# 3D-Printed Concrete Bridges: Material, Design, Construction, and Reinforcement
## 4 Construction Techniques for 3D-Printed Concrete Bridges
### 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. 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.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Printer Type** & **Nozzle Geometry** & **Innovations in the Printing System** \\ \hline
[16] & 6 DOF ABB IRB6650 robotic arm & Round with a diameter of 25 mm & - \\ \hline
[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 \({}^{\circ}\)F for rapid compressive strength development \\ \hline
[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. \\ \hline
[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. \\ \hline
[41] & 6 DOF robotic arm & - & Not mentioned \\ \hline
[18] & 6 DOF robotic arm & - & Variable-Speed Printing: Modifies nozzle speed to reduce overfilling and ensure uniform printing. \\ \hline
[24] & 6 DOF robotic arm & Backflow nozzle-rectangle measuring 60 and 80 mm by 12 mm & - \\ \hline
[25] & 6 DOF robotic arm & - & Real-Time Path Generation: Controlled via Rhino and Grasshopper platforms, enabling on-the-fly adjustments. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Printing systems used in 3D-printed concrete bridges.  
Figure 5: Printer type: (**a**) Gantry printer [40] and (**b**) Robotic arm.  
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.