# 3D-Printed Concrete Bridges: Material, Design, Construction, and Reinforcement
## 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 (3 DCP), astructure is fabricated layer by layer with a nozzle that deposits material along a defined path. Compared to conventional concrete construction methods, 3 DCP 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 [PERSON] 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.