The nature of concrete-based structures means that they generally perform very well in fire. However, concrete is fundamentally a complex material and its properties can change dramatically when exposed to high temperatures. The principal effects of fire on concrete are loss of compressive strength and spalling – the forcible ejection of material from the surface of a member.
Though a lot of information has been gathered on both phenomena, there remains a need for more systematic studies of the effects of thermal exposures. The response to realistic fires of whole concrete structures presents yet greater challenges due to the interactions of structural elements, the impact of complex small-scale phenomena at full scale, and the spatial and temporal variations in exposures, including the cooling phase of the fire.
The main aim of the project is to test the influence of the porosity of the concrete with the proposed test program as well as the thickness of component on the spalling of the concrete in case of fire. The project will analyse the newest findings published in scientific literature and the project findings will be verified using the fire tests.
The following methods will be used: Literature review and subsequent laboratory tests on specially prepared test specimens.
It is suggested to discover the influence of porosity and component humidity on the fire resistance through two test series. The tests will be based on further knowledge of literature on evaluations as well as a defined temperature. In the first phase the available information in literature, which concern the mentioned subject, will be evaluated. It is thus guaranteed that no double determination will occur. Qualitative statements could set up the right priorities. Plates with different w/z-ratio and air-content will be manufactured. These are going to be tested in fire tests on their fire resistance. The humidity of the plates are constant. Parallel to the samples of the first test, samples with higher bending resistance will be produced. Like that it is guaranteed that the influence on the concrete prescription do not have to be considered on the comparison on the results. The prescription, which show the highest fire resistance in the first test will be tested in the second test with a higher bending resistance.
The following conclusions were formulated under this project:
The behaviour of concrete in fire is not well characterised at present, and further research is required in almost every aspect of this field. The response of concrete materials to heating is fundamentally complex; for example, degradation in the physical properties of concrete varies strongly depending on the details of the concrete mix, including the moisture content, and relevant environmental parameters, such as the maximum fire temperature and fire duration. These changes are generally irreversible. Systematic studies are required on the effects of different heating conditions on concrete.
A more significant challenge arises in relating these detailed small-scale behaviours to the performance of whole structures in realistic fires. Though good progress has been made on modelling the mechanical behaviour of concrete structures, particularly when the significant role of LITS is properly accounted for, the use of detailed models to predict spalling behaviour remains a significant challenge. Moreover, capability to predict structural interactions, which may have a role in failures, is poorly developed.
Historically, very simple treatments have often been adopted to describe fire environments, referencing simple temperature-time curves or assuming homogeneous temperatures, which are poor representations of real fires. More extensive research into the effects of temporal and spatial variations in heating, on a range of concrete compositions is now required. This demands further testing of complete concrete structures in realistic fires, to observe their holistic behaviour, including the interactions between different parts of a structure, and in order to facilitate the validation of advanced computer models.
Detailed studies of the performance of concrete structures in actual fire incidents can also assist greatly in advancing knowledge of real-world behaviour.