界面弧形裂纹对混凝土开裂强度的影响研究

混凝土由于水分蒸发、干缩、泌水以及骨料与砂浆变形不一致等原因会导致骨料与砂浆的界面层中产生弧形裂纹，从而对混凝土开裂强度产生很大影响.从细观角度将混凝土视作由粗骨料和水泥砂浆组成的两相复合材料，并将界面层视为粗骨料与水泥砂浆的接触层进行分析.首先基于相互作用直推估计(interaction direct derivative, IDD)法，考虑混凝土中骨料颗粒的相互作用，将施加在混凝土表征体积元的远场外荷载等效为无限大基体中含单一骨料的等效外荷载.然后，将等效外荷载转化为最大和最小主应力，基于断裂力学理论得到界面层中弧形裂纹的应力强度因子，并根据复合型裂纹幂准则判断弧形裂纹是否发生开裂，进而来研究混凝土开裂强度的变化规律.通过与数值模拟结果的比较，验证了界面弧形裂纹应力强度因子解析解的有效性，参数分析结果表明，当裂纹与最大主应力垂直或与最小主应力呈45°夹角时，骨料周围弧形裂纹最易发生开裂破坏.随着裂纹长度增加，混凝土受拉和受压开裂强度先减小后增大，且均存在最不利的裂纹长度.混凝土开裂强度随着骨料体积分数的增加而增大，随着骨料粒径的增大而减小.在裂纹长度较小时，增大骨料的弹性模量有利于提高混凝土开裂强度.骨料周围承受同号应力可以提高混凝土的开裂强度，反之，异号应力会降低开裂强度.

Study on Effects of Interfacial Arc Cracks on Cracking Strengths of Concrete

Owing to water evaporation, dry shrinkage, bleeding and inconsistent deformation between aggregates and mortar, cracks will inevitably occur in the interfacial transition zone between aggregates and mortar and significantly affect the cracking strength of concrete. From the viewpoint of meso-scopic mechanism, concrete is regarded as a 2-phase composite material of coarse aggregates and cement mortar, and the interfacial transition zone is considered as a contact layer in the analysis. Firstly, in view of the interaction between aggregate particles in concrete, the far-field external load on the representative volume element (RVE) of concrete was simplified as an equivalent load on an infinite matrix containing a single aggregate with the interaction direct derivative (IDD) method. Then the equivalent external load was transformed into principal stresses, and the stress intensity factor (SIF) of the interfacial arc crack was obtained based on the fracture mechanics theory. The compound power law was chosen to judge the cracking of concrete, and the variation law of the cracking strength of concrete was studied. In comparison with the FEM, the analytical SIF solution of the interfacial arc crack has verified validity. The parametric analysis results show that, the arc crack is most likely to open when it is perpendicular to the maximum principal stress or at an angle of 45° to the minimum principal stress. With the increase of the crack length, the tensile and compressive cracking strengths decrease first and then increase, and they both have the most unbeneficial crack lengths. The cracking strength increases with the aggregate volume fraction and decreases with the aggregate particle size. For a small crack length, increasing the elastic modulus of aggregates can improve the cracking strength. The cracking strength will increase in the case of the same-sign stresses around aggregates, and on the contrary, will decrease in the case of different-sign stresses.