In this work, a series of concrete creep tests with different stress conditions were carried out by laboratory tests and the discrete element method (DEM). The instantaneous strain, creep strain, creep rate, as well as creep nonlinearity ratio were obtained by laboratory tests. Meanwhile, a DEM micromechanical model, including coarse aggregate with actual geometry, homogenized mortar, and interfacial transition zone (ITZ), was established and the validity of numerical results was verified by laboratory tests. On this basis, the mesostructure characteristics (i.e., contact force and force chain) were obtained by DEM micromechanical-based analysis. The results show that the creep strain increases with the increase of stress level and with the extension of load holding time, and the creep develops rapidly within 60 days and slowly after 60 days. As the applied stress level increases, the nonlinearity of creep strain becomes more obvious, all specimens achieve a stable nonlinear ratio after about 20 days. Meanwhile, the contact force in mortar and ITZ all tend to increase with the increase of stress level, resulting from the viscoelastic slippage in burger contact. The average contact force of ITZ is larger than that of mortar, and a difference of about 20% can be observed. The peak probability density of mortar occurs at = 0.6–0.8, while that of ITZ occurs approximately at = 0.5. By comparison with other models, the improved nonlinear viscoelastic-plastic creep model can well predict the creep behavior.

中文翻译：

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混凝土徐变行为的实验室试验和细观离散元建模

本工作通过实验室试验和离散元法（DEM）进行了一系列不同应力条件下的混凝土徐变试验。通过实验室试验获得了瞬时应变、蠕变应变、蠕变速率以及蠕变非线性比。同时，建立了包含实际几何形状的粗骨料、均质砂浆和界面过渡区（ITZ）的DEM微力学模型，并通过室内试验验证了数值结果的有效性。在此基础上，通过基于DEM微力学的分析获得了细观结构特征（即接触力和力链）。结果表明，蠕变应变随着应力水平的增大和保载时间的延长而增大，且蠕变在60 d内迅速发展，60 d后缓慢发展。随着施加应力水平的增加，蠕变应变的非线性变得更加明显，所有试件在大约20天后达到稳定的非线性比。同时，由于汉堡接触中的粘弹性滑移，砂浆和ITZ中的接触力都随着应力水平的增加而增加。 ITZ的平均接触力比砂浆大，可以观察到约20%的差异。砂浆的峰值概率密度出现在 = 0.6–0.8 处，而 ITZ 的峰值概率密度大约出现在 = 0.5 处。与其他模型相比，改进的非线性粘弹塑性蠕变模型能够很好地预测蠕变行为。