In recent years, many researchers have published articles about 3DPC. In this regard, the effect of different fibers on fresh and mechanical properties has been investigated. The cited bibliographic references were selected from various sources such as Scopus, Elsevier and Google Scholar. Significant articles were analyzed without considering repetitions. As the summarized statistics in Figure 1 show, more than 40% of articles are related to 2021; therefore, this statistic shows that researchers are currently interested in this field. The primary purpose of this article is to consider the effects of various types of fibers on the properties of 3DPC; therefore, academia and industry can use this review to enhance their information about the role of different fibers in 3DPC. The statistics illustrate that the researchers work with steel fibers more than other fibers; hence, it is an excellent opportunity for other scholars to use other types of fibers. Additionally, the statistics demonstrate that developed countries such as China and Australia are attending this field; thus, investigating the challenges of 3DPC could be an exciting subject for universities in other countries.

Fibers that can be used as reinforcements in 3DPC are steel, carbon, glass, polyvinyl alcohol (PVA), polyethylene (PE), and polypropylene (PP). A summary of the typical characteristics of various fibers is shown in Table 1. Different reinforcing fibers might bring various qualities to concrete and play multiple roles in enhancing the functionality of cement-based materials. High-tensile strength fibers such as steel can significantly increase the bending and tensile strength of concrete mixtures, as well as their fatigue and impact resistance. In addition, they can enhance the material’s energy absorption capacity, toughness, and strain capacity [37,38]. The downside of steel fibers is that they rust easily and are relatively expensive [39]. Carbon fibers are lightweight, strong, resistant to corrosion, high temperatures, and fatigue, and have a high strength-to-weight ratio. They can also enhance the concrete’s electrical conductivity, pressure, and magnetic sensitivity [40,41,42,43]. However, they have inadequate impact resistance and weak toughness. Glass fibers offer decent heat insulation and corrosion resistance. Still, they have low alkali resistance and must be coated when used in concrete [44,45,46]. Polymer fibers are high-strength synthetic monofilament bundle fibers. When these fibers are added to mixtures, they can effectively limit the formation and growth of fractures, as well as increase their impermeability and impact resistance [47]. PVA [48], PE [49], and PP [50] are common types of used synthetic polymer fibers. Other fibers, such as basalt, recycled, and natural, may lessen the brittle failure of composites, but their performance is not as good as that of the mentioned fibers. Therefore, less research has been conducted on using these fibers [51]. Figure 2 shows several types of fibers.

The fresh properties of 3DPC mostly pertain to workability performance. Workability performance includes fluidity, cohesiveness, and water retention. By incorporating fibers into the 3DPC mixtures, it is possible to avoid spalling, restrict deformation and achieve a uniform and continuous printing sample. Furthermore, the key to determining whether the concrete can be continuously and evenly extruded and ensure pouring molding is to manage its thixotropy and setting time due to the needs of the concrete printing process. In addition, it is essential to note that fiber content can block the nozzle when it reaches a sensitive volume [2]. The summarized information about different types of tests in order to determine the fresh concrete properties and their results of them are shown in Table 2.

The fresh properties of 3DPC reinforced with steel fibers, length of 13 mm, diameter of 0.2 mm and density of 8.7 kg/m³, at the contents of 0.25, 0.5, 0.75 and 1 wt.%, were investigated by Singh et al. [56]. In this study, the researchers used natural river sand with a particle size smaller than 1.18 mm and nano clay (nC). The results about the slump flow demonstrated that with the addition of steel fibers, the slump flow decreased. Additionally, when the fiber ratio increased, the slump flow at an early age compared with the control specimen reduced by 4.76, 5.71, 5.71 and 6.61% for specimens that contain 0.25, 0.5, 0.75 and 1 wt.% fibers, respectively. Moreover, adding nano clay increased structural rebuilding during the extrusion of 3DPC because it behaved as an interlocked web, leading to increase in friction and adhesion among particles.

Yang et al. investigated the printing quality of 3D printed ultra-high-performance fiber-reinforced concrete 3DPUHPFRC [57]. The two kinds of round copper-plated steel fibers with a length of 6 and 10 mm were used in the research. Moreover, using the fly ash improved the workability and durability of 3DPC [57]. In this study, the researchers used 24 kg/m³ nano calcium carbonate in the mix ratio of 3DPUHPFRC so as to improve the properties of the 3D printed paste. The results about the quality of printability illustrated that the 3DPUHPFRC without steel fibers had better quality than that with 1 vol.% 6 mm and 1 vol.% 10 mm steel fibers. This outcome was caused by the difficulties that 3DPUHPFRC had when extruding steel fibers from the nozzle.

Ye et al. investigated the workability and buildability of 3D printed ultra-high ductile concrete (3DPUHDC) reinforced by PE fibers (1.0, 1.5, and 2.0 vol.%) [58]. 3DPUHDC is a particular type of concrete that is made for its better tensile strain capacity [59,60,61,62,63]. In this research, spread diameter and penetration depth were used in order to evaluate workability. The spread diameter and penetration depth figures for the specimens with 1 vol.% fibers were 169 and 86 mm, respectively. The spread diameter for the samples with 2 vol.% fibers was 144 mm, and the penetration depth was 76 mm. For evaluating printability, the overall height of the 10-layer was measured. The height of the 10-layer printed samples with 2 and 1 vol.% fibers were 99 and 97 mm, respectively. It was determined that a reduction in the workability occurred because more and more twisted fiber orientation was noticed as the fiber concentration was raised to 1.5 and 2 vol.%.

Li et al., investigated the workability and buildability of 3DPC reinforced by glass fibers (up to 0.61 vol.%) [64]. In this research, 3DPC was made from coral sand. In addition, seawater was used for creating 3DPC samples. Freshwater is scarce in remote islands and coastal areas, and shipping from the mainland is prohibitively expensive, posing a significant barrier to concrete manufacturing in these isolated places. In such a case, using saltwater may be a viable option [65,66,67,68]. The flow table test was used for workability and buildability; the height of 3DPC was measured. Adding 0.6 vol.% glass fibers reduced the flow diameter and the average height of printed samples by 11 and 11.7 mm, respectively [64]. Chu et al., demonstrated that adding 0.5 vol.% glass fibers into 3D printed high-strength fiber-reinforced concrete (3DPHSFRC) decreased the slump flow and flow diameter by less than 10% [69]. 3DPHSFRC can be engineered to have high tensile strength and flexibility. Moreover, we can replace all steel bars with fibers in 3DPHSFRC [70,71,72,73].

Sun et al. conducted research that added 0.8, 1, 1.2, 1.4 and 1.6 vol.% PVA fibers in the 3D printed paste [74]. The results about the fluidity illustrated that the addition of PVA fibers decreased the fluidity of the mixture so that the fluidity of the mixture with 0.8 vol.% of PVA fibers was 21% more than that of mixtures with 1.6 vol.% of PVA fibers. In this regard, other scholars concluded that a higher amount of fibers reduced the fluidity because of interlocking and entangling among fibers in mixtures [75,76].

Zhang and Aslani added 1 and 1.5 vol.% PVA fibers and 0.5 and 1 wt.% activated carbon powder in order to determine the optimum mix design [77]. The results of the slump flow of mixtures illustrated that adding more PVA fibers reduced the slump flow. The main reason for this result is that the addition of fibers restricted the movement of aggregates; hence, the slump flow was reduced. The slump flow of mixtures with no PVA fibers was 57% and 44% higher than mixtures with 1 and 1.5 vol.% PVA fibers, respectively.

Sun et al. investigated the 3D printed lightweight engineered cementitious composites by incorporating 1.75% PVA fibers by mass ratio to cement [78]. The slump flow results illustrated that the mixtures’ slump flow with 1.75% of PVA was reduced compared to those without fibers. The slump flow of the mixtures without fibers was 80% higher than that of mixtures with 1.75 wt.% fibers. The PVA fibers also decreased the setting time in comparison with mixtures without fibers. Another scholar asserted that this matter is due to fibers overlapping, making a network with each other and causing a reduction in the setting time [79].

The rheological properties of fresh Engineered Cementitious Composite (ECC) are essential in its application in 3DPC. The rheological behaviors determine the pumping ability, extrudability and buildability of 3DP-ECC samples [80,81]. In this regard, Yu et al. evaluated the rheological properties of 3DP-ECC with 26 kg/m³ PVA fibers, 1026 kg/m³ fly ash, 345 kg/m³ silica sand and 30 kg/m³ ordinary cement [82]. The results showed that spread flow decreased with time. In this case, the results of the shape retention test demonstrate that with passing time, the ability of the mixture in buildability and shape retention test increased.

The effect of adding PP fibers on the fresh properties of 3DPC was investigated by Tran et al. [83]. In this research, the content of PP fibers varied from 1.35 to 5.4 kg/m³. This research used the Bingham model to evaluate the plastic viscosity and the dynamic yield stress [84]. The specimens with 0.22 water-to-binder ratio and 5.4 kg/m³ PP fibers had 282.3 3 Pa viscosity and 1207.6 Pa dynamic yield stress. It was stated that adding PP fibers increased the plastic viscosity and the dynamic yield stress by nearly 400% and 320%, respectively, because the friction of the particles in the fresh concrete immediately influenced the dynamic yield stress and plastic viscosity.

Furthermore, PP fibers served as the needle aggregate in fresh concrete. Thus, adding more PP fibers increased the needle aggregate, resulting in high friction among aggregates. Figure 3 shows the 3DPC samples in this research.

Chu et al. added 0.5 vol.% carbon fibers into 3DPHSFRC and discovered that slump and flow diameter decreased by less than 5% [69]. Li et al. investigated the effect of basalt fibers (0.21, 0.22, 0.42, 0.43, 0.64, 0.65, 0.85 and 0.86 vol.%) on 3DPC. The addition of 0.85 vol.% basalt fibers decreased the flow diameter from 178 to 162 mm. Conversely, the addition of 0.85 vol.% basalt fibers increased buildability from 52.3 to 52.5 mm [64].

Table 2 compares the impact of various fiber types on the 3DPC’s fresh properties. In this table, the percentage, specifications, and types of fibers are shown. In addition, tests and a summary of the results are provided in Table 2.

In most studies, the slump flow test was used for measuring the fresh properties of 3DPC because it is easy to operate and it contains valuable results. As a result, Figure 4 displays the effect of types and percentages of fibers on the slump flow test. As it is seen, the results show that the amount of slump flow for most 3DPC reinforced with fibers was in the range of 150–200 mm, which is consistent with the researchers’ findings.

Additionally, the slump flow values and fiber percentage results show that the excessive amount of fibers reduced the slump flow, which can reduce the efficiency of 3DPC, but this is not always the case. The 3DPC’s fiber type selection is crucial and may have a negative impact. Basalt and glass fibers seem to improve the slump flow, whereas steel, PE, and PVA decrease it. Based on the fiber type, the rate of changes in the slump flow will also vary. The findings indicate that PVA fibers had a larger slump flow reduction rate than PE and steel fibers. In addition, compared to basalt fibers, the rate of slump flow increased more rapidly for glass fibers.

For steel and basalt fibers, changes in the rate of slump flow were less than 5%, which shows the fiber percentage cannot substantially alter the slump flow. Regarding PE, glass, and PVA fibers, the rate of change in the slump flow was higher and in the range of 25–35%. Finally, PVA fibers seem to have the highest impact on slump flow, while steel fibers have the least impact. Moreover, in most cases, adding fibers generally improved the buildability of mixtures, which is one of the main characteristics of 3DPC.