Advancing Hybrid Fiber-Reinforced Concrete: Performance, Crack Resistance Mechanism, and Future Innovations

Abstract

This research investigates the effects of steel (ST) and synthetic (SYN) fibers on the workability and mechanical properties of HPFRC. It also analyzes their influence on the material’s microstructural characteristics. ST fibers improve tensile strength, fracture toughness, and post-cracking performance owing to their rigidity, mechanical interlocking, and robust adhesion with the matrix. SYN fibers, conversely, mitigate shrinkage-induced micro-cracking, augment ductility, and enhance concrete performance under dynamic stress while exerting negative effects on workability. Hybrid fiber systems, which include ST and SYN fibers, offer synergistic advantages by enhancing fracture management at various scales and augmenting ductility and energy absorption capability. Scanning electron microscopy (SEM) has been crucial in investigating fiber–matrix interactions, elucidating the effects of ST and SYN fibers on hydration, crack-bridging mechanisms, and interfacial bonding. ST fibers establish thick interfacial zones that facilitate effective stress transfer, whereas SYN fibers reduce micro-crack formation and enhance long-term durability. Nonetheless, research deficiencies persist, encompassing optimal hybrid fiber configurations, the enduring performance of fiber-reinforced concrete (FRC), and sustainable fiber substitutes. Future investigations should examine multi-scale reinforcing techniques, intelligent fibers for structural health assessment, and sustainable fiber alternatives. The standardization of testing methodologies and cost–benefit analyses is essential to promote industrial deployment. This review offers a thorough synthesis of the existing knowledge, emphasizing advancements and potential to enhance HPFRC for high-performance and sustainable construction applications. The findings facilitate the development of new, durable, and resilient fiber-reinforced concrete systems by solving current difficulties.

1. Introduction

Concrete has been widely recognized for its adaptability, cost-effectiveness, and ease of application in construction [1,2,3]. Concrete is extensively preferred in the construction of buildings, roads, and bridges due to its high compressive strength [4,5]. Nonetheless, despite its superior performance under compressive pressures, plain concrete is intrinsically deficient in tension, leading to low tensile strength [6,7]. This constraint renders concrete vulnerable to cracking under tensile stress, and as cracks develop over time, they compromise the material’s structural integrity and durability [8,9]. To rectify these intrinsic deficiencies, substantial progress in concrete technology has been achieved to enhance its performance, especially regarding tensile strength, toughness, and durability [10]. The creation of high-performance fiber-reinforced concrete (HPFRC) is one of the most promising advances in this area [11,12,13]. The design of HPFRC often features a low water–binder ratio, the use of superior pozzolanic materials, and a significant amount of fiber reinforcement [14,15]. These fibers, randomly dispersed within the concrete matrix, are essential for stress transmission across cracks and inhibiting their propagation, thereby substantially improving the material’s structural integrity [16,17]. Consequently, HPFRC demonstrates exceptional mechanical properties, including increased flexural strength, fracture toughness, and augmented resistance to thermal shock, impact, and fatigue loading [18,19]. Figure 1 compares plain concrete with HPFRC and highlights the advantages brought by the inclusion of fibers in HPFRC.

The efficacy of the fibers in HPFRC is determined by various aspects, including their kind, size, distribution, and orientation within the mixture [20]. This specialty concrete incorporates fibers into the mixture to improve its mechanical properties, namely to reduce fracture formation and boost the material’s overall structural performance [21]. The primary benefit of HPFRC is its capacity to merge the superior compressive strength of conventional concrete with markedly improved tensile strength, ductility, and fracture resistance [22]. The incorporation of fibers like ST [23], SYN [24], and natural fibers [25] modifies the failure mode of concrete, diminishing its brittleness and improving its resilience under tensile stress. Figure 2 illustrates the appearance of different types of fibers used in concrete blends.

Consequently, HPFRC exhibits superior crack resistance and enhanced toughness and durability across diverse environmental and mechanical situations [26]. Among these, ST fibers are the most frequently utilized because of their reinforcing capabilities and resilience to severe environmental conditions [27]. ST fibers are essential in altering the failure mode of concrete, diminishing its brittleness, and postponing crack development [28]. ST fibers mitigate fracture propagation by spanning fissures and transferring internal stresses, so augmenting the concrete’s capacity to endure tensile forces and markedly strengthening its overall toughness [29,30]. ST fibers, generally composed of carbon or stainless steel, possess a tensile strength exceeding 1100 MPa, rendering them suitable for concrete reinforcement [31]. These fibers exhibit great strength and elastic modulus, hence enhancing the mechanical properties of concrete significantly [32]. SYN fibers, including polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA), and glass (GL) fibers, contribute to reinforcement; however, their influence on overall performance is typically less pronounced than that of ST fibers [33]. Nevertheless, the integration of ST and SYN fibers in HPFRC can yield supplementary advantages, including higher fracture resistance, superior energy absorption during impacts, and improved overall durability [34]. The alignment and quantity of fibers in the concrete mixture are essential for enhancing the material’s performance. Researchers have discovered that augmenting the fiber volume or modifying the geometric characteristics of the fibers can substantially boost the concrete’s crack resistance and improve its ductility [35,36]. Furthermore, fibers must be uniformly dispersed within the matrix to guarantee optimal stress transmission and crack resistance [37]. Although ST fibers are predominantly utilized in HPFRC, research has investigated the advantages of integrating ST fibers with alternative fiber types, such as PP or GL fibers, to further improve the mechanical and physical characteristics of the material. The collaboration among various fiber types can lead to stronger crack resistance [38], increased energy absorption during impact [39], and improved long-term durability [40]. Continuous improvements in mix design and the integration of alternate fiber types are rendering HPFRC a more viable and economical option for a broader range of applications [41]. The future of HPFRC is hopeful since innovations in fiber technology and mix design persist in improving the performance and usability of this material [42]. Research is increasingly centered on enhancing fiber efficiency, examining the capabilities of advanced fibers such as hybrid fibers, and analyzing the long-term performance of HPFRC under diverse environmental circumstances [43]. With the increasing demand for durable, sustainable, and resilient infrastructure, HPFRC is anticipated to be crucial in fulfilling the requirements of high-performance applications such as bridges [44], high-rise buildings [45], and pavements [46], where superior strength and durability are imperative.

This review comprehensively examines crack resistance mechanisms, workability, mechanical properties, ductility, and toughness in fiber-reinforced concrete. A key focus is on understanding how different fiber types and their hybrid combinations influence these properties, contributing to enhanced structural performance. Additionally, the study analyzes the microstructural characteristics of fiber-reinforced concrete using scanning electron microscopy (SEM) to assess fiber–matrix interactions and crack-bridging effects. By evaluating the existing research, this review identifies the advantages and limitations of various fiber reinforcements in improving concrete durability and mechanical efficiency. Furthermore, the study explores how fiber dispersion, orientation, and volume fraction affect workability and long-term performance. A critical assessment of the literature reveals the existing gaps, particularly in optimizing fiber combinations, long-term durability studies, and performance under extreme environmental conditions. Based on these gaps, potential future research directions are discussed, including the development of advanced fiber materials, novel mix designs, and improved fiber dispersion techniques. Ultimately, this review aims to provide insights into optimizing fiber-reinforced concrete for high-performance applications while guiding future studies toward addressing unresolved challenges in the field.

2. Mechanisms of Crack Development and Growth

Concrete generally develops several micro-cracks prior to the application of external loads, a phenomenon commonly known as inherent cracking [47]. The micro-cracks result from the material’s heterogeneous nature, where the aggregate and mortar components fail to connect adequately, resulting in localized stress concentrations [48]. The aggregate–mortar interface is a crucial place where cracking often begins, as it is generally the weakest component of the composite concrete system [49]. The interfacial transition zone (ITZ), where the characteristics of the aggregate and mortar converge, frequently experiences the first failure under stress, profoundly affecting the overall mechanical performance of concrete. The ITZ is a critical region in concrete, where the interface between the aggregate and the surrounding mortar is formed. This zone is often the site of the first failure under stress, as it exhibits weaker bonding between the aggregate and the cement matrix compared to the bulk of the material. The ITZ’s properties, including its porosity and composition, influence the overall mechanical behavior of the concrete, such as strength, durability, and crack resistance. When subjected to external forces, the ITZ can become a focal point for micro-cracking and stress concentration, leading to premature failure. Therefore, the improvement in the ITZ through fiber reinforcement or other methods is essential for enhancing the performance and longevity of concrete structures [50].

The development of the ITZ results from the wall effect, which occurs due to the presence of aggregate, and the localized water accumulation effect, induced by gravitational forces, as depicted in Figure 3. Cracks in concrete typically originate in the weak interfacial transition zone (ITZ) when exposed to external loads, as it represents the most susceptible area of hardened concrete [51]. The initial breaking directly affects the mechanical properties of the material by modifying the stress distribution and impairing the load-bearing capacity [52]. Cementitious materials, like traditional concrete, exhibit very low tensile strength and restricted tensile strain capacity, rendering them susceptible to brittle failure [53]. The limited tensile strain capacity of concrete results in the rapid propagation of cracks once they initiate under additional stress. This leads to the material’s brittle characteristics, wherein cracks form and propagate practically certainly under stress circumstances [54,55]. The principal cause of crack formation is when the external force surpasses the concrete’s limited tensile strength, resulting in isolated fractures [56]. Upon the initiation of a crack, it transcends the surface and propagates through the material, forming a network of fissures that compromises the structural integrity [57]. The propagation of cracks adds to the nonlinear behavior of concrete, especially at lower stress levels, where the material exhibits considerable deformation without a proportional rise in load [58]. Moreover, crack propagation frequently results in volumetric expansion, hence intensifying the material’s vulnerability to failure as the cracks enlarge and merge [59]. An effective method to alleviate this brittle behavior and enhance the post-cracking performance of concrete is the integration of fibers [60]. These fibers are essential for bridging cracks and distributing stress throughout the matrix, hence aiding in the regulation of crack growth [61]. The fibers facilitate the redistribution of stresses, inhibiting the uncontrolled propagation of fractures and enabling the material to support loads following the initial breaking phase [62]. The capacity of fibers to transmit stress between the matrix and tensile strains at the rupture point is a crucial element in enhancing the overall toughness of concrete [63]. Consequently, fiber-reinforced concrete (FRC) demonstrates improved post-cracking performance, leading to a substantial enhancement in its toughness and resistance to additional damage [64]. This fiber reinforcement mitigates the likelihood of catastrophic collapse, enhancing the concrete’s capacity to preserve structural integrity despite the emergence of first cracks [65]. The inclusion of fibers can postpone the initiation of cracking and markedly improve the material’s energy absorption capacity, hence increasing its durability and resistance to cracking under cyclic or dynamic loading conditions [66,67].

3. Effectiveness of Fibers Before and After Crack Formation

Concrete is inherently a heterogeneous material, defined by a complex internal structure that includes pores, micro-cracks, and other discontinuities, mostly resulting from shrinkage and heat strains during the curing and hardening phases [68]. The presence of coarse aggregates and external boundary conditions generally mitigates these micro-cracks and voids, restraining their proliferation [69]. Consequently, when external loads are exerted on concrete, the material’s matrix, comprising the cement paste and aggregates, transmits a portion of the applied force to the fibers integrated into the mixture [70]. The load transmission to the fibers transpires prior to the formation of any substantial macro-cracks in the material [71]. Research indicates that the strength of concrete can be markedly improved by integrating fibers with an elastic modulus superior to that of the matrix, enabling the fibers to assist in load-bearing prior to the occurrence of extensive cracking [72]. In HPFRC, the principal advantage of fibers becomes apparent following the matrix’s initial fracture [73]. Once fissures commence in the concrete, fibers are essential in spanning these fissures and redistributing the tension throughout the fracture surfaces [74,75]. Fibers are especially effective in this capacity due to their tensile strength, which significantly exceeds that of hardened concrete [76]. Consequently, during the post-cracking phase, fibers can inhibit the spread of cracks [77], offering a means to regulate fracture formation [78] and mitigate the degree of damage [79].

The failure of an HPFRC structure is predominantly determined by the bond failure between the concrete matrix and the fibers [80]. In tensile tests, this phenomenon is frequently noted as a pull-out effect, wherein the fibers are extracted from the matrix, resulting in the ultimate failure of the composite material [81]. The efficacy of fibers in impeding crack propagation is significantly improved with the utilization of deformed-end fibers, such as two hooked-end ST fibers [82]. These fibers experience substantial plastic deformation during crack bridging, resulting in significant energy dissipation [83]. The alignment and deformation of fibers in reaction to crack propagation not only aid in preventing additional crack expansion but also enhance the overall toughness of the concrete, improving its performance after cracking [84]. This energy dissipation mechanism is crucial for enhancing the durability and load-bearing capability of HPFRC under cyclic or dynamic loading circumstances [85]. In HPFRC, both aggregates and fibers collectively contribute to crack bridging [86]. Nonetheless, upon evaluating the distinct contributions of aggregate and ST fibers to the bridging mechanism, it is clear that the influence of ST fibers is markedly more substantial [87]. Figure 3 demonstrates that ST fibers provide a far more efficient crack-bridging mechanism than the bridging effect offered by aggregates. The onset of cracks and their subsequent propagation can be categorized into three main zones: (1) a micro-cracking and macro-crack growth zone, where small fissures initiate and expand within the matrix; (2) a bridging zone, where fibers and aggregates collaborate to counteract crack opening and distribute stresses across the crack surfaces; and (3) a traction-free zone, where cracks have completely propagated, rendering the material incapable of effectively bearing load across the fractured area [88]. Figure 4 presents a schematic representation of how fibers affect the fracture behavior of concrete under tensile stress. The post-cracking behavior of fiber-reinforced concrete (FRC) is affected by various parameters, such as fiber content and the binding strength between the fibers and the matrix [89]. Increased fiber content, especially when fibers intersect the cracks abundantly, can lead to post-cracking stress beyond the initial cracking load, hence inducing strain-hardening behavior [90]. The material sustains escalating stress as more cracks develop, resulting in a multiple cracking pattern in the concrete [91]. The strain-hardening tendency is characteristic of HPFRC which can experience considerable deformation without a notable reduction in load-bearing capability [92]. Nonetheless, at standard fiber contents (below 1%), the concrete typically demonstrates a strain-softening response post-cracking [93]. Upon the initiation of the first crack, damage rapidly localizes, resulting in a gradual decline in the material’s load-bearing capability. This trend signifies a more brittle response, wherein crack propagation accelerates following the initial fracture, culminating in the material’s final failure [94].

4. Influence of Different Fibers on Concrete Behavior and Characteristics

4.1. Workability

The term “workability” describes how easily concrete can be mixed, transported, placed, and finished without segregation or excessive bleeding [95]. It is a key factor in ensuring high-quality and durable concrete structures [96]. High workability allows for easier handling and placement, particularly in areas with complex formwork or dense reinforcement [97]. On the other hand, insufficient workability can cause placement difficulties, leading to honeycombing or void formation in the final structure [98]. The incorporation of fibers often reduces workability by restricting the flow of the concrete mixture [99]. This is due to the increased internal friction between particles, which is particularly significant for fibers with a high aspect ratio. The addition of fibers to concrete generally decreases workability by limiting the movement of the mixture, making it less fluid and harder to handle [99]. This reduction occurs because fibers create a network within the mix, increasing internal friction and resistance to flow. The effect is particularly pronounced with fibers that have a high aspect ratio (length–diameter ratio), as their elongated shape leads to more entanglement and interaction between particles. As a result, the concrete mixture becomes stiffer, requiring additional effort for mixing, placing, and finishing [100]. Consequently, fiber-reinforced concrete (FRC) tends to have lower slump values than conventional concrete, requiring additional effort during mixing, transportation, and placement [101]. The decline in workability is more pronounced with long or rigid fibers, such as ST fibers, which increase the resistance to flow within the mixture [102]. However, using fibers with specialized geometries, such as crimped or straight shapes, can improve the cohesiveness of the mix and partially counteract the loss of workability [103]. Figure 4 illustrates the comparison of workability based on spreading diameters in plain concrete and HPFRC, highlighting the negative impact of fibers on concrete’s workability.

In certain instances, the incorporation of superplasticizers (SPs) or other chemical admixtures may be employed to mitigate diminished workability, thereby guaranteeing that the FRC maintains requisite flow properties for efficient placement and handling [104]. Superplasticizers (SPs) do not completely eliminate the workability loss caused by fibers. Instead, they primarily improve the overall workability of the mix, but cannot fully mitigate the impact of fibers on workability. The spreading diameter (SD), also known as slump flow (SF), is a significant measure of workability. The inclusion of fibers in the mixture often leads to a diminished standard deviation, especially when substantial quantities of long fibers are utilized, thereby reducing the fluidity of the mixture [105]. The influence of fiber content on workability is significantly contingent upon the type of fiber employed. ST fibers typically result in a more pronounced decrease in workability than SYN fibers owing to their greater density and rigidity [106,107]. The distribution and orientation of fibers in the concrete mixture also affect its workability. Fibers oriented parallel to the flow direction can increase resistance, reducing the overall slump [108,109,110]. Notwithstanding these limitations, the judicious selection of fiber type and mix design, along with the incorporation of workability-enhancing additives, can improve the workability of fiber-reinforced concrete, striking a balance between ease of placement and the mechanical advantages conferred by fibers [111,112,113].

4.2. Compressive Strength

The use of ST and SYN fibers in concrete is a prevalent method to improve its mechanical qualities. The influence of these fibers on the compressive strength of concrete is contingent upon their kind, number, size, and the regularity of their dispersion within the mixture. ST fibers, recognized for their superior tensile strength, markedly enhance fracture resistance and tensile strength in concrete, reducing the likelihood of abrupt failure, especially in high-strength applications [114]. The elevated density and rigidity of ST fibers might adversely impact the workability of the concrete mix, resulting in difficulties in attaining homogeneity [115]. This may consequently restrict the anticipated enhancement in compressive strength [116]. Conversely, SYN fibers are lighter and more flexible, rendering them less intrusive to the workability of concrete in comparison to ST fibers [117,118]. They are frequently employed to mitigate cracking and improve the durability of concrete [119,120]. Nonetheless, the impact of SYN fibers on compressive strength is typically less significant than that of ST fibers [121,122]. SYN fibers are more effective in mitigating surface cracks and enhancing concrete flexibility at lower doses [123]. At elevated dosages, they may diminish the density of the concrete, resulting in a possible reduction in compressive strength [124]. The divergent impacts of ST and SYN fibers on concrete stem chiefly from their distinct mechanical and physical characteristics. ST fibers directly enhance compressive strength and prevent cracking because of their high stiffness and tensile strength [125,126,127,128].

An irregular distribution of SYN fibers may impede the anticipated mechanical performance of the concrete [129,130,131]. Yao et al. [130] found that VA fibers at a 0.5% dosage and 12 mm length reduce the autogenous shrinkage of UHPC by 28.34% while increasing compressive strength by 20.4%. Hybrid fiber systems, utilizing both ST and SYN fibers concurrently, present a viable approach for enhancing the mechanical properties of concrete [132]. These systems integrate the superior tensile strength of ST fibers with the enhanced workability and ductility offered by SYN fibers, resulting in a more balanced performance [133]. Hybrid fiber-reinforced concrete significantly reduces cracking, hence prolonging the overall lifespan of the construction [134]. These solutions are especially appropriate for applications necessitating both great strength and longevity, including industrial floors, pavements, and bridge decks [135]. Achieving optimal outcomes from hybrid fiber-reinforced concrete necessitates meticulous attention to the ratios of ST and SYN fibers. Inadequate ratios may lead to variable density and diminished performance [136]. The diameters and configurations of the fibers significantly affect the mechanical properties of the concrete [137]. Shorter, thinner, and more pliable fibers enhance packing density and promote homogeneous distribution within the concrete matrix [138]. This improves uniformity and promotes compressive strength [139]. In contrast, longer and thicker fibers can produce increased bond resistance but may necessitate more extensive mixing to ensure uniform dispersion [140]. This heightened effort may impair workability, resulting in difficulties during mixing and positioning [141]. To achieve best performance, ST fibers are generally utilized in volumes ranging from 0.5% to 1.5% [142,143,144], whereas SYN fibers are incorporated at a rate of 0.2% to 0.5% [145,146,147]. Figure 5 presents a schematic representation of the stress–strain curves for plain concrete and HPFRC under axial loading.

4.3. Splitting Tensile Strength

Concrete possesses exceptional compressive strength but has an intrinsically poor tensile capacity due to its brittle characteristics [148]. Under tensile pressures, it is susceptible to cracking, which can significantly undermine its longevity and structural integrity [149]. Fibers are frequently included in concrete to mitigate this constraint [150]. ST and SYN fibers are two prevalent varieties, each providing distinct advantages in augmenting splitting tensile strength (STS). The incorporation of these fibers enhances the concrete’s crack resistance, tensile load capacity, and ductility in failure mode [151]. ST fibers are well known for enhancing the mechanical properties of concrete, especially its STS [152]. Owing to their superior tensile strength and rigidity, ST fibers are exceptionally proficient in preventing crack initiation and propagation [153]. When a concrete matrix experiences tensile loads, micro-cracks generally develop and propagate until they attain a critical threshold, resulting in failure [154]. ST fibers span these fissures, distributing the stress over the crack surfaces and inhibiting further widening of the crack [155]. ST fibers also improve post-cracking tensile performance, enabling the concrete to sustain loads following cracking [156]. This is essential in applications where tensile strength is of paramount importance, such as pavements [157], precast components [158], and industrial flooring [159].

The efficacy of ST fibers is contingent upon various aspects, including length, diameter, aspect ratio (length–diameter ratio), and dosage [160]. Figure 6 provides a schematic illustration comparing plain concrete and HPFRC under splitting tensile loading. An elevated aspect ratio enhances fiber efficacy by augmenting the adhesion between the fibers and the concrete matrix, allowing them to support greater loads [161]. Synthetic fibers, including polypropylene (PP), polyethylene (PE), and polyvinyl alcohol (PVA), are lightweight and flexible, and provide significant benefits in improving the splitting tensile properties of concrete [162,163,164]. In contrast to ST fibers, which are primarily intended to enhance tensile strength, SYN fibers are superior at managing micro-cracks that develop during initial loading or as a result of plastic and drying shrinkage [165]. In the literature, some researchers have reported significant increases in splitting tensile strength (STS) in fiber-reinforced concrete, while others have closely related the increases in compressive strength and STS in these concretes. Najem et al. [126] found that adding 1.5% ST fibers to high-performance concrete increases STS by 63.86%. ST fibers significantly enhance the tensile strength of concrete [166,167,168,169]. ST fibers also offer significant tensile strength and augment post-cracking performance [170], whilst SYN fibers promote ductility and regulate micro-cracking [171]. These fibers synergistically enhance one another, mitigating cracking across various scales. In hybrid systems, SYN fibers typically manage initial micro-cracks [172], mitigating their development into larger fissures [173], whereas ST fibers impede the advancement of macro-cracks under elevated tensile loads [174]. This multi-scale crack management leads to higher tensile performance [175], superior energy absorption [176], and increased durability [177]. Hybrid fibers are especially efficacious in rigorous applications, such as marine structures [178], where durability and saltwater resistance are critical; high-rise buildings [179], which necessitate superior load-bearing capacity and seismic resilience; and pavements and airport runways [180], providing resistance to substantial loads and repetitive stresses.

4.4. Flexural Strength

Concrete, as a principal construction material, provides exceptional compressive strength but is deficient in tensile strength due to its brittle characteristics. A crucial factor in enhancing the structural performance of concrete is its flexural strength (FS), which refers to its capacity to withstand bending or tensile stresses [181]. To mitigate this intrinsic constraint, fibers, particularly ST and SYN fibers, are frequently integrated into the concrete mixture [182]. These fibers enhance the material’s flexural strength and ductility by bridging fissures and redistributing stresses [183]. ST fibers are recognized for their capacity to augment the mechanical properties of concrete, especially in enhancing flexural strength. During the bending of concrete, tensile stresses arise on one side of the member, whereas compressive stresses manifest on the opposing side. In unreinforced concrete, tensile loads induce fracture formation and propagation, resulting in abrupt failure [184]. ST fibers, characterized by their superior tensile strength and rigidity, efficiently span these fissures and inhibit their expansion [185]. The incorporation of ST fibers not only enhances the ultimate flexural strength (FS) of concrete but also significantly improves its post-cracking behavior, making the material more resilient under bending stresses [186]. These fibers contribute to a more ductile failure mode by enabling the concrete to sustain loads even after cracking, thereby preventing sudden and brittle fractures. This improved load-bearing capacity in the post-cracking phase is attributed to the ability of ST fibers to bridge cracks and efficiently transfer stress across crack surfaces, thereby delaying failure and increasing the energy absorption capacity of the material [187]. The effectiveness of ST fibers in enhancing flexural performance is influenced by multiple factors, including their aspect ratio, volume fraction, orientation, and dispersion within the matrix [188]. A higher aspect ratio typically increases the fiber’s crack-bridging ability, allowing for improved stress redistribution and better resistance to crack propagation [189]. Moreover, an optimal fiber dosage is crucial, as excessive fiber content may lead to clustering, negatively affecting the workability and homogeneity of the mix [190]. Studies have shown that the incorporation of ST fibers can lead to an improvement in flexural strength depending on their concentration, alignment, and interaction with the surrounding matrix. Jodeiri et al. [190] found that adding 1% ST fibers to concrete increases its FS by 19.42% for one beam model and 6.76% for another. Danha et al. [191] found that increasing the ST fiber volume fraction from 0% to 1%, 2%, and 3% increases the FS of ultra-high-performance concrete by 156%, 261%, and 389%, respectively. Atiş and Karahan [192] concluded that while fly ash reduces strength, it improves workability and durability, and steel fiber addition enhances tensile strength and freeze–thaw resistance but reduces workability. Blazy et al. [193] highlighted the challenge of testing polymer fiber-reinforced concrete due to the lack of standardized methods. The study explored using EN 14651 to assess flexural tensile strength, reporting a 5.5–13.5% increase with 2.0–3.0 kg/m³ synthetic fibers [191,192,193]. On the other hand, SYN fibers provide distinct advantages in improving the flexural performance of concrete, although their reinforcing mechanism differs from that of ST fibers. Unlike ST fibers, which primarily contribute to post-cracking strength and load-bearing capacity, SYN fibers play a crucial role in mitigating early-stage micro-cracking and enhancing the material’s toughness by improving stress distribution within the matrix [194]. Figure 7 shows a schematic comparison of the flexural load-deflection behavior of plain concrete and HPFRC.

Their adaptability and lightweight characteristics provide improved distribution throughout the mixture, hence ensuring uniform crack management [195]. SYN fibers enhance the concrete’s resistance to crack propagation under flexural loads by inhibiting the coalescence of micro-cracks into bigger fissures [196]. Although their direct impact on final flexural strength may be less pronounced than that of steel fibers [197], they significantly enhance the material’s energy absorption capacity and post-cracking ductility [198]. SYN fibers are especially advantageous in scenarios where the management of shrinkage cracks and enhancement of impact resistance are essential, such as in thin sections [199], overlays [200], or pavements [201]. The integration of ST and SYN fibers in hybrid fiber-reinforced concrete provides a balanced method for improving flexural strength and performance [202,203,204]. In contrast, plastic fibers had a smaller impact, increasing the ultimate load by only 6.62%. These findings highlight the effectiveness of fiber combinations, especially steel and carbon, in enhancing the strength and performance of concrete beams. Hybrid fibers enhance the FS of concrete by combining the benefits of different fiber types. Macro-ST fibers contribute to increased load-bearing capacity by bridging cracks and improving post-cracking behavior, thereby enhancing the material’s toughness and durability. Meanwhile, micro-SYN fibers play a crucial role in delaying crack initiation and restricting crack propagation by distributing stresses more evenly throughout the matrix. This synergistic interaction between macro-ST and micro- SYN fibers results in improved mechanical performance, making fiber-reinforced concrete more resistant to flexural loads and structural deterioration over time. This synergy is especially effective in water tanks [205], enhancing flexural strength under hydrostatic pressure; in precast elements [206], augmenting load resistance and structural integrity; and in seismic regions [207], where superior crack management and energy absorption are critical. These characteristics render hybrid fibers suitable for rigorous applications, including retaining walls, railway sleepers, and offshore structures, where superior strength is essential [208].

4.5. Ductility and Toughness

Concrete, although a flexible and robust material, is intrinsically brittle under tensile and flexural loads, exhibiting abrupt fracture when its tensile limit is surpassed [209]. To mitigate this constraint, fibers are frequently integrated into concrete to enhance its ductility and energy absorption capacity, particularly under flexural loads [210]. ST and SYN fibers have been thoroughly examined for their capacity to alter the post-peak performance of concrete, improving its energy absorption and load-bearing capabilities following cracking [211]. Ductility is an essential factor in structural design, as it assesses a material’s potential to distort without a substantial reduction in load-bearing capability [212]. In plain concrete, cracking due to flexural loads is sudden and brittle, resulting in a swift reduction in strength [213]. When fibers are integrated, they function as crack arrestors, redistributing tensile loads across crack surfaces and enabling the material to undergo additional deformation prior to failure [214].

ST fibers significantly enhance the ductility of concrete due to their superior tensile strength and rigidity [215]. When subjected to flexural loads, these fibers play a crucial role in impeding crack propagation and preventing sudden, brittle failure. By bridging cracks and transmitting stresses effectively across the fracture plane, ST fibers enable a more gradual reduction in load-bearing capacity after peak load is reached, thereby improving post-peak performance [216]. This controlled failure mechanism contributes to a substantial enhancement in structural resilience and overall load-bearing capacity, making fiber-reinforced concrete more durable under dynamic and long-term loading conditions [217]. SYN fibers, despite their lower stiffness compared to ST fibers, offer a distinct contribution to ductility by effectively controlling the formation and growth of micro-cracks [218]. These fibers act as a first line of defense against crack initiation, delaying the development of larger fractures and promoting a more uniform stress distribution throughout the concrete matrix. This crack-controlling ability is particularly beneficial in mitigating stress concentrations, reducing the likelihood of sudden fracture, and enhancing the material’s overall ability to undergo deformation without failure [219]. Energy absorption capacity (EAC), commonly referred to as toughness, is another critical property significantly influenced by fiber reinforcement. Toughness is measured as the area under the load-deflection curve, representing the total energy a material can absorb before complete failure [220]. Plain concrete, due to its inherently brittle nature, exhibits limited energy absorption capacity. However, the inclusion of fibers drastically improves this property, leading to a more resilient composite material capable of withstanding greater mechanical stresses [221]. ST fibers excel in enhancing toughness by providing exceptional resistance to crack propagation under flexural loading conditions [222]. Once micro-cracks develop, ST fibers act as load-bearing elements that span across the cracks, redistributing stresses and preventing rapid crack widening [223]. Their strong mechanical bond with the concrete matrix facilitates efficient stress transfer, significantly improving the energy absorption capacity and prolonging the structural integrity of the material even under extreme conditions [224]. SYN fibers, while not as effective as ST fibers in sustaining post-cracking loads, contribute to toughness through a different mechanism. Their primary role lies in the early stages of fracture formation, where they prevent the rapid coalescence of micro-cracks by dispersing stress concentrations over a broader area [225]. This mechanism delays the progression of major cracks, leading to increased energy dissipation and a more ductile failure mode. Although their direct impact on post-peak strength is generally lower than that of ST fibers [226], SYN fibers act as a secondary reinforcement, further enhancing the material’s overall resistance to failure and improving its toughness in combination with ST fibers [227]. Figure 8 illustrates a comparison of the ductility and toughness between plain concrete and HPFRC.

The interplay of these effects leads to a more progressive and controlled failure, especially in slender or weakly laden portions [228]. The most notable effect of fibers on concrete is their capacity to modify its post-peak performance, especially under flexural stresses [229]. In plain concrete, the load decreases sharply following the peak load due to the swift advancement of cracks and the absence of devices to counteract additional deformation [230]. This conduct is exceedingly unfavorable in structural applications where energy dissipation and gradual failure are paramount [231]. The incorporation of ST fibers significantly alters the post-peak behavior [232]. ST fibers offer a bridging mechanism that mitigates crack propagation, leading to a gradual and regulated reduction in load-bearing capability [233]. This induces a pseudo-ductile characteristic, allowing the material to sustain considerable loads despite the presence of cracks [234]. The strength maintained post-peak load is commonly termed residual strength, and its size is contingent upon fiber dosage, aspect ratio, and distribution [235]. SYN fibers, while less efficient in preserving residual strength, improve the post-peak behavior by diminishing the width and propagation velocity of cracks [236]. This aids in alleviating abrupt failures and enhances the material’s overall energy absorption properties [237]. SYN fibers, when integrated with ST fibers in hybrid systems, can improve post-peak performance by mitigating micro-cracks, while ST fibers manage the greater tensile stresses linked to macro-cracks [238,239]. The integration of ST and SYN fibers in hybrid systems capitalizes on the advantages of both fiber types, providing an enhanced solution for augmenting ductility and energy absorption under flexural stresses [240]. SYN fibers control micro-cracks and inhibit their development into bigger fissures [241], whereas ST fibers prevent the spread of macro-cracks and offer substantial post-cracking strength [242]. Hybrid systems provide enhanced performance regarding load-deflection behavior, characterized by a more progressive reduction in strength following the peak load and an increased deformation capacity [243]. This multi-scale crack management maximizes energy absorption while preserving structural integrity [244]. The collaboration of ST and SYN fibers produces a durable material appropriate for uses including wind turbine foundations [245], dam spillways [246], and heavy-duty pavements [247], where superior strength and toughness are essential.

4.6. SEM Characterization of Fiber–Matrix Interaction in FRC

The use of ST and SYN fibers in concrete markedly modifies its internal structure, enhancing its mechanical and durability characteristics under diverse loading circumstances [248]. Scanning electron microscopy (SEM) offers critical insights into microstructural alterations, particularly concerning the interaction between fibers and the concrete matrix [249]. Comprehending the fiber–matrix interface is crucial, as it significantly affects the transmission and distribution of stresses inside the composite material, hence altering the fracture resistance [250], ductility [251], and toughness [252] of the reinforced concrete. ST fibers are recognized for their significant stiffness, tensile strength, and strong interfacial adhesion with the concrete matrix [253]. The SEM analysis of ST fiber-reinforced concrete (SFRC) emphasizes the mechanical interlocking facilitated by the rough or hooked surfaces of the steel fibers. SEM imaging plays a pivotal role in distinguishing the different roles of ST and SYN fibers in crack-bridging and stress transmission due to its high-resolution imaging capabilities, allowing for detailed observation of the fiber–matrix interface at the microstructural level. The distinct surface characteristics of ST fibers, such as their smooth and rigid texture, facilitate the direct observation of how these fibers bridge cracks and redistribute stresses across the fracture surfaces, which is crucial for understanding their contribution to post-cracking behavior and overall toughness. On the other hand, SEM allows for the examination of SYN fibers, which typically exhibit a more flexible and rough surface texture, revealing how they interact with the matrix and contribute to mitigating micro-cracking by regulating crack propagation at an early stage. Through SEM analysis, one can visualize the different mechanisms by which these two types of fibers improve the mechanical properties of concrete. Moreover, SEM provides invaluable insights into the bonding efficiency between the fibers and the concrete matrix, further elucidating how effective stress transfer is achieved, particularly in hybrid fiber systems, where the complementary roles of both fiber types contribute to improved performance. The ability to discern these microstructural details enables researchers to optimize the design and application of fiber-reinforced concrete for enhanced durability and performance under various stress conditions [254]. Figure 9 presents an SEM investigation of SFRC exposed to freeze–thaw deterioration.

This mechanical interlocking enhances the adhesion between fibers and the matrix, facilitating effective stress transmission during fracture development [255]. The crack-bridging capacity of ST fibers is a significant characteristic demonstrated via SEM, as fibers traverse crack planes, postponing additional crack propagation and enhancing post-crack load resistance [256]. Furthermore, SEM imaging frequently demonstrates that ST fibers can deflect and impede cracks, compelling them to circumvent the fibers rather than propagate directly through them [257]. This deflection mechanism elevates the energy necessary for crack propagation, hence enhancing fracture toughness and augmenting the longevity of concrete structures [258]. SYN fibers, although less rigid and robust than ST fibers, are essential for regulating micro-crack formation and enhancing the overall uniformity of the concrete matrix [259]. The SEM imaging of SYN fiber-reinforced concrete frequently demonstrates the uniform distribution of these fibers inside the matrix, hence diminishing the probability of localized stress concentrations [260]. In contrast to ST fibers, SYN fibers are often smoother and chemically inert, leading to less bonding with the matrix [261]. The SEM analysis indicates that modified or surface-treated SYN fibers have superior adhesion, hence promoting stress transmission and augmenting their reinforcing effect [262,263]. These fibers are proficient in mitigating shrinkage-induced micro-cracking, frequently identified via SEM as small cracks efficiently bridged by flexible SYN fibers [264]. SYN fibers considerably enhance the durability and service life of concrete by restricting the coalescence of micro-cracks into bigger fissures. The SEM investigation has revealed significant distinctions in the post-cracking behavior of ST and SYN fibers [265]. ST fibers generally exhibit a pull-out behavior when subjected to loading, a phenomenon that significantly influences the post-cracking performance of fiber-reinforced concrete. SEM imaging has revealed that this behavior is governed by mechanical interlocking, frictional resistance, and adhesion between the fibers and the cementitious matrix, which collectively contribute to energy dissipation during the fracture propagation phase [266]. As the fibers are gradually pulled out, frictional forces and progressive debonding mechanisms mitigate the rapid spread of cracks, thereby enhancing the material’s toughness and durability. This pull-out mechanism helps sustain an elevated post-cracking load-bearing capacity, ensuring the structural functionality of concrete despite the occurrence of cracks [267]. On the other hand, SYN fibers demonstrate a higher susceptibility to premature debonding before complete extraction [268]. While this tendency can reduce their effectiveness in crack bridging, it still facilitates energy dissipation through the redistribution of stress along the matrix [269]. However, their overall contribution to residual strength remains less significant compared to ST fibers due to their lower frictional resistance and mechanical interlocking capacity [270]. The distinct pull-out behaviors of ST and SYN fibers underscore their complementary roles in hybrid fiber-reinforced concrete, where the interplay of mechanical bonding, frictional resistance, and energy dissipation mechanisms governs the composite’s overall performance [271,272]. SYN fibers are notably proficient in preserving material integrity under cyclic or dynamic loading situations, hence assisting in the postponement of fatigue degradation. The application of SEM has further underscored the synergistic effects of ST and SYN fibers in hybrid fiber systems [273]. Combining these two fiber kinds allows for the optimization of concrete’s performance by leveraging the benefits of both [274]. The SEM pictures of hybrid fiber-reinforced concrete frequently exhibit a hierarchical crack management system, wherein SYN fibers regulate micro-crack propagation while ST fibers span macro-cracks [275]. This synergistic interaction establishes a multi-scale reinforcing network, enhancing the ductility and toughness of the composite material [276]. Moreover, the SEM imaging of hybrid systems demonstrates that improved crack-bridging and stress redistribution capacities facilitate enhanced energy dissipation under loading, yielding superior post-cracking performance [277]. Scanning electron microscopy (SEM) has provided significant insights into the interfacial transition zone (ITZ) of fiber-reinforced concrete. The interfacial transition zone (ITZ), generally spanning 30–40 μm around aggregates and fibers, exhibits increased porosity and a modified composition relative to the bulk cement matrix [278]. ST fibers considerably influence the ITZ microstructure, with twisted fibers exhibiting enhanced bonding and diminished crack widths in comparison to hooked fibers [279]. In high-quality concretes, a preferential proliferation of calcium-based compounds was noted adjacent to dolomite grains, whereas ultra-high-performance fiber-reinforced concrete displayed calcium-silicate-hydrate (C-S-H) gels with diminished Ca/Si ratios in proximity to fibers and aggregates [280]. Nanoindentation investigations demonstrated that the elastic modulus of the interfacial transition zone (ITZ) is roughly 67% of that of bulk cement, signifying its relatively diminished rigidity. While the ITZ features may not markedly affect the composite elasticity, comprehending its microstructure is essential for fracture modeling and the advancement of sophisticated concrete materials [278]. These data illustrate the significance of fiber–matrix compatibility in attaining the appropriate mechanical properties. In conclusion, ST and SYN fibers provide unique but complementary advantages to the microstructural and mechanical characteristics of concrete [281]. ST fibers offer exceptional crack resistance, improved toughness, and great post-cracking performance owing to their rigidity and effective fiber–matrix adhesion [282]. SYN fibers enhance matrix homogeneity, mitigate micro-cracking, and improve durability, especially under shrinkage or dynamic loading situations [283]. SEM analysis is an essential instrument for comprehending and enhancing these effects, providing intricate visualizations of fiber–matrix interactions, crack-bridging mechanisms, and microstructural advancements [284]. Integrating ST and SYN fibers in hybrid systems allows for the complete utilization of the synergistic advantages of both fiber types, resulting in concrete composites that exhibit durability and ductility, along with improved energy absorption and post-cracking resilience [285].

5. Addressing Knowledge Gaps and Shaping Future Research in FRC

• Investigating Hybrid Fiber Interactions:

• Future research should focus on understanding the multi-scale interactions between ST and SYN fibers in hybrid systems [286,287,288,289].
• Advanced imaging techniques like micro-CT scanning and SEM analysis can provide a deeper understanding of how these fibers interact at various scales within the matrix [290,291,292].
• Advanced imaging techniques such as micro-CT scanning and SEM analysis play a crucial role in understanding the multi-scale interactions between ST and SYN fibers in hybrid fiber-reinforced concrete (HFRC). These techniques provide detailed, high-resolution images of the concrete’s internal structure at different scales, allowing researchers to observe the fibers’ distribution, alignment, and interaction with the surrounding matrix.
• Micro-CT scanning offers three-dimensional imaging, enabling the visualization of the fiber–matrix interface and the porosity within the concrete at a microstructural level. This technique helps in identifying how fibers are distributed within the matrix, whether they are well dispersed or aggregated, and how this affects the overall performance of the material. It can also reveal the interactions between different types of fibers, such as ST and SYN, and their respective roles in improving concrete’s mechanical properties [293,294,295].

• Fiber Design Innovations:

• Research should explore novel fiber designs, such as three-dimensional geometries or chemically modified surfaces, to improve bonding properties.
• The creation of predictive models that consider fiber shape, material characteristics, and matrix composition will help optimize fiber content and placement, enhancing performance and minimizing material use [296,297].

• Integration of Functional and Smart Fibers:

• The integration of functional or smart fibers could provide additional advantages, such as self-sensing capabilities for structural health monitoring [298,299,300].
• ST fibers with electrical conductivity or SYN fibers impregnated with nanomaterials could improve the dual functionality of concrete.

• Sustainability and Eco-Friendly Materials:

• Sustainability research should focus on identifying environmentally friendly alternatives, such as recycled ST fibers from industrial by-products or bio-based SYN fibers sourced from renewable resources [301,302,303].
• These developments could reduce the environmental impact while maintaining or enhancing the mechanical and durability characteristics of HPFRC.

• Enhanced Testing Methodologies:

• A broader range of testing methodologies should be employed to simulate real-world exposure conditions, such as fire exposure [304], seismic stresses [305], or prolonged immersion in corrosive chemicals [306].
• This approach will enhance the understanding of the long-term durability of HPFRC and the contributions of ST and SYN fibers in real-world conditions.

• Synergistic Benefits of Hybrid ST and SYN Fibers in Concrete:

Enhanced Performance: The hybrid use of ST and SYN fibers combines their strengths, improving concrete’s tensile strength [307], crack resistance [308], post-cracking behavior, and ductility [309].

Synergistic Benefits: ST fibers enhance strength and crack resistance, while SYN fibers improve workability, reduce shrinkage cracking, and increase resilience under stress. Together, they offer superior performance under dynamic loading and harsh conditions [310].

Optimized Durability: The hybrid fiber system enhances structural integrity, toughness, and durability, making it ideal for high-performance fiber-reinforced concrete (HPFRC) applications [311].

Versatility and Efficiency: Hybrid fibers allow for the creation of tailored concrete mixes that are stronger, more durable, and easier to handle, making them suitable for a wide range of construction needs and contributing to more sustainable, cost-effective solutions [312].

AI-based simulations: The integration of artificial intelligence (AI) in simulating the performance of fiber-reinforced concrete (FRC) has revolutionized the way we design and optimize these materials [313]. By utilizing advanced machine learning algorithms and computational modeling techniques, AI-driven simulations enable a comprehensive analysis of the mechanical behavior, crack propagation, and long-term durability of FRC under a wide range of conditions [314]. These simulations provide insights into the optimal fiber distribution, interaction between ST and SYN fibers, and their combined impact on the material’s properties, including tensile strength, ductility, and crack resistance [315]. Furthermore, AI-based tools allow for the prediction of concrete performance under dynamic loading, environmental stresses, and other real-world conditions, thus reducing the need for costly and time-consuming experimental trials [316]. By analyzing vast datasets from previous studies and ongoing experiments, AI can identify patterns and correlations that might not be immediately apparent through traditional methods, driving innovations in mix design and material selection [317]. Ultimately, the use of AI in FRC development enhances the efficiency of designing stronger, more durable concrete mixes, leading to more sustainable and cost-effective solutions in the construction industry [318].

Large-Scale Field Tests: Large-scale field tests are crucial for validating the performance and durability of fiber-reinforced concrete (FRC) under real-world conditions. These tests involve constructing full-scale specimens or structures, such as pavements, beams, or bridges, using FRC mixes that incorporate both ST and SYN fibers. By subjecting these structures to a variety of environmental and loading conditions, such as heavy traffic loads, freeze–thaw cycles, and exposure to harsh chemicals, researchers can assess how well the concrete performs in practical applications [319]. The data collected from large-scale field tests provide valuable insights into the long-term behavior of the material, including its crack resistance, load-bearing capacity, and overall structural integrity over time [320]. These tests also help in identifying any unforeseen challenges or performance issues that might not be detected in laboratory settings, ensuring that FRC meets the necessary standards for durability and safety. Moreover, large-scale field tests offer an opportunity to fine-tune mix designs and fiber combinations, ultimately leading to more optimized and reliable concrete solutions for a variety of construction projects [321].

Numerical Modeling: Numerical modeling plays a pivotal role in the analysis and optimization of fiber-reinforced concrete (FRC) systems. Using computational methods such as finite element analysis (FEA), numerical modeling allows for the simulation of the material’s behavior under various loading conditions, without the need for extensive physical testing [322]. These models can accurately predict the stress distribution, crack initiation, propagation, and overall performance of FRC under both static and dynamic loads [323]. By simulating different fiber types, including ST and SYN fibers, and their interaction with the cement matrix, numerical models provide a detailed understanding of how the fibers contribute to mechanical properties, such as tensile strength, flexural strength, and toughness [324]. This approach also enables researchers to optimize fiber placement, mix design, and other parameters, ensuring the most efficient use of materials. Furthermore, numerical modeling can simulate the effects of environmental factors like temperature variations, humidity, and chemical exposure, offering insights into the long-term durability of FRC [325]. Through iterative simulations, this process aids in identifying potential failure modes and improving the design before field implementation, ultimately saving time and resources while enhancing the overall performance and sustainability of FRC in real-world applications.

• Final Observations:

• While ST and SYN fibers have proven effective in improving concrete performance, further research is essential to optimize their potential.
• Future studies should address the existing knowledge gaps and explore innovative solutions for developing the next generation of HPFRC materials.

6. Conclusions

• Overview of Findings:

• The review analyzed the effects of ST and SYN fibers on concrete’s workability, mechanical properties, and microstructure.
• Fiber reinforcement plays a vital role in HPFRC, enhancing durability, strength, and resistance under various loading and environmental conditions.

• Key Points:

• ST fibers reduce workability due to their stiffness and increased density, making placement and compaction challenging.
• SYN fibers have less impact on workability, but issues may arise at higher dosages.
• Fiber type, content, shape, and surface treatment influence workability and performance.

• Future Research Directions:

• Focus on admixtures, mix designs, and enhanced placement methods to reduce fiber-related workability issues.
• Explore hybrid ST and SYN fiber systems for improved crack resistance, ductility, and energy absorption.
• Investigate surface treatments for SYN fibers to improve adhesion and bonding with the matrix.

• Microstructural Insights:

• SEM analysis shows that ST fibers promote strong mechanical interlocking and crack-bridging.
• SYN fibers are effective in bridging micro-cracks but require surface modification for better adhesion.

• Long-Term Performance and Sustainability:

• More research is needed to understand FRC’s performance under prolonged mechanical, thermal, and chemical stress.
• The environmental impact of fiber production and the recyclability of FRC should be explored.
• Incorporating sustainable fibers and smart fibers for health monitoring and damage detection holds great potential.

• Collaboration and Standardization:

• Collaboration between academia and industry is crucial to convert research into practical applications.
• Standardizing testing procedures and performance criteria will encourage wider adoption of FRC.
• Evaluating the cost-effectiveness of different fiber types and doses will help identify economical solutions.
• Optimal Ratio and Distribution: Determining the optimal ratio and distribution of ST and SYN fibers within the concrete matrix is a key challenge. The differences in characteristics, such as fiber length, stiffness, and surface properties, complicate the blending process, making it difficult to fully optimize their synergistic effects. Achieving uniform dispersion of both fiber types is critical, as uneven distribution can diminish their effectiveness in enhancing concrete strength and durability.
• Economic and Environmental Considerations: The increased cost of fiber reinforcement presents a challenge in terms of the economic feasibility of using hybrid systems on a larger scale. Additionally, understanding the long-term behavior of hybrid fiber systems under various environmental conditions, including freeze–thaw cycles, chemical exposure, and extreme temperatures, is essential. Addressing these challenges will require extensive research and development to improve the effectiveness, practicality, and sustainability of hybrid fiber-reinforced concrete in construction applications.
• The use of HPFRC on infrastructure projects: High-performance fiber-reinforced concrete (HPFRC) is increasingly being utilized in infrastructure projects due to its enhanced strength and durability. It is particularly suitable for applications such as seismic retrofitting of bridge columns, strengthening of parking garage slabs, and the replacement of bridge decks. The superior resistance of HPFRC to cracking, corrosion, and environmental degradation makes it ideal for structures exposed to harsh conditions, where traditional materials may deteriorate over time. Its ability to incorporate various types of fibers, such as steel and synthetic fibers, provides added benefits like improved crack resistance and better post-cracking behavior. Additionally, HPFRC’s lightweight nature reduces the overall weight of structures, enhancing their seismic performance and reducing the load on foundations. The material’s high toughness and energy absorption capabilities also contribute to its ability to withstand dynamic and extreme loads. HPFRC can be tailored to meet specific project requirements, offering greater flexibility in design and cost-effectiveness. As the demand for more sustainable and high-performing infrastructure grows, HPFRC presents an ideal solution for constructing durable, resilient, and efficient structures.

• Final Remarks:

• This review consolidates the existing knowledge on ST and SYN fibers, highlighting their benefits, challenges, and opportunities.
• It provides valuable insights for developing advanced FRC materials tailored to meet the needs of modern construction while ensuring durability and sustainability.