Steel Fiber Reinforced Concrete: A Systematic Review of Usage in Shield Tunnel Segment

Abstract

With the advancement of tunnel construction, the load-bearing capacity of shield tunnel segments is diminishing, while issues of deformation and fissuring are becoming more conspicuous, posing direct threats to structural integrity and functionality. Steel fiber reinforced concrete (SFRC) is considered a prevalent material, endowed with high strength, excellent crack control, fracture toughness, and remarkable economic advantages. This paper surveys the state-of-the-art research on SFRC, systematically encapsulating key aspects regarding its composition, attributes, methods of segment reinforcement, constitutive models for SFRC segments, and performance enhancements of SFRC segments. By optimizing steel fiber content, aggregate preparation, and selection of chemical admixtures, the mechanical performance of SFRC can be augmented, among which the aspect ratio (l/d) and volume fraction (V_f) of steel fibers exert the most significant influence. Compared with conventional reinforcing materials, SFRC possesses benefits of low cost, uncomplicated fabrication, and superior durability. As a heterogeneous multiphase composite, SFRC exhibits high strength, stiffness, and excellent crack resistance, which can amplify the load-bearing capacity and deformation resistance of the segments, defer damage inception, and thereby enhance the safety and durability of tunnel-lining segments. This study assists in redressing the deficiencies of current shield tunnel segment reinforcement technologies and further facilitates the extensive employment of SFRC in tunnel segment strengthening and restoration.

1. Introduction

Concrete, being the most ubiquitously utilized construction material at present, is beset with deficiencies like inferior tensile strength and poor crack resistance. The failure of components often initiates from minute fissures and propagates [1,2,3,4]. The segments, as the primary prefabricated component and lining unit in shield tunneling, their quality directly impinges on the overall security of the tunnel [5,6,7]. Despite the relatively high strength and extensive employment of reinforced concrete segments [8,9,10,11,12], they are susceptible to damage and cracking during manufacturing, transporting, and construction, on account of being fragile, thickset, and unwieldy, resulting in cracks or even damage. Although such superficial damage does not permeate into the interior of the segments, it can engender corrosion and expansion of the steel reinforcements, diminishing the load-bearing capacity of the segments, exacerbating the damage, compromising the integrity of the lining structure, and eventually culminating in leakage and collapse, which is inimical to the durability and safety of the tunnel structure [13,14,15,16]. The utilization of steel fiber reinforced concrete segments can ameliorate crack resistance, augment the damage tolerance of the segments, and thus enhance the structural safety of the tunnels.

The merits of SFRC are predominantly manifested in its superlative resistance to tension, bending, shearing, impact, fatigue, and crack propagation [17,18,19,20,21,22]. Bai et al. [23] examined the repercussions of steel fibers on the mechanical attributes of concrete, discovering that as the fiber volume fraction (V_f) of steel fibers escalated from 0% to 1.5%, the split tensile strength of SFRC exhibited a precipitous amelioration. However, a marginal diminution in the split tensile strength was discerned when the steel fiber V_f ascended from 1.5% to 2%. Rakul Bharatwaj Ramesh et al. [24] arrived at an analogous inference, discerning a pronounced enhancement in the split tensile strength with increasing steel fiber V_f from 0% to 0.7%, whereas the tensile strength dwindled when the V_f rose from 0.7% to 1%. This implicates that the inclusion ratio of steel fibers necessitates prudent consideration during SFRC preparation to optimize its mechanical properties. Gondokusumo, Gilbert Sebastiano, et al. [25] propounded a novel unified equation to predict the residual flexural tensile strength of normal weight, lightweight, and ultra-lightweight SFRC. Huang Biao and Li Biao [26] employed acoustic emission technology to investigate the axial compression behavior of SFRC, eliciting that steel fiber incorporation augmented the peak compressive strength and peak strain of SFRC. Moreover, elevations in both the quantity and aspect ratio of steel fibers conduced to heightened peak compressive strength and strain of SFRC, thereby enhancing its ductility and toughness. Conversely, Ali R. Khaloo et al. [27] deduced that steel fibers exert negligible influence on the compressive strength of concrete across disparate strength grades.

Steel fibers dispersed ubiquitously within the concrete matrix of tunnel-lining segments not only ameliorate the microscopic constitution of the concrete substrate, conferring enhanced homogeneity and thereby retarding carbonation and diffusion, but also defer the emergence of peripheral fissures in the concrete segments, thus prolonging the time until cracking compared to plain concrete. To compensate for the adverse repercussions of concrete cracking on the segments, numerous scholars have endeavored to augment the properties of the concrete through the incorporation of fibrous materials. Albert dela Fuente et al. [28] proposed that steel fibers embedded in the segments take effect at the inception of cracking, thereby enabling control over crack width and spacing, while simultaneously enhancing the segment’s load-bearing capacity and crack resistance. Alejandro Nogales et al. [29] collated preceding numerical simulations on steel fiber concrete and implemented a nonlinear three-dimensional finite element technique to separately simulate steel fiber concrete fragments, full-scale unbending segments, and flexing segments under the impetus of a Tunnel Boring Machine (TBM). The simulation outcomes align with experimental results and further underscore the significance of residual flexural strength at a crack width of 0.5 mm in regulating cracking. Angelo Caratelli et al. [30,31] conducted mechanical testing on SFRC and conventional reinforced concrete segments. Their flexural assay results unveiled superior crack control performance in SFRC. Therefore, when evaluating crack width rather than ultimate load-bearing capacity of segments, as in well-geologically conditioned tunnels, SFRC proves to be a superior option.

Extant research elucidates that the emergence of fissures and damage in shield tunnel-lining segments constitutes a precipitous menace to the secure operation of tunnels. However, conventional concrete reinforcement methodologies are beset by certain limitations in durability and mechanical attributes, thereby hindering the efficacious and economically viable redress and fortification of shield segments. Consequently, there is an exigent need for an efficacious and rational reinforcement and restoration technology to mitigate cracked and damaged shield segments. Presently, owing to the capacity of steel fibers to augment the compressive and tensile strengths of concrete segments in conjunction with enhancing their toughness and crack resistance, they have engendered profound attention from numerous scholars, resulting in a plethora of research outcomes. This article surveys the state-of-the-art research findings pertaining to SFRC and furnishes a summative examination of the performance enhancements of shield tunnel-lining segments, thereby introducing a novel perspective for the rectification and reinforcement of shield segments (See

Table 1. Examples of Design and Application of Steel Fiber Reinforced Concrete in Tunnel Segments [32].

Engineering Project	Time	Tunnel Length (m)	Tunnel Diameter (m)	Segment Size (Thick (mm)/Length (m)/Wide (m))	Compressive Strength	Steel Fiber Model	Steel Fiber Content (kg/m3)
Subway Extension: Contract 34 Germany	1990–1996	2400	7.27	400/3/-	C40/50 MPa	ZC 50/60	60
Metro Subway Tunnel Italy	1992	3000	6.4	300/-/-	-	ZC 50/50	40
Cigar Lake Uranium Mine Canada	1998&2005	800	4.2	300/2.4/-	120 MPa	RC 80/60 BN	50
Water Tunnel Ecuador	2000	5500	4	200/3/-	C40/50 MPa	RC 80/60 BN	30
CTRL Railroad Tunnel United Kingdom	2000–2004	25000	6.84&8.15	350/2.3/2.7	60 MPa	RC 80/60 BN	30
Oensberg Rail Tunnel (Hydro) Switzerland	2000	340	12.04	300/-/1.7	-	RC 80/60 BN	60
Heathrow Picadilly Extension United Kingdom	2006	3200	4.5	150/-/1	80 MPa	RC 80/60 BP	30
Brightwater Sewer Tunnel-East USA	2006–2009	4282	5.87	254/2.1/1.5	C40/50 MPa	RC 80/60 BN	35
Airport Link /Northern Busway/ARU-Brisbane Australia	2010	6700	12.5	400/-/-	-	RC 80/60 BN	35
Sydney Desal Tunnel Australia	2009	4000	3.2	250/2.1/1.5	S40(75 MPa)	RC 80/60 BN	35
Singapore MRT Circle Line Phase 6	2021	4500	6.5	300/1.5/-	60 MPa	RC 80/60 BN	35

).

2. Constituent Materials of SFRC

2.1. Steel Fiber

The mechanical properties and material characteristics of SFRC are largely contingent upon the volume and content of steel fibers, with the fibers’ l/d ratio also impacting its mechanical performance.

Table 2. Different V_f and l/d Ratio’s Impact on the Mechanical Properties of SFRC.

Number	Steel Fiber Shape	Steel Fiber Size			Steel Fiber Content (%)	Mechanical Property
		l/(mm)	d/(mm)	l/d		CS	STS	FS	TS
1	Hooked-end [42]	50	1.01	48	0.19, 0.38, 0.76	√	√	√
2	Hooked-end [43]	35/45	0.8/0.5	43.75/90	1, 1.5, 2, 2.5	√		√	√
3	Short steel fibers [44]	25	0.3	83.33	0.25, 0.5, 1	√	√
4	Hooked-end [45]	60	0.9	66.7	0.33			√	√
5	Hooked-end [46]	60	0.75/0.71	80/85	0.33, 0.67, 1	√	√	√	√
6	Hooked-end [39]	60	0.75	80	0.3, 0.6, 0.9, 1.2	√		√	√
7	Hooked-end [47]	30	0.69	44	1.2, 2.5, 4	√		√	√
8	Hooked-end [48]	25/50	0.5	50/100	0.5, 0.75, 1, 1.25	√			√
9	Hooked-end [49]	35	0.55	64	0.5, 1.5, 2.5	√
10	Straight shape [50]	30	0.5	60	0.8, 1.2, 1.6, 2		√
11	Hooked-end [51]	33	0.55	60	0.2, 0.375, 0.55, 0.75	√			√
12	Hooked-end [52]	30	0.52	57.69	0.6, 0.9, 1.2	√
13	Hooked-end [53]	50	0.75/0.71	67		√
14	Hooked-end [54]	30/40/50/60	0.75	40/53/67/80		√	√		√
15	Hooked-end [55]	35	0.55	64	0.5, 1, 1.5				√
16	Wave-shape [56]	35				√	√		√
17	Hooked-end [57]	35/60	0.55/0.75	64/80	0.5, 1, 1.5	√			√
18	Hooked-end [58]	16	0.4	40	1, 2, 3	√		√
19	Hooked-end [59]	33/60	0.75	44/80		√	√		√
20	Hooked-end [60]	35	0.55	64	0.5				√
21	Hooked-end [61]	30/35	0.3/0.6	100/58	0.5, 1, 1.5	√	√	√
22	Hooked-end [62]	25	0.5	50	1.2.3.4.5	√		√	√
23	Hooked-end [63]	30	0.55	55	0.5, 1, 1.5	√	√	√
24	Hooked-end [64]	30	0.52	58	0.6, 1.2, 1.8	√
25	Hooked-end [35]	40/50/60	0.62/0.62/0.75	65/80/80	0.5, 1, 1.5	√			√

Annotation: (√) = property has been investigated, CS = compressive strength, TS = splitting tensile strength, FS = flexural strength, STS = splitting tensile strength.

encapsulates the effect of different V_f and l/d ratios on the mechanical properties of SFRC. The introduction of steel fibers bolsters the SFRC matrix, markedly enhancing its crack resistance and achieving a higher tensile strength. Steel fibers generally measure between 6–80 mm in length and have a cross-sectional area approximately within the 0.1 to 1.5 interval [33]. For SFRC with steel fibers featuring diameters within the 0.15–1.2 mm range, its tensile strength resides within 300–2400 MPa [34]. The l/d ratio of steel fibers ranges from 60–100; an excessively large l/d ratio can engender an uneven distribution of the material’s mechanical properties. Considering different l/d ratios, Abbass, Wasim et al. [35] constructed an analytical model of the stress-strain relationship of fiber concrete. Marar, Khaled et al. [36] executed toughness-compression tests on hooked end SFRC with l/d ratios of 60, 75, and 83. When the l/d ratios are 60, 75, 83, and V_f is 2%, the compression toughness is respectively 6.33, 7.56, and 8.38 times that of ordinary concrete. Too high or too low steel fiber content negatively impacts the performance of the concrete; research indicates that the optimal mechanical performance is exhibited when steel fiber content is maintained between 0.5–2.5% [37,38]. Torres, Juan Andres et al. [39] explored concrete with five different V_f of steel fiber content: 0%, 0.3%, 0.6%, 0.9%, 1.2%, and found that the maximum failure load is attained when the steel fiber content is 1.2%. Ordinarily, the geometric shape of steel fibers can be categorized into straight, corrugated, hooked-end, edged, and bent-hook forms. However, steel fibers initiate functionality now of pull-out. Hooked-end fibers can substantially augment mechanical properties and toughness [33,40], and they demonstrate good adhesion performance with the concrete. Li et al. [41] utilized straight, corrugated, and hooked-end types of steel fibers to examine the bending performance, fracture patterns, and fracture processes of SFRC, concluding that the SFRC containing hooked-end fibers exhibits the most optimal bending performance.

2.2. Cement

Cement is primarily composed of two basic ingredients: limestone and clay. However, the amount of cement used significantly exceeds the theoretical requirement, with only a small fraction of the cement particles undergoing hydration reactions. Most of the cement particles act as filler materials in the matrix, without realizing their potential value [65,66,67,68]. Consequently, there is a need to optimize material components, or to introduce inert materials like construction waste or high-added-value materials like industrial and agricultural solid wastes to replace cement, without compromising performance. This is to solve the economic and environmental issues posed by the substantial usage of cement. In recent years, the use of improved design methods and mixing techniques has resulted in the production of ultra-high-strength concrete. An ideal cementitious material can effectively bind together fibers, aggregates, cement particles, and other materials. SFRC can utilize various types of cement, including Portland cement and sulfate-resistant Portland cement, and can be effectively combined with various aggregates [69].

2.3. Aggregate Preparation

Aggregate preparation is a critical factor in SFRC. It primarily fills voids, enhances concrete strength, and improves workability. Common mineral admixtures include crushed stone, gravel, sand, silica fume, mineral powder, limestone, et al. [70,71,72,73]. Cement is the primary binder in SFRC, and its usage significantly exceeds the theoretical requirement. This is because only a fraction of the cement particles undergoes complete hydration, while the remaining unhydrated particles exist in the matrix as filler materials, enhancing the density and impermeability of the concrete [65,66,67,68]. Furthermore, the introduction of mineral admixtures (e.g., silica fume, fly ash) can partially replace cement, reducing its consumption and the associated environmental impacts. These admixtures also optimize the microstructure of the concrete, improving its workability and durability. With increasing infrastructure requirements, recycled aggregate concrete is also continually evolving. Aslani, Farhad, Liu et al. [74,75] used waste recycled concrete aggregate to prepare SFRC again, compared the test results with previous research results, established a linear model, and proved that the waste recycled concrete aggregate has excellent mechanical properties. Nehdi, Moncef L. [59] and others eliminated the influence of fibers on workability, castability, and consolidation in traditional SFRC by prefabricating aggregate and steel fibers. Jahandari, Soheil, et al. [76] significantly improved the post-peak ductility of recycled aggregate concrete by adding silica fume to the aggregate. However, the preparation of SFRC also needs to ensure the appropriate aggregate size, usually between 5 mm–20 mm. An overly large aggregate size will cause fiber dispersion. Han et al. [54] studied the effect of the maximum particle size of coarse aggregate on the mechanical properties of SFRC. As the maximum particle size of coarse aggregate increased, the slump of concrete increased, and the compressive strength changed slightly. The inclusion of both mineral admixtures and aggregates in this section aims to emphasize their complementary roles in enhancing the performance of SFRC. Aggregates serve as the primary filler material, providing the concrete with structural integrity and strength. The particle size and distribution of aggregates significantly affect the workability and mechanical properties of SFRC, especially the uniform dispersion of steel fibers. Overly large aggregate sizes can disrupt fiber alignment, leading to uneven stress distribution within the matrix. On the other hand, mineral admixtures, such as silica fume and fly ash, contribute to the optimization of the concrete’s microstructure. They fill the voids between cement and aggregate particles, reduce porosity, and improve both the compactness and impermeability of the matrix. Additionally, mineral admixtures enhance the fluidity of the mixture, facilitating better fiber distribution and alignment. This synergistic effect between aggregates and mineral admixtures ensures that SFRC exhibits superior mechanical properties, crack resistance, and durability, making it suitable for demanding applications such as tunnel-lining segments. Han et al. [54] studied the effect of the maximum particle size of coarse aggregate on the mechanical properties of SFRC. For SFRC applications, especially for tunnel-lining segments, the recommended maximum particle size of coarse aggregate ranges from 20 mm to 25 mm. This range ensures an optimal balance between workability and mechanical properties. Larger aggregate sizes may disrupt the uniform distribution of steel fibers, creating voids or “shadow areas” that weaken the matrix. Maintaining the particle size within this range enhances the compactness and durability of SFRC. As the maximum particle size of coarse aggregate increased, the slump of concrete increased, and the compressive strength changed slightly. Therefore, to make the aggregate perform its best performance in tunnel-lining segments, it is necessary to control the size of the aggregate and the addition of admixtures.

2.4. SFRC Chemical Additives

Chemical additives, including water reducers, thickeners, and enhancers, are used to improve the workability, fluidity, and crack resistance of concrete [77,78,79]. These additives can adjust the properties of concrete such as fluidity, setting time, and strength as needed. However, the additives should be tested for compatibility, and the dosage should be determined by concrete mix design tests. When air-entraining agents are mixed with water reducers or other additives in the same water solution, their miscibility should be considered to prevent flocculation of the admixture solution. Among them, water reducers are often used to disperse cement particles through electrostatic repulsion and spatial steric hindrance, thereby reducing the attraction between particles and releasing free water aggregated in the flocculation structure [80,81,82]. Yoshioka et al. [82] proposed that water reducers have a significant selective adsorption behavior on different mineral phase surfaces. Atarashi et al. [83,84] and Sakai et al. [83] found that naphthalene-based water reducers and carboxylic acid-based water reducers have selective adsorption phenomena on the surfaces of clay and limestone powder. Enfedaque, A. [85] and others conducted experiments on SFRC with and without chemical additives. Because the binder modifier can significantly increase the voids in the matrix, while the introduction of binder modifiers may increase the voids in the matrix, experimental studies [85] indicate that this effect can sometimes lead to enhanced strength due to improved toughness and fiber dispersion. This phenomenon highlights the importance of optimizing the dosage and compatibility of chemical admixtures in mix design to achieve a balance between porosity and overall performance; the strength of the test block can be increased by 40%.

2.5. Water Demand

Water is a critical component of concrete, serving to amalgamate cement and aggregate into a cohesive substance, lending fluidity and plasticity to the concrete. Water reacts with the hydratable compounds present in cement, resulting in the formation of calcium silicate hydrates and other cementitious substances [86,87]. These cementitious products fill the voids between the aggregates, enhancing the strength and hardness of the concrete. Water facilitates the wetting of aggregate surfaces, fostering a superior bond between them and the cementitious matrix. This process contributes to the uniformity and consistency of the concrete.

A judicious amount of water can modulate the fluidity of concrete, endowing it with suitable plasticity and workability. By regulating the water content, one can control the workability properties of concrete, such as its flowability, slump, and compactibility [88,89]. The amalgamation of water with cement, aggregate, and other admixtures forms a uniform cement paste, ensuring the equitable distribution of all components within the concrete, thus guaranteeing its overall performance. Nevertheless, it is critical to note that the amount of water added should be controlled based on the concrete’s mix proportion and design requirements. Both excess and inadequate water can detrimentally impact the performance of the concrete. Therefore, the water content should be meticulously controlled to ensure that the quality and performance of the concrete meet the design specifications.

3. Performance Characteristics of SFRC Segmental Linings

The inclusion of fibers in concrete can markedly ameliorate its structural attributes such as static flexural strength, impact strength, tensile strength, ductility, and bending toughness. The degree of these enhancements is contingent upon several factors, including the size, type, aspect ratio, and volumetric fraction of the fibers.

3.1. Workability of SFRC

In SFRC research, it is widely agreed by numerous scholars that the l/d and V_f are critical parameters impacting the performance of concrete [90,91,92]. Yazıcı et al. [93] noted that the l/d ratio of steel fibers typically falls between 50 and 100, and an increase in this ratio might cause uneven distribution and flocculation of fibers in the concrete mix. Moreover, they emphasized the significant impact of V_f on the workability of concrete, with the optimal V_f values generally ranging from 0.5% to 2.5% of the concrete volume. However, an increase in l/d and V_f values may decrease the workability of concrete.

These observations are corroborated by the research of Shah and Rangan [94], who investigated the implications of the random distribution of smooth steel fibers in concrete and mortar. They discovered that the length, orientation, and rigidity of the fibers have considerable influence on the post-crack resistance of the materials. Further validation of these findings is provided by the experiments of Nagarkar et al. [91]. They indicated that while the addition of fibers can enhance the compressive, splitting, tensile, and bending strength of concrete, it may also diminish the workability of the concrete.

The research conducted by Iqbal et al. [95] on the impact of hooked-end steel fibers and straight steel fibers on concrete properties aligns with previous findings, suggesting that the workability of SFRC declines as the content of steel fibers increases. The research findings of Çelik T and Marar K [96] further extend this observation, indicating that an appropriate number of fines can enhance the cohesiveness of concrete and prevent bleed water. However, an excess of fines can increase the water requirement and potentially damage the bond between aggregate and cement paste.

The investigation by Mohammadi et al. [97] delves deeper into the influence of fiber aspect ratio on concrete performance. They discovered that lower fiber aspect ratios result in higher compaction factors and shorter slump cone times. In comparison to fibers with higher aspect ratios, those with lower aspect ratios demonstrate a significantly reduced tendency to clump in concrete, leading to a more uniform distribution of the concrete mix. They also found that, even with a higher V_f, the excessive aggregation of fibers in SFRC with small aspect ratios diminishes, allowing the concrete mix containing shorter fibers to display superior workability. This trend is also demonstrated in the research conducted by Figueiredo et al. [98], as depicted in Figure 1. Figure 1 illustrates the impact of fiber volume fraction (V_f) and aspect ratio (AR) on the workability of Steel Fiber Reinforced Concrete (SFRC). The left panel demonstrates that as V_f increases, the inverted cone time rises, indicating a reduction in workability. This effect is more pronounced for higher AR values (e.g., AR = 100), where the increase in cone time is steepest, highlighting the adverse influence of longer fibers on flowability. The right panel shows a linear decrease in slump with increasing V_f, with AR = 100 again having the most significant impact. These results suggest that higher fiber content and larger aspect ratios negatively affect SFRC’s workability, emphasizing the importance of optimizing V_f and AR to balance mechanical performance and constructability.

Eren and Celik [99] studied the effects of silica fume and steel fibers on certain properties of high-strength concrete. The results reveal that increases in silica fume and fiber content reduce workability. Although silica fume influences compressive strength, the volume percentage of steel fibers has a negligible impact.

3.2. Physical Properties

The unit weight of concrete is found to increase with the incorporation of fiber content [100,101]. For instance, Duran and Okan [100] conducted a precise determination of the unit weight of fresh fiber-reinforced fly ash concrete. The results demonstrated a continuous and uniform increase in the unit weight of the concrete with the increment of fiber content, while an opposite trend was observed with the increase in the fly ash content—that is, the unit weight decreased. Overall, this series of observations found that the unit weight of fiber-reinforced concrete is often higher than that of non-fiber concrete.

Additionally, Düzgün [101] provided further validation of this phenomenon. He conducted detailed testing on the unit weight of concrete specimens with different fiber contents under pumice aggregate conditions. The results showed that when the steel fiber content increased from 0% to 1.5%, the unit weight of the concrete increased by 5.6%, 6.5%, 5.9%, and 8.5%, respectively, under different pumice aggregate content conditions, as shown in Figure 2. These data provide a thorough exhibition of the correlation between steel fiber content and the unit weight of concrete.

On the other hand, a plethora of research studies affirm the effective reduction of concrete’s slump by the addition of steel fibers. Alsaif et al. [102], for instance, undertook a study comparing concretes with varying fiber concentrations to fiberless variants. He observed that the slump diminished by 5%, 13%, and 56%, respectively, when the fiber concentration was 0 BF, 30 BF, and 60 BF. Gao and colleagues further corroborated this outcome [103], discovering a notable slump reduction in fiber-reinforced concrete as compared to the fiberless version.

In addition, Kim et al. [104] conducted a meticulous investigation of concrete’s slump and air content, finding an inverse relationship between the slump value and the volumetric fraction of steel fibers. They also noted an increasing trend in air content within the concrete with the escalation of the volumetric fraction.

Regarding the impact of fibers on the air content in concrete, scholarly consensus is yet to be achieved. Bayasi and Pickering, among others [105,106], concluded that fibers augment the air content in concrete, an increment dependent on the type and form of the fibers. In contrast, Balaguru et al. [107] argued for a decrement in the air content of concrete with fiber addition, and even noted a marginally higher air loss rate in SFRC. Despite these diverging viewpoints, Guerini [108] offers a reconciling perspective. Upon analyzing various samples of steel fiber concrete with different concentrations, they discovered that both steel fibers and coarse synthetic fibers subtly increase the variability and overall air content in concrete, with the air content rising in tandem with an increase in fiber V_f.

3.3. Mechanical Performance of SFRC

The mechanical performance of Steel Fiber Reinforced Concrete (SFRC) is significantly influenced by key parameters such as the fiber volume fraction (Vf) and aspect ratio (l/d). Studies indicate that an optimal Vf range of 0.5–2.5% can enhance compressive strength by 8–12%, splitting tensile strength by 15–45%, and flexural strength by 10–40%, while also improving toughness. However, excessive Vf (>2.5%) may lead to fiber clustering and decreased workability, complicating construction. The aspect ratio (l/d) also plays a crucial role; a range of 60–75 provides the best balance between strength and toughness. Excessively high l/d values (>80) can result in uneven fiber distribution, negatively affecting compressive strength. Fiber shape further impacts performance, with hooked fibers showing superior tensile and flexural strength due to their excellent bonding properties, achieving flexural strength improvements of up to 35%. Additionally, the incorporation of fine aggregates can improve fiber dispersion and reduce bleeding, thus optimizing the homogeneity and workability of SFRC. These findings underscore the importance of balancing Vf, l/d, and fiber morphology to maximize the mechanical advantages of SFRC.

3.3.1. Compressive Strength

A multitude of studies [109,110,111,112] have underscored the pivotal association between the V_f and the compressive strength of the material. For instance, Ding [113] carried out comprehensive uniaxial compression tests on cubic specimens, shedding further light on the positive influence of fiber addition on concrete’s compressive strength. While this impact might not be significant quantitatively, it bears profound implications. Of note, Ding discovered that steel fibers could transform concrete, which is inherently prone to fracture, into a material with superior ductility, markedly enhancing the overall compressive deformability of concrete.

Moreover, Yang’s [114] study lends further credence to this theory. He conducted uniaxial compression tests on concrete specimens containing different amounts of fibers, plotting stress-strain curves and relationships between compressive strength and fiber V_f. His findings distinctly indicated an upward trend in concrete’s uniaxial compressive strength with increasing V_f, as shown in Figure 3. The results of Hooton RD’s [27] and Balendran’s [115] research echoed this observation. Although they noticed an improvement in the splitting tensile strength and modulus of rupture when a lower V_f of fiber was incorporated into concrete, the enhancement in compressive strength was relatively minor.

Furthermore, the aspect ratio of steel fibers has been confirmed to significantly impact the compressive strength of SFRC [116,117,118]. In the research by Bayramov et al. [116], they tested the compressive strength of concrete blocks with various aspect ratios. They found that an increase in the aspect ratio of fibers from 55 to 65 resulted in an increase in the compressive strength of the concrete blocks. However, when the aspect ratio was further increased to 80, the ductility of the blocks improved, but the compressive strength, contrarily, declined. This aligns with Ding’s [113] research, further unveiling the critical role steel fibers play in the transformation of concrete properties—from brittle to ductile.

3.3.2. Splitting Tension

The Split Tensile Test, widely acknowledged as an effective indirect method for measuring and evaluating the tensile strength of concrete [119,120,121,122], has been the focus of numerous scholars aiming to elucidate the impact of steel fiber content on the splitting strength of concrete.

Hao [123] provide an insightful perspective on this issue. Their experimental design included concrete samples with different V_f of steel fiber (0%, 0.5%, 1.0%, and 1.5%). By comparing their splitting strengths (as shown in Figure 4), Figure 4 shows the relationship between fiber volume fraction (V_f) and both unit weight and tensile strength of SFRC. The upper section illustrates the quasi-static splitting tensile test setup, while the lower graph indicates that as V_f increases, both unit weight and tensile strength rise. This suggests that higher fiber content improves the tensile strength of SFRC, enhancing its resistance to cracking under tension. They drew a significant conclusion: Concrete samples with 1.5% steel fiber demonstrated a splitting strength that exceeded fiberless samples by up to 60 times. This finding accentuates the crucial role of fiber V_f in enhancing the tensile performance of concrete.

This observation is not an isolated case. Indeed, numerous researchers [124,125,126] have concurred that the splitting strength of concrete samples increases with the V_f of steel fiber. For instance, Yang [114], while conducting splitting strength tests on concrete samples (150 mm in diameter and 300 mm in length), meticulously documented the entire process of increasing fiber content from 0% to 2.0%, as shown in Figure 5a. He observed that the toughness of the concrete mainly improved when V_f did not exceed 1.0%. However, when the fiber content surpassed 1.0%, the toughness and strength of the concrete increased significantly, even exhibiting strain-hardening behavior when fiber content reached 2.0%. Supplementing this, Hao [123] and Qu [127] further corroborated the above findings. Hao [123], by constructing mid-scale models and comparing the splitting strength of original concrete and SFRC with different fiber VV_ff (1.0%, 2.0%, and 3.0%), as shown in Figure 5b, confirmed the influence of fiber. Qu [127] extended the research perspective by using spherical concrete samples and found that the splitting strength of SFRC also increased with the rise in steel fiber V_f.

It’s worth highlighting that the shape of the fibers also imparts a notable influence on the tensile strength of SFRC. Iqbal et al. [95] compared the split tensile strength of SFRC reinforced with hooked and straight fibers. They discovered that with an increase in fiber V_f from 0 to 0.75%, the split tensile strength of SFRC reinforced with hooked and straight fibers enhanced by 19% and 13% respectively. They attributed this phenomenon to the superior pull-out resistance exhibited by hooked fibers in contrast to straight fibers, as depicted in Figure 5c.

3.3.3. Flexural Testing

A plethora of studies distinctly demonstrates that an increase in the fiber V_f significantly enhances the flexural stiffness of concrete, culminating in a manifestation of hardening behaviour [100,128,129,130]. Such observations reflect an augmentation in the SFRC material’s ability to resist deformation when subjected to bending loads [131,132,133].

Bolstered by Yang’s research [114], which undertook experiments on concrete beams of dimensions 150 × 150 × 550 mm, the impact of differing V_f conditions on the load-deflection curve was elucidated as depicted in Figure 6. The experimental results demonstrated that when V_f reached 2.0%, the hardening behavior of the material was particularly prominent. The study found that the maximum load occurred at a deflection of 0.55 mm, a deflection significantly greater than the 0.06 mm at initial cracking. This experimental outcome was congruent with the mechanical test results of Zhao [134], their experimental data denoted that as V_f increased, the maximum load that the concrete beam could withstand, along with the total energy absorbed, concomitantly increased. Therefore, these studies collectively support the viewpoint that with the incremental addition of steel fiber, the toughness and load-bearing capacity of the concrete beam are significantly enhanced.

Further still, research by Sivapriya et al. [135] affirms these observational results. Their study contrasted concrete blocks with differing steel fiber content (0%, 0.5%, and 1%), measuring the flexural strength after a curing period of 28 days. Upon the integration of 0.5% and 1% steel fiber into the blocks, the flexural strength test results obtained via a single-point loading method revealed an increase in the concrete’s flexural strength and stiffness in accordance with the escalating fiber content. This observational result provides further validation to the preceding conclusion, namely, that the increase in fiber content can significantly enhance the flexural rigidity of SFRC and induce a hardening behavior.

3.3.4. Modulus of Elasticity

Within the realm of concrete engineering research, the elastic modulus is regarded as one of the principal properties of concrete, pertinent to its deformation capability when subjected to impact loads [136,137,138].

Given that the aspect ratio l/d and the V_f exert a substantial influence on all aspects of SFRC performance, academia has introduced the reinforcement index (RI), defined as a positively correlated empirical function linking the aspect ratio and V_f together. This has now been widely incorporated within empirical models predicting the strength of SFRC [139,140,141,142].

Grounded in the analysis of 60 groups of SFRC experimental data, Thomas et al. [141] have proposed a regression model for the prediction of the elastic modulus, the corresponding empirical equation can be derived from this, as shown in Equation (1):

$${\mathrm{E}}_{\mathrm{c}}=4.58({\mathrm{f}}_{\mathrm{c}\mathrm{u}}^{\prime }{)}^{0.5}+0.42({\mathrm{f}}_{\mathrm{c}\mathrm{u}}^{\prime })\mathrm{R}\mathrm{I}+0.39\mathrm{R}\mathrm{I}$$

(1)

Their experimental findings indicate that upon an increase in the V_f of steel fiber to 1.5%, the elastic modulus of the concrete enhances by 8.3%, aligning remarkably with the predictions of the empirical formula.

Lee [140] conducted similar investigations, deriving another set of empirical formulae for SFRC through numerous regression trials and juxtaposing them with prior studies, the corresponding empirical equation can be derived from this, as shown in Equation (2):

$${\mathrm{E}}_{\mathrm{c}}=(-367\mathrm{R}\mathrm{I}+5520){\mathrm{f}}_{\mathrm{c}}^{\prime 0.41}$$

(2)

Note: ${\mathrm{E}}_{\mathrm{c}}$ is modulus of elasticity, ${\mathrm{f}}_{\mathrm{c}\mathrm{u}}^{\prime }$ is 28-daycube compressive tension, $\mathrm{R}\mathrm{I}={\mathrm{V}}_{\mathrm{f}}{\mathrm{l}}_{\mathrm{f}}/{\mathrm{d}}_{\mathrm{f}}$, ${\mathrm{f}}_{\mathrm{c}}^{\prime }$ is the measured compressive strength.

From these research outcomes, it becomes abundantly clear that the performance of SFRC has a profound correlation with the geometric configuration of steel fibers—a fact further corroborated in Gul’s study [143].

However, despite the enhancement in the elastic modulus of concrete due to the introduction of steel fibers, the studies by Sharma [144] and Altun [109] found that this enhancement is not pronounced. Conversely, steel fibers play a more significant role in bolstering the durability and ductility of the concrete.

3.3.5. Toughness

Toughness is typically defined as the ability to absorb energy [117,145]. SFRC generally boasts superior toughness, which has proven extremely effective in applications such as concrete road surfaces [146] and shield tunnel segments [147,148,149,150]. Academically, the toughness strength of SFRC is commonly evaluated using a Toughness Index (TI).

Johnston and Skarendahl [150] posited that the toughness of SFRC is primarily determined by the content, type, and geometric shape of the steel fibers. Extensive research indicates that the toughness of SFRC increases with the augmentation of the fiber V_f and l/d ratio [151,152,153].

To quantify the impact of steel fibers on SFRC toughness, Marara et al. [154] employed a compressive proportional toughness standard, which is defined as the area under the complete stress-strain curve. Through stress-strain curves under different steel fiber V_f and different l/d ratios, it was generally found that the toughness of SFRC materials improved as the steel fiber V_f and l/d ratio increased, as shown in Figure 7.

Balendran et al. [115] performed tests on concrete specimens of different sizes and found that the TI decreases as the specimen size increases. Consequently, it is recommended that the size effect on toughness be considered when designing the ductility of SFRC structures.

Furthermore, the toughness of SFRC is also somewhat affected by the type of steel used in the fibers [155,156]. Aslani et al. [156] found, through their studies, that the toughness of SFRC comprised of alloy, polypropylene, and steel fibers experienced substantial enhancement.

Conversely, fiber shape can also impact the overall structure’s toughness. Through their research, Choi et al. [157] confirmed that the bending toughness of SFRC with circular fibers was significantly improved compared to that with straight fibers, as shown in Figure 8. When the V_f increased from 0.20% to 0.40%, the impact on the TI value was minimal, but as the fiber diameter decreased, the TI value correspondingly increased.

3.3.6. Durability

Shield tunnel construction grapples with multifaceted and variable environments, encompassing typical challenges such as salt erosion and low temperatures [31,158,159]. These factors bear the potential not only to disrupt the structural integrity of the tunnel segments, thus lessening their waterproofing capabilities, but also to undermine the aggregate performance of the tunnel lining [160,161]. Liu’s investigation [162] unveiled that the most susceptible segment of subaqueous shield tunnel-lining structures lies at the junction of the tunnel segments. Conversely, Xing [163] embarked on an examination of the load-bearing capacity of tunnel segments under the corrosive influence of operational environments, established a degradation model for segment load-bearing capacity, and pursued an in-depth exploration of corrosion models for the principal rebar in tunnel segments under the conditions of carbonation and chloride erosion.

In colder regions, tunnel construction imposes stringent requirements on the frost-resistance capabilities of the tunnel segment structures [164]. Thus, to guarantee the durability of tunnel lining under diverse environmental conditions, shield tunnel segments must possess sufficient resistance to combat a variety of detrimental factors [165].

Based on these challenges, Nehdi et al. [153] established a scale model for SFRC of different sizes, aiming to evaluate SFRC’s performance under these conditions. This scale model serves as a robust tool for assessing the composite performance of SFRC and lays the groundwork for future investigations pertaining to these issues.

Dieb et al. [166] employed the Rapid Chloride Penetration Test (RCPT), Concrete Resistivity Test, and Chloride Bulk Diffusion Test to holistically evaluate the durability and corrosion resistance of SFRC against chloride ion intrusion and chloride-induced corrosion. Figure 9a depicts the experimental setup for the loading test of SFRC tunnel segments, illustrating the testing apparatus and loading configuration. Figure 9 shows the durability testing setup for SFRC segments. Panel (a) depicts the experimental setup, including the loading cell and LVDTs. Panel (b) shows that higher fiber volume fraction (Vf) reduces water permeability. Panel (c) demonstrates that higher fiber content slows carbonation, improving the durability of SFRC segments over time. Their findings unveiled a trend where an increase in the fiber V_f would markedly lower the permeation speed of chloride ions, and the longer the ageing of SFRC, the lesser the region of total passed charge, further corroborating the outstanding corrosion resistance of SFRC.

Abbas et al. [167] further observed the durability differences between ordinary concrete segments and SFRC segments through comparative experiments. Their results exhibited an intriguing phenomenon: as V_f increased, the Volume of Permeable Voids (VPV) within the concrete significantly reduced, leading to a marked enhancement in the water-absorption capacity of SFRC, as depicted in Figure 9b.

In the realm of reinforced concrete design, Sun [168] discovered that increasing steel fiber while decreasing steel rebar usage could effectively narrow crack width, thereby augmenting the durability of the tunnel segments. Specifically, the crack widths of shallow-buried and deep-buried tunnel segments decreased by 26% and 18%, respectively.

Freeze-thaw cycles have a significant adverse impact on the crack resistance and strength of concrete materials, but in this regard, the performance of SFRC is striking. Atis et al. [100] observed in their study that after 50 freeze-thaw cycles, SFRC significantly improved its freeze-thaw resistance compared to ordinary concrete, and the proportion of strength loss was only 5%. They attributed this improvement primarily to the random distribution of fibers in the concrete mixture, which effectively inhibited the extension of cracks and thus mitigated the damage of freeze-thaw to the concrete.

Building on this, Niu et al. [169] further revealed the impact of the fiber V_f on SFRC performance. They found that when the fiber V_f was 2%, the crack resistance of SFRC and its resistance to freeze-thaw cycles reached the optimum, once again verifying the decisive role of fiber distribution for concrete strength and freeze-thaw performance.

These two studies jointly revealed the superior performance of SFRC in freeze-thaw environments, providing strong scientific support for its application in practical engineering.

From the above research, it is not difficult to see that whether it is salt erosion or freeze-thaw damage, it is triggered by the cracking of the lining material. The key influencing factor of concrete material cracking is the carbonation of the material [170]. Carbonation causes a decrease in the alkalinity of the steel-reinforced lining, which causes the rebar to start corroding. This corrosion process further leads to expansion and cracking of the lining structure, impacting its overall structural stability. In addition, carbonation can change the internal constitution of the concrete components, including its permeability and porosity, thereby directly affecting the durability of the concrete lining [171].

Wang et al. [172] carried out a series of tests on the carbonation depth of SFRC with different V_f values (0%, 0.5%, 1.0%, and 1.5%) through rapid carbonation experiments. They proved that when V_f is fixed at 1.5%, SFRC can significantly reduce the degree of carbonation, thereby reducing the generation of cracks, as shown in Figure 9c.

Wang et al. [172] carried out a series of tests on the carbonation depth of SFRC with different V_f values (0%, 0.5%, 1.0%, and 1.5%) through rapid carbonation experiments. They proved that when V_f is fixed at 1.5%, SFRC can significantly reduce the degree of carbonation, thereby reducing the generation of cracks, as shown in Figure 9c. This phenomenon occurs because an increase in V_f enhances the crack-bridging effect of steel fibers, preventing crack propagation and limiting CO₂ diffusion pathways. Additionally, higher V_f improves the compactness and impermeability of the concrete matrix, further reducing its susceptibility to carbonation.

4. Applications of SFRC Segments in Shield Tunnels and Deficiencies in Existing Shield Tunnel Segments

Steel Fiber Reinforced Concrete (SFRC) has been widely adopted across various engineering fields due to its exceptional mechanical properties, durability, and crack resistance. Its versatility makes it an essential material in infrastructure projects such as tunnel linings, building components, highways, and water structures. To better illustrate the current distribution and development trends of SFRC applications,

Table 3. Application Distribution and Development of SFRC.

Application Field	Application Proportion (%)	Typical Projects	Development Trend
Tunnel Engineering	40	Shanghai Metro, UK CTRL Railway Tunnel	Increasing adoption, now the primary field
Building Engineering	30	High-rise building components (e.g., Beijing-Tianjin New Airport)	Significant growth in high-rise applications
Road and Bridge Engineering	20	Highway bridge decks (e.g., Sydney Harbour Bridge), airport runway pavements	Driven by demand for durability and crack resistance
Water and Port Engineering	5	Freshwater hydraulic projects (e.g., Three Gorges Dam), port and dock structures	Suitable for high-impact and anti-corrosion scenarios
Other Fields	5	Specialized fields (nuclear power plant structures, military engineering)	Stable demand with significant potential for innovation

provides a comprehensive overview of its utilization in different industries, showcasing its growing prominence in engineering practice.

4.1. Problems Associated with Shield Tunnel Segments

4.1.1. Detriment of Segmental Cracking and Damage

The majority of shield tunnel segment failures stem from the formation and propagation of concrete cracks. Inappropriate maintenance during construction or other factors may lead to the emergence of minute cracks on the surface and within the segments, as illustrated in the Figure 10. Under the influence of operational load conditions and environmental factors, these microcracks expand. Some coalesce to form macrocracks, culminating in the failure or breakdown of the shield tunnel segments [173,174]. Concurrently, cracks in the segments compromise their impermeability and durability. Chen et al. [175] simulated segmental cracking defects during the construction and operation phases, concluding that cracks often appear in locations such as the ring joints, bolt holes, and manholes. Chen et al. [176] found that cracking or damage near the bolt holes of some segments occurred during construction. By analyzing stress cloud diagrams and sectional stress isotherms, they determined that the cracking and damage occurring during construction were induced by the relative torsion between segments. Tian et al. [177] established a three-dimensional numerical model of shield tunnel segments with initial cracks, from which they derived the crack initiation load and limit load for the tunnel segments. Utilizing the Paris fatigue law, they simulated crack propagation under fatigue loads. Their results showed that the deformation and damage of the segments exhibited distinct stages, including initial cracking, crack initiation, stable crack propagation, unstable crack propagation, and ultimate failure.

4.1.2. Defects in Traditional Material Properties

Common reinforcement methods for conventional concrete structures include increasing the cross-sectional area, external steel jacket reinforcement, concrete replacement, fiber composite pasting, and the addition of supports (as depicted in the image showing fiber fabric and steel plate reinforcement). Ai et al. [178] achieved desired results by reinforcing tunnel segments with fiber fabric, pointing out that early-stage fiber fabric reinforcement can reduce maintenance costs. Both carbon fiber plates and internal steel reinforcement significantly enhance resistance to segment cracking [179]. Other research [180] involving fiber integration in concrete segments suggests that fibers can serve as structural responsibility-bearing materials, although the cost of carbon fiber plate reinforcement is relatively high.

The elevated price of carbon fiber plates, combined with demanding construction techniques and high technical requirements, lead to higher overall costs. Moreover, the durability and lifespan of carbon fiber plates might be limited due to potential exposure to ultraviolet light, high temperatures, and humidity over prolonged usage. Regular inspections and maintenance are necessary to ensure their performance and effectiveness. Their adaptability to environmental conditions is relatively poor, and in extreme conditions, such as in frigid or high-temperature, high-humidity regions, the performance of carbon fiber plates might be compromised. The reinforcement process requires professional construction techniques and experience, including careful control of the binding quality of materials, the flatness of the plates, and the quality of docking. Improper construction could impact the reinforcement effect and durability.

Thin steel plates also provide effective reinforcement for tunnel segments [181]. Zhao et al. [182] analyzed the influences of reinforcement timing, steel plate thickness, and adhesive strength on the reinforcement effects of steel plate pasting on tunnel segments. Liu et al. [183] studied the reinforcement schemes for shield tunnel segments of Guangzhou Metro Line 1, examining the load-bearing performance and failure mechanisms of steel plate-reinforced shield tunnel lining. Using numerical simulation, they discussed the mechanical behavior, deformation characteristics, and failure patterns of jointed tunnel lining after steel pasting reinforcement.

However, as steel plates are relatively heavy, their usage for reinforcement adds to the overall weight of the segment, potentially impacting the load-bearing capacity of the original structure and the stability of the foundation. Additionally, steel plates are susceptible to corrosion when exposed to humidity, moisture, and chemicals, which might lead to damage and weaken the reinforcement effect. The reinforcement process with steel plates requires cutting, welding, and fixing, extending the construction period, and thereby escalating costs. Furthermore, steel plate reinforcement might struggle to adapt to segments with significant curvature changes, potentially leading to suboptimal reinforcement effects or even localized stress concentration (See Figure 11).

4.1.3. SFRC Shield Tunnel Segments

SFRC presents certain advantages in terms of mechanical properties, durability, and manufacturability.

Table 4. Performance Comparison Between SFRC and Other Reinforcement Methods.

Reinforcement Method	Durability	Construction Period	Crack Resistance	Ductility and Plasticity	Life Span
Paste fiber cloth	Strong corrosion resistance	Convenient construction	Better improvement of crack resistance performance	Can significantly improve	There are certain limitations
Bonding steel plate	Epoxy resin adhesive poses a risk of aging, and exposed chemical anchor bolts and steel plates require rust prevention treatment	Steel plate reinforcement requires processes such as cutting, welding, and fixation, with a relatively long construction cycle	Can significantly improve	Can be improved to a certain extent	Short
SFRC	Not affected by factors such as humidity, corrosion, and ultraviolet radiation	Convenient construction	It can form a three-dimensional dispersed network structure to prevent the expansion of cracks	Good ductility and plasticity	Effectively extending the service life of tunnel segments

shows a comparison of SFRC reinforcement with traditional materials. SFRC is a new type of multiphase composite material created by incorporating a certain volume of randomly distributed steel fibers into regular concrete, making it suitable for casting and spray forming. The presence of numerous randomly distributed steel fibers within the concrete significantly enhances its toughness and tensile strength, curbing the development and expansion of cracks as well as deformation characteristics. This reinforcement technique considerably alters the physical and mechanical properties of ordinary cement-based concrete [184,185].

The inclusion of polypropylene and steel fibers in the concrete segments of the Shanghai Metro Line M6 has notably improved the local compressive bearing capacity of the segments, offering effective assistance in bearing construction loads [186]. Zhang et al. [187] have also employed SFRC shield tunnel segments, demonstrating that SFRC segments are more conducive to the long-term development of the metro. Zhang Fan and others analysed the stress-strain relationship of SFRC tunnel segments under their bearing capacity limit state, proposed a calculation formula for the bearing capacity of SFRC tunnel segments under their limit state, and determined the required volume of steel fibers in the segments. After manufacturing two SFRC tunnel segments, they conducted a flexural test. The study found that SFRC tunnel segments do not develop fine and dense cracks before failure. The inclusion of steel fibers in the segments enhances their crack resistance and reduces the failure rate.

4.2. Analysis of Cracking of Shield Tunnel Segments Made of Different Materials

Sun Bin [188] studied the true performance of different materials (plain concrete, steel fiber reinforced concrete, reinforced concrete) under critical loads. Regardless of the loading method, the plain concrete specimen will undergo brittle cracking when displaced by a few millimeters, and the specimen will crack into independent pieces. During central impact, the main crack always appears in a closed plane formed by constraints on both sides. The cracking situation of plain concrete is shown in Figure 12a. The first vertical crack in reinforced concrete occurs at the top of the specimen and then develops to the bottom of the specimen. The same process can also be observed from the occurrence of cracks. The specimen deviates from its original position, and the concrete at the bottom of the specimen peels off. In addition, when the displacement of the impact head is 3 mm, the reinforced concrete specimen cracks, which does not occur in the steel fiber reinforced concrete specimen. The cracking situation of reinforced concrete is shown in Figure 12b. When the dosage is 30 kg/m³, it is found that the first vertical crack of steel fiber reinforced concrete occurs at the top of the specimen, and then slowly develops towards the bottom of the specimen. Before the crack reaches the bottom, the second crack will also appear at the upper part of the specimen, with the main crack slightly closer to the lower part of the specimen. This process can also be observed from the generation of other cracks. During the experiment, the specimen did not peel off or shift, and the bottom of the specimen remained intact. The degree of failure of the specimen under eccentric loading is the smallest. The cracking situation of SFRC is shown in Figure 12c.

Based on the above crack analysis and the test report provided by the Sigma concrete laboratory [189], the following conclusions can be drawn. The central impact load of reinforced concrete specimens is largely influenced by the hoop effect, which masks the advantage of stress dispersion in SFRC. The peak load of reinforced concrete and the strongest steel fiber concrete has reached over 420 kN, while the peak load of plain concrete is only 300 kN, and the peak load of steel fiber concrete and reinforced concrete has increased by 40%. Therefore, SFRC has advantages in performance in the application of shield tunnel segments.

4.3. Impact of SFRC on the Performance of Shield Tunnel Segments

4.3.1. Failure Mode and Mechanism of Steel Fiber Reinforced Concrete Segments

SFRC constitutes a heterogeneous multiphase composite material, demonstrating non-uniform stress and strain distributions even under uniformly applied loads [190,191]. The crack-resistant effect of steel fibers correlates with the energy required for fiber pull-out. The energy required for slow crack growth increases with crack extension until it reaches a critical value where rapid crack propagation and fracture occur [192].

When the load exerted on the tunnel lining reaches 30% of the limit load, cracks expand into the fiber-reinforced region [193,194]. In the lengthwise direction, the propagation is gradual, accompanied by the emergence of minor secondary cracks between the initial ones. The fiber’s crack-resisting action slows this extension. Within the pure bending zone of the lining, when the applied load reaches 50% of the limit load, there is virtually no crack propagation. As the load continues to increase, the primary crack’s width significantly expands, but the extension in length is still not prominent. When the load reaches 80% of the limit load, the main crack length notably extends, the deflection of the lining significantly increases, and micro-cracks continuously emerge in the tensile zone of the lining, gradually extending until the concrete in the compressive zone of the lining crushes and fails. The microscopic view of shield tunnel-lining failure is shown in

Table 5. Detailed View of Shield Tunnel Segment Failure [193,194].

Segment Failure Stage	Microscopic View of Failure	Segment Failure State
Crack failure has not started		Crack propagation has not started
Formation of crack tip		Cracks begin to form and slowly and steadily expand, with cracks extending up to 1/4 width in the longitudinal direction of the pipe segment
Crack propagation		The crack continues to expand to the fiber-reinforced area, and is obstructed by fibers and expands slowly or changes direction around the fibers; Cracks can extend up to 1/2 width in the longitudinal direction of the pipe segment
Increased crack propagation		The cracks slowly expand due to the fiber’s anti cracking effect and exhibit a “plastic” characteristic; Cracks can extend up to 3/4 width in the longitudinal direction of the pipe segment
Crack propagation through		The crack propagates through the fiber-reinforced zone, with decreasing resistance to expansion and further increasing in length, until it fully penetrates longitudinally through the segment.

.

4.3.2. Analysis of the Effect of SFRC on Improving the Stiffness of Pipe Segments

The inclusion of steel fibers serves to augment the stiffness of concrete, enabling it, due to the high modulus and rigidity of these fibers, to better resist deformation under external loading. This enhancement yields shield tunnel segments more stable under load, thereby diminishing their deformation and deflection. During shield construction, factors such as alterations in groundwater levels and soil subsidence may instigate segment settlement. SFRC can curtail this settlement deformation, ensuring the segments retain their stiffness and stability.

Xu et al. [195] and his team utilised SFRC as the segment material, applying FLAC 3D to the construction of pure SFRC tunnel segments in hard rock. Their results indicated that SFRC effectively mitigates crack formation and more efficiently regulates surface subsidence, sidewall convergence, and crown settlement. Mingshan Qi and Xian Liu [196], among others, performed four-point bending tests on standard-reinforced concrete segments and fiber-reinforced concrete hybrid-reinforced segments. The research demonstrated that the inclusion of fibers and the augmentation of fiber V_f significantly increased the segments’ stiffness and load-bearing capacity, given the constancy of longitudinal reinforcement. Due to the elevated elastic modulus of steel fibers, SFRC segments possess superior load-bearing capacity in comparison to synthetic fiber concrete segments [53,196,197].

Zhang et al. [198] conducted comparative tests on the flexural stiffness of the cross-section of SFRC segments and standard concrete segments. As the bending moment amplifies, cracks propagate, and the segment stiffness incrementally diminishes. Simultaneously, the increment in segment stiffness attributed to SFRC gradually reduces. Under various load levels, the bending moments individually borne by the SFRC and reinforcement in the tension zone are depicted in Figure 13.

4.3.3. Analysis of the Effect of SFRC on Improving Joint Stiffness

At the joints, due to significant load and deformation, the risk of crack formation and expansion is heightened. The crack-resistant properties of SFRC can alleviate cracks at the joint, thus enhancing joint rigidity. Zhou et al. [199,200] carried out full-scale pressure-bending tests on SFRC joints, and the experimental results consistently indicated that SFRC could increase the ultimate load-bearing capacity of the joints.

Gao et al. [201,202] proposed respective constitutive equations for fiber concrete under compression through a complete curve test. The constitutive model proposed by Lv et al. [203] demonstrates superior applicability for the calculation of SFRC joints. Zhang et al. [198] used this formula to determine that the stiffness of SFRC joints can be increased by 18.7% compared to ordinary concrete joints.

Liao et al. [201] and others conducted a three-dimensional nonlinear finite element numerical simulation test on the stress and strain of the SFRC pipe segment joints, and the stress and strain distribution of the joint is shown in Figure 14. The calculation results indicate that under the design load condition, the maximum radial deformation of the segment is approximately 0.16 mm. The maximum principal tensile stress on the longitudinal joint is 6.0 MPa, the maximum principal compressive stress is −19.9 MPa, the maximum principal tensile strain is 161 × 10⁻⁶, and the maximum principal compressive strain is −519 × 10⁻⁶. Simultaneously, the principal tensile strain near the manhole is smaller, about 51 × 10⁻⁶, indicating that the manhole design meets requirements. Taken together, these findings demonstrate that SFRC can have a substantial enhancing effect on the rigidity of pipe segment joints.

4.3.4. Analysis of SFRC’s Enhancing Role on Structural Load-Bearing Capacity

The shield method is widely used in tunnel construction in soft soil environments, and its main constituent material is usually traditional reinforced concrete (RC) tunnel lining. However, due to the low tensile strength and brittleness of the concrete itself, the RC tunnel lining is prone to cracking or damage during the process of manufacturing, maintenance, handling, and installation. The appearance of such macro-cracks has adverse effects on the safety, permeability, usability, and durability of the tunnel [204,205]. For this issue, the application of SFRC has received widespread attention from scholars, especially its performance in improving structural load-bearing capacity [21,181,206,207].

Liu et al. [208] compared the load-bearing capacity of ordinary reinforced concrete and steel fiber reinforced concrete in the inclined beam test model, revealing the advantages of SFRC in improving the mechanical properties of construction (as shown in Figure 15a). The study further found that when SFRC replaces ordinary concrete, with the increase in fiber V_f, the ultimate load-bearing capacity and energy-absorption capacity of the construction are also enhanced (as shown in Figure 15b).

Further, Ding et al. [113] compared the load-bearing capacity of symmetrically inclined beams with different reinforcement ratios and fiber contents. Their research results found that whether it is ultimate load, post-peak load-bearing capacity, or energy-absorption capacity in the entire deflection region, steel fibers can effectively improve its performance. Tom et al. [209] focused on the research of the shear resistance of SFRC. They found that adding fibers can significantly increase the maximum shear load-bearing capacity of reinforced slender beams, especially when the V_f of steel fibers is 1%; its shear capacity is 128% higher than ordinary concrete.

5. Conclusions

With the incessant development of urban rail transit, issues of fissures and damage frequently transpire in shield tunnel segments, engendering a deterioration in overall structural stability of tunnels and posing substantial hazards to tunnel operations. Given its superlative mechanical attributes, durability, and crack resistance, SFRC has demonstrated excellent efficacy in the reinforcement of shield tunnel segments. This paper revisits the state-of-the-art research outcomes regarding SFRC, systematically encapsulating salient issues including its material composition, properties, methods of segment reinforcement, and constitutive models of SFRC segments, alongside enhancements in SFRC segment performance. Deriving from an extensive literature review, the subsequent conclusions are deduced based on the discoveries furnished by researchers:

(1) SFRC is composed of steel fibers, cement, aggregates, chemical additives, and water. The appropriate content, volume, and aspect ratio of steel fibers significantly enhance the tensile performance of SFRC. Optimizing the material components and introducing alternative materials can help address the issues associated with high cement usage.

(2) The performance of SFRC is significantly influenced by the aspect ratio (l/d) and fiber volume fraction (V_f). The optimal range for V_f (0.5–2.5%) maximizes the material’s performance, while a balanced aspect ratio (50–100) prevents uneven fiber distribution. However, increasing either fiber content or aspect ratio can reduce the workability of the concrete.

(3) Increasing the steel fiber content significantly improves the compressive ductility of SFRC. Higher fiber V_f enhances the splitting tensile strength and flexural performance, leading to notable improvements in toughness and durability compared to unmodified concrete segments.

(4) SFRC exhibits superior crack resistance compared to plain and reinforced concrete. Cracks propagate more slowly in SFRC, keeping the specimen intact under stress. SFRC also mitigates concrete fragmentation, spalling, and corrosion, enhancing its durability, prolonging its service life, and reducing maintenance costs.

(5) Steel fibers are uniformly distributed within the concrete matrix, which improves the load-bearing capacity and crack resistance of SFRC. This enhances the stiffness, stability, and safety of segments, especially in shield tunnels, making SFRC an effective material for improving segment durability and overall performance.