A comprehensive mesoscopic investigation has been conducted to examine the evolution of the fracture process zone (FPZ), using notched plain concrete beams subjected to three-point bending as a generic representation. The concrete beams are modelled as random heterogeneous materials containing three components, namely coarse aggregates, mortar and the interface transition zone (ITZ). To better represent the fracture process in concrete, a coupled cohesive-contact interface approach is employed to model the crack initiation, crack propagation and the friction mechanisms during the fracture process. Thus, the FPZ is naturally captured in the simulation as the zone composed by microcracks along the ITZ or through the mortar matrix in the mesoscale model. The macroscopic response of load-deformation curves as well as the shapes and sizes of FPZ calculated from numerical results are validated against experimental observations and good agreement is achieved. Subsequent modelling results demonstrate that the FPZ tend to exhibit a main crack and multiple secondary microcracks. During the growth of the main crack, new microcracks initiate while some microcracks formed earlier stop growing and even close. The evolution path of the FPZ is strongly irregular due to the random spatial distribution of the aggregate particles with weak ITZs. The influence of the size effect on the FPZ is also investigated from the numerical simulation. Results show that the width of the FPZ is insensitive to the beam size but the length of the FPZ is strongly dependent on the beam size, and such characteristics of the FPZ are deemed to be intrinsic reasons for the overall size effect phenomenon in concrete structures.

已经进行了全面的介观研究，以检查经过三点弯曲的带缺口普通混凝土梁作为一般表示，以检查断裂过程区（FPZ）的演变。将混凝土梁建模为随机的异质材料，其中包含三个成分，即粗骨料，砂浆和界面过渡区（ITZ）。为了更好地表示混凝土中的断裂过程，采用耦合内聚-接触界面方法对断裂过程中的裂纹萌生，裂纹扩展和摩擦机理进行建模 。因此， FPZ在模拟中自然捕获为由沿ITZ或通过中尺度模型中的砂浆基质的微裂缝组成的区域。根据实验结果验证了载荷-变形曲线的宏观响应以及根据数值结果计算出的FPZ的形状和尺寸 并达成良好的协议。随后的建模结果表明，FPZ倾向于显示一个主裂纹和多个次级微裂纹。在主裂纹扩展期间，会出现新的微裂纹，而较早形成的一些微裂纹则停止生长，甚至关闭。由于具有弱ITZ的聚集颗粒的随机空间分布，FPZ的演化路径非常不规则。还通过数值模拟研究了尺寸效应对FPZ的影响。结果表明，FPZ的宽度对梁的大小不敏感，但是FPZ的长度强烈取决于梁的大小，并且FPZ的这种特性被认为是混凝土结构中整体尺寸效应现象的内在原因。