# Experimental Study of 3D Concrete Printing Configurations Based on the Buildability Evaluation
## 3 Experimental Program
### Configuring Methods  
#### 3.3.1 Building Index (\(Bl\))  
Buildability is about the ability of the PO to maintain the printed geometry itself with fast-setting and early strength to bear its own weight and that from subsequent layers. As said in Section 1, neither the maximum number of layers nor the height of the PO can be used to evaluate buildability since they are highly dependent on the configuration of the PD. In this study, the building index (\(BI\)) was defined by how many layers/loops can be finished within the initial setting time duration \(T\). \[BI=\frac{T}{t_{loop}} \tag{2}\]  
where \(t_{loop}\) stands for the loop time, indicating the (average) time for one-layer fabrication to combine the printing scale (total length of the filaments in one layer) of the PO and the printing speed of PD. As shown in Figure 5, for a hollow cylinder with a diameter of \(D\) at a printing speed (nozzle motion speed) of \(V\), the loop time can be determined as:  
\[t_{loop}=\frac{D\times\pi}{V} \tag{3}\]  
and the \(BI\) can be further expressed as:  
\[BI=\frac{T}{t_{loop}}=\frac{T\times V}{D\times\pi} \tag{4}\]  
The \(BI\) includes the initial setting time of PM, the printing scale of the PO, and the printing speed of the PD. It is easy to understand that: (1) for one fixed printing scale, if the printing speed is increased, the PO is more likely to collapse, since more layers are stacked on; (2) for a fixed printing speed, the smaller the printing scale, the more layers would be stacked on the previous printing layer, and the more likely the PO will collapse; (3) if the \(t_{loop}\) is fixed in one printing activity, the higher the \(T\), the more likely the PO will collapse. Thus, lowering \(BI\) would lead to 3D printing success. Figure 4: (**a**) Illustration of inter-support from adjacent filaments and variable motion speeds and (**b**) method for determining whether the printed result is a Pass or Fail. To evaluate the \(Bl\), three groups of specimens were prepared with the designated layer height, filament width, and total height of 5.0 mm, 15 mm, and 30 layers, respectively. The first group with different SAC contents was prepared, and they were expected to show different initial setting times. The specimens were 100 mm diameter cylinders made of OSS1, OSS2, and OSS3 at a printing speed of 50 mm/s and marked as D100V50 OSS1, D100V50 OOSS2, and D100V50 OOSS3. The numbers 100 and 50 after the letters D and V show the diameter of the specimen and the printing speed, respectively. The second group of specimens of identical size was prepared at different printing speeds, ranging from 20 mm/s to 50 mm/s with an increment of 10 mm/s. They were made of mix OSS3 and marked as D100V20 OSS3, D100V30 OSS3, D100V40 OSS3, and D100V50 OSS3. The third group of specimens was prepared with different cylinder diameters ranging from 50 mm to 200 mm with an interval of 50 mm. They were printed with the mixture OSS3 at a printing speed of 40 mm/s and marked as D50V40 OSS3, D100V40 OSS3, D150V40 OSS3, and D200V40 OSS3. In addition, other supplementary specimens were produced to fulfill the orthogonal test requirements for diameter, speed, and initial setting time, as well as to provide more data for analysis, as listed in Table 3.