# Recycling 3D Printed Concrete Waste for Normal Strength Concrete Production
## 3 Results and Discussion  
### Compressive Strength  
#### 3.1.1 Influence of Replacement Level  
Following the testing of specimens at 28 days to determine both their individual and mean compressive strength, the equations specified in EN 206:2013 [25] are applied. These calculations lead to the determination of the measured average compressive strength value of the specimens (\(f_{cm,cube,test,H}\)), a crucial parameter used for classifying the concrete strength class according to BV-MI 01:2005 (H) [15]. The specific classifications are detailed in Table 4, providing a clear reference for the concrete's strength characteristics based on industry standards. This systematic approach ensures that the concrete production process is aligned with the established guidelines and results in accurate strength class classifications. These mixes are utilized to investigate the impact of replacement levels on compressive strength. A comparison is made between the measured average compressive strength value of specimens (\(f_{cm,cube,test,H}\)) and the corresponding minimum required compressive strength value (\(f_{cm,cube,H}\)) for both C40/50 and C30/37 at the age of 28 days, which are specified as 67 MPa and 49 MPa respectively [15]. The results clearly indicate that the measured compressive strength average value for each concrete mix surpasses the mean compressive strength requirement value stipulated for the concrete strength class. This observation holds true for instances of mixed curing. In other words, the measured mean compressive strength values consistently meet or exceed the specified requirement values for the respective concrete strength classes. This alignment underscores the robustness of the concrete mixes, confirming their adherence to the established standards [20; 21; 22; 23; 24; 25; 26; 27] and emphasizing the reliability of the mean compressive strength results under the conditions of the study. Figures 7 and 8 provide a comprehensive depiction of the variations in compressive strength and density with respect to the replacement levels for the mix series, respectively. The inclusion of error bars, based on standard deviation, enhances the precision and insightfulness of the presented data. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mix Designation**} & **Density** & \(f_{cm,cube,test,H}\) & **Strength Class [15]** \\  & **(kg/m\({}^{3}\))** & **(MPa)** & \\ \hline C40-RA0 & 2377 & 69 & C40/50 \\ C40-RA50 & 2220 & 75 & C45/55 \\ C40-RA67 & 2174 & 75 & C45/55 \\ C30-RA0 & 2363 & 57 & C30/37 \\ C30-RA50 & 2155 & 49 & C30/37 \\ C30-RA67 & 2165 & 58 & C30/37 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Concrete strength with variable RA replacement. Focusing on the C40 series, Figure 7 reveals a noteworthy observation: the compressive strength at both 50% and 67% replacement levels is identical, and notably higher than that at the no replacement level (0%). Simultaneously, Figure 8 illustrates a consistent trend of decreasing density with an increasing replacement level. In the context of the C30 series, Figures 7 and 8 show a significant trend where the 50% replacement level exhibits lower compressive strength and density compared to both the 0% (no replacement) and 67% replacement levels. This indicates that a 50% replacement negatively impacts the compressive strength and density of the concrete mix. One potential explanation for this is the high percentage of fine aggregate and poor compaction during casting. The significant finding is that the 67% replacement level emerges as particularly effective. Remarkably, it not only aligns with the 0% (no replacement) scenario but even surpasses the compressive strength exhibited by the reference specimens. The primary reasons for low strength of the reference specimens are attributed to the smooth surface of the quartz gravel and the poor grading of the natural aggregate, as depicted in Figures 1 and 6, respectively.