Kannangara, Thathsarani (2024) An investigation of the physio-chemical transformations and moisture migration processes occurring in concrete during spalling. PhD thesis, Victoria University.

Abstract

Concrete spalling, which is the flaking, or exploding of concrete layers during fire, can be catastrophic, resulting in collapsing of the structures in some situations. While several theories have been put forward with a view to explaining this undesirable phenomenon, none of these have been able to explain the phenomenological evolution of spalling with an acceptable level of certainty. However, the migration and pooling of considerable amounts of water from the unexposed surface during fire has been established to be key contributor of spalling. Although numerous studies are available, due to the complex physio-chemical transformations occurring to concrete structures under heat/fire, it has proved to be difficult to experimentally analyse the phenomenon of moisture migration. Therefore, it is evidently an area that requires investigation on a much deeper level. Consequently, there is a knowledge gap in the available literature, which warrants a systematic and quantitative approach to investigating, especially, the theories behind the transportation of moisture during spalling. The current project was primarily aimed to examine the mass loss, migration of moisture through the micro flow-channels and the associated transportation processes. For this purpose, three types of mix designs that were based on ordinary Portland cement (OPC), and one based on geopolymer paste, were chosen. The latter was primarily selected for comparison purpose as geopolymer-based concrete elements are relatively resistant to the phenomenon of spalling. Here, in order to address the physio-chemical mechanism(s) of spalling of concrete structures in actual fire scenarios using an appropriate testing regime, both small- and medium- and large-scale investigations were carried out. Furthermore, during the course of the investigation, several synthetic polymeric components, such as, polypropylene, polyester and cotton fibres, were employed as ‘sacrificial’ agents in mitigating the spalling behaviour of test samples. These agents were subjected routine analyses, which included, morphological (Scanning Electron Microscope with Energy Dispersive provision: SEM/EDS), spectroscopic (Fourier transform Infrared: FT-IR), thermal (Thermo-gravimetric Analysis: TGA) and calorimetric tests (Differential Scanning Calorimetry: DSC and Pyrolysis Combustion Flow Calorimetry: PCFC), etc. To realize the above goals, an appropriate methodological approach to the problem was carefully employed. This included bespoke analyses, both at the small- and medium-scale, and the former primarily included: TGA, X-ray diffraction technique (XRD), solid-state Nuclear Magnetic Resonance spectroscopy (NMR), SEM imaging. Furthermore, porosity measurements (based on Rapid Chloride Penetration test and BET theory) were carried out, with a view to identifying the pores/micro-channels within the test samples. Here, measurements relating to the moisture content of the mixtures, cured specimens, and the test samples during the various analyses, were conducted. In addition, with a view to gaining insights into the chemical environments of hydrated calcium centres, before and after spalling, in some cubical test specimens, small-angle neutron scattering studies (SANS) were utilized. Furthermore, a high-resolution optical microscopy was used to scan the surfaces of the cured and heat-treated samples. Finally, appropriate mechanical testings on samples were carried out, primarily, to measure the compressive strengths. Here, a short analysis on the difference in compressive strength using different types of specimens, namely cubical and cylindrical, was also investigated. Finally, the set of empirical data, collated through the above experiments, was systematically analysed with a view to elucidating the various physio-chemical transformations that lead to spalling. The current investigation clearly demonstrated that the incorporation of polymeric components, as sacrificial agents, resulted in a lesser degree of spalling in OPC-based cubical specimens. The fundamental physio-chemical process(es) responsible for this effect can be attributed to the preferential melting/degradation of the polymeric elements within the concrete matrix, thus relieving the built-up pore pressure owing to the generation of water within the samples upon exposure to heat/ fire. The same effect was also observed in the case of large-scale test specimens. In the case of cubical samples, the exposure to heat and/or carbon dioxide (through accelerated carbonation tests) had an effect on their mechanical properties albeit to different degrees. On a comparative scale, the geopolymer-based cubical specimens, as expected, only suffered a nominal degree of spalling, if at all, and in general the effect of carbonation/heat treatment on the mechanical properties were minimal.

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