Effects of Steel Fiber Percentage and Aspect Ratios on Fresh and Harden Properties of Ultra-High Performance Fiber Reinforced Concrete

: Steel ﬁbers and their aspect ratios are important parameters that have signiﬁcant inﬂuence on the mechanical properties of ultrahigh-performance ﬁber-reinforced concrete (UHPFRC). Steel ﬁber dosage also signiﬁcantly contributes to the initial manufacturing cost of UHPFRC. This study presents a comprehensive literature review of the effects of steel ﬁber percentages and aspect ratios on the setting time, workability, and mechanical properties of UHPFRC. It was evident that (1) an increase in steel ﬁber dosage and aspect ratio negatively impacted workability, owing to the interlocking between ﬁbers; (2) compressive strength was positively inﬂuenced by the steel ﬁber dosage and aspect ratio; and (3) a faster loading rate signiﬁcantly improved the mechanical properties. There were also some shortcomings in the measurement method for setting time. Lastly, this research highlights current issues for future research. The ﬁndings of the study are useful for practicing engineers to understand the distinctive characteristics of UHPFRC.


Introduction
Concrete is a very popular construction material (its annual consumption is about 25 billion tons) because of its low cost, availability, long durability, easily given shape and size, and ability to be sustained in extreme weather conditions [1]. On the other hand, concrete has a number of problems that cause considerable concern in the construction industry. Concrete is a brittle material with low tensile strength (approximately 1/10 of its compressive strength) [2]. In addition to its low tensile strength, concrete fracture toughness is at least 100 times less than that of steel. Moreover, concrete elements have a low capacity for resisting cracks under dynamic loads [3]. Consequently, these factors allow concrete structures to easily grow cracks during service life, create access for deleterious agents, and ultimately lead to steel bar corrosion [3][4][5][6][7][8]. Conventional concrete with low strength and a brittle nature creates concerning issues such as the durability and large section sizes of RC and prestressed structures [9][10][11][12]. To overcome the limitations of conventional concrete, a new scope of research was created to develop a cementitious composite having ultrahigh compressive strength, low porosity, and high ductility. Substantial research has been carried out to develop this technology, and it is known as ultrahigh-performance fiber-reinforced concrete (UHPFRC).

Effect of Steel Fiber Percentages and Aspect Ratio on Setting Times
The setting time of concrete is crucial for construction work, as it is associated with the removal of formwork, the construction schedule, etc. The outer surface of an UHPFRC specimen (exposed to weather) has a much faster evaporation rate of water than that of the inner part. This rapid evaporation of surface moisture misrepresents measurements of UHPFRC setting time, following the ASTM C403 [41] penetration resistance testing method. Yoo et al. [42] applied various methods and materials and suggested that using paraffin oil might resist water evaporation during the penetration resistance test. Consequently, accurate initial and final setting times for UHPFRC were found to be 10.8 and 12.3 h, respectively, as shown in Figure 1. Zhang et al. [43] demonstrated that for an identical mix design, initial setting times were delayed from 670 to 1165 min, and final setting times were delayed from 1010 to 1915 min when the volumetric steel fiber ratio increased from 1% to 3%, as shown in Table 1. Results indicated that the increased steel fiber amount had a negative effect on setting time. Very limited research has been performed to understand the effect of the fiber percentage on the setting time.
12.3 h, respectively, as shown in Figure 1. Zhang et al. [43] demonst tical mix design, initial setting times were delayed from 670 to 1165 times were delayed from 1010 to 1915 min when the volumetric stee from 1% to 3%, as shown in Table 1. Results indicated that the increa had a negative effect on setting time. Very limited research has been stand the effect of the fiber percentage on the setting time.

Effects of Steel Fiber Percentages and Aspect Ratios on Slump or Wor
Owing to its low w/b ratio, UHPFRC demonstrates low flow difficulty during casting. In addition, an increase in steel fibers affe UHPFRC. Svec and Pade [44] observed that the flow of UHPFRC si with the increase in steel fiber percentage, as depicted in Figure 2. was encountered when the steel fiber percentage reached 8%. Simil searchers also reported that an increased fiber percentage negatively [45][46][47]. Another study by Yu et al. [48] revealed that relative slu creased with an increase in steel fiber percentage (0.5% to 2.5%; Figu for the reduced slump is summarized as follows: • An increased steel fiber amount creates interlocking among mately leads to reduced workability [48]; • If the length of fiber is more than the aggregate size, the sur becomes larger and creates a cohesive force between fibers [48

Effects of Steel Fiber Percentages and Aspect Ratios on Slump or Workability
Owing to its low w/b ratio, UHPFRC demonstrates low flowability, which causes difficulty during casting. In addition, an increase in steel fibers affects the workability of UHPFRC. Svec and Pade [44] observed that the flow of UHPFRC significantly decreased with the increase in steel fiber percentage, as depicted in Figure 2. An interlocking issue was encountered when the steel fiber percentage reached 8%. Similarly, several other researchers also reported that an increased fiber percentage negatively impacted slump flow [45][46][47]. Another study by Yu et al. [48] revealed that relative slump flow linearly decreased with an increase in steel fiber percentage (0.5% to 2.5%; Figure 3). The main reason for the reduced slump is summarized as follows: • An increased steel fiber amount creates interlocking among steel fibers and ultimately leads to reduced workability [48]; • If the length of fiber is more than the aggregate size, the surface area of the fiber becomes larger and creates a cohesive force between fibers [48].

Effects of Steel Fiber Percentages and Aspect Ratios on Slump or Workability
Owing to its low w/b ratio, UHPFRC demonstrates low flowability, which causes difficulty during casting. In addition, an increase in steel fibers affects the workability of UHPFRC. Svec and Pade [44] observed that the flow of UHPFRC significantly decreased with the increase in steel fiber percentage, as depicted in Figure 2. An interlocking issue was encountered when the steel fiber percentage reached 8%. Similarly, several other researchers also reported that an increased fiber percentage negatively impacted slump flow [45][46][47]. Another study by Yu et al. [48] revealed that relative slump flow linearly decreased with an increase in steel fiber percentage (0.5% to 2.5%; Figure 3). The main reason for the reduced slump is summarized as follows: • An increased steel fiber amount creates interlocking among steel fibers and ultimately leads to reduced workability [48]; • If the length of fiber is more than the aggregate size, the surface area of the fiber becomes larger and creates a cohesive force between fibers [48].    Hoang and Fehling [49] investigated the effects of fiber percenta 3%) and aspect ratio (13/0.175, 20/0.25, and 9/0.15) on t500 (the time req to spread 500 mm dia) and slump flow (Figures 4 and 5). Fiber percenta (L/D) increments reduced slump flow and increased t500. Several oth reported that an increased fiber aspect ratio negatively impacted slump et al. [20] reported that UHPFRC mixtures containing steel fibers with (6 mm long and 0.15 mm diameter) were more workable, and steel fi up to 10%. Rossi [51] found that 12 mm long and 0.15 mm diameter fib 3% of the total concrete volume could be used without affecting mixtu ternatively, Wu et al. [52] and Yu et al. [53] reported that considering a of steel fiber, a higher aspect ratio (both researcher teams used 13/0.2 creased flowability compared to that of steel fibers with low aspec 6/0.16).  Hoang and Fehling [49] investigated the effects of fiber percentage (0%, 1.5%, and 3%) and aspect ratio (13/0.175, 20/0.25, and 9/0.15) on t 500 (the time required for UHPFRC to spread 500 mm dia) and slump flow (Figures 4 and 5). Fiber percentage and aspect ratio (L/D) increments reduced slump flow and increased t 500 . Several other researchers also reported that an increased fiber aspect ratio negatively impacted slump flow [47,50]. Wille et al. [20] reported that UHPFRC mixtures containing steel fibers with a low aspect ratio (6 mm long and 0.15 mm diameter) were more workable, and steel fibers could be used up to 10%. Rossi [51] found that 12 mm long and 0.15 mm diameter fibers that used up to 3% of the total concrete volume could be used without affecting mixture workability. Alternatively, Wu et al. [52] and Yu et al. [53] reported that considering an identical amount of steel fiber, a higher aspect ratio (both researcher teams used 13/0.2) demonstrated increased flowability compared to that of steel fibers with low aspect ratios (6/0.2 and 6/0.16). Hoang and Fehling [49] investigated the effects of fib 3%) and aspect ratio (13/0.175, 20/0.25, and 9/0.15) on t500 (th to spread 500 mm dia) and slump flow (Figures 4 and 5). Fib (L/D) increments reduced slump flow and increased t500. S reported that an increased fiber aspect ratio negatively impa et al. [20] reported that UHPFRC mixtures containing steel (6 mm long and 0.15 mm diameter) were more workable, a up to 10%. Rossi [51] found that 12 mm long and 0.15 mm d 3% of the total concrete volume could be used without affec ternatively, Wu et al. [52] and Yu et al. [53] reported that con of steel fiber, a higher aspect ratio (both researcher teams u creased flowability compared to that of steel fibers with 6/0.16).

Effects of Steel Fiber Percentages and Aspect Ratios on Compressive Behavior
Experimental studies regarding the effects of steel fibers on the compressive strength of UHPFRC have not been in agreement. Some studies reported that the compressive strength of UHPFRC was not influenced or was negatively influenced by an increase in fiber percentage [45,54,55]. The researchers highlighted that due to the lack of a homogeneous distribution of steel fiber, a large amount of steel fibers encountered fiber bundling that eventually led to weak spots in the mixture. This decreased the efficiency of the concrete matrix and ultimately resulted in reduced compressive strength [45,55,56]. For instance, Hoang and Fehling [49] tested UHPFRC samples having various steel fiber percentages and aspect ratios and expressed that the addition of fiber percentages to UHPFRC negatively impacted compressive strength, as depicted in Figure 6. This research underlined that an increased fiber percentage might lead to a reduction in the compressive strength of UHPFRC of up to 7% in contrast to the no-fiber case, owing to improper fiber distribution in the concrete matrix.

Effects of Steel Fiber Percentages and Aspect Ratios on Comp
Experimental studies regarding the effects of steel fiber of UHPFRC have not been in agreement. Some studies r strength of UHPFRC was not influenced or was negatively fiber percentage [45,54,55]. The researchers highlighted tha neous distribution of steel fiber, a large amount of steel fibe that eventually led to weak spots in the mixture. This decre crete matrix and ultimately resulted in reduced compressi stance, Hoang and Fehling [49] tested UHPFRC samples h centages and aspect ratios and expressed that the addition FRC negatively impacted compressive strength, as depicted derlined that an increased fiber percentage might lead to a strength of UHPFRC of up to 7% in contrast to the no-fiber distribution in the concrete matrix.  On the other hand, several researchers reported that the compressive strength of UHPFRC substantially increased (see Figure 7) with an increase in steel fibers [47,48,[57][58][59]. Steel fibers delay concrete crack propagation and formation and ultimately result in higher strength properties [60,61]. Kazemi and Lubell [62] found a positive effect from the incorporation of steel fibers when following the ASTM C109/C109M-07 [63] and ASTM C39/C39M-10 [64] standards. In addition to compressive stress improvement, researchers also reported that an increased fiber percentage improved peak strain, which indicated ductile behavior and altered the failure pattern [65]. On the other hand, several researchers reported that the co UHPFRC substantially increased (see Figure 7) with an increase in 59]. Steel fibers delay concrete crack propagation and formation a higher strength properties [60,61]. Kazemi and Lubell [62] found a p incorporation of steel fibers when following the ASTM C109/C10 C39/C39M-10 [64] standards. In addition to compressive stress imp also reported that an increased fiber percentage improved peak s ductile behavior and altered the failure pattern [65].  Previous studies have revealed that the aspect ratio of steel fibers notably influenced the compressive strength of UHPFRC. With a constant steel fiber amount, a higher steel fiber aspect ratio corresponded to better compressive strength [47,50,52,53]. Steel fibers with a high aspect ratio resisted large cracks and thus improved compressive strength; in contrast, steel fibers with a low aspect ratio could only control the opening and propagation of microcracks. Su et al. [66] used two types of steel fibers for different aspect ratios (MF06 = 50, MF15 = 125, TF03 = 100, TF05 = 60). The experiment showed that with a constant steel percentage (2.5%), steel fibers with a lower aspect ratio had lower compressive strength compared to that of steel fibers with a higher aspect ratio. The behavior was the same for both twisted and straight steel fibers, as illustrated in Figure 8a,b.
Appl. Mech. 2021, 2, FOR PEER REVIEW 6 with a high aspect ratio resisted large cracks and thus improved compressive strength; in contrast, steel fibers with a low aspect ratio could only control the opening and propagation of microcracks. Su et al. [66] used two types of steel fibers for different aspect ratios (MF06 = 50, MF15 = 125, TF03 = 100, TF05 = 60). The experiment showed that with a constant steel percentage (2.5%), steel fibers with a lower aspect ratio had lower compressive strength compared to that of steel fibers with a higher aspect ratio. The behavior was the same for both twisted and straight steel fibers, as illustrated in Figure 8a,b.
(a) (b) Several researchers have examined the effect of steel fiber percentage on the dynamic compressive behavior of UHPFRC under different strain rates [57,67]. Rong et al. [57] and Lai and Sun [67] examined the dynamic compressive behavior of a steel fiber dosage of up to 4%. Test results revealed that (1) the stress-strain relationship of UHPFRC was linear until ultimate strength was reached, as shown in Figure 9a; (2) an increased steel fiber amount improved the peak strain, maximal compressive strength, and elastic modulus of the concrete specimens, as shown in Figure 9b. Those experiments also found that a higher strain rate was associated with improved compressive strength.
Several researchers have examined the effect of steel fiber percentage on the dynamic compressive behavior of UHPFRC under different strain rates [57,67]. Rong et al. [57] and Lai and Sun [67] examined the dynamic compressive behavior of a steel fiber dosage of up to 4%. Test results revealed that (1) the stress-strain relationship of UHPFRC was linear until ultimate strength was reached, as shown in Figure 9a; (2) an increased steel fiber amount improved the peak strain, maximal compressive strength, and elastic modulus of the concrete specimens, as shown in Figure 9b. Those experiments also found that a higher strain rate was associated with improved compressive strength. Different strain rates also influence the compressive strength of UHPFRC with different steel fiber aspect ratios. For instance, Su et al. [66] examined the impact of the aspect ratio of steel fibers for different strain rates (60 and 80 s -1) with a constant steel fiber percentage. Samples with a higher aspect ratio exhibited improved compressive strength; in contrast, samples having steel fibers with a lower aspect ratio exhibited notably reduced compressive strength, as shown in Figure 10. Different strain rates also influence the compressive strength of UHPFRC with different steel fiber aspect ratios. For instance, Su et al. [66] examined the impact of the aspect ratio of steel fibers for different strain rates (60 and 80 s −1 ) with a constant steel fiber percentage. Samples with a higher aspect ratio exhibited improved compressive strength; in contrast, samples having steel fibers with a lower aspect ratio exhibited notably reduced compressive strength, as shown in Figure 10.

Effects of Steel Fiber Percentages and Aspect Ratios on Flexural Strength
Several researchers also examined the effect of fiber percentage on the flexural strength of UHPFRC and found that it significantly improved flexural UHPFRC strength [47,58,59,68]. An increased fiber dosage provides a larger bonding area between the concrete matrix and fibers and results in the effective control of crack propagation [47,69]. Yu et al. [48] established a relation between flexural strength and fiber percentage, as shown in Figure 11. The relation between flexural strength and steel fiber percentage was parabolic. Figure 11 shows that there was sharp flexural strength improvement for specimens with more than 1.5% steel fiber. Kazemi and Lubell [62] tested the flexural strength of UHPFRC according to ASTM C1609/C1609M-10 [70] and ASTM C1018-97 [71] with 2% to 5% of steel fiber. Flexural strength was improved by up to 107% when fiber percentage increased from 2% to 5%. However, Yoo et al. [45,69], Park [68], and Wu [72] observed no significant change in the first cracking load for specimens with increased fiber percentage. This happened because the first cracking stress was determined on the basis of the matrix strength of UH-PFRC. At that stage, steel fibers had no influence because there was no fiber bridging effect, as Figures 12a and 13a show. After cracking, the flexural strength capacity increased in a pseudolinear way with the increase in fiber percentage, as shown in Figures 12b and 13b. This behavior can be attributed to increased fiber bridging capacity.
(b) Figure 9. Stress-strain response at different strain rates: (a) fiber percentage (0%, 3%, 4%); (b) fiber percentage (0%, 3%, 4%) [57,67]. Different strain rates also influence the compressive strength of UHPFRC with different steel fiber aspect ratios. For instance, Su et al. [66] examined the impact of the aspect ratio of steel fibers for different strain rates (60 and 80 s -1) with a constant steel fiber percentage. Samples with a higher aspect ratio exhibited improved compressive strength; in contrast, samples having steel fibers with a lower aspect ratio exhibited notably reduced compressive strength, as shown in Figure 10.

Effects of Steel Fiber Percentages and Aspect Ratios on Flexural Strength
Several researchers also examined the effect of fiber percentage on the flexural strength of UHPFRC and found that it significantly improved flexural UHPFRC strength [47,58,59,68]. An increased fiber dosage provides a larger bonding area between the concrete matrix and fibers and results in the effective control of crack propagation [47,69]. Yu et al. [48] established a relation between flexural strength and fiber percentage, as shown in Figure 11. The relation between flexural strength and steel fiber percentage was parabolic. Figure 11 shows that there was sharp flexural strength improvement for specimens with more than 1.5% steel fiber. Kazemi and Lubell [62] tested the flexural strength of UHPFRC according to ASTM C1609/C1609M-10 [70] and ASTM C1018-97 [71] with 2% to 5% of steel fiber. Flexural strength was improved by up to 107% when fiber percentage increased from 2% to 5%. However, Yoo et al. [45,69], Park [68], and Wu [72] observed no pl. Mech. 2021, 2, FOR PEER REVIEW significant change in the first cracking load for specimens with increased fibe This happened because the first cracking stress was determined on the basis strength of UHPFRC. At that stage, steel fibers had no influence because fiber bridging effect, as Figures 12a and 13a show. After cracking, the flex capacity increased in a pseudolinear way with the increase in fiber percenta in Figures 12b and 13b. This behavior can be attributed to increased fiber bri ity. Figure 11. Relation between fiber dosage and flexural strength [47].  In addition to examining the influence of steel fiber dosage on the flexural strength of UHPFRC, researchers have also investigated the effects of the aspect ratios of steel fibers on the flexural strength of UHPFRC. Steel fibers with a high aspect ratio demonstrated much improved flexural strength in contrast to that of smaller-aspect ratio steel fibers [38,53,68]. This behavior can be attributed to the larger bonding area in the concrete matrix for longer steel fibers, which leads to increased fiber pullout or slip capacity [39,73]. For instance, Wu et al. [52] measured flexural strength according to GB/T 17671-1999 [74], considering 2% steel fibers with different aspect ratios of 6/0.20 and 13/0.2. Steel fibers with a higher aspect ratio had much better flexural strength than that of steel fibers with a low aspect ratio. In addition, several research groups found that the fiber aspect ratio had negligible impact before the first cracking, as shown in Figure 13a  Appl. Mech. 2021, 2, FOR PEER REVIEW 9 [35,45,69,75,76]. However, over a certain aspect ratio, the impact of a higher aspect ratio was negative in the post-cracking load condition, as shown in Figure 13b, where specimens SS, SM, SL, HL, and TL had aspect ratios of 65, 97.5, 100, 80, and 100, respectively. A group of researchers examined the impact of fiber percentage through the weightdrop test and found that the maximal load-bearing capacity increased with an increase in fiber percentage [77,78]. Farnam et al. [78] examined the effects of different fiber percentages through impact load and noted that two samples containing 2% steel fiber content broke after 15 strikes, whereas specimens with 4% steel fibers broke after 30 strikes. On the other hand, Ngo et al. [79] examined the impact of different steel fiber percentages with varying strain rates (6.94 and 13.83 m/s loading rate) and observed that an increase in fiber percentage resulted in increased average peak load (Table 2).
Several researchers also investigated the effect of steel fiber aspect ratio and reported that a larger steel fiber aspect ratio resulted in increased flexural capacity and number of strikes [53,79]. For example, Yu et al. [53] examined dynamic flexural strength using the Charpy impact test and saw that steel fibers with an aspect ratio of 13/0.2 absorbed more impact energy than steel fibers with a 6/0.16 aspect ratio, although steel fibers were constant. Ngo et al. [79] tested flexural strength using different strain rates (6.94 and 13.83 m/s) and noted that higher-aspect-ratio steel fibers (19/0.2) resulted in improved load resistance capacity in contrast to steel fibers with a smaller aspect ratio (13/0.2), as shown in Table 2.  In addition to examining the influence of steel fiber dosage on the flexural strength of UHPFRC, researchers have also investigated the effects of the aspect ratios of steel fibers on the flexural strength of UHPFRC. Steel fibers with a high aspect ratio demonstrated much improved flexural strength in contrast to that of smaller-aspect ratio steel fibers [38,53,68]. This behavior can be attributed to the larger bonding area in the concrete matrix for longer steel fibers, which leads to increased fiber pullout or slip capacity [39,73]. For instance, Wu et al. [52] measured flexural strength according to GB/T 17671-1999 [74], considering 2% steel fibers with different aspect ratios of 6/0.20 and 13/0.2. Steel fibers with a higher aspect ratio had much better flexural strength than that of steel fibers with a low aspect ratio. In addition, several research groups found that the fiber aspect ratio had negligible impact before the first cracking, as shown in Figure 13a. However, it had a remarkable effect in the post-cracking condition, as illustrated in Figure 13b [35,45,69,75,76]. However, over a certain aspect ratio, the impact of a higher aspect ratio was negative in the post-cracking load condition, as shown in Figure 13b, where specimens SS, SM, SL, HL, and TL had aspect ratios of 65, 97.5, 100, 80, and 100, respectively.
A group of researchers examined the impact of fiber percentage through the weightdrop test and found that the maximal load-bearing capacity increased with an increase in fiber percentage [77,78]. Farnam et al. [78] examined the effects of different fiber percentages through impact load and noted that two samples containing 2% steel fiber content broke after 15 strikes, whereas specimens with 4% steel fibers broke after 30 strikes. On the other hand, Ngo et al. [79] examined the impact of different steel fiber percentages with varying strain rates (6.94 and 13.83 m/s loading rate) and observed that an increase in fiber percentage resulted in increased average peak load ( Table 2). Several researchers also investigated the effect of steel fiber aspect ratio and reported that a larger steel fiber aspect ratio resulted in increased flexural capacity and number of strikes [53,79]. For example, Yu et al. [53] examined dynamic flexural strength using the Charpy impact test and saw that steel fibers with an aspect ratio of 13/0.2 absorbed more impact energy than steel fibers with a 6/0.16 aspect ratio, although steel fibers were constant. Ngo et al. [79] tested flexural strength using different strain rates (6.94 and 13.83 m/s) and noted that higher-aspect-ratio steel fibers (19/0.2) resulted in improved load resistance capacity in contrast to steel fibers with a smaller aspect ratio (13/0.2), as shown in Table 2.

Effects of Steel Fiber Percentages and Aspect Ratios on Split Tensile Strength
Several groups of researchers have observed that an increased dosage of steel fiber has a positive effect on the split tensile strength of UHPFRC [47,58,59,66,[80][81][82]. Su et al. [66] reported that the split tensile strength of a specimen significantly increased when the steel fiber amount increased from 1% to 2.5%, as illustrated in Figure 14a. An experimental program carried out by Shehab El-Din et al. [81] showed that the split tensile strength increased by 66% when the fiber dosage was increased from 0 to 3%.
Appl. Mech. 2021, 2, FOR PEER REVIEW 10 [66] reported that the split tensile strength of a specimen significantly increased when the steel fiber amount increased from 1% to 2.5%, as illustrated in Figure 14a. An experimental program carried out by Shehab El-Din et al. [81] showed that the split tensile strength increased by 66% when the fiber dosage was increased from 0 to 3%. Several researchers have examined the impact of the aspect ratio of steel fibers on split tensile strength [66,81,82]. The experimental study carried out by Su et al. [66] revealed that specimen MF 15 (aspect ratio 125) demonstrated higher split tensile strength than that of specimen MF06 (aspect ratio 50), as shown in Figure 14b. Su et al. [66] also observed that larger-aspect ratio steel fibers (MF15 and TF03) had a narrow crack and better resistance capabilities than those of smaller-aspect ratio steel fibers (MF06 and TF05), as shown in Figure 15. Shehab El-Din et al. [81] noted that steel fibers with a high aspect ratio had a significant effect when the specimens were casted with a smaller steel fiber amount (1%), as shown in Table 3. However, it followed a downward trend with an increased steel fiber amount, as shown in Table 3.  Several researchers have examined the impact of the aspect ratio of steel fibers on split tensile strength [66,81,82]. The experimental study carried out by Su et al. [66] revealed that specimen MF 15 (aspect ratio 125) demonstrated higher split tensile strength than that of specimen MF06 (aspect ratio 50), as shown in Figure 14b. Su et al. [66] also observed that larger-aspect ratio steel fibers (MF15 and TF03) had a narrow crack and better resistance capabilities than those of smaller-aspect ratio steel fibers (MF06 and TF05), as shown in Figure 15. Shehab El-Din et al. [81] noted that steel fibers with a high aspect ratio had a significant effect when the specimens were casted with a smaller steel fiber amount (1%), as shown in Table 3. However, it followed a downward trend with an increased steel fiber amount, as shown in Table 3.

Conclusions and Future Research Needs
This work conducted an extensive literature review on the distinctive characteristics of ultrahigh-performance fiber-reinforced concrete (UHPFRC). According to the state-ofthe-art review and discussions, the following summary can be drawn: 1. Steel fiber amount and aspect ratio significantly influence the workability of UHP-FRC. Higher steel fiber percentages and aspect ratios cause interlocking among fibers and ultimately lead to reduced slump flow; 2. The compressive strength of UHPFRC is gradually increased with increased steel fiber dosage for the capability of delaying crack formation and propagation. Nevertheless, improper fiber distribution in a concrete mixture may lead to the decreased compressive strength of UHPFRC; 3. Steel fibers with a high aspect ratio resist crack opening and improve compressive strength. In contrast, steel fibers with a low aspect ratio can only control microcrack opening and propagation; 4. The effects of aspect ratios were examined for flexural and split tensile strength.
Higher-aspect ratio steel fibers could improve flexural and split tensile strength with a constant fiber percentage, owing to the large bonding surface between fibers and the concrete matrix; 5. A faster strain rate significantly improved the mechanical properties of UHPFRC.
In addition to the aforementioned conclusions, the following aspects need to be addressed in future research: 1. Extensive research should be carried out to understand the effect of fiber percentage or aspect ratio on the setting time of UHPFRC; 2. Owing to the low water-to-cement ratio, the outer surface of UHPFRC has a much faster evaporation rate than that of the inner surface. Therefore, an accurate measure-

Conclusions and Future Research Needs
This work conducted an extensive literature review on the distinctive characteristics of ultrahigh-performance fiber-reinforced concrete (UHPFRC). According to the state-ofthe-art review and discussions, the following summary can be drawn:

1.
Steel fiber amount and aspect ratio significantly influence the workability of UHPFRC. Higher steel fiber percentages and aspect ratios cause interlocking among fibers and ultimately lead to reduced slump flow; 2.
The compressive strength of UHPFRC is gradually increased with increased steel fiber dosage for the capability of delaying crack formation and propagation. Nevertheless, improper fiber distribution in a concrete mixture may lead to the decreased compressive strength of UHPFRC;