# 3D-Printed Concrete Bridges: Material, Design, Construction, and Reinforcement
## 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. [PERSON] 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. [PERSON] et al. [25] used a similar approach, embedding steel bars within cavities during printing and tensioning them post-curing to strengthen the bridge. [PERSON] 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. [PERSON] 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-strengthsteel 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]. [PERSON] et al.