Determination of the size effect on the spalling of concrete in case of fire

混凝土结构具有较高的耐火性，与其他建筑材料组成的结构相比，在发生火灾时，混凝土结构具有更持久的承载能力。然而，它们的表面会突然剧烈地剥落，导致横截面减小，从而降低其承载能力。这种现象中最危险的类型是发生在火灾的前30分钟的爆炸性剥落。直到今天，爆炸性剥落背后的确切机制仍然不完全清楚，尽管有大量的研究（例如：RILEM TC SPF）和一些最近发展的理论。爆炸剥落研究的难点在于，由于混凝土的工艺性能和环境因素的不同，可能产生的变量较多，导致综合影响。剥落的一个决定性的环境因素是结构元件的几何形状和在火灾试验中使用的试样的几何形状。该项目的主要目的是分析不同尺寸的试样，以找到可能的剥落尺寸效应。将分析六种不同的混凝土混合物，以便涵盖混凝土技术参数。在初步测试中，我们能够显示出试样尺寸与剥落深度以及与高温相关的高强度混凝土混合物损伤之间的相关性。该项目将被细分，以调查五个主要感兴趣的领域。第一个子项目的目的是验证混凝土的技术性能，并确定所选混凝土混合物的平衡水分含量。以下三个子项目反映了三种不同的规模水平：小规模、半全尺寸和全尺寸。这些子项目各由四个工作包组成，包括试样制备、试样说明、防火试验和损伤分析。在这里，主要关注的是通过诸如剥落深度或剥落体积等参数定量确定剥落，以及与温度相关的损伤。在这项研究中，每个样品的内部和外部结构需要在火灾试验之前，期间和之后通过合适的方法确定，包括摄影测量，声发射分析和计算机断层扫描。最后一个超级项目的目的包括数据分析，将每个混凝土混合物的试件尺寸和剥落参数相关联。考虑到混凝土的主要特性，这些相关性旨在为混凝土剥落建立一般尺寸效应定律。该尺寸效应规律可作为实施和制定火灾条件下混凝土结构剥落试验标准方法的依据。

Concrete structures have a high fire resistance which provides them with a more durable load-bearing capacity in the event of a fire compared to structures composed of other building materials. However, their surfaces can suddenly and violently spall which leads to a reduction of the cross section and therefore, a reduction of their load-bearing capacity. The most dangerous type of this phenomenon is explosive spalling which takes place in the first 30 minutes of a fire. Until today the exact mechanism behind explosive spalling remains incompletely understood, despite intensive research (e.g.: RILEM TC SPF) and a number of recently developed theories. The difficulty in researching explosive spalling derives from the large amount of possible variables leading to combined effects due to varying concrete technological properties as well as environmental factors. One decisive environmental factor for spalling is the geometry of a structural element and of the specimen used during a fire test, respectively.The primary objective of this project is to analyse specimens of different sizes to find a possible size effect of spalling. Six different concrete mixtures will be analysed in order to cover concrete-technological parameters as well. In preliminary tests we were able to show a correlation of the specimen size on spalling depth as well as high-temperature-related damage to a high-strength concrete mixture.The project will be sub-divided in order to investigate five main areas of interest. The aim of the first sub-project is to verify concrete technological properties and to determine the equilibrium moisture content of the selected concrete mixtures. The following three sub-projects reflect three different size levels: small-scale, semi full-scale, and full-scale level. These sub-projects each consist of four work packages which comprise specimen preparation, specimen description, fire tests and damage analysis. Herein, the quantitative determination of spalling by parameters such as spalling depth or spalling volume, and further, temperature-related damages are of main interest. For this investigation, the internal and external structure of each specimen needs to be determined before, during and after a fire test by suitable methods, including e.g. photogrammetry, acoustic emission analysis and computer tomography. The aim of the last sup-project involves the data analysis, correlating specimen size and spalling parameters for each concrete mixture. These correlations are intended for the development of a general size effect law for concrete spalling, taking into account the main concrete properties. The size effect law can serve as a basis for the implementation and establishment of a standard test method for spalling of concrete structures in case of fire.