The effect of the boundary conditions, defined as the thermal expansion restraint imposed by the external uniaxial compressive load in plain concrete subjected to high temperatures, is discussed. A numerical mesoscopic investigation uncoupled the load induced thermal strain (LITS) into three main components (aggregates and matrix strain contributions and the effect of the thermal expansion restraint). The analyses demonstrate that LITS depends on the materials properties (viscous-plastic behavior) and also on the boundary conditions, which is function of the test setup and seems to be not related to the material properties. In this paper, the role of the boundary conditions on 1-phase (homogeneous) and 2-phase (matrix + aggregates) numerical analyses are investigated. A LITS semi-empirical model is incorporated in the numerical modeling to account for the stiffness degradation. It is demonstrated that in 1-phase models, the effect of the boundary conditions is absent, which is not true for mesoscale analyses. In this case, the effect of the boundary conditions should be taken into account and subtracted from the results to avoid an overestimation of the material response. It is important mentioning that the analyses of concrete behavior at elevated temperatures and transient creep experimental procedures completely neglect the role of the boundary conditions.

中文翻译：

---

高温单轴压缩素混凝土边界条件的影响

讨论了边界条件的影响，定义为外部单轴压缩载荷对高温素混凝土施加的热膨胀约束。数值细观研究将载荷引起的热应变 (LITS) 分解为三个主要组成部分（骨料和基体应变贡献以及热膨胀约束的影响）。分析表明，LITS 取决于材料属性（粘塑性行为）以及边界条件，这是测试设置的函数，似乎与材料属性无关。在本文中，研究了边界条件在 1 相（均质）和 2 相（矩阵 + 聚集体）数值分析中的作用。一个 LITS 半经验模型被纳入数值模型以解释刚度退化。结果表明，在 1 相模型中，边界条件的影响不存在，这对于中尺度分析是不正确的。在这种情况下，应考虑边界条件的影响并从结果中减去，以避免高估材料响应。值得一提的是，混凝土在高温下的行为分析和瞬态蠕变实验程序完全忽略了边界条件的作用。应考虑边界条件的影响并从结果中减去，以避免高估材料响应。值得一提的是，混凝土在高温下的行为分析和瞬态蠕变实验程序完全忽略了边界条件的作用。应考虑边界条件的影响并从结果中减去，以避免高估材料响应。值得一提的是，混凝土在高温下的行为分析和瞬态蠕变实验程序完全忽略了边界条件的作用。