Experimentally-validated stochastic model for freeze-thaw microstructural degradation and damage of hardened cement paste

混凝土结构的可持续性在很大程度上受到其在环境暴露方面的耐久性的影响，例如冻融(FT)侵蚀。到目前为止，文献中还没有足够准确的模型来正确预测和量化耐久性，即混凝土因循环冻结和解冻而退化。原因有很多：一方面，负载，即英国《金融时报》的风险敞口是一个高度可变的过程，英国《金融时报》事件的总体非常难以分类。另一方面，冻结和解冻导致材料性质的局部变化(最终导致剥落)，由于其空间范围很小，很难检测和量化，性质上高度可变，很大程度上取决于混凝土中的水分含量以及冻结条件。我们通过引入一个随机的、空间分辨的混凝土FT降解模型来解决这两个方面。我们集中研究了硬化水泥浆体(HCP)作为关键材料，并构造了一个典型的体积单元(RVE)，该单元由直径在10微米到100微米之间的较大毛细孔的混合物组成，嵌入在均匀的各向同性固体中，具有亚微米级的孔隙率和结构特征。对这种RVE的机械和物理性能进行了数值模拟，并使用统计纳米压痕和核磁共振测试(以及其他技术)对其进行了校准，以表征固体和微压痕的性能，从而表征整个RVE的性能。详细研究了由于FT作用而产生的RVE性质的演化。我们在数值和实验上对HCP样品进行了FT测试，具有高的空间分辨率，并具有不同的孔隙率、孔径分布和最低温度。这项研究将为结构的分层模型奠定基础。至于目前的技术状态，(I)到目前为止，这种测量仅限于整体(非空间分辨)测量，并且(Ii)没有测量实际的机械性能，而是使用诸如超声运行时间的辅助参数。在这里，我们显然希望在对拟议的设置的了解方面取得显着进展。此外，实验工作和数值模拟之间的强大联系将允许对潜在过程有更完整的理解，为实验结果的随机外推奠定基础。项目中的工作将在Michael Beer教授和Matteo Broggi博士之间密切合作进行，他们将专注于随机模型和分析，以及Michael Haist教授，他们将进行实验工作和物理模型。这项工作将得到4个国际合作者的大力支持。

The sustainability of concrete structures is highly influenced by their durability with regard to environmental exposures, such as freeze-thaw (FT) attack. So far, no sufficiently accurate model is available in the literature, which correctly predicts and quantifies the durability, i.e. the concrete degradation due to cyclic freezing and thawing. The reasons for that are numerous: on the one hand, the loading, i.e. the FT exposure is a highly variable process and the population of FT events is extremely difficult to classify. On the other hand, freezing and thawing cause very localized changes in the materials properties (which eventually lead to spalling), which due to their very small spatial extent, are hard to detect and to quantify and which are highly variable in nature, depending very much on the moisture content in the concrete as well as the freezing conditions. We address both these aspects by aiming to introduce a stochastic, spatially resolved model for the FT degradation of concrete. We focus our investigations on hardened cement paste (hcp) as the key material and formulate a representative volume element (RVE) consisting of a mixture of larger capillary pores with diameters between 10 µm to 100 µm, which are embedded in a homogeneous, isotropic solid, with sub-micron size porosity and structural features. The mechanical and physical properties of this RVE are simulated numerically, calibrated using statistical nanoindentation and NMR testing (amongst other techniques) characterizing the properties of the solid and µCT and microindentation, characterizing the properties of the entire RVE. The evolution of the properties of this RVE due to FT action is closely studied. We perform FT testing on hcp samples, numerically and experimentally, with high spatial resolution and with various porosities, pore size distributions and minimum temperatures. This study will form the basis for a layered model of the structure. As for the current state of the art, (i) such measurements until now were limited to bulk (non spatially resolved) measurements only and (ii) did not measure the actual mechanical properties but rather used auxiliary parameters such as ultrasound runtime. Here we clearly expect pronounced progress in knowledge with the proposed setup. Also, the strong interlink between experimental work and numerical simulation will allow for a much more complete understanding of the underlying processes, building the basis of a stochastic extrapolation of the experimental findings. The work in the project shall be carried out in close collaboration between Prof. Michael Beer and Dr. Matteo Broggi, who will focus on the stochastic model and analysis, and Prof. Michael Haist, who will carry out the experimental work and the physical modelling. The work will be greatly supported by 4 international collaborators.