# 3D-Printed Concrete Bridges: Material, Design, Construction, and Reinforcement
## 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 [PERSON] et al. [31], the anisotropy in compressive strength of 3D-printed concrete (3 DPC) 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 [PERSON] 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. Figure 4: Different cases of loading direction and specimen configuration for (**a**) compressive and (**b**) flexural strength [32]. 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].