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Fig. 6. Morphology of specimens after subjected to a temperature of 800 °C

Figure 6 Morphology of specimens after subjected to a temperature of 800 °C

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ratio of these fibers varies in the range of 9.57-300, with an average value of 111.24 (Figure 2c). The average length of CRSF was 21.87 mm, with 2 mm and 47 mm as lower and upper limits (Figure 2d). The aspect ratio of these fibers also varies in the range of 9.66-372, with an average value of 102.66 (Figure 2e). The tensile strength of fibers is also 2488 MPa.
ratio of these fibers varies in the range of 9.57-300, with an average value of 111.24 (Figure 2c). The average length of CRSF was 21.87 mm, with 2 mm and 47 mm as lower and upper limits (Figure 2d). The aspect ratio of these fibers also varies in the range of 9.66-372, with an average value of 102.66 (Figure 2e). The tensile strength of fibers is also 2488 MPa.
Fig. 3. Electric heating furnace Before determining the mechanical behavior of concrete, the specimens were exposed to 200 °C and 600 °C inside an electric heating furnace (Figure 3). To obtain the desired temperature, a heating rate of 1-4 °C/min was applied for all specimens. Then, they were kept at a constant target temperature for 1 hour and a half to ensure uniform temperature distribution in the specimen volume. The specimens were heated, cooled in a furnace to room temperature, and tested the next day.
Fig. 3. Electric heating furnace Before determining the mechanical behavior of concrete, the specimens were exposed to 200 °C and 600 °C inside an electric heating furnace (Figure 3). To obtain the desired temperature, a heating rate of 1-4 °C/min was applied for all specimens. Then, they were kept at a constant target temperature for 1 hour and a half to ensure uniform temperature distribution in the specimen volume. The specimens were heated, cooled in a furnace to room temperature, and tested the next day.
Table 1. The proportions of mix composition (kg/m?)
Table 1. The proportions of mix composition (kg/m?)
Fig. 4. Setup for: a) Compressive; b) Tensile; and c) Flexural test where P;: is the maximum tensile force, Le: is the length of the cylinder (200 mm), and
Fig. 4. Setup for: a) Compressive; b) Tensile; and c) Flexural test where P;: is the maximum tensile force, Le: is the length of the cylinder (200 mm), and
Fig. 5. Color change of specimens at different temperatures In order to investigate the influence of fibers on cracking and spalling behavior, the specimens were heated in an electric heating furnace at a rate of 4 °C/min to a temperature of 800 °C. Then the specimens' color, cracking, and spalling were checked at different temperatures. As shown in At 800 °C, no explosive spalling occurred for the specimens. The morphology of all specimens after exposure to 800 °C is shown in Figure 6. The results show that using RSF can reduce the intensity of cracking and spalling of the surface of the specimens. Textiles and rubbers attached to the surface of the fibers melt completely at 800 °C. They can create open pores inside the concrete, which leads to easier evaporation of water vapor and helps reduce internal pore pressure. Deep cavities, macro-cracks, and spalling are visible on the surface of the specimens. The depth of the control specimens' cavities is more significant than those reinforced with recycled fibers. The specimens reinforced with CRSF and ISF showed similar and close performance.
Fig. 5. Color change of specimens at different temperatures In order to investigate the influence of fibers on cracking and spalling behavior, the specimens were heated in an electric heating furnace at a rate of 4 °C/min to a temperature of 800 °C. Then the specimens' color, cracking, and spalling were checked at different temperatures. As shown in At 800 °C, no explosive spalling occurred for the specimens. The morphology of all specimens after exposure to 800 °C is shown in Figure 6. The results show that using RSF can reduce the intensity of cracking and spalling of the surface of the specimens. Textiles and rubbers attached to the surface of the fibers melt completely at 800 °C. They can create open pores inside the concrete, which leads to easier evaporation of water vapor and helps reduce internal pore pressure. Deep cavities, macro-cracks, and spalling are visible on the surface of the specimens. The depth of the control specimens' cavities is more significant than those reinforced with recycled fibers. The specimens reinforced with CRSF and ISF showed similar and close performance.
At the temperature of 600 °C, the highest mass loss is related to RSFI-0.5%, with a value of 8.80%. At this temperature, all
At the temperature of 600 °C, the highest mass loss is related to RSFI-0.5%, with a value of 8.80%. At this temperature, all
At a temperature of 600 °C, a sharp drop in strength can be seen for all specimens. The lowest value of residual compressive strength is related to NC, with a value of 12.87 MPa, the highest value is CRSF- 0.5%, with a value of 19.67 MPa. The residual compressive strength of RSFI- 0.5% is 6.53% higher than NC at this temperature. The positive role of the presence of rubber and textiles with fibers can be seen at high temperatures. Rubber and textiles melt at a temperature of 600 °C and create a network of open pores inside the concrete, which helps escape the formed water vapor more efficiently and can reduce cracking and spalling caused by pore pressure (Wu and Zhou, 2020; Hossain et al., 2013). Of course, it is essential to mention that the complete melting of the rubber attached to the surface of the fibers reduces the bond between the cement paste and the fibers. Therefore, this issue can also justify the reduction of the compressive strength of specimens reinforced with RSFI compared to CRSF and ISF. The results of the average compressive strength of the specimens at different temperatures are presented in Table 2. At ambient temperature, the compressive strength of specimens reinforced with CRSF and ISF is close. However, at a temperature of 200 °C, the compressive strength of CRSF-0.5% is 7.44% lower than ISF-0.5%. Compared to RSFI, CRSF is cleaner fibers but still contains less rubber and textiles. Textiles close to the surface of the specimen burn at 200 °C and can lead to micro-cracks and porosity and, as a result, decrease in strength compared to ISF-0.5%
At a temperature of 600 °C, a sharp drop in strength can be seen for all specimens. The lowest value of residual compressive strength is related to NC, with a value of 12.87 MPa, the highest value is CRSF- 0.5%, with a value of 19.67 MPa. The residual compressive strength of RSFI- 0.5% is 6.53% higher than NC at this temperature. The positive role of the presence of rubber and textiles with fibers can be seen at high temperatures. Rubber and textiles melt at a temperature of 600 °C and create a network of open pores inside the concrete, which helps escape the formed water vapor more efficiently and can reduce cracking and spalling caused by pore pressure (Wu and Zhou, 2020; Hossain et al., 2013). Of course, it is essential to mention that the complete melting of the rubber attached to the surface of the fibers reduces the bond between the cement paste and the fibers. Therefore, this issue can also justify the reduction of the compressive strength of specimens reinforced with RSFI compared to CRSF and ISF. The results of the average compressive strength of the specimens at different temperatures are presented in Table 2. At ambient temperature, the compressive strength of specimens reinforced with CRSF and ISF is close. However, at a temperature of 200 °C, the compressive strength of CRSF-0.5% is 7.44% lower than ISF-0.5%. Compared to RSFI, CRSF is cleaner fibers but still contains less rubber and textiles. Textiles close to the surface of the specimen burn at 200 °C and can lead to micro-cracks and porosity and, as a result, decrease in strength compared to ISF-0.5%
‘The rate of resistance reduction compared to the ambient temperature
‘The rate of resistance reduction compared to the ambient temperature
Fig. 9. Compressive strengths of specimens at different temperatures Fig. 8. Compressive failure mode of specimens: a) Ambient temperature; b) 200 °C; and c) 600 °C
Fig. 9. Compressive strengths of specimens at different temperatures Fig. 8. Compressive failure mode of specimens: a) Ambient temperature; b) 200 °C; and c) 600 °C
ig. 10. Comparison of stress-strain curves of specimens at different temperatures: a) 20 °C vs 600 °C; b) 200 °C vs 600 °C; and c) All temperatures
ig. 10. Comparison of stress-strain curves of specimens at different temperatures: a) 20 °C vs 600 °C; b) 200 °C vs 600 °C; and c) All temperatures
Fig. 11. The secant modulus of elasticity of the specimens at different temperatures Figure 12 shows how tensile cracks are formed in specimens at _ different temperatures. All cracks occurred in the middle of the section, close to the diameter. In NC and RSFI-0.5% at 600 °C, the cracks were deeper, and near the cross-section wall, the cracks have spread in the form of branches. Steel fibers (regardless of theit type) have partially prevented the branching erowth of cracks at hish temperatures.
Fig. 11. The secant modulus of elasticity of the specimens at different temperatures Figure 12 shows how tensile cracks are formed in specimens at _ different temperatures. All cracks occurred in the middle of the section, close to the diameter. In NC and RSFI-0.5% at 600 °C, the cracks were deeper, and near the cross-section wall, the cracks have spread in the form of branches. Steel fibers (regardless of theit type) have partially prevented the branching erowth of cracks at hish temperatures.
Fig. 12. Tensile failure mode of specimens at different temperatures
Fig. 12. Tensile failure mode of specimens at different temperatures
Fig. 13. Splitting tensile strength of specimens at different temperatures At a temperature of 200 °C, the residual tensile strength of ISF-0.5% has decreased to 12.37%. This is while the average strength reduction of specimens reinforced with recycled fibers reaches 22.67%. The recycled fibers' short length and small diameter and the presence of rubber and textiles can be the reason for the more significant decrease in residual tensile strength at this temperature compared to industrial fibers.
Fig. 13. Splitting tensile strength of specimens at different temperatures At a temperature of 200 °C, the residual tensile strength of ISF-0.5% has decreased to 12.37%. This is while the average strength reduction of specimens reinforced with recycled fibers reaches 22.67%. The recycled fibers' short length and small diameter and the presence of rubber and textiles can be the reason for the more significant decrease in residual tensile strength at this temperature compared to industrial fibers.
Fig. 14. Cracking of prism beams at 200 °C: a) NC; b) RSFI-0.5%; c) CRSF-0.5%; and d) ISF-0.5% According to the — load- deflection diagrams, the slope of the tangent to the curve is defined as flexural stiffness. In general, the results show that the flexural stiffness of all specimens decreases with increasing temperature. This behavior can be related to the decomposition of calcium hydroxide in cement paste and the release of chemical bonding water at higher temperatures (Memon et al., 2019). The most significant decrease in flexural stiffness is related to NC. In contrast, the Using Eq. (2), MOR was calculated for specimens at different temperatures. The results show that steel fibers (regardless of their type) have significantly increased the MOR compared to the control specimen a all temperatures. As shown in Figure 16, at ambient temperature, the highest MOR corresponds to CRSF-0.5% with a value of 5.99 MPa, and the lowest MOR corresponds to NC with a value of 3.05 MPa. RSFI-0.5% also significantly improved MOR with the presence of rubber and textiles and increased MOR at ambient temperature by 44.59% compared to NC. However, it still performs less than ISF-0.5% and CRSF- 0.5%. ISF-0.5% at ambient temperature has increased MOR by 51.47% compared to NC. hydroxide in cement paste and the release of
Fig. 14. Cracking of prism beams at 200 °C: a) NC; b) RSFI-0.5%; c) CRSF-0.5%; and d) ISF-0.5% According to the — load- deflection diagrams, the slope of the tangent to the curve is defined as flexural stiffness. In general, the results show that the flexural stiffness of all specimens decreases with increasing temperature. This behavior can be related to the decomposition of calcium hydroxide in cement paste and the release of chemical bonding water at higher temperatures (Memon et al., 2019). The most significant decrease in flexural stiffness is related to NC. In contrast, the Using Eq. (2), MOR was calculated for specimens at different temperatures. The results show that steel fibers (regardless of their type) have significantly increased the MOR compared to the control specimen a all temperatures. As shown in Figure 16, at ambient temperature, the highest MOR corresponds to CRSF-0.5% with a value of 5.99 MPa, and the lowest MOR corresponds to NC with a value of 3.05 MPa. RSFI-0.5% also significantly improved MOR with the presence of rubber and textiles and increased MOR at ambient temperature by 44.59% compared to NC. However, it still performs less than ISF-0.5% and CRSF- 0.5%. ISF-0.5% at ambient temperature has increased MOR by 51.47% compared to NC. hydroxide in cement paste and the release of
Fig. 15. Load-deflection curve of specimens at different temperatures: a) 20 °C; b) 200 °C; and c) 600 °C
Fig. 15. Load-deflection curve of specimens at different temperatures: a) 20 °C; b) 200 °C; and c) 600 °C
Fig. 16. MOR of specimens at different temperatures £0 YTS /U 6 The residual resistance factors Rs 49 and Rio,20 Show the average resistance level maintained after the first crack as a percentage of the resistance of the first crack. 100 values represent entirely plastic behavior (ASTM, 1997). Low performance is indicated by lower values (ASTM, 1997). The control specimen has a zero residual resistance factor (ASTM, 1997). Table 4 shows the values of the residual resistance factors Rsi9 and Ryo29. At ambient temperature, CRSF-0.5% showed plastic behavior. The ductility of the specimens was evaluated based on ASTM C1018 (1997). The toughness indices Is, I49, and Iz9 for fiber-reinforced specimens are shown in Figure 17. The control specimens fail suddenly, and their deflection is slight; Hence, their toughness indices values are ignored. The results show that increasing Table 3. Average MOR of specimens at different temperatures (MPa)
Fig. 16. MOR of specimens at different temperatures £0 YTS /U 6 The residual resistance factors Rs 49 and Rio,20 Show the average resistance level maintained after the first crack as a percentage of the resistance of the first crack. 100 values represent entirely plastic behavior (ASTM, 1997). Low performance is indicated by lower values (ASTM, 1997). The control specimen has a zero residual resistance factor (ASTM, 1997). Table 4 shows the values of the residual resistance factors Rsi9 and Ryo29. At ambient temperature, CRSF-0.5% showed plastic behavior. The ductility of the specimens was evaluated based on ASTM C1018 (1997). The toughness indices Is, I49, and Iz9 for fiber-reinforced specimens are shown in Figure 17. The control specimens fail suddenly, and their deflection is slight; Hence, their toughness indices values are ignored. The results show that increasing Table 3. Average MOR of specimens at different temperatures (MPa)
Table 4. Residual resistance factors at different temperatures 3.6. Microstructure of Concrete at Ambient and High Temperatures
Table 4. Residual resistance factors at different temperatures 3.6. Microstructure of Concrete at Ambient and High Temperatures
Fig. 18. SEM observations for specimens at 600 °C: a) NC; b) RSFI; C) ISF; and d) CRSF
Fig. 18. SEM observations for specimens at 600 °C: a) NC; b) RSFI; C) ISF; and d) CRSF
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