Effects of intermediate drying periods on the scaling behavior of concrete under freeze-thaw deicing salt attack

目前正在通过实验室试验验证混凝土结构对冻融除冰盐侵蚀的足够抵抗能力，或者根据几十年的经验，某些混凝土成分被认为是满足的。然而，已知的许多情况下，没有通过实验室测试的混凝土成分在实践中没有显示出损坏。造成这种情况的主要原因是德国实验室和服务环境之间的差异。而混凝土样品在实验室暴露在恒定的水分供应，现场条件相对干燥。前期DFG项目试验表明，中间干燥期对盐结垢的演化有显著影响。然而，潜在的机制尚未解决。因此，所提出的研究将有助于更好地了解盐结垢的基本机制，并改善混凝土结构在实际条件下的评价。实验工作分为六个工作包。在第一个包装中，将量化第一次冻融攻击前不同储存条件的影响。这些研究能够确定无预损伤和盐结晶的干燥期的作用。在第二个工作包中，分析了间歇干燥期对结垢演变有显著影响的原因。为了检测具体的机理，使用了成熟的方法（XRD， EDX，重量测量，氯化物含量测定）以及新的高分辨率技术，如单面核磁共振和激光烧蚀。在第三个工作包中，详细研究了重复干燥周期的影响。由此，可以分析干燥周期的积极影响的有效性。这对实际工程中混凝土损伤演化的预测具有重要意义。数值研究将在工作包4中使用CHTM模型进行。在第五个工作包中，在大型混凝土构件上验证了先前结果的可转移性。为此，申请人在高速公路上储存了10个冬天的混凝土样品（暴露条件XF2和XF4）。这些样本以及其他样本将被彻底调查。由此建立了室内试验与现场条件的联系，并对数值分析进行了验证。附加数值参数研究（虚拟调查）的结果为损伤发展预测的简化工程模型的发展提供了重要的见解（包6）。最后，将从研究中得出特征参数，这些参数可以预测混凝土结构在冻融除冰盐侵袭下的使用寿命。这使得XF2和XF4暴露等级的混凝土结构的环境和经济优化设计成为可能。

Adequate resistance of concrete structures against freeze-thaw deicing salt attack is currently being verified by laboratory tests or is considered fulfilled for certain concrete compositions based on decades of experience. However, numerous cases are known in which concrete compositions, which had failed the laboratory test, showed no damage in practice. A main cause for this is the difference between laboratory and service climate in Germany. While concrete samples in laboratory are exposed to constant moisture supply, field conditions are comparatively dry. Experiments in a previous DFG project showed that intermediate drying periods had a significant effect on the evolution of salt scaling. However, the underlying mechanisms are unresolved. Therefore, the proposed investigations will provide better understanding of the basic mechanisms of salt scaling and improve the evaluation of concrete structures under practical conditions. The experimental work is divided into six work packages. In the first package, the effect of different storage conditions before the first freeze-thaw attack will be quantified. These studies enable the determination of the role of drying periods without pre-damage and salt crystallization. Within the second work package, the causes for the significant effect of intermittent drying periods on scaling evolution are analyzed. In order to detect the specific mechanisms, well-established methods (XRD, EDX, gravimetric measurements, determination of chloride content) as well as new, high-resolution techniques such as single-sided NMR and laser ablation are used. In the third work package, the effect of repeated drying periods is investigated in detail. Herewith, the effectiveness of the positive impact of drying periods can be analyzed. This is very important for the prediction of damage evolution of concrete in practice. The numerical study will be carried out in work package 4 using CHTM models. In the fifth work package, the transferability of the previous results is verified on large-scale concrete components. For this, the applicant has stored concrete samples for ten winters on a highway (exposure condition XF2 and XF4). These samples as well as additional ones will be thoroughly investigated. Through this, a link from laboratory experiments to field conditions is established and the numerical analysis is validated. The results of additional numerical parameter studies (virtual investigation) provide important insights for the development of a simplified engineering model for the prediction of damage development (package 6). Finally, characteristic parameters will be derived from the investigations which enable service life prediction of concrete structures exposed to freeze-thaw deicing salt attack with intermediate drying periods. This enables environmentally and economically optimal design of concrete structures in the exposure classes XF2 and XF4.