Influence of high temperature on the mechanical properties of hybrid fibre reinforced normal and high strength concrete
Introduction
Within the context of the most usual construction materials for building and civil infrastructures, concrete stands out because of its excellent behaviour when exposed to high temperatures and fire condition. All the variables that assess this performance are defined in International Standards [1], [2] which also include some advices to fulfil the structural requirements in these cases, such as minimum concrete covers for each particular fire resistance, simplified methods for the determination of the structural resistance of each type of section, or even some basis for advanced numerical modelling methods. Reinforced concrete became a structural material during the second half of 19th century. One of the main reasons was actually its good behaviour during accidental fires, especially when compared to the other usual construction material of that age, e.g. cast iron. Even today concrete presents better fire performance than modern structural steel. Two appropriate examples of this assertion are the Windsor Tower fire in Madrid in 2005 and the Grenfell Tower fire in London in 2017.
In the 1970′s concrete research focus shifted towards the structural behaviour of the material under high temperatures [3], [4], [5], [6], because of the development of nuclear power plants, whose reactors were initially and currently made in concrete. In the late 1980′s the focus was turned towards high strength concrete (HSC), with a more compact microstructure than normal strength concrete (NSC) [7], [8], [9], [10], [11], [12]. This dense internal structure makes HSC more sensitive to the splitting of the concrete cover in the hottest elements. One of the causes of structural spalling, as this phenomenon is known, is the thermal gradients and the thermal incompatibility of the different components of the reinforced concrete elements: the cement paste, the aggregates, the steel rebars and tendons, which have different coefficients of thermal expansion. Another cause of spalling is the dehydration of the C-S-H gel and the portlandite, which generates water steam and provokes an increase of the pore pressure within the concrete. Spalling is one of the greatest concerns when dealing with concrete structures exposed to fire. In this regard, notable fire catastrophes can be mentioned, such as fire in the Eurotunnel of the English Channel in 1996, the Tauern tunnel in 1999 (Austria) and the Mont Blanc tunnel in 1999 (France) [13], [14], [15], [16]. In these examples, the thermal exposure in a confined environment, and with a high combustible charge, caused a fast temperature rise, which led to severe concrete damage and explosive spalling. Thus, the structural section is rapidly reduced and inner layers become exposed, hence the structural safety may be compromised for the design load values.
In the last two decades research has been focused on studying the influence of the addition of fibres to the concrete subjected to high temperatures. Polypropylene fibres, which melt at approximately 170 °C, have been proven adequate to control the spalling of concrete; their melting creates an additional capillary pore network inside the concrete, which allows the release of the water steam generated by dehydration reactions [17], [18]. Thus, the main objective of polypropylene fibres is not the structural reinforcement but the improvement of the fire resistance of concrete. Instead, steel fibres can be used in order to increase the residual strength of concrete after exposure to elevated temperatures [19], [20]. However, some research [21] argues that the benefit of adding steel fibres may not be so obvious, since they contribute to speeding up the heating of the concrete and may be prone to producing internal micro-cracking because of thermal incompatibility with the concrete matrix. Recent researches [21], [22], [23] have studied hybrid fibre reinforced concrete, which incorporates both polypropylene and steel fibres. Experimental results suggest a synergy between both types of fibre in enhancing the mechanical properties of concrete subjected to high temperatures.
There are three different types of tests for concrete at high temperatures: in stressed tests specimens are prestressed to a certain fraction of their ultimate load and then are exposed to high temperature until failure, either at a constant load or after loading at high temperature [9], [24], [25], [26], [27]; in unstressed tests specimens are heated and then tested at high temperature [4], [5], [7], [19], [28]; and finally in residual tests specimens are heated to maximum temperature and then cooled and tested at ambient temperature [17], [21], [29], [30], [31].
The current version of the European standard for concrete structures [2] gives tabulated data for the evolution of the mechanical properties of NSC subjected to elevated temperatures, accounting for the case of quartz aggregates and for the case of calcareous aggregates. However, the standard acknowledges the lack of sufficient data for the case of high strength concrete (HSC), making no distinction between the use of different types of aggregate. Furthermore, this standard gives no provisions for fibre-reinforced concrete at high temperatures.
This paper presents the experimental results obtained on six batches of normal and high strength fibre reinforced concretes subjected to high temperatures up to 825 °C. The mechanical properties studied are the compressive strength, the tensile splitting strength, the dynamic modulus of elasticity and the flexural strength and ductility after natural cooling, i.e. residual mechanical properties. According to the existing literature [32], residual properties are always lower than their high temperature counterparts, hence these results would be conservative. The main objective is assessing the influence of hybrid fibre content with a low volume fraction on the evolution of the mechanical properties of concrete subjected to elevated temperatures, especially for high strength concretes made with limestone aggregates.
Section snippets
Materials and specimen preparation
Six different fibre reinforced concrete (FRC) batches were prepared, divided in two different types of concrete: three of them were designed to obtain normal strength concrete (NSC), while the other three were high strength concretes (HSC). Table 1 includes the dosages of this two reference concretes, NSC and HSC. Limestone aggregates were used for both coarse and fine fractions; CEM II/B-M (S-L) 42.5R according to UNE-EN 197-1:2001 was used for NSC, while a combination of silica fume and CEM I
Results and discussion
The evolution of the compressive strength (fc), the tensile strength (fct), the density (ρ) and the dynamic modulus of elasticity (Ec,dyn) are given in Table 3, whereas the mechanical properties measured in the bending tests of the 15 × 15×60 cm specimens are given in Table 4: fct,LOP corresponds to the concrete flexural strength at the limit of proportionality (LOP), which is the highest stress attained before reaching a crack mouth opening displacement (CMOD) of 0.05 mm; fct,R1 and fct,R3
Conclusions
The current paper reports an experimental campaign for the study of the behaviour of hybrid fibre reinforced normal and high strength concrete after being exposed to high temperatures. The effect of high temperature on the residual mechanical properties of hybrid fibre reinforced concretes, tested in this research, was less severe than in steel fibre reinforced concretes found in previous references.
Polypropylene fibres proved appropriate for controlling the explosive spalling during heating
Acknowledgements
The authors want to acknowledge SIKA, Grupo Ferroatlántica, Aridur Hormigones S.L. and Prefabricados Saval Hermanos S.L. for their indispensable collaboration in this research.
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