# Additive Manufacturing as an Alternative to Core Sampling in Concrete Strength Assessment
## 2 Materials and Methods
### Fabrication of Structures and Sampling for Control Specimens  
In laboratory conditions, at the Additive Technologies Section of the Competence and Technology Transfer Centre in Construction of the D. Serikbayev East Kazakhstan Technical University (EKTU), a wall-type structural fragment (Type 1 block) was printed using each of the developed concrete mixtures. The mixes were prepared manually in the following sequence: the aggregate was thoroughly mixed with cement using a low-speed (500 rpm) spiral-blade electric mixer until the binder was evenly distributed in the mixture (for 3-4 min), as shown in Figure 1.  
Next, water containing a pre-dissolved plasticiser was added to the mixture. After adding the water, mixing was continued for another 5-6 min.  
To control the workability of the fresh concrete mix, a portion of the batch was placed into a standard slump cone, and the slump depth was measured in centimetres (Figure 2).  
If the slump met the printer manufacturer's requirements--specifically, a slump between 8 and 10 cm--the mixture was loaded into the screw-type hopper of the printer's  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Concrete Mix** & **Mix 1** & **Mix 2** & **Mix 3** & **Mix 4** & **Mix 5** \\  & **C:P = 100** & **C:P = 150** & **(C+TMF):P = 150** & **C(P+TMF) = 150** & **(C+TMF):P+TMF) = 150** \\ \hline \multicolumn{6}{c}{**Component Consumption**} \\ \hline Cement CEM 1 52.5H, kg & 10.34 & 12.73 & 11.58 & 12.73 & 11.58 \\ \hline Sand, kg & 36.37 & 36.37 & 36.37 & 26.55 & 26.55 \\ \hline TMF (techno-mineral filler), kg & – & – & 1.15 & 9.82 & 10.97 \\ \hline Water, kg & 6.33 & 8.43 & 7.30 & 8.66 & 7.18 \\ \hline Plasticiser, g & 76 & 51 & 69 & 69 & 64 \\ \hline \multicolumn{6}{c}{**Mixture Properties**} \\ \hline Water-to-cement ratio (W/C) & 0.61 & 0.66 & 0.63 & 0.68 & 0.62 \\ \hline Workability class & P2 & P2 & P2 & P2 & P2 \\ \hline Slump cone (cm) & 8 & 9 & 7 & 9 & 9 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Material consumption per batch (3D-printer-hopper fill) and mixture characteristics (adjusted final mixes).  
Figure 1: Mixing of the concrete mixture.  
printhead, and printing of the designated structural element was initiated according to the defined dimensions (Figure 3).  
At the midpoint of the printing cycle, a sample of the concrete mixture was taken to fabricate control specimens. Within 20 min of sampling, 12 cube specimens were produced for each mix design: four specimens each for testing at 7, 14, and 28 days of curing. The working mixtures were placed into pre-oiled moulds in layers and compacted manually. Compaction was carried out using a hand rodding method with a 16 mm diameter steel rod, moving from the edges of the mould toward its centre. Excess material on top was removed and smoothed with a towel. The resulting specimens are shown in Figure 4.  
After 24 h, the specimens were demoulded (Figure 5). All specimens were stored next to the printed structures under ambient conditions, without strict curing control, since it is difficult to maintain an optimal curing environment (20 \(\pm\) 2 \({}^{\circ}\)C, 95 \(\pm\) 5% relative humidity) for large-scale printed elements [36; 37]. Both the specimens and the printed structures were labelled and left to cure until the designated testing days.  
Figure 3: Printing process of the concrete element.  
Figure 2: Workability test of the fresh concrete mixture.  
Figure 4: Sampling and preparation of control specimens.  
Figure 5: Demoudling of concrete cube specimens and their curing conditions: (**a**) demoulded specimens immediately after removal from moulds; (**b**) covered specimen during early-stage curing; (**c**) storage of demoulded cubes alongside printed structural elements under ambient laboratory conditions.  
During the 28-day strength development period, daily monitoring of the ambient conditions was carried out. The temperature ranged between 18 and 24 \({}^{\circ}\)C and the relative humidity remained between 80 and 90%.