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Concrete In Australia : September 2014
50 Concrete in Australia Vol 40 No 3 FEATURE: CONCRETE PERFORMANCE IN FIRE Performance of concrete in fire Vinh T.N. Dao, School of Civil Engineering, The University of Queensland The behaviour of concrete in fire depends on its mix constituents and proportions, and is determined by complex physico- chemical transformations during heating. In this paper, relevant features of these transformations are detailed to form the basis for discussing key materials measures to enhance concrete performance in fire. The limitation of inconsistent thermal boundary conditions in conventional tests, which accounts in part for the wide variability in observed behaviour, is also highlighted. An improved test method that effectively addresses this limitation, together with an ongoing research to better characterise concrete performance in fire, is then outlined. Outcomes of this research are expected to support the transition to performance-based fire engineering with greater degrees of flexibility to enable more efficient fire design of concrete structures. 1.0 INTRODUCTION The outbreak of fire in buildings and civil engineering structures can have disastrous consequences, including severe structural damage, total loss of contents, and loss of life (Torero, 2011). Adequate design for fire is thus an important and essential requirement in the design process. The current approach to design for fire resistance is to ensure that none of the following three main fire-related limit states is reached in less than a specified fire-resistance period: • The strength limit state is entered when a structural member, with reduced load-carrying capacity due to elevated temperatures, can no longer carry the sustained loads during the fire. • A fire-separating member will reach the limit state of integrity when flames and hot gases can pass through it. • The limit state of insulation is considered to be reached when the temperature on an unexposed face of a fire-separating member reaches a level at which combustible material in contact with the surface would ignite. Concrete has favourable inherent characteristics with respect to fire: (1) It has low thermal conductivity and high heat capacity; (2) It is non-combustible; and (3) It does not emit smoke or toxic gases. High levels of fire resistance for traditional concrete structures can generally be achieved by adopting certain member dimensions and cover to reinforcement (Standards Australia, AS 3600, 2009). Nevertheless, concrete also exhibits some less attractive aspects when exposed to elevated temperatures: degradation of material properties and spalling – consequently, both the load-carrying and separating/insulating functions of concrete structures could be compromised. Despite much research in the past several decades, the predominant approaches adopted in current codes of practice remain largely prescriptive, requiring either standard details or testing to demonstrate equivalent fire resistance. This prescriptive approach is generally conservative, rigid and restrictive. The current trend is thus to further transition toward performance-based specifications (Croce, et al., 2008; Kodur, et al. 2012), which allow greater flexibility and more efficient designs. The success of such a transition, however, is dependent on the correct modelling of major influencing factors. These factors include fire loadings, reliability of fire safety measures/ equipment, and models for concrete performance at elevated temperatures – many of which remain inadequately quantified. This paper will first provide an overview of concrete performance at elevated temperatures, with a focus on (1) the degradation of concrete properties and (2) spalling. On that basis, possible measures to enhance the fire performance of concrete are discussed, mainly from the concrete materials’ viewpoint. Finally, ongoing research to better characterise concrete performance in fire using an improved test method that effectively addresses the limitation of inconsistent thermal boundary conditions in conventional tests is outlined. 2.0 DEGRADATION OF CONCRETE PROPERTIES Concrete is a complex composite made from cementitious materials, water, aggregates and admixtures. These component ingredients interact to form hardened concrete that typically consists of three phases: aggregates, hydrated cement paste and the interfacial transition zone between those two phases. The behaviour of concrete during both heating and cooling is therefore dependent not only on the physico-chemical changes of each individual phase but also on the complex interaction between these changes. A detailed discussion of the physico-chemical transformations of concrete at elevated temperatures can be found elsewhere (Khoury, 2000; Schrefler et al., 2002). Some typical changes are shown in Figure 1(a) (Khoury, 2008). Notable features relevant to our discussion here are highlighted as follows: • The degradation of concrete properties upon heating is influenced by a significant number of factors, both materials (e.g . mix design, curing) and environmental (e.g . heating/ cooling rate, load during heating, moisture condition). This complexity is further magnified by the inconsistent test conditions with limited repeatability, as discussed in Section 4. As a result, concrete properties at elevated temperatures vary over a wide range (Phan, 1996). Data for a given type of concrete should therefore be discussed in the context of its 50-53 - Vinh.indd 50 50-53 - Vinh.indd 50 26/08/14 10:23 AM 26/08/14 10:23 AM