# Experimental Study of 3D Concrete Printing Configurations Based on the Buildability Evaluation
## 4 Results and Discussion
### Building Index  
Figure 12a shows the effect of diameters, initial setting times, and printing speeds on the \(BI\) values. The \(BI\) values increased with decreasing diameter, increasing initial setting time, and increasing printing speed. However, not all of the specimens were printed successfully, as shown in Figure 11. The failure and success cases are indicated as red and black dots in Figure 12a, respectively. The printing failed at \(BI=76.4\) for material mix OSS2 in group D100V50T@, at \(BI=124.2\) for the printing speed at 50 mm/s in group D100V@OSS3, and at \(BI=198.7\) for the cylinder diameter at 50 mm in group D@V40 OSS3. The printing succeeded at \(BI=19.1\) for material mix OSS2 in group D100V50T@, at \(BI=99.4\) for the printing speed at 50 mm/s in group D100V@OSS3, and at \(BI=99.4\) for the cylinder diameter at 50 mm in group D@V40 OSS3. Therefore, printing would definitely succeed with \(BI\) lower than the success points and fail with \(BI\) higher than the failing points for each group. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **V = 20 mm/s** & **V = 30 mm/s** & **V = 40 mm/s** & **V = 50 mm/s** \\ \hline D = 50 mm & 3.13 min & 2.12 min & 1.60 min & 1.10 min \\ D = 100 mm & 7.52 min & 5.03 min & 4.17 min & 2.68 min \\ D = 150 mm & 11.82 min & 8.03 min & 5.80 min & 4.78 min \\ D = 200 mm & 15.98 min & 10.85 min & 8.15 min & 6.53 min \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The printing times for each sample according to its diameter and printing speed. Figure 11: Photos of some of the printed hollow cylinders. Figure 12b shows the relationship between \(BI\) and the loop time, based on different initial setting times for mixes OSS1, OSS2, and OSS3. It can be seen from the figure that the longer the initial setting time, the wider the acceptable range of \(BI\). The mix OSS3 with 13 min of initial setting time had the largest acceptable \(BI\) range of 99.4, whereas the mix with the shortest initial time only showed a \(BI\) range of 19.1. As shown in Figure 12a, results from the three independent groups show that the lower \(BI\) could guarantee successful printing. It could be explained from the \(BI\) definition in Equation (4) that: the printed filaments would have more time to obtain strength to maintain geometry before the next layer is stacked on, leading to better buildability with a lower printing speed \(v\), larger printing scale \(D\), or shorter initial setting time \(T\). Therefore, for one printing application with a known printing scale \(D\), if the printing speed of the 3D printer is fixed, the material should be developed with a low initial setting time in the acceptable range of the material printability. If the material mix proportion is fixed, the printing speed should be lower in the acceptable range of the total printing time of the application. For the PD in this study, the \(BI\) ranges of the OSS1, OSS2, and OSS3 were 0-19.1, 0-61.1, and 0-99.4, respectively. A \(BI\) of around 50 was chosen to print different sizes of cylinders with mix OSS3 as the PM. Four cylinders with diameters of 50 mm, 100 mm, 150 mm, and 200 mm were sketched and sliced with different printing speeds to guarantee a \(BI\) of around 50. Each printed cylinder consisted of 30 layers. All four printings succeeded without collapse or obvious slump, as shown in Figure 13. Table 5 summarizes the required printing time and the calculated \(BI\). The loop time in Equation (3) can be controlled to be equal to or longer than the initial setting time, meaning \(BI\leq 1\). In this case, the printed filament could have enough time to gain early strength and maintain its geometry without collapse. However, the time for layer bonding \(t_{bonding}\), as illustrated in Figure 5, would be extended reversely, decreasing the interface bonding strength [28; 29] and the mechanical properties of the PO. To obtain a good bonding, the printing speed needs to be higher, leading to a bigger \(BI\) and a high mechanical property of the PO.