# Experimental Study of 3D Concrete Printing Configurations Based on the Buildability Evaluation
## 3 Experimental Program
### Configuring Methods
#### 3.3.2 Height Reduction Test (HRT)  
In traditional concrete research, the slump test is used to evaluate the workability of the mortar (about 10 Kg for the slump experiment). However, in a 3 DCP process, the amount of material being printed/extruded is relatively small (about 300 g for 100 mm length of printed filament); thus, the ordinary slump test for a large amount of mortar is not suitable to characterize the PM. In this study, a height reduction test (HRT) was proposed to quantify the height loss of the extruded filaments and find the critical value.  
The extruded fresh mortar should fill the gap between the nozzle tip and the upper surface of the previously printed layer, as shown in Figure 6a. Otherwise, the printed line will not follow the designated route, especially at turning points, as shown in Figure 6c,d. In a printing process, \(h\) is defined as the designed layer height and \(H\) as the gap height reserved for a layer. The \(H\) would be larger than \(h\), due to the slump of previous layers, thus creating more room for the subsequent layer. The gap height tolerance (GHT) \(\delta\) is defined as: \(\delta=(H-h)/h\,,H\geq h\). [PERSON] et al. (1998) suggested that the printing materials with low- to zero- slump should be used, then \(H\to h\), meaning \(\delta\to 0\), and based on the authors' experience, for printing ordinary geometry, the \(\delta\) should be less than 0.2, meaning that \(H\) should be less than 1.2\(h\). Otherwise, the extruded filament cannot land in the designated plane by free fall, as indicated in the red circle shown in Figure 6d.  
The printing technology has been developed for complex components that have many turns in the printing route. misplacement would lead to inconsistency with the designed geometry and, eventually, a collapse failure, as shown in Figure 7, as the accumulated height error increases. It is essential since most of the printing toolpath has the same fixed height for each layer, marked as same-layer-height printing.  
The extruded material would have a slight slump. To determine the slumping requirement, a parameter, height reduction ratio (\(HRR\)) \(r\), was proposed as defined in Equation (5). It is easy to conduct an HRT to obtain the height ratio \(x\), which is the measured real height of a printed filament divided by its designed height \(h\). Then, the \(HRR\) can be expressed as:  
\[HRR=r=1-\frac{\text{{real height measured}}}{h}=1-x \tag{5}\]  
Three groups of HRTs have been conducted, with a time gap of 10 min, 20 min, and 15 h, respectively. The criteria of the \(HRR\)\(r\) were theoretically calculated and verified by a printing simulation, which was conducted via Microsoft Excel with VBA (Visual Basic  
Figure 6: The layer stacking process. (**a**) A typical layer-stacking operation, (**b**) the layer stacking with reserved height H, (**c**) an L-turn for layer stacking operation with GHT, and (**d**) inaccurate filament landing.  
for Applications) program. Detailed calculation and simulation of \(HRR\) for the conditions when \(BI=1\) and \(BI>1\) are provided in later sections.