Analysis of fire resistance of concrete with polypropylene or steel fibers
Introduction
Given the importance of concrete as a structural material and the significance of preserving its stability in case of fire, advancing in the research of materials and systems that improve their behavior has become a need, because fire reduces concrete resistant capacity and rigidity and generates deformations imposed during the fire and the cooling phase [1], [2].
Concrete subjected to early stages of fire has no noticeable loss of qualities, and rather, it increases its mechanical strength when up to 280 °C, maintaining a grey appearance. From 280 °C onwards, the mechanical ability of concrete decreases, reaching a compressive strength loss of 15% at around 400 °C. This decisively influences the way to work and the type of exposure to fire of the structural element [3].
In addition to resistance, the decrease of the longitudinal elasticity module of concrete is of utmost importance, decreasing up to 75%, when the temperature rises from 20 °C to 400 °C. In addition, this thermal gradient produces expansion differences between the different faces of the structural element regarding their exposure to fire, resulting in differences in strain and high tensile stresses, that produce concrete cracking [4], [5], [6], [7].
The addition of steel fibers modifies the nonlinear behavior of structural concrete, especially its tensile strength, preventing the opening and propagation of cracks and increasing its ductility [8], [9], [10], [11], [12].
Concrete reinforced with polypropylene fibers, due to the physical and mechanical properties of the fibers, reduces the permeability and capillar porosity blocking the pores in the concrete. These improvements are achieved with the optimal amount of polypropylene of 0.7 kg/m3 [13], [14], [15], [16].
The addition of metallic fibers in concrete, when subjected to fire, produces a positive influence, improving energy absorption and reducing cracking [17], [18], [19]. In the case of the addition of polypropylene fibers, the ability to reduce cracking is due to fact that concrete permeability increases suddenly between 80 °C and 130 °C, and polypropylene, once it has reached the melting point, flows through the cracks and produces channels allowing the water vapor and gases to be evacuated releasing the pore pressure [20], [21], [22], [23], [24], [25], [26], [27], [28]. Although there are numerous research works on the behavior of concrete elements to fire, no literature has been found about concrete elements subjected to compression in the range of temperatures studied in the present work, neither when subjected to a direct fire test, nor comparisons performance to fire of concrete with steel and polypropylene fibers.
Based on the above premises, the aim of this research is to compare the mechanical behavior of concrete with addition of metallic fibers, with concrete with polypropylene fibers, when exposed to direct fire action, with maximum temperatures of around 400 °C. Likewise, the mechanical behavior of these concretes will be compared with concrete without fibers, both exposed to fire and at room temperature.
Section snippets
Materials
For the development of the experimental work, the following equipment and materials listed below were used.
Materials used:
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Cement type CEM II/BL 32,5 for the three batches, according to the standards UNE-EN 197-1:2011 and RC-08 [29], [30].
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Washed fine river sand with 0–4 mm granulometry fraction, of washed siliceous nature, according to the standard UNE-EN 13139/AC:2004 [31].
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Coarse aggregate with 4–20 mm granulometric fraction, of washed siliceous nature, and a maximum aggregate size of 12 mm,
Results and analysis
Results obtained in the compression test of concrete at room temperature are shown in Fig. 5, using the most representative graph for each case, and showing the strength-strain evolution in the specimens without addition, and in the ones with different addition percentages. As can be seen, strengths values are higher in specimens with additions. Similar behaviors can be observed for 1 and 2% percentages of each addition, and at the same time, a better behavior is achieved with the addition of
Discussion
The analysis of the compression test results displayed in Table 4 show that the specimens once subjected to the direct fire test increase their maximum strength (σmax), while diminishing the ultimate strength (σu). In terms of deformation and strain energy, both the ultimate and maximum ones, suffer an increase in concrete specimens after the fire test.
Fig. 9 allows us to compare the mean values corresponding to maximum strength and maximum strain, before and after being subjected to direct
Conclusions
From the result analysis shown in this study, the following conclusions can be drawn:
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Concrete with addition of polypropylene fibers and steel fibers in percentages of 1 and 2% by weight achieve greater compression strength than without additions, but they lose ductility after the maximum strength has been reached.
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Under normal environmental conditions, concrete with addition of polypropylene fibers reaches higher strength values than those with steel fibers, for percentage rates of 1 and 2% by
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