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

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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.