# Experimental Study of 3D Concrete Printing Configurations Based on the Buildability Evaluation
## 4 Results and Discussion
### Building Index
Fortunately, the layer bonding would be enhanced by other techniques, such as inserting rebars [30] or nails [31]. Therefore, the \(BI\) needs to be chosen carefully to balance successful printing with the mechanical properties of the printed results. Figure 12: The \(BI\) results. (**a**) Results separately based on initial setting time group D100V50T@, printing speed group D100V@OSS3, and cylinder diameter group D@V40 OSS3, and (**b**) the relationship between \(BI\) and the loop time \(t_{loop}\), based on different initial setting time T.  
In normal printing activities, the \(t_{loop}\) may choose the average loop time of the printing component, or the whole printing time divided by the layer amount, as in Table 5. Those data can be found in the TPG software. For solid components, such as in the sketch in Figure 4a, the loop time for one layer could be:  
\[t_{loop}=\frac{\textit{material mass of one layer }(gram)}{\textit{the nozzle extruding speed }(gram\ \textit{per}\ s)}\]  
Besides the \(t_{loop}\), there are many time data in the 3 DCP, related to the yield stress, slumping, setting, and bonding behavior of the fresh material, among which only the initial/final setting time are universal characterizing methods, indicating a material starting to lose plasticity and completely losing plasticity, respectively. The final setting time is later than the open time, out of the printability range. Thus, the initial setting time was the suitable parameter in the \(BI\) at this stage. Although some materials have zero slump and there is no height reduction in each printed layer, the printing failure still would happen due to the elastic buckling and plastic collapse [32; 33], since the distribution of the individual material, after being stirred by the blades, extruded by the screw, and sheared by the nozzle tip, is not sufficiently uniform. More studies should be conducted on the timing data to upgrade the \(BI\) equation by balancing the yield stress, slumping, setting, and bonding behavior. Nevertheless, compared to the existing buildability characterizing methods that simply quantify layer amount [10; 13], height achievement [8], or height reduction [7; 20], the \(BI\) in Equation (4) considers and balances the printing speed and printing size and results in a dimensionless value. \(BI\) is related to the material and its PD and the nozzle tip specified, and it evaluates the working range of PM and PD. In other words, \(BI\) characterizes not only the material but also the printing system. For one developed PM, \(BI\) could be one configuration for generating the best toolpath from variable sizes of PO. Furthermore, the \(BI\) in this study was only for an extruding mechanism stirring, shearing, and pushing the material out of the nozzle tip, as utilized in this study and other similar studies [34; 35], and may not be suitable for piston-like extruders [36; 37; 38]. Figure 13: The four cylinders printed with the same \(BI\) of around 50. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Unit** & **D = 50 mm** & **D = 100 mm** & **D = 150 mm** & **D = 200 mm** \\ \hline Printing speed \({}^{\star}\) & mm/s & 10.1 & 18.6 & 27.9 & 37.2 \\ \(t_{loop}\) & s & 15.5 & 16.9 & 16.9 & 16.9 \\ \(BI\) & & 50.2 & 46.2 & 46.2 & 46.2 \\ \hline \end{tabular}  
* The printing speed is the average value. The TPG generated the speed of each toolpath according to the given maximum speed and the CAD file.  
\end{table} 
Table 5: Data and calculation of the printed cylinders with material mix OSS3.