Effects of various fibres on high-temperature spalling in high-performance concrete
Introduction
The use of high-performance concrete (HPC) in infrastructure elements has recently increased because it is stronger and less permeable than conventional concrete materials, which is attributable to its characteristically dense microstructure. HPC shows a very low water-to-binder (w/b) ratio and cannot hold enough water to maintain the capillary-pore saturation needed to sustain both continuous cement hydration and the pozzolanic reaction. The behaviour of HPC at high temperatures is very complex and, in turn, affects the global behaviour of heated HPC-based structures.
Concrete exposed to fire spalls owing to two phenomena: (1) the restrained thermal dilation of the water inside the concrete pores, which generates biaxial compressive stress parallel to the heated surface, subsequently leading to tensile stress developing in the direction perpendicular to the heated surface (Fig. 1) [1]; and (2) the build-up of pressure in the concrete pores as a consequence of the physically/chemically bound water in the cement vaporizing, thereby loading tensile stress in the heated concrete microstructure (Fig. 2) [2]. The second mechanism has recently been investigated by comparing the vapour pressure of water inside concrete pores with the saturated vapour pressure (SVP) of water in the specimens [3], [4], [5]. Several studies have examined the synergistic effect of various combinations of fibres on the behaviour of HPC exposed to fire [6], [7] and found that some combinations of fibres increased the fire-resistance of the HPC. Researchers have also reported how various fibres affected the mechanical properties of cement-based materials at high temperatures [8], [9], [10], [11], [12].
Adding synthetic fibres, especially polypropylene (PP) fibres, to HPC is a widely used and effective method of preventing explosive spalling [13], [14], [15], [16], [17], [18], [19], [20].
Fig. 3 shows how spalling is reduced when PP fibres are mixed into concrete. Under normal conditions, the PP fibres mixed into concrete exist in a dispersed condition in the concrete. If the concrete surface gets heated during a fire, surface cracking occurs first. Then, as the fire continues to burn, and the temperature of the concrete rises, the free water existing within the concrete turns into vapour, forming vapour bubbles. It is thought that, in the absence of the PP fibres, the vapour pressure within these vapour bubbles results in tensile stress. If the tensile stress exceeds the tensile strength of the concrete, spalling results. However, when the PP fibres are present, they melt at 165–170 °C and form a vapour pressure dissipation network, which effectively dissipates the vapours within the concrete and prevents spalling. Silva et al. [21], [22] studied the mechanical properties of natural-sisal-fibre-reinforced cement-based materials. They found that the morphology of the sisal fibres plays an important role in determining the bond strength. An average adhesive bond strength as high as 0.92 MPa has been reported for the fibre shape that results in the best interfacial performance.
Filho et al. [23] investigated the thermomechanical behaviour of continuous-sisal-fibre-reinforced cement-based materials. The composites exhibited extensive degradation at 250 °C, most probably owing to the degradation of the sisal fibres. Nevertheless, the strength and toughness at temperatures of 200 °C and lower were very similar to those obtained at room temperature.
Sim et al. [24] determined the feasibility of using basalt fibres as a material for strengthening structural concrete members by studying how the durability, mechanical properties, and flexural strength of such structural concrete members changed when such fibres were added to concrete. Two layers of a basalt fibre sheet were found to result in greater strengthening. In addition, the strengthening effect did not need to extend over the entire length of the flexural member.
Adding water-soluble polyvinyl alcohol (WSPVA) fibres to refractory castables is an effective method of preventing explosive spalling [25]. Although researchers have experimentally determined the permeability of heated PP-fibre-reinforced HPC [26], [27], [28], [29], [30], few studies have investigated how adding natural and synthetic fibres such as jute and WSPVA to HPC might prevent HPC from spalling. Our previous studies had examined the relation between the spalling behaviour of HPC reinforced with natural jute and WSPVA fibres and the vapour pressure of the water inside the HPC [31], [32]. In this study, we investigated how PP, jute, and WSPVA fibres mitigated the increase in vapour pressure inside HPC heated to high temperatures. We compared the measured vapour pressures of the water inside the concrete pores with the SVP of water in the specimens. Permeability tests were also performed on the concrete specimens containing jute or WSPVA fibres and on a control specimen at 100, 200, 300, and 400 °C.
Section snippets
Materials and methods
Fire endurance tests were performed on a fibre-less control specimen and on specimens containing jute, WSPVA, or PP fibres.
Results of temperature measurements
Fig. 11 shows the temperature curves for the control and jute specimens. The curves for all the specimens exhibit the same trend. The curves for the control and jute samples show a small plateau, i.e., a perturbation. The control specimen explosively spalled when the temperature plateaued from 120 to 160 °C 10 mm below the surface of the heated specimen. The temperatures of the jute specimen plateaued at 200 °C 10 mm below the surface of the heated specimen. The evaporation of water in concrete was
Conclusions
The following conclusions can be drawn based on the results presented in this study:
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The jute, WSPVA, and PP concrete specimens did not spall explosively. The maximum vapour pressures of water within the jute, WSPVA, and PP specimens were 2.5, 1.5, and 1.0 MPa, respectively. However, the permeabilities of the heated WSPVA and jute specimens increased. The normalised permeabilities of the WSPVA and jute specimens were lower than those of the PP specimen. We hypothesize that the straw-like
Acknowledgements
We wish to thank Japan Insulation Co., Ltd. for supporting this study. We thank Tesac Co., Ltd. for providing the jute fibres and Kurray Co., Ltd. for providing the WSPVA fibres. This study was also supported by Showa Concrete Industry Co., Ltd. We also acknowledge the contribution of the Institute for Research in Construction of the National Research Council of Canada (NRC-IRC), where the DSC and TGA tests were performed.
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