Influence of elevated temperature on the engineering properties of ultra-high-performance fiber-reinforced concrete

The chemical composition based on X-ray fluorescence (XRF) analysis and physical properties of Portland ASTM International Type-I cement and silica fume used in this study are shown in Table 1. The sand used was dense quart sand with a maximum particle size of 2 mm. The mixture proportions are presented in Table 2. All concrete mix designs had a cement content of 900 kg/m³ and 222 kg/m³ of micro silica. The sand content was 1000 kg/m³ in all mixes. All mixes had a w/b (cement and silica fume) ratio of 0.146. The workability of concrete was maintained using a superplasticizer at a dosage of 30 kg/m³. This work evaluated three types of fiber: steel, PVA, and PP fibers, as shown in Figure 1. Table 3 summarizes the properties of all fibers. The steel fibers used were hook-ended with a length of 30 mm and 0.5 mm in diameter. The tensile strength of the steel fibers was 1225 MPa, and the modulus of elasticity was 200 GPa. The PVA fibers were 30-mm straight fibers with a tensile strength of 900 MPa, and elasticity modulus of 23 GPa. The PP fibers were 12-mm straight fibers with a tensile strength of 550 MPa and modulus of elasticity of 4 GPa. Table 4 shows the percentage by volume of fibers used in each mix design.

Fig. 1.

Different concrete mixtures were evaluated in this study, as shown in Table 4. To examine the impact of incorporating various types of fibers, the hybrid UHPC mixes were classified into five groups. It was ensured that the overall volume fraction of fibers remained constant at 1% by volume across all mixes. Out of six mix designs, the first mix was a UHPC control mix with no fibers (M0-NF), and the remaining five mixes included fibers (i.e., UHPFRC). The second mix (M1-S1) is UHPFRC made with 1% by volume of steel fibers. The third mix (M2-S0.7-PVA0.3) is UHPFRC made with a hybrid fiber system of 0.7% by volume of steel fibers and 0.3% by volume of PVA fibers. The fourth mix (M3-S0.7-PVA0.2-PP0.1) was made of a hybrid system consisting of 0.7% by volume of steel fibers, 0.2% by volume of PVA fibers and 0.1% by volume of PP fibers. A hybrid fiber made of 0.7% by volume of steel fibers and 0.2% by volume of PVA fibers was used in the fifth mix and 0.5% by volume of steel fibers and 0.5% by volume of PVA fibers were used in the sixth mix. Foreach of the six mixes, twenty-four 100 mm × 200 mm cylinders and eight 150 mm × 150 mm × 600 mm prisms were cast, as shown in Table 5. The cylinders were used for compressive and split tensile strength testing, while the concrete prisms were used for flexural testing. For each mix design, six cylinders and three prisms were tested for each exposure temperature considered in this study (i.e., room temperature at 26°C, 100°C, 300°C, 400°C, and 500°C).

For all concrete cylinder samples, plastic molds were used, and the concrete was filled in a single layer. Concrete prisms were created by pouring concrete into steel molds. The concrete surface was finished with a steel trowel after casting. After 24 hours of humid curing at ambient temperature, the concrete prisms and cylinders were removed from the molds and then cured in water for 28 days.

At the end of the 28-day water curing, the specimens of concrete were air dried at ambient temperature for 28 days to ensure the concrete was dry before exposure to highly elevated temperatures. This was to allow the drying shrinkage to occur before the sudden increase in temperature. An electric oven with clear dimensions of 1000 mm × 1000 mm × 1000 mm was used. The concrete samples were exposed to the heated regime at a rate of 8°C/min. Heating curves for each heating regime (i.e., 100°C, 300°C, 400°C, and 500°C) are shown in Figure 2. This figure also includes the curve of heating as per ISO 834 [47]. The temperature of the oven was controlled using a Type-K thermocouple. Once the oven temperature reached the maximum temperature (i.e., 100°C, 300°C, 400°C, and 500°C), the temperature was sustained for three hours. After being heated, all types of concrete samples were cooled down to ambient temperature before testing.

Fig. 2.

The fresh concrete properties were measured using slump flow according to ASTM C143 [48], and the concrete unit weight was evaluated according to ASTM C138 [49]. The concrete hardened properties were evaluated using compressive and splitting tensile strength as well as flexural strength. The concrete compressive strength was evaluated as per ASTM C39 [50]. Before testing, the concrete cylindrical samples were capped with sulfur capping to ensure a leveled testing surface. During the compressive strength testing, axial strain was measured using a compressometer made of two linear variable differential transformers (LVDTs), as shown in Figure 3. The elastic modulus was evaluated as per ASTM C469 [51]. The splitting tensile strength was evaluated in accordance with ASTM C496 [52]. The flexure strength was measured using a three-point bending test based on ASTM C78 [53]. The flexural testing was deflection controlled with a 0.25 mm/min displacement rate until failure. The mid-span deflection was measured using (LVDTs), as shown in Figure 4. The load and deflection were recorded using a data acquisition system. The mass loss of the samples was determined based on the weight of specimens before and after exposure to elevated temperature using a precise weighing scale. Furthermore, microstructural analysis was also performed through SEM micrographs to evaluate the microstructure of the UHPGC specimens using the instrument (JOEL JSM-7600F Scanning Electron Microscope) available at King Saud University.

Fig. 3.

Fig. 4.

The fresh concrete properties, including the slump flow and unit weight, are presented in Table 6. All mixes showed good workability with no bleeding or fiber segregation. The slump flow of the control mix without fibers (i.e., M0-NF) was 280mm, which was greater than the slump of the UH-PFRC mixes that ranged from 250 mm to 257 mm. The lower slump in the UHPFRC mixes is due to the existence of fibers. The unit weight of all mixes ranged from 2285 kg/m³ to 2363 kg/m³.

Figures 5 and 6 present the concrete specimens after exposure to high temperatures. Concrete samples heated to 300°C, 400°C, and 500°C showed discoloration, whereas no discoloration was observed in specimens heated to 100°C. For concrete samples heated to 100°C, no spalling or cracking was observed. Concrete specimens made without fibers (i.e., M0-NF) and heated to 300°C showed thermal cracking at the surface, which aligns with the findings of other research in the literature [54, 55]. On the other hand, UHPFRC specimens showed no thermal cracking due to fibers that contributed to the bridging effect of cracks. At 400°C, concrete specimens made without steel fibers (i.e., M0-NF) and concrete specimens made with steel fibers alone (i.e., M1-S1) showed spalling at the end and the corner surfaces of the specimens. This spalling can be due to the dense microstructure of the concrete, which makes it difficult for the water vapor to escape, causing significant internal pressure at these locations. Previous research has indicated that concrete spalling can be attributed to the accumulation of internal pressure, thermal stresses, and thermal cracking [56]. The absence of PP and PVA fibers could also contribute as these synthetic fibers melt and evaporate at about 270^°C and 170°C, respectively, leaving venting tunnels to minimize the existing internal vapor pressure [4]. The concrete samples made with PP and PVA fibers showed significantly lower spalling and cracking compared to concrete specimens made without PVA and PP fibers. At 500°C, all concrete specimens showed significant spalling, as demonstrated in Figure 6. Therefore, no testing could be conducted on concrete after exposure to 500^°C. It was observed from the protruding steel fibers from the failing specimens that the steel fibers were well distributed.

Fig. 5.

Fig. 6.

The results of the residual compressive strength are given in Table 7. Figure 7 presents the concrete specimens after the compressive strength testing. Every single value in this table shows the average of three consecutive samples. This table also shows the percentage loss of compressive strength to the initial compressive strength before exposure to high temperatures. As mentioned previously, concrete samples exposed to high temperature at 500°C could not be tested due to the significant degradation during heating. Concrete specimens exposed to high temperatures had minimal residual compressive strength compared with samples that had not been subjected to high temperatures. It was noted that the residual compressive strength of the concrete mixes dropped as the temperature rose, regardless of fiber content or type.

Fig. 7.

For concrete samples exposed to ambient temperature, steel fibers increased the compressive strength by 16% in comparison with bare mix (i.e., mix M0-NF). As demonstrated in Table 6, concrete samples produced with a hybrid matrix comprising steel, PP, and PVA fibers increased their compressive strength by about 10%. In comparison to the control mix, the compressive strength improved by 7% to 12% when a combination of PVA and steel fibers was utilized (i.e., mix M0-NF).

For concrete samples exposed to high temperatures between 100°C and 400°C, the use of a combination of fiber systems effectively increased the residual compressive strength after being subjected to high temperatures with respect to concrete samples containing no fibers. This is due to fibers’ capacity to regulate thermal cracking and internal vapor pressure venting caused by the melting and evaporation of PVA and PP fibers at high temperatures [57, 58].

It was observed that concrete specimens exposed to 100°C temperatures showed a 1% to 4% increase in compressive capacity, which was identical to the same concrete specimens exposed to ambient temperature. This can be attributed to the additional hydration reactions as a result of heating to 100°C. The exposure to 100°C increases the ionic movement of ions and water vapor, which causes a portion of the unhydrated cement particles to react, causing increased hydration products and thus increased strength. In a study by Hager et al. [59], similar observations were reported; they stated that the reduction in strength may be caused by the cracking of the cement paste around quartz aggregates, which expand at higher temperature such as 400–500°C due to the transformation from β quartz to α quartz. Similar findings and explanations were observed in earlier studies [59], where this occurrence was attributed to the phenomenon of internal autoclaving. Moisture is transformed into water vapor as a result of heating. Under these conditions, chemical and physical changes may take place. The process of simultaneously exposing the material to high pressure and temperature is a well-known technology in the prefabrication of concrete. This procedure may activate changes in the microstructure of hydrates and often increases concrete strength. Another study by Peng at al. [60] showed that the degree of C-S-H decomposition depends on the heating temperature and duration. At higher temperatures, the C-S-H gel decomposed into crystalline phases such as calcium oxide (CaO) and silica (SiO₂), leading to a loss of strength in the material. As the temperature continued to increase, these crystalline phases underwent further reactions leading to the formation of calcium aluminosilicate and wollastonite. After exposure to high temperature at 300°C, however, the control mix (i.e., M0-NF) without fibers showed approximately 13% reduction in strength under compression, which was the greatest reduction observed compared to other concrete mixes made with fibers. Concrete specimens exposed to high temperature at 300°C showed a decrease of approximately 6% when steel fibers were used (i.e., M1-S1) compared to the same mix exposed to room temperature. Concrete samples made with a combination of steel and synthetic PVA fibers reveal a reduction in the compressive strength ranging from 3% to 9%. Compared to the same mix exposed to room temperature, no significant reduction in the compressive strength was found in concrete samples prepared using a combination of steel, PVA, and PP fibers when exposed to 300°C compared to the same mix exposed to room temperature.

At 400°C, the control mix without fibers showed an approximately 27% reduction in compressive strength. A similar reduction in the compressive strength was reported in concrete samples containing steel fibers only. The residual compressive strength was enhanced by using a hybrid fiber system instead of steel fibers. The compressive strength of concrete samples made with a combination of PVA, and steel fibers was reduced by 18% to 24%. The compressive strength of concrete samples composed of hybrid fibers (i.e., steel, PVA, and PP) was reduced by about 22%.

The compressive strength of all mixes showed that the exposure to high temperature at 400°C caused significant strength loss compared to exposure to a temperature of 300°C. Compared to concrete with no fibers, the use of fibers caused a significant increase in the residual compressive strength after exposure to 300°C compared to concrete without fibers. However, at 400°C the use of fibers was less effective in increasing the residual strength compared to what was observed at 300°C. In general, using a combination of steel, PVA, and PP fibers was more effective in increasing the concrete residual compressive strength after exposure to high temperatures at 300°C and 400°C. Schein-herrová et al. [61] performed another study and found that reactive powder concrete with 2% Cu-Zn-coated steel fibers had the highest residual compressive strength after being exposed to high temperatures up to 800°C. This was compared to samples without fibers and samples with uncoated steel fibers. The Cu-Zn layer on the steel fibers prevented the fibers from rusting and made it easier for the fibers to stick to the matrix. This made the material better at resisting loss of compressive strength at high temperatures.

3.3.1.

Stress-strain relationships

It’s crucial to assess the stress-strain response of concrete, especially when it’s subjected to high temperatures. The stress-strain plots of concrete samples exposed to 26°C, 300°C, and 400°C, respectively, are shown in Figures 8, 9, and 10. From the above-mentioned figures, it can be noted that concrete specimens exposed to ambient temperature showed greater elastic modulus compared to similar concrete mixes exposed to high temperatures at 300°C and 400°C. Concrete mixtures exposed to 400°C showed the lowest modulus of elasticity. On the other hand, concrete mixes exposed to 300°C and 400°C showed higher ductility and energy dissipation compared to similar concrete mixes exposed to room temperature. Furthermore, it was observed that for UHPFRC samples exposed to high temperature at 400°C, the stiffness reduction was more pronounced than the reduction in the compressive strength. This could be attributed to the degradation of the concrete matrix and the development of thermal cracks [62]. UH-PFRC made with steel fibers showed considerable energy absorption and better elastic modulus (i.e., area under the plots of stress-strain) compared to concrete samples made with no fibers or UHPFRC made with a hybrid system. This is expected, because steel fibers exhibit greater modulus of elasticity and tensile strength compared to PVA and PP fibers. In addition, steel fibers have better high-temperature resistance compared to PVA and PP fibers. On the basis of previous studies [63, 64], UHPC and UHPFRC have different stress-strain behaviors due to the presence of different types of reinforcements. UHPC has a high elastic modulus and exhibits strain-softening behavior after peak stress, while UHPFRC has a similar elastic modulus, but shows a plateau or gradual reduction in stress after peak stress due to the fiber bridging effect. The addition of fibers to UHPFRC also results in improved ductility and crack resistance, making it more suitable for structural applications that require high-performance concrete.

Fig. 8.

Fig. 9.

Fig. 10.

Figures 11 through 13 present the load versus mid-span deflection curves for concrete specimens exposed to ambient temperature, 300°C, and 400°C. Figure 14 shows the failure modes of the flexural specimens. All concrete mixes showed a linear load-displacement response before reaching the peak load. As expected, concrete specimens made without fibers (i.e., UHPC) showed a brittle failure, while concrete mixes made with fibers (i.e., UHPFRC) showed a ductile behavior and a greater load-carrying capacity and toughness. This can be attributed to the capability of fibers to bridge the micro and macro cracks.

Fig. 11.

Fig. 12.

Fig. 13.

Fig. 14.

Table 7 shows the flexural strength of all concrete mixes exposed to room temperature, 100°C, 300°C and 400°C. At room temperature, concrete specimens made without fibers (i.e., UHPC) showed a flexural strength of 10.3 MPa. In comparison, concrete specimens made with fibers showed a flexural strength that ranged from 16.9 MPa to 19.8 MPa, as shown in Table 7, which corresponds to an approximately 65% to 90% increase in the flexural strength. This can be attributed to the presence of fibers that increased the cracking resistance of the concrete. Concrete made with steel fibers showed the greatest increase in flexural strength (i.e., 90%) compared to the control mix without fibers. The concrete mix made of 0.7% steel fibers and 0.3% PVA fibers showed an approximately 85% increase in flexural strength compared to the control mix without fibers. The remaining three concrete mixes made of a combination of either steel, PVA, and PP fibers or steel and PVA fibers showed approximately the same flexural strength at room temperature.

When the concrete was subjected to 100°C, the flexural strength was slightly increased by approximately 1% to 4% in all mixes. This can be due to the increased compressive strength that was previously reported when the concrete was exposed to temperatures of 100°C. After exposure to higher temperatures at 300°C and 400°C, concrete specimens made without fibers showed approximately 15% and 27% reduction in flexural strength, respectively, compared to the same concrete exposed to room temperature. The decrease in the flexural strength in concrete mixes made with fibers was approximately 4% to 9% after the exposure to 300°C and 11% to 25% when exposed to 400°C. Concrete mix made with a combination of steel, PP, and PVA fibers showed the lowest reduction in flexural strength when exposed to high temperature at 300°C and 400°C.

Concrete specimens made with fibers (i.e., UH-PFRC) showed significantly greater ductility at failure and greater load-carrying capacity compared to concrete specimens without fibers (i.e., UHPC). The failure mode in concrete specimens without fibers was brittle, while a ductile failure with fewer and more distributed cracks was observed in concrete specimens made with fibers, as shown in Figure 14. Concrete specimens made with steel and PVA fibers showed extended softening behavior compared to concrete specimens made with steel fibers only. This indicates that using a hybrid system made with steel and PVA fibers was more efficient in increasing the cracking resistance of concrete compared to using steel fibers alone. In addition, the hybrid system made of steel and PVA fibers was more effective in controlling the cracking propagation compared to a hybrid system made with steel, PVA, and PP fibers. The results of this study are consistent with previous research on the effect of fibers on the flexural strength of concrete. For example, a study by Wu et al. [67] found that adding steel fibers to UHPC increased the flexural strength by up to 83%. Another study by Jiao et al. [70] found that the flexural strength of UHPC decreased by up to 27% after exposure to 400°C. Overall, the study’s findings suggest that adding fibers to UHPC can significantly increase its flexural strength and ductility, and using a hybrid system made with a combination of fibers can provide better cracking resistance and control of cracking propagation.

The results of the splitting tensile strength study are provided in Table 7. Concrete specimens exposed to room temperature showed a splitting tensile strength between 6.6 MPa and 12.3 MPa. Concrete specimens without fibers showed the lowest splitting tensile strength of 6.6 MPA. Concrete specimens made with fibers showed significantly greater splitting tensile strength between 11.3 MPa and 12.3 MPa, representing a 74% to 86% improvement. This is expected as the presence of fibers improved the cracking resistance of the concrete resulting in a higher splitting tensile strength. Concrete specimens without fibers revealed a brittle failure, while concrete made with fibers showed a ductile failure.

As observed previously with the compressive and flexural strength testing results, the exposure to an elevated temperature at 100°C caused an approximately 3% increase in the splitting tensile strength. This was attributed to the increase in the compressive strength due to the additional hydration reactions that occurred, causing increased strength. Furthermore, the elevated temperature exposure may also influence the pore structure and microstructure of the concrete. The heat can lead to the expansion of moisture within the concrete, creating internal pressure that further contributes to the densification of the material. This densification reduces the occurrence of voids and pore connectivity, which positively affects the strength of the concrete [59, 71]. When the concrete was exposed to high temperatures of 300°C and 400°C, the control mix made without fibers showed approximately 14% and 27% decrease in the splitting tensile strength compared to the same concrete mix exposed to room temperature. Alternatively, concrete mixes made with fibers showed lower reductions in the splitting tensile strength when subjected to temperatures at 300°C and 400°C. After exposure to 300°C, the reduction in the splitting tensile strength was 8% in the concrete mix made with steel fibers and 3% to 7% in concrete mixes made with steel and PVA fibers compared to those exposed only to room temperature. Concrete mix made with hybrid fibers consisting of steel, PVA, and PP fibers showed a 6% reduction when the concrete was exposed to 300°C. After exposure to 400°C, the reduction in the splitting tensile strength was 13% in the concrete mix made with steel fibers only, 13% to 22% in concrete mixes made of a combination of steel and PVA fibers, and 14% in concrete made with a hybrid of steel, PVA, and PP fibers compared to concrete exposed to room temperature. The experimental results of these findings match previous studies exploring the effects of fiber reinforcement and high temperatures on concrete properties. Doo et al. [72] and Doo and Shin [73] found that fibers enhance splitting tensile strength and crack resistance, consistent with current findings. Similarly, Chen et al. [45] showed that high temperatures decrease splitting tensile strength more in concrete without fiber reinforcement, in line with the experimental results.

To evaluate the effect of fiber content and type on the mechanical characteristics of UHPC, the compressive, flexural, and splitting tensile strength results were normalized (i.e., the results at high temperature were divided by the results obtained at room temperature). The impact of high temperatures exposure on the normalized compressive, flexure, and splitting tensile strengths are presented in Figures 15 to 17, respectively.

Fig. 15.

In Figure 15, it can be observed that the addition of fibers caused a significant enhancement in the residual compressive strength after the exposure to high temperatures of 300°C and 400°C. It can also be observed from this figure that steel fibers were the most efficient in maintaining the compressive strength after the exposure to 300°C compared to the other hybrid fiber system. At 400°C, however, the hybrid fiber system, which included steel, PP, and PVA fibers, proved to be the most effective in maintaining the compressive strength compared to concrete specimens with steel fibers alone or a combination of steel and PVA fibers.

In Figure 16, it can be observed that hybrid systems consisting of steel and PVA fibers and steel, PVA and PP fibers were more effective in maintaining flexural strength after exposure to 300°C. Hybrid fiber consisting of steel, PP, and PVA fibers (i.e., mix M3-S0.7–PVA0.2–PP0.1) and mix M4-S0.7–PVA0.2 were the most efficient in maintaining the flexural strength after exposure to 400°C followed by steel fibers alone (M1-S1).

Fig. 16.

Fig. 17.

For the splitting tensile strength study, a combination of 0.7% steel fibers and 0.3% PVA fibers was the most effective in maintaining the splitting tensile strength after exposure to 300°C. All the fiber systems evaluated in this study showed approximately similar effectiveness in retaining the splitting tensile strength after exposure to 400°C.

In Figures 18 to 20, it can be observed that using steel fibers alone was the most effective in maintaining the mechanical characterization of UHPC exposed to high temperatures at 300°C and 400°C. Hybrid fibers made of steel, PP, and PVA fibers were the second most effective system, followed by a combination made of steel and PVA. Because of the melting and evaporation of PP fibers at high temperatures, the inclusion of PP fibers improves the mechanical characteristics of UHPC by providing venting channels for lowering the vapor pressure inside the samples. The test results of UHPC made with hybrid fibers under elevated temperatures in this study are consistent with findings in the literature. The addition of fibers, particularly steel fibers, significantly enhanced the residual compressive strength after exposure to high temperatures of 300°C and 400°C, which is in line with the results reported in previous studies [74–76].

Fig. 18.

A mass reduction is expected when concrete is subjected to elevated temperatures due to free and chemical water loss. The initial mass was measured before placing the test specimen in the furnace. The final mass of the samples was considered to be the mass after reaching target temperature. To calculate the percentage of mass loss of the UHPC and UHPFRC specimens due to elevated temperatures, the weights of the specimens were recorded before and after exposure to the elevated temperature. Because of rapid drying, the weight loss from UHPC increases as the severe temperature increases. The variation in mass loss and temperature is depicted in Figure 18. All samples were heated to the target temperature with an exposure duration of 3 hours. The findings indicated that the mass loss increases with increasing temperature and the extreme increase at 400°C occurs within a 3-hour exposure time. It can be noted that the concrete specimens containing a higher content of steel fiber show less mass loss at higher temperature exposure than specimens with a higher content of synthetic fibers, as a result of the melting of polypropylene and polyvinyl alcohol fiber. Several studies have investigated the mass loss of UHPC made with different types of fibers when exposed to elevated temperatures. A study by Li et al. [74] investigated the effect of different types of fibers, including steel fibers, polypropylene fibers, and hybrid fibers, on the mass loss of UHPC exposed to high temperatures of up to 1000°C. The study found that UHPC with steel fibers exhibited the highest mass loss, followed by UHPC with polypropylene fibers and hybrid fibers. The addition of hybrid fibers reduced the mass loss of UHPC compared to UHPC with only steel or polypropylene fibers. The study also found that the addition of hybrid fibers improved the residual compressive strength and ductility of UHPC after exposure to high temperatures. A study by Chen et al. [45] investigated the effect of hybrid fiber on the mass loss of UHPC exposed to high temperatures up to 1200°C. The study found that UHPC with hybrid fibers exhibited lower mass loss than UHPC without fibers or with only steel fibers. The study also found that the addition of hybrid fibers improved the thermal stability of UHPC and reduced the rate of strength loss at elevated temperatures. Another finding by Yang et al. [76] investigated the effect of hybrid fiber on the mass loss of UHPC exposed to high temperatures up to 1000°C. The study found that UHPC with hybrid fibers exhibited lower mass loss than UHPC without fibers or with only steel fibers. The study also found that the addition of hybrid fibers improved the spalling resistance of UHPC and reduced the severity of spalling.

In addition to the mechanical properties evaluated in this study, microstructure analysis using scanning electron microscopy on fracture surfaces was used to evaluate the microstructure of each mix before and after exposure to high temperature at 400°C. One sample was extracted from each of the six mixes before and after exposure to 400°C. Once the sample was removed from the cylinder, it was stored in a sealed container containing soda-lime and silica gel to prevent any carbonation or further hydration. At the testing time, a fresh-fracture surface was exposed from each sample before SEM testing. Figure 19 presents secondary electron images of concrete mixes before exposure to high temperature. Figures 20 and 21 present secondary electron images of concrete mixes after exposure to high temperature at 400°C.

Fig. 19.

Fig. 20.

Fig. 21.

In Figure 19, it can be observed that the microstructure is dense as a result of the lower water-to-cement ratio and the use of micro silica, causing additional hydration products that densify the microstructure. The microstructural analysis also showed well distributed fine aggregates with no entrapped air voids. The exposure to high temperature at 400°C caused degradation at the interface between the hydrated cement matrix and the steel fibers. The decomposition of the cement matrix caused this degradation at the interface and the yielding of steel fibers. The decomposition of the cement matrix includes the complete decomposition of ettringite and the loss of chemically bonded water. Figure 20b shows a gap between the hydrated cement matrix and the steel fibers caused by this degradation. Concrete made with PVA fibers showed empty spaces occupied by the PVA fibers before melting and evaporating, as shown in Figure 21. The shape confirmed this and the size of the empty voids, because the PVA fibers had a circular cross-section of 0.06 mm. The network of voids that were previously occupied by PVA fibers aided in releasing the vapor pressure build-up due to the exposure to 400°C temperatures [77]. Studies by Zhang et al. [78] and Li et al. [74] revealed that thermal decomposition of ettringite and C-S-H gel breakdown cause significant microstructural changes in UHPC. UHPC with PVA or polypropylene fibers showed more damage, while UHPC with steel fibers caused fiber/matrix interface debonding and cracking. The fiber type and exposure temperature determine fiber effects.