# Overview of the Development of 3D-Printing Concrete: A Review
## 3 Performance Requirements of 3D-printing Concrete Materials
### Printability of 3D-printing Concrete Materials
#### 3.1.3 Buildability  
Buildability refers to the degree of deformation and overall stability of the 3D printed cement-based material after extrusion under its own weight and subsequent extrusion and gravity of the printed layer. Because the 3D-printing concrete technology does not have templates, it is easy to have interlayer defects or settlement of the printing layer. The previous printing layer must support the subsequent printing layer with better bonding ability with the subsequent printing layer, and the components do not collapse or deform. Thus, the printing material needs to have interlayer support and shape retention. One of the basic requirements of 3D printed concrete is good buildability. The buildability is an index to measure the early stiffness of the structure, the foundation of the integrity of the printed components, and the key to the stability of the layers. Research on the buildability can ensure the printed building stability. Buildability can be improved by increasing the amount of aggregate and adding mineral admixtures and additives. [PERSON] et al. [20] prepared the slurry by mixing 2% cement to replace silica fume and nanoclay, which greatly improved the construction  
Figure 4: Test sample to evaluate extrudability. (**a**) Long strip evaluation chart. (**b**) Stacking evaluation chart. Reprinted with permission from ref. [16]. Copyright 2012, RILEM. Reprinted with permission from ref. [27]. Copyright 2019 MDPI. performance of the slurry, and the construction height was increased from the original 72 mm to 260 mm. [PERSON] et al. [29] added 4.5% lithium carbonate to the mixture to promote rapid coagulation of the printing mixture, thereby achieving the purpose of improving buildability. [PERSON] et al. [18] found that the buildability of 3D printed concrete is determined by its yield stress, while the yield stress of concrete depends on the ratio of cement paste and aggregate in the mixture. With the same aggregate content, the finer the concrete, the higher the concrete yield stress, which is easier to buildability. [PERSON] et al. [30] controlled the proportion of cement-based materials by controlling a single variable of microcrystalline cellulose (MCC). The detailed parameters are shown in Table 1. The study found that, as the addition of microcrystalline cellulose increases, the slump rate decreases. The buildable height is increased, and the buildability is significantly improved, as shown in Figure 5.  
The buildability test can directly explore the non-deformation of the stacking and accumulation of concrete through traditional construction test methods. [PERSON] et al. [30] used a print nozzle with a diameter of 20 mm and a printing speed of 80 mm/s to prepare a 5-layer hollow structure with a size of 400 mm \(\times\) 150 mm \(\times\) 100 mm, as shown in Figure 6. The structure slump was used to characterize the buildability of the mixture. [PERSON] et al. [31] used a loading device to detect the deformation of the 20-layer printing structure. The loading rate was set according to the printing construction plan. When the deformation was less than 0.2%, it was deemed to meet the construction requirements. [PERSON] et al. [32] printed a 300 mm \(\times\) 300 mm rectangular hollow thin-walled  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \begin{tabular}{c} **Mix** \\ **No.** \\ \end{tabular}  & \begin{tabular}{c} **Cement** \\ **(g)** \\ \end{tabular}  & **SF (g)** & **FA (g)** & \begin{tabular}{c} **Sand** \\ **(g)** \\ \end{tabular}  & **w/b** & \begin{tabular}{c} **MCC** \\ **(g)** \\ \end{tabular}  & \ \begin{tabular}{c} **HRWRA** \\ **(g)** \\ \end{tabular}  & \begin{tabular}{c} **Li\({}_{2}\)CO\({}_{3}\)** \\ **(g)** \\ \end{tabular}  \\ \hline M-1 & 780 & 130 & 390 & 1300 & 0.35 & 0 & 4.55 & 13 \\ \hline M-2 & 780 & 130 & 390 & 1300 & 0.35 & 6.5 & 4.55 & 13 \\ \hline  
structure, stacked 20 layers vertically. Then the difference between the actual build height and the design height was calculated.