The relevant studies and main findings of HFECC with steel fiber are summarized in Table 2.

Steel fiber is the most widely used reinforcing material in cementitious composites, effectively enhancing the strength and shrinkage crack resistance of ECC [42]. Synthetic fibers, on the other hand, offer significant advantages such as low cost and reduced density. Consequently, considerable research has focused on steel-synthetic hybrid fiber ECC for producing high-strength, low-shrinkage materials.

Since relatively low volumes of PE fiber can achieve high ultimate tensile strain, early studies often utilized PE-based systems, leading to investigations into steel-PE HFECC. Ahmed and Maalej [19,41] examined the uniaxial tensile and flexural behaviors of steel-PE HFECC. Their results indicated that steel fibers enhance the tensile and flexural strength, while PE fibers contribute to higher ultimate tensile strain and deflection. Similarly, Xie et al. [33] found that steel fibers greatly improve resistance to crack growth but reduce the material's ability to dissipate energy and its axial tensile strain. When added in appropriate amounts, however, they provide superior crack control by limiting microcrack width, which enhances durability. Increasing the length of PE fibers (e.g., from 12 to 18 mm) and incorporating fly ash (at 50% cement replacement) were found to improve strain hardening and multi-cracking capacity. However, higher fine aggregate content was shown to degrade strain hardening performance.

Zhao et al. [42] developed 3D-printed steel-PE hybrid fiber ECC, which exhibited compressive strength exceeding 90 MPa and interfacial split tensile strength more than 50% higher than that of mono-PE fiber ECC. This improvement is largely attributed to the high stiffness of steel fibers, which tend to align during printing, enhancing crack-bridging efficiency. Thus, even a small amount of steel fiber in 3D-printed ECC significantly improves structural integrity—a crucial factor for printed concrete applications. It should be noted, however, that the uniaxial tensile strain capacity decreased by approximately 19% with the hybrid system. Therefore, the mix proportion of hybrid fibers must be carefully optimized according to specific engineering requirements.

Given that PVA fiber is more cost-effective than PE fiber (approximately one-eighth the price) and provides toughening effects adequate for most engineering applications, increasing attention has been directed toward steel-PVA HFECC. Ahmed et al. [41,45] investigated the flexural performance of steel-PVA HFECC. Under a constant total fiber volume of 2.5%, a mix with 1.5% PVA and 1% steel fibers exhibited the highest flexural strength and deformation capacity. While steel-PVA HFECC showed superior flexural strength compared to steel-PE systems, the latter demonstrated better post-peak load retention and deformation capability [41]. Soe et al. [43] studied the elastic modulus, flexural, and uniaxial tensile properties of steel-PVA HFECC using fiber volumes of 1.75% PVA with 0.58% steel and 1.5% PVA with 0.5% steel. Their findings indicated that increasing steel fiber content significantly enhances composite strength, whereas higher PVA fiber content does not markedly improve ultimate tensile strain. Some studies, however, reported limited strength improvement with steel fiber addition [30], highlighting the importance of compatibility between the matrix and fiber type.

In recent years, research has expanded to examine the behavior of steel-PVA HFECC with different matrix designs to achieve multifunctional composites and broaden application potential. Zhang et al. [44] studied the tensile and flexural properties of steel-PVA HFECC with a high-strength matrix. By incorporating mineral admixtures, a matrix strength of up to 108 MPa was achieved, resulting in a composite with a tensile strength of 8.1 MPa and an ultimate tensile strain of 0.5%. Their results confirmed that increasing steel fiber content in high-strength matrices improves both strength and strain capacity; however, to avoid significant degradation in tensile strain, the steel fiber content should be limited to around 1%. Additionally, steel fiber incorporation contributed to reduced crack width. Li et al. [37] investigated the mechanical properties of nano-silica modified steel-PVA HFECC. By adjusting superplasticizer dosage, the incorporation of nano-silica and hybrid fibers did not markedly reduce the workability of fresh mortar. Microstructural analysis revealed that although nano-silica increased overall porosity, it reduced pore size. Three-point bending tests showed that nano-silica enhanced strength without significantly compromising deformability, though it led to wider cracks. The hybrid steel-PVA fibers effectively controlled crack width, with the mix containing 2.5% PVA and 1.4% steel fibers exhibiting optimal flexural performance.

The relevant studies and main findings of HFECC with Synthetic Fibers of Different Sizes and Properties are summarized in Table 3.

Ahmed and Mihashi [46] investigated the influence of hybrid PVA fibers with varying lengths and thicknesses on the flexural fracture behavior of ECC with normal fine aggregate and lightweight aggregate matrices. Their results demonstrated that mixes containing 2% 12 mm long/thick PVA fiber plus 1% 6 mm long/thin PVA fiber, and 2% 24 mm long/thick PVA fiber plus 1% 6 mm long/thin PVA fiber exhibited optimal mechanical performance, achieving the highest flexural tensile strength, the maximum crack mouth opening displacement (CMOD) at peak load, and superior multiple cracking capacity. The flexural strength of ECC with normal aggregate was higher than that of lightweight aggregate ECC, while fine lightweight aggregate further outperformed coarse lightweight aggregate. Tosun-Felekoglu and Felekoglu [47] and Pakravan et al. [48] separately studied the flexural behavior of PVA-polypropylene (PP) hybrid fiber ECC. Both studies concluded that the hybrid fiber system did not significantly enhance the flexural strength compared to mono-PVA fiber ECC but improved the composite’s ductility, particularly in post-peak deformation capacity. Additionally, Tosun-Felekoglu and Felekoglu [47] emphasized that the matrix plays a decisive role in the mechanical performance of ECC, indicating that compatibility between the hybrid fiber system and matrix properties is critical.

Although PVA fiber is the most widely used fiber in ECC, high-quality PVA fibers are produced by only a few manufacturers in Japan, resulting in high costs that hinder large-scale engineering application [28]. To address this, several researchers have explored combining high-grade PVA fibers with lower-cost alternatives to reduce production expenses. Yu et al. [29] examined the feasibility of using recycled polyester fibers to partially replace PVA fibers in waterproof ECC. Flexural and uniaxial tensile tests indicated that replacing 50% of PVA fibers with recycled polyester fibers still yielded satisfactory mechanical performance, confirming the viability of this approach.

Domestically produced PVA fibers in China typically have less effective oil-coating treatment, leading to excessively strong fiber-matrix bonding and relatively poor strain-hardening behavior. However, their production cost is lower—approximately one-fifth that of Japanese fibers. Pan et al. [28] studied the mechanical properties of ECC incorporating hybrid fibers made from low-cost Chinese PVA fibers and Japanese PVA fibers. Through experimental investigation, they proposed three ECC mix proportions offering different mechanical performances and cost levels, providing practical options for engineering applications based on specific requirements.

The relevant studies and main findings of HFECC with Other Hybrid Fiber System are summarized in Table 4.

The addition of fibers is an effective method for reducing the brittleness of concrete. For decades, elastic fibers such as glass fibers have been incorporated to enhance strength; in recent years, the use of hyperelastic fibers like PP fiber has enabled significant strain-hardening behavior, leading to the development of ECC. While improving a single property such as strength is relatively straightforward, designing a multipurpose cementitious composite that balances both strength and deformation requires careful micromechanical design.

Shabakhty et al. [34] investigated cementitious composites reinforced with hybrid elastic/hyperelastic fibers (glass/PP) at volume ratios of 100/0, 75/25, 50/50, 25/75, and 0/100, and fiber lengths of 12, 25, 50, and 100 mm, under uniaxial tension. The results showed that PP fibers outperformed glass fibers in tensile strength, particularly as fiber length increased. Glass fiber composites exhibited an increase in tensile strength from 4.7 to 6.5 MPa when fiber length was increased from 12 to 100 mm, whereas PP fiber composites showed a more significant improvement from 3.7 to 11.3 MPa. In terms of strain capacity, mixes with 12 mm glass fibers reached 2.5% strain, while those with 100 mm PP fibers achieved up to 24% strain along with pronounced strain hardening.

In an attempt to develop ECC with high toughness and ductility at a low water-to-binder ratio of 0.25, Luo [49] incorporated high-modulus and high-strength carbon fibers hybridized with PVA fibers. However, test results indicated a reduction in both flexural toughness and ductility, with no significant change in strength. Similarly, Wang et al. [30] observed that adding basalt fibers or steel fibers to PVA-based ECC did notably improve uniaxial tensile strength but only moderately enhanced load-bearing capacity in the small-deformation stage. These additions considerably reduced the uniaxial tensile deformation capacity. It should be noted that the flexural deformation capacity was not significantly compromised with steel fiber incorporation. Conversely, the use of calcium sulfate allowed a reduction in PVA fiber content, lowering cost while maintaining deformation capacity and effectively increasing composite strength. Differing findings were reported by Tian et al. [50], who noted that adding basalt fibers to PE-based ECC increased crack strength by up to 40% at 1% fiber volume and tensile strength by up to 15% at 0.5% volume. However, when the basalt fiber content exceeded 0.5%, the ductility and multi-cracking capability of the hybrid system decreased, which is attributed to the differing reinforcement mechanisms of PE and basalt fibers under tension.

Ali et al. [26] incorporated shape memory alloy (SMA) fibers into a PVA fiber-reinforced ECC system and observed a significant improvement in the direct tensile capacity of the composite due to the addition of SMA fibers. Specimens containing 2% PVA and 1% SMA fibers exhibited the highest tensile capacity among all mixtures at the same testing age.

ECC with different hybrid fiber systems exhibits notable differences under varying strain rate loadings. Specifically, Maalej et al. [52] and Soe et al. [43] studied the dynamic tensile behavior of steel-PE and steel-PVA hybrid fiber ECC, respectively, under different strain rates. The results indicated that steel-PE HFECC maintained good strain hardening and multiple cracking properties within the strain rate range of 2 × 10⁻⁶ to 0.2 s⁻¹, showing no significant degradation compared to quasi-static loading. In contrast, steel-PVA HFECC demonstrated pronounced strain rate dependence: the first-crack strength and tensile strength increased with higher strain rates, while the deformation capacity decreased, accompanied by a reduction in strain hardening and multiple cracking capabilities. This distinct behavior may be attributed to differences in the bond properties between PVA or PE fibers and the matrix, warranting further investigation.

Zhang et al. [53] examined the effects of drop-weight impact on reinforced concrete, steel fiber-reinforced concrete, and steel-PE HFECC plates. The study confirmed that HFECC plates exhibited reduced damage extent, along with significantly improved ductility and energy dissipation capacity after repeated impacts. Ali et al. [26] reported in their drop-weight tests that the incorporation of SMA fibers enhanced the impact resistance of ECC. The mixture with 2% PVA and 1% SMA fibers showed the highest impact resistance among all specimens. However, exceeding this fiber volume led to issues such as fiber clustering, disrupted matrix continuity, increased porosity, and consequently reduced impact performance. Heat treatment activated the SMA fibers, introducing a local prestressing effect that further improved impact resistance, despite some damage caused to the PVA fibers during heating.

Similar conclusions were drawn by Maalej et al. [52] and Soe et al. [54] based on high-velocity projectile impact tests. HFECC significantly enhanced energy absorption and impact resistance through distributed multi-cracking mechanisms. Li et al. [55] investigated the influence of steel fibers on the impact compression performance of steel-PVA HFECC under different strain rates.

Wan [56] conducted low- and high-speed impact tests on steel-PVA hybrid fiber ECC and steel fiber ECC. Drop-weight impact tests demonstrated that steel-PVA HFECC beams exhibited significantly better flexural impact resistance than those with only PVA or steel fibers. However, high-speed impact tests using a Hopkinson bar indicated that steel fiber-reinforced cementitious composites outperformed those with only PVA or hybrid fibers in terms of impact resistance. Clearly, steel-PVA HFECC displays markedly different behaviors under low- and high-speed impacts. Overall, the incorporation of steel fibers enhances the impact resistance of ECC to varying degrees.

In summary, the impact resistance of hybrid fiber ECC is generally improved compared to single-fiber ECC. Hybrid systems such as steel-PVA/PE and SMA-PVA fibers exhibit significant positive synergistic effects. However, their performance varies noticeably under different strain rate conditions.

For engineering structures in cold regions, the freeze-thaw resistance of concrete materials is critically important. Yun [57] investigated the mechanical properties of PVA-PE HFECC after 300 freeze-thaw cycles and found that the multiple-cracking characteristics tended to degrade after cyclic freezing and thawing. This phenomenon deserves attention from both researchers and engineers, particularly in cold climates. In contrast to conventional ECC, which often show reduced tensile ductility at subzero temperatures, steel-PE HFECC exhibited improved ductility under the same conditions [58]. This enhancement can be attributed to two main factors: the increased slip capacity of steel fibers, which dominate the hybrid system, and the reduced fracture toughness of the matrix. Together, these effects contribute to the improved tensile ductility of HFECC at low temperatures. Wen and Cao [31] developed seawater sea-sand HFECC using hybrid calcium carbonate whiskers, stainless steel fibers, and PVA fibers. Similar to conventional HFECC, the addition of stainless steel fibers increased the strength but reduced the deformation capacity. In contrast, calcium carbonate whiskers were found to improve both strength and deformability.

Adjusting the fiber ratio can also improve the volume shrinkage of ECC, reduce cracking, and enhance long-term durability. Deng and Xue [59] investigated the shrinkage resistance of HFECC incorporating Chinese PVA fiber PVA fibers blended with cellulose fibers. Test results indicated that, at a water-cement ratio of 0.40, the hybrid fiber system more effectively controlled shrinkage deformation compared to systems using only Chinese or Japanese PVA fibers alone. According to Huang et al. [60], PVA fiber addition postpones crack initiation in ECC. Specifically, the cracking age of ECC containing 2% PVA fibers reaches 28.2 days, which is later than that of ECC mixes with PP or PE fibers. While Cao et al. [61] demonstrated that steel fibers effectively restrict drying shrinkage, whereas PVA fibers and CaCO₃ whiskers are more efficient against plastic shrinkage, respectively. Hence, a hybrid approach combining steel and synthetic fibers is recommended for comprehensive shrinkage control in ECC. Expansive agent can be used to further reduce the drying shrinkage of steel-PE HFECC [33].

Due to their low melting points, synthetic fibers are susceptible to degradation at high temperatures, whereas steel and basalt fibers exhibit superior high-temperature resistance. Therefore, hybrid systems combining steel/basalt with synthetic fibers can provide HFECC with higher residual strength after fire exposure compared to composites reinforced solely with synthetic fibers. For example, Li et al. [62] studied the high-temperature performance of PVA-steel HFECC and observed that the compressive strength remained largely unchanged up to 400°C, while the flexural strength, toughness, and stiffness gradually degraded with increasing temperature. This is primarily due to the melting of PVA fibers, which diminishes their bridging capacity, whereas the high thermal resistance and stiffness of steel fibers help maintain higher compressive strength within a certain temperature range. Similarly, Xiong et al. [63] reported that basalt fibers effectively inhibited crack development in PE-basalt HFECC even after exposure to high temperatures. The incorporation of basalt fibers increased the residual compressive strength by 5.3% after exposure to 200°C, and improved the tensile strength by 16.6%, 17.1%, and 56.3% at 200°C, 400°C, and 600°C, respectively. Consistently, Rawat et al. [64] also confirmed that the addition of basalt fibers enhances the residual strength of HFECC after high-temperature exposure.

A strong bond between steel reinforcement and the concrete matrix is essential for composite action and full utilization of the reinforcement’s mechanical properties. Shan et al. [2] investigated the bond behavior between steel-PE HFECC and steel bars, revealing significantly superior interfacial bond performance compared to that of ordinary concrete. During pull-out tests, the acoustic emission historical index displayed more intense peak signals, indicating enhanced bond integrity. However, the difference in bond behavior between HFECC and single-fiber ECC remains unclear.

In reinforced concrete members, reinforcement corrosion is a primary cause of durability degradation and reduced service life. A key factor in determining the widespread use of HFECC in reinforced structures is its ability to mitigate corrosion. Mihashi et al. [65] studied the chloride corrosion resistance of steel-PE HFECC reinforced beams. Their findings demonstrated that the hybrid steel-PE fibers significantly reduced chloride-induced corrosion of the reinforcement. After a long-term exposure of 52 weeks, the corrosion resistance of the HFECC beams was markedly better than that of mortar and mono-PE fiber ECC beams.

For high-strength, high-density concrete, spalling during fire exposure can compromise structural integrity, leading to reduced load-bearing capacity or even complete failure. Thus, researchers have examined the anti-spalling performance of reinforced HFECC slabs. Rawat et al. [64] investigated the spalling resistance of both single and hybrid PE fiber-reinforced ECC under fire conditions. Replacing 12 mm, 0.75% PE fibers using basalt fiber did not improve the spalling resistance of HFECC panels. Enhanced performance was achieved with longer PE fibers (18 mm) or through the incorporation of PP fibers. A hybrid mixture containing 12 mm 0.3% PP, 1.25% PE, and 0.75% basalt fibers demonstrated the best spalling resistance. Further analysis suggested that fiber melting may not be the main mechanism for improving spalling resistance. Instead, fiber distribution and bonding within the matrix play a critical role in forming an effective network for dissipating vapor pressure.

Given that the material cost of ECC is significantly higher than that of conventional concrete, its use is more advantageous in critical structural regions where enhanced toughness is required. For instance, in seismic design, the principle of “strong joints, weak members” is widely adopted, making ECC a potentially viable material for beam-column connections [56]. Precast concrete (PC) beam-column joints are critical components influencing the seismic performance of precast structures. Poor joint performance has led to the collapse of numerous PC buildings during past earthquakes. To address this, Ghayeb et al. [66] conducted a study on novel hybrid PC beam-column joints fabricated with HFECC containing 1.75% hybrid fibers (1% PVA and 0.75% hooked-end steel fibers). Quasi-static cyclic tests on three specimens—including one monolithic joint—revealed that the use of HFECC in the joint and adjacent beam regions prevented brittle failure due to the superior mechanical properties of ECC. The hybrid joints exhibited better seismic performance than the monolithic joint in terms of load-displacement capacity, ductility, strength, energy dissipation, and failure mode. The application of HFECC promoted ductile failure in the beam, thereby avoiding shear failure. Han et al. [67] further confirmed that ECC effectively suppresses the formation and propagation of shear cracks in beam-column joints, significantly improving load-carrying capacity, ductility, and energy dissipation. HFECC beam-column joints demonstrated better seismic performance both before and after peak loading compared to those with mono-PVA fiber ECC. Therefore, the use of steel-PVA HFECC in beam-column joints in high seismic risk regions represents a feasible design strategy.

Wu et al. [68] investigated the mechanical behavior of HFECC-normal concrete (NC) composite beams. A comprehensive analysis was carried out to evaluate the effects of fiber type, reinforcement ratio, and steel fiber content on the flexural performance of the composite beams. The results indicated that the stiffness of composite beams with 1.7% PVA fibers increased by 14.7% compared to NC beams, while those with 1.7% PE fibers showed a 26.1% increase. The addition of steel fibers at volume fractions of 0.6% and 1.0% enhanced the flexural capacity of PVA-based composite beams by at least 8.1%.

In addition to enhancing the toughness of new structures, existing engineering structures often require strengthening and rehabilitation to improve mechanical performance and durability, thereby extending their service life. For example, Maalej et al. [69] presented experimental results from quasi-static and low-velocity impact tests on unreinforced masonry (URM) walls strengthened with ECC. Under quasi-static loading, the ECC retrofit system significantly enhanced the out-of-plane resistance of masonry walls. The strengthened specimens exhibited increases in ultimate load capacity and deflection capacity by factors of 6.5–22 and 4.2–15.9 in Test Series I and II, respectively. Incorporating a steel mesh within the ECC layer further increased the ultimate load-bearing capacity by 40%–68%, although it reduced deflection capacity by 17%–74% compared to ECC-only specimens. The tests also confirmed that ECC exhibits strain-hardening behavior and multiple micro-cracking in masonry strengthening applications. Unlike URM walls, which failed catastrophically upon initial impact, ECC-strengthened walls sustained repeated low-velocity impacts without sudden collapse. The use of ECC layers considerably reduced fragmentation under impact, thereby lowering the risk of injury from flying debris in blast or explosion scenarios. In summary, Maalej’s study demonstrates that ECC-based strengthening systems significantly improve the impact resistance of URM walls and prevent abrupt failure. These results suggest that ECC-retrofitted masonry can help mitigate damage in events such as blasts or explosions. Li et al. [61] showed that reinforced concrete columns with an HFECC protective layer could prevent spalling and mitigate cracking under high temperatures, effectively enhancing the residual load-bearing capacity of the columns after fire exposure.

This review has systematically examined the recent advances in Hybrid Fiber Engineered Cementitious Composites (HFECC), with emphasis on micromechanical design theories, mechanical performance under quasi-static and dynamic loading, durability, and structural applications. The following key conclusions can be drawn:

(1) The incorporation of hybrid fibers, particularly combining high-modulus (e.g., steel) with low-modulus polymeric fibers (e.g., PVA, PE), enables a more tunable mechanical performance compared to single-fiber ECC. Through micromechanical design, HFECC can achieve improved multi-cracking and strain-hardening behavior, allowing designers to balance strength, ductility, and cost according to engineering requirements.

(2) The critical fiber volume theory provides a fundamental basis for optimizing hybrid fiber proportions. Both strength-based and energy-based criteria must be satisfied to ensure robust strain-hardening. Hybrid fiber systems often exhibit synergistic effects, enhancing crack control and composite toughness beyond what is achievable using individual fibers.

(3) Under quasi-static loading, steel-PVA and steel-PE hybrid systems significantly improve flexural and tensile strength while retaining high deformation capacity. The incorporation of high-volume fly ash and nanomaterials such as nano-silica can further refine microstructure and interface properties, contributing to higher performance and better crack width control.

(4) HFECC exhibits pronounced strain-rate sensitivity. Under low to high strain-rate loading, hybrid fibers effectively improve impact and blast resistance. Steel-PE systems demonstrate better retention of ductility under dynamic tension compared to PVA-based hybrids. The combination of shape memory alloy fibers with PVA also shows promising improvements in energy dissipation under impact.

(5) Durability performance—especially under freeze-thaw cycles, high temperatures, and chloride exposure—requires further investigation. Initial studies indicate that hybrid fibers can mitigate spalling under elevated temperatures and reduce shrinkage cracking. However, long-term durability under combined mechanical and environmental loads remains inadequately studied.

(6) Structural applications—including reinforced beams, beam-column joints, and composite members—demonstrate that HFECC can enhance load capacity, energy dissipation, and damage tolerance. Its use in critical regions (e.g., joints in seismic designs) and in repair/strengthening applications shows significant potential for improving structural resilience.

(7) Despite these advantages, high material costs—mainly due to the dependence on high-performance imported PVA fibers—still hinder large-scale implementation. The development of low-cost domestic fibers and optimized hybrid combinations remains essential for broader adoption.

Despite promising progress, several challenges and research opportunities remain for HFECC:

(1) Cost Reduction and Localization: There is an urgent need to develop and promote cost-effective domestic fibers and hybrid combinations. Further research should explore the use of recycled and natural fibers in hybrid systems to reduce cost and environmental impact without compromising mechanical performance.

(2) Dynamic and High-Strain Rate Behavior: The underlying mechanisms governing rate-dependent behavior in different hybrid systems are not fully understood. Further experimental and numerical studies are needed to clarify fiber-matrix interactions under dynamic loads and to establish standardized impact-resistant design guidelines.

(3) Durability under Multi-Action Environments: Comprehensive studies are needed to evaluate HFECC performance under coupled conditions—such as mechanical loading plus freeze-thaw cycles, chloride attack, or carbonation—especially for structures in harsh environments.

(4) Full-Structural Application Validation: Most existing studies focus on material-level properties. More large-scale experiments and real-world demonstrations are necessary to validate the structural performance and economic feasibility of HFECC in beams, columns, joints, and composite systems.

(5) Multi-Scale Modeling and Design Integration: Advanced numerical models that link micro-mechanical behavior to structural response should be developed. Integration of machine learning for mix-proportion optimization and performance prediction could further accelerate the design of tailored HFECC.

(6) Standardization and Specification Development: The absence of standardized testing and design codes for HFECC limits its regulatory acceptance. Efforts should be made to establish unified material specifications and design guidelines to support practical engineering application.

In summary, HFECC offers significant potential for next-generation resilient infrastructure. With continued research focusing on performance optimization, sustainability, and real-world validation, HFECC is expected to play an increasing role in advanced construction and rehabilitation projects.