Steel fibers in 3D-printed concrete: a brief review of feasibility and research gaps

Concrete is one of the most widely used construction materials and plays a crucial role in infrastructure development. Ordinary concrete is characterized by low tensile strength compared to its compressive strength, resulting in a brittle mode of failure at a structural scale. In turn, steel exhibits excellent tensile and compressive behavior. Therefore, the combination of these two materials represented a breakthrough, leading to the development of reinforced concrete (Kloft et al., 2024). However, continuous reinforcement, such as bars, limits the geometric freedom of structural forms. To overcome this challenge, distributed reinforcement in the form of fibers was introduced, enabling the development of materials with increased resistance to cracking and broader design possibilities.

UHPFRC represents an advanced class of cementitious materials characterized by superior mechanical performance and durability. Its application in additive manufacturing, particularly in spray-based 3D printing, offers significant potential for the production of complex and customized structural elements.

The aim of this paper is to present a literature review focused on the effect of steel fiber reinforcement on the fresh-state properties of UHPFRC, including workability, pumpability, and printability, as well as on its hardened mechanical performance, such as compressive and tensile strength, in the context of spray-based 3D printing.

Due to the significant variability in mix compositions, fiber types, and testing methodologies reported in the literature, direct quantitative comparison between studies remains limited. Therefore, this review adopts a qualitative approach, emphasizing the identification of consistent trends and relationships between material composition and performance.

The novelty of this study lies in a focused analysis of spray-based 3D printing using UHPFRC, with particular attention to the role of steel fibers in shaping both fresh-state behavior and hardened performance. By synthesizing and critically evaluating existing research, this paper identifies key limitations related to rheology, pumpability, and process optimization, and highlights specific research gaps that need to be addressed to enable more reliable and standardized application of UHPFRC in 3D printing.

The broad family of fibre reinforced cementitious materials includes Engineered Cementitious Composites (ECC), of which Ultra High-Performance Fibre Reinforced Cementitious composite (UHPFRC) represents a specific type. UHPFRC is an advanced cement-based material characterized by a very low water-to-binder ratio, typically equal to or lower than 0.2. It exhibits outstanding mechanical performance, with compressive strength commonly exceeding 150 MPa, tensile strength reaching approximately 10 MPa - 15 MPa with distinctive strain hardening due to the incorporation of steel fibers. UHPFRC is usually made of fine particles with a maximum size of 1 mm (Abdolpour et al., 2022; Smarzewski, 2025). Additionally, an improvement in durability is also observed due to a reduced water penetration coefficient and limited diffusion of chloride ions, which directly translates into increased durability of structures (Abdolpour et al., 2021). In turn, ECC is characterized by tensile deformability significantly higher than that of ordinary concrete. Moreover, depending on the composition, a stress-strain behaviour including strain-hardening may be observed, which significantly enhances the durability and operational performance of concrete elements, while maintaining a relatively low fiber content (Li et al., 2020). However, a disadvantage of these materials is the high cost of their production, which is mainly related to high cement consumption and the incorporation of expensive steel fibers (Abdolpour et al., 2021).

Various types of fibers are used in UHPFRC and ECC, including basalt, polypropylene, and steel fibers, which may also differ in length and geometry resulting in distinctive mechanical properties of the obtained material (Yoo & Yoon, 2015). Moreover, steel fibers can be coated with calcium carbonate to increase bonding to the cementitious matrix, allowing a reduction in the amount of fibers in the mixture while maintaining or even improving the mechanical properties of the material (Kim et al., 2025). Ordinary steel fiber-reinforced concrete is widely used in such fields as hydraulic engineering, bridge engineering, and military applications, due to its very high impact resistance (Zhang et al., 2025). Increased steel fiber content also results in higher flexural strength, owing to the positive influence of fibers on mechanical and structural performance of materials and elements. Steel fibers improve tensile strength, impact resistance, and post-cracking behaviour (Abbass et al., 2018).

Despite overall sustainability of UHPFRC application in the long term thanks to durability and reduction of mass of elements, the material production generates an additional carbon footprint and increased material costs as compared to ordinary concrete (Bertola et al., 2026). Therefore, the use of recycled fibers is becoming increasingly popular e.g., fibers obtained from used car tires (Abdolpour et al., 2022). Recycled steel fibers (RSF) can serve as a partial substitute for conventional steel fibers in the reinforcement of concrete elements (Abdolpour et al., 2021). With regard to the environment, a negative impact of concrete production is observed due to the manufacturing of cement and aggregates, which are non-renewable raw materials. Furthermore, the production of 1 ton of cement results in emissions of 600 kg – 700 kg of CO₂ which, accounts for 5 % – 8 % of global greenhouse gas emissions. Therefore, partial or complete replacement of classical cement composite constituents with sustainable materials is increasingly observed (Niewiadomski et al., 2022).

In recent years, 3D concrete printing (3DCP) has increasingly become the subject of academic research and industrial interest due to its significant potential for applications in the construction sector (Mechtcherine et al., 2019). Due to the continuous demand for new buildings and the shortage of qualified labor, automation and digitalization is considered an appropriate solution in the construction sector (Sawicki et al., 2026). Various process types are distinguished, with extrusion-based printing being the most widely used method, while alternative techniques such as selective particle binding or spraying are also described.

The extrusion method consists in the layer-by-layer deposition of a cementitious material through a printing nozzle along a predefined printing path (Buswell et al., 2020). According to research, the use of 3D concrete printing in construction can reduce the amount of waste by 60 %, labor costs by 70 %, and production time by 70 % (Singh et al., 2022). In comparison with traditional casting methods, 3D printing is characterized by greater design flexibility and the ability to create complex geometries (Bi et al., 2022). Printed concrete is based on interdisciplinary cooperation combining various fields of science. Moreover, 3D printing is widely applied in the construction of bridges, buildings, infrastructure, as well as landscape elements (Liu et al., 2025). Examples include the first multi-story apartment building constructed in the Netherlands in 2015 and an office building built in Dubai in 2016 (Zhang et al., 2019).

In additive manufacturing with concrete, the use of formwork during the forming process is eliminated, which also has a positive environmental impact by reducing CO₂ emissions (Xiao et al., 2022). In traditional construction, reinforcement must be placed in the formwork before concrete placement. However, in 3DCP technology this process can be digitally controlled and carried out at various stages of manufacturing. Such an approach increases both design and material freedom, as a result of which reinforcement is no longer treated merely as an additional structural element but becomes an integral part of the entire system (Kloft et al., 2024).

However, the extrusion-based printing method exhibits certain limitations, among which weak interlayer bonding (cold joins) and difficulties in the integration of steel reinforcement are the most significant. To overcome them, a digitally controlled sprayed concrete process inspired by conventional shotcrete principles was developed at TU Braunschweig, resulting in Shotcrete 3D Printing (SC3DP). This method consists of applying the concrete mix under compressed air pressure onto the target surface, which enables the deposition of concrete layers at various angles due to the controlled printing path and jetting parameters. An additional advantage is the possibility of combining printing with the assembly of steel reinforcement (Kloft et al., 2024). Within the framework of digital fabrication, SC3DP constitute one of the approaches enabling formwork-free construction and flexible integration of reinforcement. Layer formation is created by spraying leads to improved compaction of the material and strong interlayer adhesion. Research has confirmed that the Shotcrete 3D Printing (SC3DP) method allows for high printing accuracy and mechanical strength (Dörrie et al., 2025). Nevertheless, this method is not free from limitations, as the control of the spraying process is significantly more complex than in the case of extrusion. Moreover, to ensure appropriate pumpability, stability, and sprayability, it is necessary to design mixtures with a very precise selection of components (Heidarnezhad & Zhang, 2022).

Wet-mix sprayed UHPFRC is an established technology for the repair and strengthening of engineered structures. It enables the execution of ultra-thin, monolithic strengthening shells with high mechanical performance and durability, while reducing material consumption and construction time. The spraying process promotes mix compaction and favorable fiber orientation within the plane of the sprayed surface, enhancing mechanical performance, particularly under tensile stresses. This technology allows effective strengthening without significantly altering the geometry or mechanical behavior of existing structures, making it well suited for applications with limited accessibility (Huynh et al., 2017).

While these applications are primarily limited to surface strengthening, the underlying principles of controlled material deposition and fiber alignment provide a basis for further technological development.

In this context, shotcrete 3D printing extends the spraying process from two-dimensional surface application to spatial material deposition, enabling the fabrication of three-dimensional structural elements. A schematic drawing illustrating the principles of the shotcrete-based 3D printing method considered in this study is presented in Figure 1.

Fig. 1.

Schematic illustrating the main topics addressed in the literature review (own research)

In 3DCP, fibers significantly influence both the processability and mechanical performance of printed elements. Slump flow tests have shown that an increase in fiber content leads to a deterioration of the concrete mix workability and flowability, which may negatively affect subsequent printing attempts. For a fiber content of 0.9 %, the incorporation of steel fibers led to a reduction in flowability by 24.1 % – 44.5 %, due to their high density and tendency to interlock (Xia et al., 2025). Fibers significantly improve the adhesion between successive concrete layers during 3D printing; therefore, increasing their content enhances the buildability of printed structures. In addition, fibers increase resistance to deformations caused by self-weight or extrusion during early age setting (Li et al., 2020). Buildability, i.e., the ability of printed concrete to maintain its height, increases after addition of fibers, with steel fibers exhibiting the greatest effect. This enhancement is attributed to an increase in viscosity and yield stress of the mixture. In the study, this behavior was observed for fiber contents ranging from 0.5 % to 2 %. In printed concretes containing steel fibers, the anisotropy of compressive strength is less dependent on fiber length but increases with fiber content (Xia et al., 2025). Using X-ray computed tomography (X-CT), it was determined that fiber addition in printed concrete may lead to increased porosity during printing (Singh et al., 2022). The sample without steel fibers exhibited a porosity of 6.1 %, whereas the sample with 3 % steel fibers reached 7.6 %. These results suggest that the inclusion of steel fibers leads to an increase in the porosity of UHPFRC (Yang et al., 2021).

Within shotcrete-based 3D concrete printing, increasing attention has been directed toward high-performance cementitious materials capable of meeting enhanced structural requirements. In this context, UHPFRC applied using shotcrete technology exhibits mechanical behaviour strongly influenced by the spraying process and steel fiber content. The incorporation of steel fibers transforms the compressive failure mode of shotcrete from brittle to ductile and significantly enhances peak strain and compressive toughness. However, due to the process-related limitations, the fiber content in shotcrete UHPC is typically restricted to around 1.5 %, since higher fiber volumes can cause clogging of the spraying system. (Xiao et al., 2024). In contrast, conventional UHPFRC mixtures usually require at least 2 % – 3 % steel fibers to achieve required mechanical and durability properties. This highlights the need for the development of optimized mixtures that combine densely packed matrix composition and high fiber content with adequate pumpability and sprayability. UHPFRC represents a promising material for shotcrete-based 3D printing, offering favorable sprayability, low rebound and enhanced mechanical and durability-related properties resulting from bridging and process-inducted fiber orientation (Chen et al., 2023).

Current studies focus mainly on the influence of fiber size on mechanical properties (Xia et al., 2025) and ductility of UHPFRC and ECC (Li et al., 2020), as well as on the introduction of more environmentally friendly solutions that may positively affect the environment (Abdolpour et al., 2021; Niewiadomski et al., 2022). Although numerous studies concern the printing of concrete, the majority of them focus on extrusion-based technologies (Singh et al., 2022; Xia et al., 2025; Zhang et al., 2019). In contrast, the number of publications that precisely describe shotcrete-based 3D printing using concrete reinforced with steel fibers remains limited. This is confirmed by the set of publications identified through a Google Scholar search engine. In the context of the planned research project, the present study is intended to verify whether UHPFRC will be suitable for application in the shotcrete-based 3D printing method. For this purpose, a preliminary literature survey was conducted in order to identify existing research gaps, to formulate appropriate research objectives, and to develop a proper research program.

To illustrate the research interest in the topics of concrete, fibers, 3D printing and shotcrete, the number of publications published since 2000 was analyzed using keyword-based searches in the Google Scholar database. The results of this analysis are presented in Table 1.

The search included individual keywords such as “concrete”, “fibers”, “3D printing”, and “shotcrete 3D printing”, as well as their combinations. The results show that while a large number of publications include single keywords, the number of relevant studies decreases significantly when these terms are combined.

Additionally, long-term research trends were analyzed for the keywords “steel fibers in concrete”, “fiber-reinforced concrete”, and “ultra-high performance concrete for 3D printing”. The number of publications published over the past 65 years was evaluated based on keyword searches in Google Scholar, considering occurrences in titles, abstracts, and keywords. The results are presented in Figure 2.

To further assess the research gap in shotcrete-based 3D printing, a comparative analysis was performed using the Google Scholar database. The number of publications containing the keywords “shotcrete-based 3D printing” and “extrusion-based 3D printing” was identified for the period since 2000. The results are presented in Figure 3.

No additional filtering or selection criteria (e.g., based on abstracts, study type or methodology) were applied, as the aim of this analysis was to assess general research trends rather than perform a detailed systematic literature review.

Fig. 2.

Number of papers published over 65 years (Google Scholar)

Fig. 3.

Comparison of articles on shotcrete-based and extrusion-based 3D printing (Google Scholar)

As the authors intend to focus their further research on shotcrete-based 3D printing, one of the elements of the state-of-the-art review is a comparison of two concrete 3D printing methods: spraying and extrusion. However, to provide a comprehensive overview of printed concrete technologies, the dominant 3D printing methods were compared, allowing the identification of their advantages and positive aspects, as presented in Table 2. Importantly, the review shows that there are no studies that isolate and compare only the effect of the process type i.e., spraying vs. extrusion. As a result, the comparison is qualitative, since differences in material, scale, and practical implementation cannot be separated from the influence of the process itself.

Table 2 indicates that shotcrete-based 3D printing offers greater geometric flexibility and reinforcement integration, whereas extrusion-based 3D printing provides more predictable process control and surface quality, highlighting complementary rather than competing characteristics of both approaches.

An additional outcome of the conducted state-of-the-art review is Table 3 – a comparison between a concrete mix reinforced with steel fibers and a mix without fibers. The aim of this comparison is to highlight the beneficial effect of fibers on the properties of the concrete mix and to indicate its high potential for application in 3D printing technology.

The comparison presented in Table 3 indicates that the incorporation of fibers significantly affects the fresh-state properties of the mixture, which is particularly relevant for shotcrete-based processes. Reductions in workability and flowability, as well as changes in porosity and air content depending on fiber type and dosage, may influence the stability of the spraying process and therefore require careful mixture design. These results highlight the need to balance process-related requirements of shotcrete-based technologies with the modification of UHPC through fiber incorporation.

Table 4 also presents a comparison of the properties of hardened concrete with and without steel fiber reinforcement. The aim of this comparison was to provide a broader insight into the mechanical properties of fiber-reinforced hardened concrete, which constitutes the subject of the authors’ further research in the context of shotcrete-based 3D printing.

Table 4 indicates that fiber reinforcement significantly modifies the mechanical behaviour of UHPC, leading to a transition from brittle to more ductile failure behavior. The presence of fibers enhances tensile strength, impact resistance and post-cracking load-carrying capacity, while promoting the development of multiple, distributed cracks prior to crack localization. In UHPC, crack localization tends to occur rapidly and is primarily controlled by stress concentration within the cementitious matrix.

Based on the conducted state of the art review, the authors identified the current state of research. However, at the same time, numerous research questions were formulated and several significant research gaps were identified.

Although the interest in 3D printing using fiber reinforced cementitious materials in the construction sector is steadily increasing, these technologies have not yet been sufficiently investigated. Current studies focus primarily on the composition of concrete mixtures. However, there is still a lack of information regarding an optimal mixture specifically tailored for shotcrete-based 3D printing. The main research gap identified in the presented literature review is the lack of studies directly comparing the mechanical and physical properties of mixtures with identical compositions produced using shotcrete-based 3D printing and extrusion techniques. Furthermore, the reviewed literature does not clearly distinguish between UHPFRC and UHPC, and the boundary of steel fiber content between these materials is not consistently defined.

Fig. 4.

Research gaps identified based on literature review (own research)

As a result, quantitative analysis is not feasible due to differences in mixture compositions across studies. Therefore, only a qualitative analysis was conducted. Moreover, comprehensive studies analyzing the exact properties of UHPC reinforced with steel fibers in direct comparison with concrete without fibers are still lacking. The use of recycled mixtures for the shotcrete-based 3D printing method shows promise due to their favorable mechanical properties. To increase stability and reduce costs, the use of coarser aggregates in concrete mixtures could be considered. In this case, however, the issue of material rebound occurring during spray-based printing would need to be addressed. Future research should also focus on the relationship between the nozzle parameters, including the nozzle standoff distance, nozzle travel speed, pumping speed, as well as the resulting layer width and depth, as summarized in Figure 4.

This study evaluated the possibility of application of UHPFRC in SC3DP, with particular focus on the influence of steel fibers on fresh-state properties and hardened mechanical performance. The analysis indicates that steel fiber reinforcement plays a key role in enhancing tensile behavior, crack resistance, and post-cracking load-carrying capacity, while also affecting rheological properties such as pumpability, stability, and printability during the spraying process. These properties are critical for achieving stable and controlled spatial material deposition in SC3DP. Compared to extrusion-based methods, spray-based 3D printing offers advantages such as improved adaptability to complex geometries, enhanced interlayer bonding, and more effective material compaction due to the nature of the spraying process. At the same time, the results highlight significant challenges related to mix design optimization, rheological control, and process parameter adjustment. The variability in mixture compositions and testing methodologies across existing studies limits the possibility of direct quantitative comparison, confirming the need for a more standardized and systematic research approach. From an application perspective, SC3DP enables the fabrication of complex three-dimensional structural elements without the need for formwork, offering new possibilities for architectural and engineering design. From an environmental standpoint, this technology may contribute to more sustainable construction by reducing material consumption, minimizing waste, and improving resource efficiency through optimized material use. Overall, the findings confirm the strong potential of SC3DP with UHPFRC for advanced construction applications, while also identifying key limitations that must be addressed to enable its broader and more reliable implementation.