3D-Printed Concrete Bridges: Material, Design, Construction, and Reinforcement

Abstract

3D Concrete Printing (3DCP) technology is rapidly gaining popularity in the construction industry, particularly for transportation infrastructure such as bridges. Unlike traditional construction methods, this innovative approach eliminates the need for formwork and enhances both economic efficiency and sustainability by lowering resource consumption and waste generation associated with formwork. This paper examines current research on 3D-printed concrete bridges, highlighting key areas such as concrete mixtures, design processes, construction techniques, and reinforcement strategies. It delves into computational methods like topology optimization and iterative “design by testing” approaches, which are crucial for developing structurally efficient and architecturally innovative bridges. Additionally, it reviews specific admixtures or additives within the concrete mix, assessing how they improve essential properties of printable concrete, including extrudability, buildability, and interlayer bonding. Moreover, it shows that the primary construction approach for 3DCP bridges involves prefabrication and on-site assembly, with robotic arm printers leading to scalability and precision. Reinforcement continues to be challenging, with the most commonly used strategies being post-tensioning, hybrid techniques, and fiber reinforcement. This paper offers insights into the advancements and challenges in 3D-printed concrete bridge construction, providing valuable guidance for future research and development in this field.

1. Introduction

Additive manufacturing (AM), also known as three-dimensional (3D) printing, is a group of emerging techniques for manufacturing 3D structures directly from a digital model in successive layers [1]. The adoption of AM techniques in the construction industry has the potential to revolutionize this industry, transforming its image from a labor-intensive field often associated with safety challenges and sustainability concerns into a more efficient, precise, and environmentally friendly practice [2]. As the construction industry shifts towards AM, cementitious material extrusion has emerged as one of the most widely adopted techniques [3]. Its popularity stems from the affordability and accessibility of open-source extrusion-based printers and the widespread use of Portland cement, which offers reliable mechanical properties at a low cost. Cementitious materials provide a cost-effective alternative to metallic materials and greater functionality than polymers for structural applications [3]. Among these materials, concrete stands out as particularly well-suited for extrusion-based systems. In 3D Concrete Printing (3DCP), a structure is fabricated layer by layer with a nozzle that deposits material along a defined path. Compared to conventional concrete construction methods, 3DCP offers several significant advantages [4]: the reduction of construction costs through the elimination of expensive formwork; improved safety by reducing the need for dangerous jobs; faster construction times due to the constant operational rate of 3D printing; minimized errors through highly precise material deposition; enhanced sustainability by reducing formwork waste; and greater architectural freedom, enabling the creation of more sophisticated and innovative designs.

Bridges make up a significant portion of operational 3D-printed concrete structures globally. Currently, all existing 3D-printed bridges are designed specifically for pedestrian and bicycle traffic. However, researchers are optimistic that future advancements will enable the development and exploration of 3D-printed bridges that can support motor vehicle traffic as well [5]. The first 3D-printed concrete bridge was built in 2017 in Gemert, Netherlands [6,7]. This 8-m-long bicycle bridge results from a collaboration between BAM Infra and the Technological University of Eindhoven (TU/e). It spans 3.5 m in width and has a thickness of 0.9 m, constructed from 800 layers of 3D-printed concrete. In January 2019, a 26.3-m long 3D-printed concrete pedestrian bridge was completed at Wisdom Bay Park in Shanghai [8]. This bridge stands as the second longest of its kind in the world. Designed by Professor Xu Weiguo from Tsinghua University, it features handrails that resemble flowing ribbons along its arch. With a width of 3.6 m, the bridge was constructed using two robotic arms in a process that took over 450 h to complete. That same year, the world’s longest 3D-printed concrete bicycle bridge was unveiled in Nijmegen, Netherlands, measuring an impressive 29 m in length. This project was a collaboration between the TU/e and the 3D printing center at Saint-Gobain Weber Beamix, incorporating multiple BAM robotic arms. The design showcased rounded, natural shapes, seamlessly integrating with its outdoor environment [9]. In July 2021, the “Striatus” bridge was constructed in Venice, Italy (see Figure 1a). This project, a collaboration between the Block Research Group and the Computation and Design Group at Zaha Hadid Architects, resulted in a 12-by-16-m arched footbridge located in a park. Notably, the bridge was built entirely without reinforcement, utilizing printed concrete blocks arranged to form an arch, which echoed the appearance of traditional masonry bridges [10]. Two years later, in 2023, the “Phoenix” Bridge, shown in Figure 1b, was constructed as an evolved iteration of the “Striatus” bridge. This 3D-printed concrete masonry structure was constructed using 10 tons of recycled materials, including aggregates derived from the original Striatus blocks. The project was a collaborative effort among Holcim, the Block Research Group at ETH Zurich, the Computation and Design Group at Zaha Hadid Architects, and incremental3D [11,12].

This paper provides a literature review on research related to 3D-printed concrete bridges, offering valuable insights for designers and researchers aiming to advance this emerging field. The review focuses on key aspects, including material properties, design processes, construction techniques, and reinforcement strategies. Specifically, the paper addresses the following critical questions:

• Design process: What are the design processes for 3D-printed concrete bridges?
• Material: What additives/admixtures are utilized in the mixture, and how do they enhance the performance and printability of 3D-printed concrete for bridges?
• Construction methods: What construction techniques are implemented, including prefabricated and in situ approaches, and what 3D printing systems, such as gantry-based or robotic arm setups, are used in bridge construction?
• Reinforcement: What reinforcement strategies, such as embedded bars or fiber-reinforced concrete, are applied in 3D-printed concrete bridges, and how is reinforcement integrated?

By addressing these questions, the paper aims to provide a foundational understanding of the current research in 3D-printed concrete bridges and guide future advancements in this innovative field.

2. Design Process of 3D-Printed Concrete Bridges

The conceptual design of a bridge relies heavily on the designer’s intuition and their ability to understand how various structural components interact to transfer weight and loads to the ground efficiently and safely [15]. Specifically, in the context of 3D-printed concrete bridges, the design process demands additional considerations due to the unique fabrication methods that impact material behavior, geometric possibilities, and structural performance. This section explores the design approaches discussed in the literature, emphasizing how designers leverage the capabilities of additive manufacturing to develop innovative and efficient bridge designs.

A common computational approach in the design of 3D-printed concrete bridges is Topology Optimization (TO). This technique aims to optimize material distribution within a specified design domain to achieve objectives such as maximizing structural performance or minimizing material usage while adhering to predefined constraints [16]. Vantyghem et al. [16] applied the density-based TO approach to the design and manufacture of a simply supported girder subjected to a uniform load. The density-based structural topology considers a collection of density values ranging between 0 (void) and 1 (material) at discrete points within the design domain. To this end, the beam was discretized into a grid of square finite elements, and the initial domain was initialized with a uniform density value of 0.5, while the top surface density was fixed at 1 to maintain the integrity of the loading area. The setup for the topology optimization is shown in Figure 2a. The post-tensioning tendon was incorporated into the design as an initial geometric constraint. The optimization aimed to minimize displacements at the top surface by iteratively adjusting density values to determine the most efficient material distribution while satisfying constraints on volume fraction and support locations. After optimization, the conceptual 2D design was translated into a practical 3D-printable structure through an extensive post-processing phase. Key modifications included shaping the lower chord circularly to ensure even encasement of the post-tensioning cable, widening the upper chord to create a pedestrian deck surface while limiting its width to reduce transverse tensile forces, and incorporating end blocks to anchor post-tensioning forces. Lastly, a 3D Finite Element (FE) analysis was conducted to validate the design, providing insights into stress distribution and the required reinforcement layout under various loading conditions. This foundational use of TO establishes a basis for efficient material usage and structural performance, linking computational methods to practical applications. The final design is shown in Figure 2a, with black parts representing concrete, white representing void, and cyan representing the tendon.

Building upon this work, Ooms et al. [17] used a similar girder design that integrated a wider top surface for improved bridge applications. The design featured two longitudinal, topology-optimized, post-tensioned concrete girders supporting a continuous bridge deck that was supported at four corners.

The FloatArch bridge was also designed using topology optimization, specifically employing the Bi-directional Evolutionary Structural Optimization (BESO) method to distribute concrete and steel within a defined design space efficiently [18]. This approach leverages concrete’s compressive strength and steel’s tensile capacity by iteratively refining material placement within a defined design space, concentrating concrete in compression zones and steel in tension zones. In their final design, concrete components had hollow sections to reduce self-weight while maintaining structural efficiency, and steel cables were simplified and anchored to the concrete via steel nodes (see Figure 2b). This example transitions TO principles to bridges with mixed-material systems, showcasing the versatility of computational design.

The Castilla-La Mancha Park footbridge in Alcobendas, Madrid, is another example of how topology optimization can be applied to form-finding in the design of 3D-printed bridges [19]. This project employed the D-shape, particle-bed printing method that differs from conventional extrusion-based 3D concrete printing. Parametric design played a central role in optimizing material distribution, minimizing waste through recycling raw materials during production, and enhancing structural performance by placing material only where necessary. Additionally, generative algorithms were used to maintain porosity while enabling the creation of complex geometries, demonstrating the synergy between advanced computational tools and innovative manufacturing techniques [20].

The “Striatus Bridge” and its evolution, the “Phoenix Bridge”, exemplify innovative, iterative design processes that integrate form-finding, performance-based optimization, and rigorous structural analysis to address both aesthetic and structural challenges in 3D-printed concrete bridges [13,21]. Both bridges share a common goal of minimizing tensile stress in unreinforced concrete by utilizing arch forms, which function as transverse span structures designed to transfer loads primarily through compression (see Figure 3a) [22]. The design of the Striatus Bridge began with a skeletal graph connecting its five supports, which served as the foundation for generating a 2D mesh. This mesh was refined using Thrust Network Analysis (TNA) to establish a 3D thrust network of compressive forces in equilibrium with predefined loads, as shown in Figure 3b. The resulting form-found mesh was discretized into individual voussoir blocks, with joints carefully positioned to align with the compressive force flow. Stability was verified through Discrete Element Modeling (DEM), which evaluated the dry-assembled structure under various conditions. Iterative adjustments to block interfaces and thicknesses ensured that the final design met both aesthetic and structural requirements [13]. Integrating form-finding and structural analysis demonstrates the iterative refinement necessary for compression-dominant designs.

Building on the Striatus design, the Phoenix Bridge employed a similar iterative approach while incorporating traditional masonry-inspired techniques [21]. The design process emphasized optimizing the arch axis to minimize bending moments, ensuring that the cross-section primarily experienced compressive stress. Finite Element Analysis (FEA) was used in evaluating stress distribution, allowing refinements to the arch geometry and block dimensions to balance structural performance with aesthetic considerations.

The “Design by Testing” methodology has also been a key approach in several projects. This methodology can complement TO by ensuring experimental validation of design choices and linking computational and physical testing. Salet et al. [23] utilized the “Design by Testing” approach to create a 3D-printed bicycle bridge. This approach involved comprehensive testing of materials and structures at every phase, which included characterizing materials (such as assessing compression, tension, modulus of elasticity, creep, and shrinkage), conducting destructive tests on a 1:2 scaled model, and carrying out assembly trials. The design phase began with understanding the constraints their 3D concrete printer posed. Although the entire bridge could theoretically fit within the printer’s build space, the team opted for a modular design to ease transportation and assembly. This modularity also enabled optimization of the bridge’s cross-section, exploiting the printer’s features. The insights gathered during testing led to notable enhancements, such as a 4% reduction in both print path length and material consumption.

Ahmed et al. [24] implemented a comparable “Design by Testing” strategy to construct the world’s longest bicycle bridge. Their approach merged experimental testing with parametric design to enhance printability, material performance, and structural demands. Tests on material properties verified the structural integrity and evaluated long-term behavior, establishing a foundation for design endorsement. Parametric design enabled the creation of digital models that allowed iterative optimization, resulting in a nature-inspired double-curved deck spanning tapered columns, which appeared to sprout organically from the deck. Zhan et al. [25] also adopted this method to design a 3D-printed prestressed concrete bridge. Their process began with material research and testing to guarantee both printability and structural soundness. Critical properties were assessed, including a maximum overhang angle for inclined printing. Computational tools were later utilized for digital design and optimization, encompassing toolpath refinement to achieve continuous extrusion and focusing material in high-stress regions. Section optimization was also executed to ensure a compressive state during prestressing.

Another innovative approach called “Minimass” technique was developed by Net Zero Projects (NZPs) [26]. This approach focuses on creating bending structures, such as beams, with optimized geometries using 3DCP. The design principles prioritize axial tension and compression, forming a “truss-like” structure rather than relying solely on material bending strength. A concrete top chord resists compression, while a steel cable bottom chord (standard post-tension tendons) resists tension. These elements are separated by concrete webs, with the arrangement of webs and cable geometry tailored to the applied loads. In this application, the concrete top chord was reinforced with mild steel, enabling it to function as a beam column rather than just an axial compression member. The final design included primary beams spanning the bridge’s length, with thinner slabs placed between them. Prefabricated lattice slabs provided permanent formwork for an in situ topping slab and a safe working platform during construction. To fully utilize 3DCP, both the primary beams and lattice slab elements were 3D-printed, applying principles of efficient composite construction commonly seen in conventional T-beam designs.

In summary, various computational and experimental design approaches have been employed to optimize material usage and enhance structural efficiency in 3D-printed concrete bridges. Topology optimization (TO) has been extensively used to reduce material waste and improve structural efficiency, but its reliance on predefined constraints and the need for extensive post-processing can limit design flexibility. Parametric design enables greater geometric adaptability and iterative refinement, making it ideal for complex freeform structures, such as the Castilla-La Mancha Park footbridge. Meanwhile, form-finding techniques like Thrust Network Analysis (TNA) focus on generating compression-optimized geometries, which have been crucial in masonry-inspired designs such as the Striatus and Phoenix Bridges. The ‘Design by Testing’ approach bridges the gap between digital design and physical validation, ensuring real-world feasibility through experimental material characterization and full-scale structural testing, as seen in the world’s longest 3D-printed bicycle bridge. Finally, the Minimass technique represents a shift toward efficient axial force-based structures, integrating 3D printing with post-tensioning for improved structural behavior under bending loads.

These methods are not mutually exclusive but can be combined to maximize the advantages of 3D-printed bridge design. For example, TO can be integrated with parametric modeling to refine printability constraints, and form-finding can be complemented by structural testing to validate real-world performance. As computational and experimental methodologies evolve, future 3D-printed bridge designs will likely leverage hybrid approaches, combining the precision of algorithm-driven optimization with performance-based testing to achieve more sustainable and structurally efficient designs.

3. Materials Used in 3D-Printed Concrete Bridges

The concrete material used for printing purposes has the features of self-compacting concrete (i.e., no need to vibrate) and sprayed concrete (i.e., expelling fresh concrete from a nozzle) to conform to the essential requirements of a freeform construction system. Printable concrete should meet certain fresh properties to be printable [27]. The most critical of these properties are extrudability and buildability [28]. Extrudability is the ability of concrete to move through the pipes and nozzles at the printing head and is influenced mainly by the workability (consistency) of the concrete and mix proportions (i.e., cementitious binder–aggregate ratio, water–binder ratio, admixture usage). After passing the extrudability criteria, self-compacting filaments can be produced. The printed filaments should experience minimum deformations under the weight load of subsequent layers. Also, the bottom filaments should bond to the top ones to build monolithic elements. At this stage, the printed concrete needs buildability which refers to the capacity to print a certain number of layers or heights. Buildability is also related to workability and mix proportions, especially the dependency of workability with time, i.e., open time. The open time is defined as the period in which the workability of fresh concrete is at a level where extrudability is preserved and is crucial for maintaining a balance between structural stability and layer adhesion during the printing process [28]. Achieving these properties requires a careful balance of mix design, ensuring that the concrete maintains the necessary flowability and stability during extrusion while providing sufficient strength and durability in its hardened state. This balance is achieved by strategically incorporating various additives and admixtures, which enhance specific properties of the concrete.

One of the critical challenges in 3D concrete printing is the weak interlayer bonding or the occurrence of “cold joints” [29] between layers. In multilayered structures, the hardened properties of the interlayer are typically weaker than those of the bulk material, forming potential weak links in the structure. Unlike conventional cast concrete, these weak interlayer zones introduce mechanical anisotropy, which can significantly impact structural performance and durability [30]. Some studies have investigated this anisotropy by performing mechanical tests such as compression and flexure in different loading directions relative to the printed layers, as shown in Figure 4 [23,24]. According to a review by Wang et al. [31], the anisotropy in compressive strength of 3D-printed concrete (3DPC) is relatively low, regardless of mix design variations. However, flexural and tensile strengths exhibit the highest anisotropy, with significant reductions in strength perpendicular to the printing direction compared to parallel directions (see Figure 4b). This trend was also observed in the work of Salet et al. [23], where flexural strength decreased by approximately 32%, from 1.9 MPa to 1.3 MPa. This reduction in strength is a crucial consideration in the design of 3D-printed concrete bridges, as it directly influences load-bearing capacity and long-term structural integrity.

The pronounced anisotropic behavior is largely attributed to poor interlayer adhesion and the formation of microvoids during the printing process [33]. One of the primary factors influencing interlayer bonding strength is the time interval between layer depositions, commonly referred to as “delay time” [2]. Delay time plays a crucial role in determining the surface moisture of the printed concrete, which directly impacts bond strength [4]. A higher surface moisture content provides an aqueous medium that enhances interlayer adhesion, allowing fresh concrete in successive layers to bond more effectively [2]. Surface moisture is influenced by both printing parameters (e.g., extruder type, nozzle pressure, and deposition speed) and material properties (e.g., mix composition, bleeding rate, and evaporation rate) [34].

In summary, achieving strong interlayer bonding is crucial for minimizing the anisotropic behavior of 3D-printed concrete, thereby enhancing its mechanical performance and structural integrity.

Table 1. Materials used in 3D-printed concrete bridges.

Ref.	Additive/Admixture	Purpose	Property Enhanced
[16]	Water retention agent	Improves water retention, provides thixotropic behavior, and prevents pressurized bleeding	Thixotropy
[35]	Superplasticizer	Enhances flowability and reduced water demand	Workability, extrudability
[36]	Superplasticizer	Enhances flowability and reduces water demand	Workability, extrudability
	Deforming agent	Improves workability and prevents segregation	Consistency, stability
	Retarder	Delays setting time to extend the workability window	Open time, buildability
	Viscous agent	Enhances thixotropic behavior	Buildability, layer adhesion
[21]	Polypropylene (PP) fibers	Improves tensile strength and crack resistance	Printability
[17,37]	Limestone powder	Improves workability and reduces shrinkage	Flowability, dimensional stability
	Polycarboxylate ether (PCE)	Enhances flowability and reduces water demand	Pumpability, extrudability
	Hydroxypropyl methylcellulose (HPMC)	Enhances viscosity and thixotropy	Buildability, layer adhesion
	Alkali-free shotcrete accelerator (ACC)	Controls setting time for rapid layer stacking	Buildability
[38]	Water-reducing agent	Enhances flowability and reduces water content	Pumpability, extrudability
	Silica ash and fly ash	Improves workability and durability	Workability, dimensional stability
	Polypropylene (PP) fibers	Enhances interlayer bonding and reduces cracking	Interlayer adhesion, crack resistance
[25]	Silica fume	Enhances strength and durability	Compressive strength, durability
	Metakaolin	Improves thixotropy and extrusion properties	Buildability, extrusion stability
	Superplasticizer	Improves flowability and reduces water demand	Workability, extrudability
	PVA fiber	Enhances tensile strength and crack resistance	Interlayer adhesion, crack resistance

presents the specific additives and admixtures incorporated in mix designs for 3D-printed concrete bridges, emphasizing their role in optimizing fresh properties, improving interlayer adhesion, and ensuring the successful execution of large-scale 3D printing applications.

4. Construction Techniques for 3D-Printed Concrete Bridges

This section reviews the printing systems and construction techniques used in 3D-printed concrete bridges, highlighting their applications, advantages, and challenges.

4.1. Printing System

Large-scale 3D printers used in bridge construction can be broadly categorized into two primary types: robotic arm printers and gantry printers. Each type offers unique advantages and limitations, making them suitable for different project requirements. Robotic arm printers are known for their mobility and flexibility, enabling the creation of intricate designs due to their six degrees of freedom (see Figure 5b). However, they are constrained by smaller print sizes, the need for fine aggregates, and the requirement for advanced programming skills. In contrast, gantry printers shown in Figure 5a are more suitable for producing larger structures, accommodating coarse aggregates, and achieving longer element lengths with simpler software. However, these systems are bulkier, less mobile, and demand significant on-site setup [39]. The choice between robotic arm and gantry printers is ultimately dictated by the specific needs of a project, including size, complexity, and material requirements. This section reviews the different printing systems used in the construction of 3D-printed concrete bridges, as presented in

Table 2. Printing systems used in 3D-printed concrete bridges.

Ref.	Printer Type	Nozzle Geometry	Innovations in the Printing System
[16]	6 DOF ABB IRB6650 robotic arm	Round with a diameter of 25 mm	-
[35]	Self-developed System with horizontal and vertical actuators mounted on a raptor track drive	Rectangular (12 inch in length, 1 inch in thickness, and 1.5 inch in height per layer) with side plates to control material flow	Accelerated Heat Curing: Expedites printing with heat curing at 150 °F for rapid compressive strength development
			Mobile Platform: Raptor track drive enhances scalability and allows for complex element printing
[23]	4-DOF gantry robot	Rectangle measuring 40 mm by 10 mm	Hybrid down/back-flow nozzle: Combining down-flow and back-flow nozzles allows for cable integration while ensuring effective bonding between layers.
[21]	6 DOF KUKA R120–2500 robotic arm	Rectangle measuring 60 mm by 15 mm	Spatial Path Fitting Technology: Adjusted robot posture for spatially curved cavity and uneven stacking.
			Planar Path Fitting Technology: Controlled nozzle rotation for consistency with travel path tangent vector.
			Sensor-Based Real-Time Monitoring: Sensors on screw pump and printer for real-time parameter adjustments.
[41]	6 DOF robotic arm	-	Not mentioned
[18]	6 DOF robotic arm	-	Variable-Speed Printing: Modifies nozzle speed to reduce overfilling and ensure uniform printing.
[24]	6 DOF robotic arm	Backflow nozzle-rectangle measuring 60 and 80 mm by 12 mm	-
[25]	6 DOF robotic arm	-	Real-Time Path Generation: Controlled via Rhino and Grasshopper platforms, enabling on-the-fly adjustments.

. It focuses on the types of printers and nozzle geometries used alongside key innovations in printing technologies that have enhanced these systems’ scalability, precision, and efficiency.

The review of printing systems for 3D-printed concrete bridges highlights the dominance of robotic arm printers, which offer flexibility and precision for creating intricate designs. These systems, enhanced by innovations such as spatial and planar path fitting technologies and real-time path generation, are well-suited for complex geometries and controlled environments. While gantry printers are better suited for larger structures and coarse aggregates, their use in bridge construction remains limited.

4.2. Construction Techniques

This section reviews the construction methods used in 3D-printed concrete bridges, focusing on prefabrication and on-site assembly. Prefabricated methods involve printing structural components off-site in a controlled environment, allowing for precise quality control and optimized material usage. These components are transported to the construction site and assembled into the final structure.

The Striatus Bridge demonstrates a prefabrication approach involving segmenting a digitally modeled arch into 53 individual voussoirs using a stereotomy process [13,41]. Their technique ensured that contact surfaces experience primarily compressive forces, aligning with the principles of unreinforced masonry. Prefabricated segments were printed off-site and transported to the site for assembly. The construction process included preparing a foundation system with steel footings connected by tension ties supported by ground screws tailored to site constraints. A temporary scaffolding system accommodated the curved profile, supporting the segments during assembly. The segments were placed sequentially, with tension ties pre-tensioned before final positioning. The decentering process transferred structural loads from temporary supports to the ties and concrete segments, creating a stable, self-supporting structure.

Another example of prefabrication is constructing a 4.7-m-long prestressed concrete bridge [25]. The bridge was divided into six equal-length units to facilitate transportation and printing logistics, each weighing approximately 80–90 kg. During assembly, the prefabricated units were supported on a simple brick bracket, and mortar was used to fill the joints between them, ensuring proper force transmission.

The FloatArch Bridge also utilized prefabrication but integrated hybrid construction techniques to achieve a complex free-form design [18]. Segmented into nine components shown in Figure 6a, the design used principal stress trajectories to ensure contact surfaces experienced perpendicular compressive forces. After curing, segments were transported and assembled with temporary and permanent supports. Steel cables served as tensile members, connecting the end segments cast conventionally. The absence of grout or mortar allowed for disassembly and potential reuse, emphasizing sustainability.

Vantyghem et al.’s girder [16], which consisted of 18 prefabricated segments joined on-site with cast end blocks, is another example of a hybrid construction approach (the segments and their assembling are shown in Figure 5b). These segments were printed off-site. Traditional casting was utilized for the two end blocks, essential for anchoring the post-tensioning forces and joining the prefabricated segments. These components were fabricated on-site with conventional formwork, ensuring structural integrity in areas requiring reinforcement. Similarly, the Nijmegen 29-m bridge span was divided into five simply supported parts [24]. Prefabricated elements were transported to the site, glued together, and reinforced with traditionally cast concrete anchor blocks, while the columns utilized 3D-printed concrete “lost formwork.” The road bridge in Ukraine was also constructed using both prefabrication and traditional in-site casting [26]. To optimize material usage and structural efficiency, the bridge utilized prefabricated minimass beams and lattice slab concrete panels, topped with a cast–in situ slab. The minimass beams relied on axial tension and compression rather than bending strength, while the lattice slabs served as permanent formwork and a safe working platform during construction.

The construction of the 3D-printed concrete bicycle bridge in Gemert, Netherlands, further exemplifies this approach [23]. Due to facility limitations, the bridge’s 6.5-m span was printed at the TU/e 3D concrete printing facility in six individual elements. The segments were designed to be rotated 90° after printing and assembled on-site. To ensure structural integrity, synthetic epoxy-based interface material was applied between the assembled elements. The post-tensioning system was anchored in traditionally cast and reinforced concrete bulkheads, while the bridge rested on two conventional abutments with pile foundations. Additionally, a steel parapet spanning the full bridge length was supported independently on the foundations.

Lastly, prefabricated and masonry assembly methods were used to construct the “Phoenix” bridge, a 9.5-m-long pedestrian bridge [21]. The bridge’s primary arch ring was subdivided into 53 individual blocks of four different types, while the guardrails were divided into 40 pieces of 10 different types. The masonry assembly method was employed on-site, with the prefabricated blocks sequentially laid and bonded using an epoxy-based mortar. Temporary earthwork arch formwork, made of compacted soil, supported the primary arch ring during assembly, beginning at the arch foot and symmetrically progressing towards the arch crown. Each block was rotated 90 degrees during installation to ensure the printed layers were perpendicular to the arch axis. The bridge was further supported by gravity abutments and embedded foundations.

To conclude this section, prefabrication and on-site assembly are the most commonly employed methods for constructing 3D-printed concrete bridges. This approach offers significant benefits while also presenting certain challenges. Below is a summary of the advantages and drawbacks of prefabricated construction techniques:

Advantages:

-

Reduced Construction Time: Prefabrication of structural components combined with rapid on-site assembly can reduce construction time by 50% to 80% compared to traditional cast-in-place methods [35,42,43].

-

Improved Quality Control: Off-site fabrication of bridge segments in controlled environments ensures consistent conditions, reducing variability and enhancing accuracy and reliability in the final structure [23,35].

-

Minimized On-Site Disruptions: Prefabrication reduces on-site activities, minimizing traffic congestion, noise, and environmental impact, which is particularly advantageous in urban or ecologically sensitive areas [19].

-

Enhanced Sustainability: Modular prefabricated components allow for disassembly, reuse, and potential recycling, contributing to more sustainable construction practices [13,18].

Disadvantages:

-

Transportation and Handling Challenges: The transportation of large, prefabricated segments can be logistically challenging and often requires specialized equipment.

-

Connection Complexity: Robust design and execution of connections between prefabricated segments are essential to ensure efficient load transfer, structural integrity, and long-term durability of the bridge.

5. Reinforcement in 3D-Printed Concrete Bridges

Reinforcement is essential for nearly all concrete structural components due to the material’s inherent weakness in tension. However, in 3D-printed concrete structures, conventional reinforcement methods, such as rebars, present challenges because of the layered fabrication process. This section reviews the reinforcement methods employed in 3D-printed concrete bridges, as documented in the literature.

Post-tensioning is a widely used method for reinforcing 3D-printed concrete bridge components, addressing the need for enhanced structural capacity and longer spans [16,25,38,44]. This method involves embedding steel tendons or cables within the printed concrete and applying tension after the concrete hardens, inducing compressive stresses to counteract tensile forces. Vantyghem et al. [16] combined post-tensioning with steel rebars to effectively integrate reinforcement into a 3D-printed girder. High-strength steel cables were optimized through topology design to align with the girder’s geometry and were anchored in specially designed end blocks to withstand concentrated forces. Steel rebars provided supplementary reinforcement, enhancing tensile strength and ensuring robust force transfer. Their reinforcement positioning is shown in Figure 6b. Zhan et al. [25] used a similar approach, embedding steel bars within cavities during printing and tensioning them post-curing to strengthen the bridge. Mitrović et al. [44] employed post-tensioning to connect printed segments into a single structural element, strategically tensioning steel bars to enhance shear capacity and overall structural integrity.

The “Minimass” bridge design [26] represents another innovative application of post-tensioning, combining reinforced concrete and steel cables. The design features beams acting as trusses, with a reinforced concrete top chord handling compression, a steel cable bottom chord resisting tension, and concrete webs separating the two. The cables were encased in grease-filled ducts, providing corrosion resistance and allowing for re-stressing. Prefabricated lattice slabs and an in situ topping slab ensured seamless load transfer between the printed elements.

Salet et al. [23] also employed post-tensioning in their 3D-printed bicycle bridge using Dywidag-system tendons. These tendons were embedded in the printed elements, stressed, anchored at both ends, and then released. To address shear and torsion, high-strength steel cables were integrated into the concrete filaments during printing. A specialized embedding device was developed to ensure proper integration, maximizing bond strength and structural performance.

Some other projects tackled the challenges of embedding reinforcement during printing by opting for post-printing installation methods. For instance, reinforcement in 3D-printed bridge columns [43] involved installing reinforcement cages into 3D-printed hollow cylindrical formwork. The reinforcement cage comprised steel rebars, ties, and corrugated ducts that were installed within the formwork. The ducts were pathways for energy-dissipating (ED) steel and Fe-SMA rebars, which were inserted and grouted post-assembly. Fe-SMA rebars provided prestressing, with smooth midsections bonded to grout and threaded ends anchored in the footing and top loading block. Once the reinforcement cages were in place, cast concrete was poured into the formwork to encase the steel elements. The segments were stacked over a precast footing, bonded with cement paste, and grouted with the rebars.

The use of fibers within the concrete mix offers another reinforcement strategy for 3D-printed concrete bridges, which can improve tensile strength, ductility, and interlayer bonding. Polypropylene fibers, for example, were used in the 3D printing mortar for a bridge column to enhance crack resistance and interlayer adhesion [38]. Javed et al. [35] employed 2% steel fibers by volume in their mix to increase the strength and ductility of the printed layers, demonstrating the potential of fiber-reinforced concrete as a viable alternative to traditional reinforcement methods. Additionally, Zhan et al. [25] utilized 13 mm polyvinyl alcohol (PVA) fibers in their fiber-reinforced high-strength cementitious mortar to further enhance the structural properties of the printed elements. These examples underscore the versatility and effectiveness of fiber reinforcement in addressing the mechanical challenges associated with 3D-printed concrete structures.

Recent studies have demonstrated the feasibility of embedding FRP grids within printed concrete layers, improving structural integrity while maintaining printability [45,46,47]. This method has not yet been applied in 3D-printed concrete bridges, but it has potential for future applications, particularly in lightweight, corrosion-resistant structures where conventional reinforcement may be impractical. While FRP-based reinforcement in 3DPC offers numerous benefits, challenges remain, particularly in ensuring adequate bonding between FRP elements and the printed concrete matrix, optimizing grid spacing for uniform load distribution, and addressing long-term durability concerns. Future research should explore hybrid reinforcement techniques, where FRP is combined with steel or discrete fiber reinforcement to maximize performance while maintaining ease of printing.

Hybrid approaches that combine innovative and traditional techniques have also been explored. For example, the Striatus bridge is a 3D-printed masonry arch bridge that eliminates internal reinforcement in the printed blocks by relying on compression-only principles [41,48]. The structural stability of the arch is achieved through its geometry, while the foundation employs reinforced concrete pads to anchor steel footings and ground screws, demonstrating a blend of traditional and modern construction methods.

In conclusion, post-tensioning was the most used reinforcing approach, combining steel cables, rebars, and optimized designs to enhance structural capacity and stability. However, fiber-reinforced polymer (FRP) reinforcement can be a promising alternative, offering a lightweight and corrosion-resistant solution that can be directly integrated into the printing process. Additionally, post-printing methods, such as installing reinforcement cages in printed formwork, provide flexibility for incorporating advanced materials like Fe-SMA rebars. Hybrid approaches, such as the compression-only Striatus bridge, highlight the potential for integrating traditional and modern techniques in 3D-printed bridge construction.

6. Conclusions

The review indicates that 3D-printed concrete bridges have substantial potential to transform construction practices through enhanced material efficiency, design flexibility, and sustainability. The following conclusions and recommendations were drawn from this research:

Design process: The design processes for 3D-printed concrete bridges demonstrate the potential of computational and experimental methodologies in modern construction. Techniques such as topology optimization, parametric design, and thrust network analysis enable efficient material usage, reduce waste, and create innovative forms while leveraging the unique capabilities of 3D printing. Iterative methodologies, like “design by testing,” ensure structural integrity and reliability and validate computational models. Moreover, incorporating advanced numerical methods, such as Discrete Element Modeling (DEM) and Finite Element Analysis (FEA), can lead to more sustainable and efficient solutions in bridge construction.

Materials: Printable concrete must maintain a balanced fresh state to ensure successful 3D printing and long-term performance. To achieve this, various admixtures and additives have been incorporated into the mix design of different 3DCP bridges. These include superplasticizers, water retention agents, polypropylene fibers, metakaolin, etc., all of which work to enhance workability, extrudability, buildability, and mechanical strength. Future research should focus on optimizing eco-friendly materials to enhance the sustainability of 3D-printed concrete structures. This includes developing materials with lower carbon footprints, incorporating recycled or renewable components, and improving the efficiency of material usage. Such advancements could significantly reduce the environmental impact of construction while maintaining or even improving structural performance and durability.

Construction techniques: The review of printing systems used for 3DCP bridges revealed a significant preference for robotic arm printers over gantry printers. Known for their flexibility and precision in producing intricate designs, robotic printers can leverage advanced technologies such as spatial and planar path fitting, real-time path generation, optimized nozzle configurations, and real-time monitoring.

Prefabrication and on-site assembly are the most practical and widely accepted methods for constructing 3D-printed concrete bridges. This approach enables components to be manufactured in controlled environments, enhancing precision and consistency and minimizing material waste. However, ensuring the bridge’s structural integrity requires meticulous attention during the assembly and connection of individual components. This attention is especially critical for bridges intended to support substantial loads, such as vehicles, as poorly executed connections can create weak points that compromise the overall stability and durability of the structure.

Reinforcement strategies: Post-tensioning has emerged as an effective method for reinforcing 3D-printed concrete bridges. This technique combines steel cables, rebars, and optimized designs to enhance both structural capacity and stability. Practical post-printing approaches, such as embedding reinforcement cages within printed formwork, further extend the applicability of traditional reinforcement strategies. Additionally, fiber reinforcement can offer the potential for enhancing tensile strength, ductility, and interlayer bonding. However, challenges remain in addressing reinforcement perpendicular to layer-to-layer interfaces—an aspect crucial for maintaining structural integrity. This gap presents an exciting opportunity for future innovation. Integrating perpendicular reinforcement during the printing process, possibly through the use of auxiliary robots, expands the structural capabilities of 3D-printed concrete.

7. Recommendations for Future Research

While significant progress has been made in the field of 3D-printed concrete bridges, several key research areas remain underexplored and require further investigation:

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Investigating advanced numerical modeling approaches. Coupled multi-scale models and machine learning-based simulations could optimize structural design under real-world conditions.

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Exploring the role of anisotropy in mechanical performance. Further research is required to quantify the impact of anisotropic behavior and interlayer bonding on long-term structural integrity.

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Developing standardized guidelines and building codes. A lack of regulatory standards hinders the widespread adoption of 3D-printed bridges. Future research should contribute to the establishment of industry-wide guidelines for structural assessment and approval.

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Eco-friendly cementitious materials. Future research should develop low-carbon cement alternatives for 3D-printed concrete bridges.

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Automated quality control and real-time monitoring. Future research should focus on AI-driven defect detection and adaptive extrusion systems to enhance precision during printing.

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Scaling up in situ 3D printing. While most 3D-printed bridges are prefabricated, advancements in mobile printing technologies could enable on-site, large-scale printing for infrastructure projects.

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Developing hybrid reinforcement strategies. To improve structural resilience, research is needed on integrating pre-tensioning, post-tensioning, and embedded reinforcement techniques.

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Optimization of fiber-reinforced polymer reinforcement. Investigating FRP’s role in 3D-printed bridges could expand its application in corrosion-resistant, lightweight structures.