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Concrete In Australia : September 2014
32 Concrete in Australia Vol 40 No 3 FEATURE: CONCRETE PERFORMANCE IN FIRE Experimental parametric study on the effectiveness of polypropylene fibres at mitigating heat-induced concrete spalling Cristian Maluk, Postdoctoral Research Associate, School of Engineering, University of Edinburgh, UK Luke Bisby, Arup Professor of Fire and Structures, School of Engineering, University of Edinburgh, UK Giovanni Pietro Terrasi, Head, Mechanical Systems Engineering, EMPA Dübendorf, Zurich, Switzerland Many modern concrete structures incorporating high-strength concrete may be susceptible to heat-induced explosive concrete spalling during fire. This presents a major challenge for contemporary concrete designers who often wish to use high-strength concrete in various structural engineering applications. The concrete industry is only just beginning to grapple with the implications of increased spalling of modern concrete mixes. There is widespread disagreement on the relative importance of the physical mechanisms which may trigger or exacerbate heat-induced concrete spalling, and while it has been shown that introducing polypropylene (PP) fibres into the fresh concrete reduces the likelihood of spalling during furnace tests, the reasons for PP fibre effectiveness remain a matter of debate within the research community. This article presents a comprehensive experimental study on heat-induced concrete spalling, with an emphasis on assessing the effectiveness of various types and doses of PP fibres. High-strength, self-consolidating concrete mixes in which PP fibre type, cross section, length, supplier, and dose were varied, were tested under simulated standard furnace exposures. It is shown that increased PP fibre dose, which is currently the sole parameter prescribed by available design guidelines, mitigates spalling. However PP fibre cross-section and length may also be important for PP fibre effectiveness in mitigating heat-induced concrete spalling during fire. 1.0 HEAT-INDUCED CONCRETE SPALLING The building design industry has traditionally, and to a large extent justifiably, relied on the perceived ‘inherent’ fire safety features of concrete (e.g . non-combustible, non-flammable, high thermal inertia) to assure the structural fire safety (fire resistance) of concrete structures (Bilow & Kamara, 2008). Fire resistance design of concrete structural elements currently relies almost universally on prescribing minimum member dimensions and minimum concrete cover to the steel (or other) reinforcement and presumes satisfactory response during fire on this basis (typically without explicit consideration of full structural response in fire by structural designers). Advances in concrete technology, driven mainly by factors other than fire safety (i.e. architectural design, economics, ease and speed of construction, sustainability, etc.) have promoted the use of new structural systems and construction techniques, many of which use modern, high-strength concrete mixes. Such mixes tend to suffer from an increased propensity for heat- induced concrete spalling (Bentz, 2000) as compared with more traditional, normal-strength concrete mixes. Failure to account for this increased propensity for spalling in modern building designs could potentially lead to unexpected structural failures during fires, if not properly considered during building design, construction and operation. Heat-induced concrete spalling is a phenomenon wherein heated concrete separates from the heat-exposed surface in a more or less violent manner (see Figure 1). This results in a reduction of the concrete cover to the internal reinforcement and a reduction in the effective size of the overall cross section and adversely affects its fire resistance. More than a century of research studies into the occurrence of heat-induced concrete spalling have led to the conclusion that spalling is a sudden and stochastic phenomenon characterized by its dependency upon multiple influencing parameters (Maluk, 2014). For instance, spalling is known to be influenced by (at least): concrete strength, moisture condition, age, aggregate type and grading, certain admixtures, mechanical loads, mechanical restraint, heated area, element thickness, severity of thermal exposure and Figure 1: Photograph showing heat-induced concrete spalling on the underside of a reinforced concrete slab after a standard furnace test. PHOTO : IEUAN RICKARD CIA 40-3 FINAL.indb 32 CIA 40-3 FINAL.indb 32 26/08/14 9:19 AM 26/08/14 9:19 AM