Empowering 3D printed concrete: discovering the impact of steel fiber reinforcement on mechanical performance

The construction industry needs to adopt technological innovations to remain competitive in a rapidly evolving environment. Implementing advanced technologies can boost productivity, elevate quality standards, reduce costs, and ensure efficient use of resources. Integrating advanced technologies such as digitalization, automation, artificial intelligence, and building information modeling (BIM) offers significant opportunities to enhance productivity, operational efficiency and economic viability. As part of the ongoing digital transformation, a suite of automated methods for concrete shaping has developed, encompassing digital casting, slipforming, and diverse forms of 3D printing [1], [2], [3]. Among these, the extrusion-based 3D concrete printing (3DCP) has recently gained notable attention from researchers and industry for its potential to reshape traditional construction practices [4], [5], [6], [7]. 3D printing technology in construction offers several advantages over traditional construction. It allows for extensive design flexibility without requiring formwork and enhances construction efficiency by lowering labor costs, shortening construction timelines, and minimizing waste [8], [9], [10], [11], [12]. These potential advantages make 3DCP a compelling automated construction method for infrastructure development compared to traditional construction techniques [13].

Incorporating reinforcement into the concrete matrix during the printing process remains one of the most significant challenges to address for advancing the applications of 3D printing technology in construction. Conventional steel bar reinforcement is difficult to implement because the automated extrusion process is not designed to accommodate manual placement or embedment of rebar without interrupting the printing sequence. Continuous reinforcement strategies, such as inserting steel cables or wires during printing, have been investigated, but issues such as nozzle obstruction, misalignment, and lack of anchorage compromise efficiency [14]. The absence of standardized reinforcement strategies also hinders the adoption of 3DCP in structural applications, as current codes and design guidelines are tailored for conventionally cast reinforced concrete. The inclusion of fibers in the concrete matrix provides crack-bridging capability and residual tensile strength, thereby markedly improving the post-cracking behavior of concrete [15], [16], [17]. Adding short, discrete fibers to the printing materials offers promising potential due to its self-reinforcement capability, reducing complexity and time consumption [18], 19]. Research has demonstrated that inclusion of fibers in 3DPCM brings favorable improvements in its characteristics. Panda, Paul and Tan [20] reported that the mechanical properties of 3D-printed geopolymer mortar improve with the incorporation of short glass fibers, with optimal enhancement observed at additions up to 1 % of the total weight. An experimental investigation was conducted on the flexural behavior of 3D-printed concrete reinforced with different lengths and dosages of steel fibres. The findings revealed that specimens incorporating 6 mm steel fibers at volume fractions of 0.75 % and 1 % exhibited the greatest improvement in flexural strength [21]. However, problems like fiber agglomeration, reduced printability, and anisotropy in mechanical response due to preferential fiber orientation during extrusion exist. Therefore, developing reinforcement approaches compatible with automated deposition, enhancing multi-directional strength, and ensuring structural reliability remains one of the foremost barriers to scaling 3DCP for load-bearing structures.

This review paper examines the effect of steel fiber reinforcement on the fresh and hardened properties of 3DPCM. All aspects include a comprehensive literature synthesis of recent experimental work, emphasizing the effect of steel fiber type, geometry, volume fraction, and orientation on fresh state properties such as workability, yield stress, extrudability and buildability of the 3DPCM. This review also critically evaluates the impact of fiber content, geometry, and orientation on the hardened state properties, including compressive and flexural strength. Particular focus is on establishing the trade-off between the printability and structural performance, because post-cracking behavior can usually be enhanced by fiber addition, but may be at the detriment of flowability and extrusion stability. The scope is limited to steel fibers, excluding other discrete reinforcements such as synthetic or natural fibers, in order to provide an in-depth understanding of steel fiber-concrete interaction within the 3D printing process. The main objective of the review is to identify optimum fiber dosages and configurations reported in the literature that balance printability and mechanical performance. Challenges and limitations associated with steel fiber integration in 3DPCM, including anisotropy, fiber clustering, and printability-performance trade-offs, are also discussed.

The widespread adoption of 3DCP technology is still limited by key material shortcomings, particularly the brittle nature of cementitious materials and the lack of conventional reinforcement in printed elements. This review highlights the critical significance of steel fibers as a practical solution to these challenges. The significance of this review lies in the need to understand the effects of steel fiber addition on both fresh and hardened behavior of 3DPCM. Understanding these effects is important to bridging the gap between laboratory research and large-scale applications. This review explores how steel fibers affect rheology, printability, strength, cracking behavior, and durability, offering a clear guide for optimizing fiber-reinforced 3DPCM. These insights will help researchers, engineers, and industry professionals create better material mixes and design guidelines, supporting the wider use of 3D printing in construction.

In 3D concrete printing, the rheological requirements are contradictory. Concrete must exhibit high workability during pumping to facilitate extrusion, yet shift to low workability with high thixotropy afterwards to enhance buildability [22], [23], [24], [25]. Various research studies examined the relationship between the fresh and hardened properties of the mix and their impact on the robustness of the printing process, particularly in the context of large-scale adoption of concrete 3D printing [4], 26]. During deposition, the mix must retain its shape, which requires adequate yield stress in the fresh concrete. The thixotropic behavior of cement particles contributes to the buildup of yield stress at rest, enabling each filament to support subsequent layers [4]. The flowability of fresh concrete is influenced by the particle size distribution of the binder and sand, with a broader distribution proving more favorable [27], 28]. Achieving optimal print quality depends on the fresh properties of the mix in combination with the print path, which is primarily determined by the scale and complexity of the target geometry.

For 3DPCM, the mix design is a key factor determining printability, strength, and durability of the printed element. It should be precisely tailored to provide adequate flowability and smooth extrusion through the printing nozzle. Mix design also affects the mechanical performance of printed concrete, such as its compressive strength, flexural strength, and durability. In 3DPCM, adding fibers requires a balance between flowability, extrudability, and buildability, making it essential to identify optimal mix proportions. Traditional mix design methods for conventional concrete are unsuitable for 3DPC. Although recent studies have proposed empirical approaches, a standardized mix design method has not yet been established because of the limited research and application history of 3DPC. In many cases, mixture proportions are reported without a clear design methodology or explanation of how the parameters were determined [29], 30]. Most 3DPCM uses a fine-aggregate mortar-like mix with no coarse aggregates. Instead of traditional slump-based workability testing, rheology-based and extrusion tests are used to proportion the mix. Superplasticizers enhance flowability without increasing water content, while viscosity-modifying agents are commonly added to prevent segregation. Accelerators or retarders are used to regulate the setting time [31]. Recent studies agree that 3DPCM mix design requires iterative optimization, where material composition (binder type, fine aggregates, SCMs, fibers) and admixture dosage are balanced to meet three key properties, namely extrudability, buildability and interlayer bond strength, with extrudability and buildability being the most important fresh state properties [29], 32]. (Table 1).

Incorporating steel fibers into 3D printable concrete mixtures significantly influences their rheological behavior, particularly in terms of flowability. In the fresh state, 3DPCM requires a delicate balance between adequate flow for extrusion and sufficient buildability for layer stability. Due to their high stiffness and aspect ratio, steel fibres interact with the cementitious matrix through mechanical interlocking and frictional drag, which generally increases resistance to flow [44], [45], [46]. Xia, Geng [38] observed a significant decrease in the flow diameter of 3DPCM as fiber content increased from 0.1 to 0.9 %. This effect is more pronounced with longer steel fibers [38]. Longer fibers are more prone to bridging and entanglement, which increases resistance to flow, especially in narrow flow channels during the spread test. They also increase the likelihood of fiber clustering. The effect of steel fibers on flow diameter, as reported in the literature, is presented in Figure 1. It can be observed that in almost all studies, the inclusion of steel fibers reduced the flow diameter. However, the reduction rate varies significantly between studies due to differences in fiber length, geometry, aspect ratio, mix design, and superplasticizer dosage. In some studies, such as Xia (2025), effect is high, whereas in some studies such as Zhang, Zhu [47] and Singh, Liu [48], it is moderate or even flat curve is observed by Chen, Li [43]. Possible reason for sharp decrease in Xia’s results may be the absence of a viscosity modifier. In investigations, where viscosity modifier was used, the inclusion of fibers didn’t affect flow diameter significantly. The observed differences across studies underline the importance of tailoring fiber dosage and mix design to the printing system’s specific requirements, especially the optimized utilization of the viscosity modifier.

Figure 1:

Effect of steel fiber content on flow diameter of 3DPCM [38], 39], 43], [47], [48], [49].

The inclusion of steel fibers affects the fresh mix’s yield stress and plastic viscosity. Higher fiber content typically increases both, which can improve buildability but may hinder extrudability. Jia, Zhou [40] investigated how different shapes and amounts of steel fibers affect the rheological characteristics of fresh mixtures. As shown in Figure 2(a), the static yield stress of the fresh mixture was found to increase with higher fiber content, with a more pronounced rise observed beyond 1.0 vol% steel fibers. Up to 1.0 vol%, fibre shape didn’t affect static yield stress much but above this, flat end fibers resulted in a more prominent increase than hooked-end fibers. However, in another study by Xia, Geng [38], a significant increase in static yield stress was observed even at lower fiber dosage (<1.0 %) (see Figure 2(b). It was also observed that increasing the fiber length from 6 mm to 12 mm sharply increased static yield stress. It may be possible that longer fibers span larger matrix volumes, increasing mechanical obstruction during flow. This results in higher internal resistance, reflected as increased yield stress. Authors argued that incorporating steel fibers generates a three-dimensional framework within the concrete, similar to scaffolding, which impedes the material’s flow and elevates its static yield stress. Unlike Jia’s results, there’s a consistent linear trend suggesting that fiber length affects rheology more predictably than fiber geometry.

Figure 2:

Effect of steel fiber content on static yield stress of 3DPCM as reported by (a) Jia, Zhou [40]; (b) Xia, Geng [38].

Dynamic yield stress is the minimum stress required to initiate flow under shearing conditions (typically during pumping or extrusion). Steel fibers increase the dynamic yield stress of 3DPC proportionally to their content, length, and aspect ratio. While beneficial for mechanical integrity, this effect can significantly impair printability beyond certain thresholds. Arunothayan, Nematollahi [49] evaluated the yield stress buildup of 3DPCM with varying fiber content, i.e. 0 %, 1 % and 2 %. Steel fiber inclusion in 3DCP led to a marked increase in static yield stress at all resting periods, as shown in Figure 3. This increase is largely due to the rigidity of the steel fibers, which reinforce the composite’s internal resistance to flow. Substantial yield stress increase at 2 % fiber content suggests aggressive thixotropic stiffening. The high initial resistance may have resulted from fiber entanglement, surface friction, and alignment effects.

Figure 3:

Effect of fibers on yield stress buildup of 3DPCM with rest time [49].

The variation of dynamic yield stress with increasing fiber content, as reported in various research studies, is shown in Figure 4. At lower fiber contents (0–1 vol%), in the studies of Altheoy, Zaid [39] and Arunothayan, Nematollahi [49] show low yield stress values. However, beyond 2 vol%, a sharp increase is evident. The results of Xia, Geng [38] show a positive linear correlation between fiber content and dynamic yield stress at all fiber lengths. However, increased fiber length has a more significant effect, likely due to forming a more interconnected network that increases internal friction and resistance to flow. Differences in aspect ratio, stiffness, or surface texture may be the reason for such diverse trends. Longer or more rigid fibers increase yield stress due to higher entanglement and the blockage effect. Different base mixes (cement content, additives like silica fume, viscosity modifiers) may also affect the ability of fibers to alter rheology. Yield stress increases with fiber content, but the rate and threshold of increase are study-specific, influenced by fiber length, mix design, and processing.

Figure 4:

Dynamic yield stress versus varying fiber content [38], 39], 43], 49].

Printability of 3DPCM is referred to the ability of a cementitious mixture to be smoothly extruded, maintain its shape after deposition, and build up multiple layers without collapse or deformation. Despite the absence of standardized guidelines for evaluating the printability of 3DPCM, extrudability and shape stability are commonly acknowledged in the literature as essential criteria for characterizing the printability of cementitious mixtures [50], [51], [52]. Steel fibers can improve shape retention by increasing cohesion. However, excessive fiber content often leads to nozzle clogging and discontinuities in printing. It should be kept in mind that the balance between stiffening and flowability is essential for successful layer deposition.

The dosage of steel fibers has a critical and nonlinear effect on the printability of 3DPCM. Altheoy, Zaid [39] observed improved buildability and sufficient extrudability when using double-hooked steel fibers at 0.75 % in ultra-high-performance 3DPC. Li, Wei and Khayat [53], evaluated the extrudability of fiber reinforced 3DPCM by printing a single layer of 300 mm length and 40 mm width with various fiber volume contents ranging from 0.5 % to 2.5 %. It was revealed, as shown in Figure 5, that the filament with 2 % 6-mm steel fibers was extruded continuously and without clogging, maintaining a shape consistent with the desired width. At 2.5 % steel fiber content, the filament exhibited significant irregularities and interruptions, likely caused by nozzle clogging due to the high fiber concentration. Increased fiber volume restricts flow and promotes physical interlocking, making clogging more probable [53].

Figure 5:

Effect of steel fiber content on width of printed filament [53].

Xia, Geng [38] reported that 0.9 vol% steel fibers resulted in a more uniform filament width than polypropylene and basalt fibers (see Figure 6). They argued that the higher rigidity of steel fibers compared to polypropylene and basalt fibers is responsible for smooth extrusion due to the reduced possibility of clump formation. It was also noted that the increase in fiber content resulted in filament width closer to the designed initial width. This may be attributed to the interlocking effect of fibers and reduced fluidity.

Figure 6:

Extrudability of 50 cm long filament with different types of fibers [38].

To evaluate the buildability of 3D printed cementitious materials (3DPCM), researchers employ various methods, with the layer stability test being one of the most common and straightforward approaches. Some researchers have described the layer stability as the percentage of deformation in the bottom layer caused by the weight of the subsequent layer on top of it. At the same time, other researchers regarded shape stability as the deformed height as a percentage of the initial height of all printed layers. Zhang, Zhu [47] observed that shape stability improves with higher steel fiber content, reaching 80.0 % shape retention at 2.0 % fibers compared to 43.0 % in the control mix. This enhancement is attributed to greater interlocking between steel fibers and aggregates, forming a denser and more rigid skeletal network that enhances the deformation resistance of the 3DPCM.

However, in another study, the opposite trend was observed, as shown in Figure 7, where increasing the fiber dosage resulted in increased deformation of the bottom layer (18 % with 2.5 % fiber vol. compared to only 5 % of the control mix) [37]. It is important to note that the higher deformation in fiber reinforced 3DPCM may be due to a higher dosage of water reducing agent used to counter the negative effects of steel fiber on flow properties. In the study of Zhang, Zhu [47], the amount of water reducing agent was kept constant across all fiber contents. Additionally, higher sand-to-binder ratios of 1.4 and 1.8 were used by Giwa, Game [38], compared to 1 used by Zhang, Zhu [47].

Figure 7:

Effect of steel fibers on bottom layer deformation (a) sand/binder = 1.4 (b) sand/binder = 1.8 [37].

The outcomes of various research studies that evaluated the effects of steel fibre content on printability are summarized in Table 2.

The printability of 3DPCM incorporating steel fibers is significantly influenced by fiber length due to its effects on rheology, extrusion, and interlayer bonding. Shorter fibers generally enhance smooth extrusion and minimize nozzle clogging, whereas longer fibers improve layer interlocking but may induce fiber entanglement and reduce flowability. Yang, Wu [36] observed that adding 1 vol% of 6 mm fibers slightly reduced uniformity but still maintained a close layer thickness (5 mm ± 0.5 mm), though minor pits appeared due to nozzle drag. When 10 mm fibers were used at the same volume, extrusion became more uneven due to greater difficulty in extracting the mortar through the nozzle, with fibers occasionally clumping and extruding together, leading to inconsistent layer thickness as observable in Figure 8.

Figure 8:

Effect of steel fiber length on printability of 3DPCM [36].

Xia, Geng [38] also observed that steel fibers longer than 15 mm led to intermittent nozzle clogging, while fibers less than 10 mm length maintained continuous filament flow with minimal defects.

Incorporating steel fibers into 3DPCM has a distinct impact on compressive strength, primarily influenced by fiber volume fraction and the interaction between fibers and the matrix. Most published studies report an enhancement in compressive strength of 3DPCM with the addition of short steel fibers. Because printed specimens are produced without vibration and formwork, so their compressive strength is lower than traditionally mold-cast specimens. Generally, the compressive strength of 3DPCM enhances when fibers are utilized at an optimum volume percentage, which ranges between 0.5 % and 2 %. The higher fiber contents (>2.0 %) can reduce strength due to poor dispersion, fiber clustering, and increased porosity. Due to the inherent anisotropic behavior introduced during the 3D printing process, the compressive strength of 3DPCM is usually determined by loading applied along 3 different directions. Although different research studies intend to define independent loading directions, in this review study the loading directions defined are presented in Figure 9. Loading direction X is defined as when load is applied along the printing plane and perpendicular to print/layer direction. When load is applied along the printing plane and parallel to print/layer direction, it is defined as loading direction Y. Whereas, the loading direction Z is defined as the load applied normal to printing plane. If, in any research study cited here, the loading direction is defined different from this definition, it is converted to match the definition defined here.

Figure 9:

Loading directions for compressive testing defined in this review study.

As reported in the literature, the effects of varying steel fiber content on the compressive strength of 3DPCM are summarized and presented in Figures 10–12. The plot reveals significant variability in the influence of steel fiber content on compressive strength across different studies. It can be observed that in most of the studies, the incorporation of steel fibers tends to increase the compressive strength of 3DPCM. Zhang, Zhu [47] explains that, when a crack is initiated, the stress is transferred from the cement-based matrix to the fibers, which helps arrest crack propagation and enhances compressive strength through the fiber pull-out mechanism. As the fiber content increases, the spacing between fibers decreases, leading to greater fiber overlap and increased friction. This further restricts crack growth and contributes to improved compressive strength.

Figure 10:

Effect of steel fiber content on compressive strength in X direction [21], 33], 34], 36], 38], 40], 41], 47], 55], 56].

Figure 11:

Effect of steel fiber content on compressive strength in Y direction [21], 33], 34], 36], 38], 40], 41], 55], 56].

Figure 12:

Effect of steel fiber content on compressive strength in Z direction [21], [33], [34], [35], [36], [37], [38], [39], [40], [41, 47], 55], 56].

Singh, Liu [48] reported that the compressive strength is higher in the Z direction than in the X and Y directions due to the horizontal layers functioning like small beams stacked vertically, allowing the load to be distributed over a larger area. Another key factor is the interlayer adhesion, which can enhance the concrete’s overall strength by improving the bonding between layers. Fibers can form an energy-absorbing mechanism through mechanical anchorage, contributing to higher compressive strength and better structural integrity. Research by Giwa, Game [37] and Xia, Geng [38] shows an initial increase followed by a plateau, particularly beyond 1.5–2.0 % fiber content. This indicates a saturation point, beyond which fibers no longer contribute significantly or even impair performance due to clustering, workability loss, or void formation.

Figure 13 shows the percentage improvement in compressive strength of 3DPCM due to fiber reinforcement. The optimum fiber content values are also plotted. It can be observed that the Strength gain varies significantly with loading direction, suggesting that the alignment or orientation of fibers within the matrix plays a crucial role. A non-linear relationship is evident between fiber content and strength gain. While some increase in fiber volume results in higher strength, saturation or inefficiency appears beyond 2.5–3 %. It can also be noted that the loading Direction Z is the most favorable for compressive strength enhancement, likely due to better load transfer through fiber alignment.

Figure 13:

Percentage improvement in compressive strength and optimum fiber content [21], [33], [34], [35], [36], [37], [38], [39], [40], [41, 47], 55], 56].

Optimum fiber content lies between 1 % and 2 %, balancing mechanical benefit and material workability. However, in the case of 3DPCM, steel fiber content alone is not a deterministic factor. Its interaction with print process, material design, and fiber characteristics governs compressive strength outcomes.

Steel fiber inclusion in 3DPCM consistently enhances flexural strength by improving crack-bridging, post-crack load capacity, and energy absorption. Across studies, the magnitude of improvement depends on fiber dosage, geometry, and distribution within the printed layers. As shown in Figure 14, most studies show an increasing trend in flexural strength with higher steel fiber content, confirming that steel fibers enhance crack-bridging and post-crack load resistance. It can be observed that the high flexural strength of 45 MPa was achieved by Yang, Wu [36] with the inclusion of 1 vol% steel fibers. Shahzad, Abbas [56] also observed a constant increase in flexural strength reaching 40 MPa at 3 vol%. This curve suggests the mix tolerates very high fiber loading with minimal workability loss. Studies, including those by Yang, Wu [36], observed an initial increase up to a dosage of 0.5 vol% followed by a decline at higher dosages. It was observed that the flexural strength of high-strength or ultra-high-strength mixes, such as Yang, Wu [36] and Shahzad, Abbas [56], increased more steadily compared to normal strength mixes due to better anchoring of fibers in a dense matrix. Conventional mixes [37], 57] start low, and the strength gain is limited even with fibres.

Figure 14:

Effect of steel fiber content on flexural strength of 3DPCM [21], [34], [35], [36], [37], [38, 41], [55], [56], [57].

The percentage improvement in flexural strength and corresponding fiber volume content from various studies is presented in Figure 15. It can be observed that the gain in flexural strength is more pronounced than in compressive strength. The optimum fiber content to achieve the highest flexural strength varies across studies. Shahzad, Abbas [56] study shows the highest gain in flexural strength with 3 vol% steel fibers. While other researchers observed significant improvement in flexural strength at lower dosages as well such as 0.5 % fiber yielded a ∼50 % increase, suggesting that baseline material was relatively weak and therefore benefited noticeably from a small amount of crack-bridging reinforcement. Several factors, such as optimized rheology, consistent fiber orientation during extrusion, and strong interfacial bonding between fibers and cement paste, affect the performance of fiber-reinforced 3DPCM.

Figure 15:

Percentage improvement in compressive strength and optimum fiber content [21], [34], [35], [36], [37], [38, 41], [55], [56], [57].

Steel fibers are known to improve the post crack performance of concrete significantly. In 3DPCM, cracking is often governed by weak interlayer bonding and the brittle nature of the cementitious matrix. Unlike cast concrete, where reinforcement is embedded continuously, 3DPCM relies heavily on fiber reinforcement to improve ductility and toughness. The inclusion of steel fibers has been shown across multiple studies to transform brittle post-cracking failure into a more ductile, energy-absorbing behavior. However, the degree of improvement depends strongly on fiber content, orientation, and distribution. Without steel fibers, brittle fracture behavior was observed in 3DPCM samples by Zhang, Zhu [47] as compared to fiber reinforced 3DPCM. As shown in Figure 16(a), there is minimal residual strength and essentially no capacity to carry load after peak, confirming that plain 3DPCM behaves like a typical quasi-brittle cementitious material with poor crack resistance. However, adding 0.5 % steel fibers significantly modifies the curve (Figure 16b). The post-crack response shows a slight plateau after the peak load, indicating the crack bridging effect. This effect is more pronounced at higher fiber dosage as indicated by the strain-hardening response in Figure 16c and d, where load-carrying capacity after cracking is maintained and sustained over extended deflections. This performance confirms that higher fiber volume significantly enhances ductility and toughness.

Figure 16:

Effect of steel fibers on post-crack performance of 3DPCM (a) without fibers; (b) with 0.5 % steel fibers; (c) 1.0 % steel fibers; (d) 2.0 % steel fibers [47].

Similar findings were reported by [54] (shown in Figure 17), where both mold cast and 3DPCM matrix (without fibers) display sudden failure with negligible post-peak capacity. This reflects the brittle nature of matrix. Whereas the composite, i.e. with inclusion of fibers, exhibits enhanced post-crack behavior. It is evident that fibers fundamentally change the post-crack response. While all matrix specimens fail abruptly, composites exhibit ductile behavior with significant energy absorption. Steel fibers dramatically enhance flexural strength and ductility, converting brittle failure into ductile, strain-hardening behavior.

Figure 17:

Comparison of post-crack behavior of 3d printed matrix (without steel fibers) and composite (with steel fibers) [54].

However, challenges remain with anisotropy (orientation along print path) and printability at higher dosages. Still, overall, steel fibers provide the most practical reinforcement solution for improving the structural reliability of 3DPCM in its post-cracking regime.

The microstructural analysis of steel fiber reinforced 3DPCM, primarily focusing on the interfacial transition zone (ITZ) between the fiber and cementitious matrix, is very important for understanding performance. A strong ITZ means fibers can develop full pull-out resistance, directly improving flexural strength, toughness, and post-crack ductility. In 3DPC, extrusion flow can cause preferential fiber orientation and may even trap micro-voids around fibers, weakening the ITZ compared to cast concrete. This fact was observed by Jia, Zhou [40] as shown in Figure 18. During extrusion, mismatched movement between the fibers and the mortar matrix can lead to the formation of gaps around the fibers, particularly in the case of hooked-end fibers. However, the formation of gaps in the ITZ may also be influenced by the preparation process, particularly factors such as the exact cutting position of steel fibers and the methods employed for cutting and polishing. Further investigation is warranted to validate this observation.

Figure 18:

Morphology of steel fiber-matrix ITZ: (a) Mold cast with hooked fibers; (b) 3DPC with hooked fibers; (c) mold cast with straight fibers (d) 3DPC with hooked fibers [40].

In a research study, using a mineral modifier (blend of sand, clayey rocks and chalk) was reported to have improved the fiber-matrix ITZ [58]. In the obtained fiber-reinforced concrete, the interfacial transition zone appeared as a cohesive monolith, free from pores, voids, cracks, or cavities. It was marked by strong fiber-to-matrix adhesion, further densified by newly formed hydration products, as shown in Figure 19. Consequently, the mineral modifier markedly enhances the bond between the hardening binder and the fibers.

Figure 19:

Fiber-matrix ITZ with mineral modifier in 3DPCM [58].

Xia, Geng [38] noted from microanalysis that the steel fibers exhibit a pull-out behavior during flexural failure as compared to rupture of basalt fibers evident from Figure 20. Authors argued that unlike PP and basalt fibers, steel fibers tend to fail through pull-out rather than rupture because of their higher stiffness and strength. This stronger mechanical interlocking with the cement matrix enhances the overall mechanical performance of the specimens. The pronounced interlocking effect dissipates considerable energy during cracking, providing a substantial reinforcement contribution.

Figure 20:

Micromorphology of fiber-3DPCM matrix ITZ after failure: (a) Pullout of steel fibers; (b) rupture of basalt fibers [38].