粗骨料形状越圆滑，剔选效果越好。微波辐照下粗骨料的剔选率可以达到50~70%，混凝土中的粗骨料在微波辐照下几乎没有损伤。微波辅助剔选粗骨料效果较好，在实际生产时应使用大功率微波并选择合适的辐照时间进行辅助剔选。

      （4）分析了单骨料混凝中的电场、温度以及应力分布随辐照时间变化，重点研究了混凝土中各处的温度梯度以及应力分布演化规律，揭示了微波辅助剔选粗骨料的细观机理。微波辐照下，粗骨料无论是环向还是径向均受拉，而砂浆在环向受压，径向受拉，且砂浆和粗骨料交界面上的粘结性能最弱，因此交界面在径向拉应力作用下开裂。

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 efficiency, 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.