# Recycling 3D Printed Concrete Waste for Normal Strength Concrete Production
## 2 Materials and Methods
### Particle Size Distribution  
The meticulous attention to adhering to established particle distribution grading curves underscores the importance of maintaining compatibility with industry standards, particularly in the pursuit of sustainable and performance-enhancing concrete formulations. To determine the extent of recycled aggregate (RA) replacement, a precise sieve analysis was performed on the RA sourced from 3D-printed concrete and the river sand in accordance with the specifications outlined in EN 933-1:2012 [22]. This standardized procedure ensures the accurate assessment and comparison of particle size distribution for both types of aggregates. The resulting grading curves for each aggregate are visually represented in Figures 2 and 3, providing a clear depiction of the size distribution characteristics. Notably, river sand predominantly functions as a fine aggregate; however, a more detailed examination, illustrated in Figure 2, reveals the presence of 7% coarse aggregate with a particle size of 4 mm by mass. This observation requires careful consideration during the mix design phase when proportioning the various ingredients for the overall construction material. By addressing these intricacies in the mix design, the goal is to achieve a composition that aligns with the specified requirements and ensures the desired performance characteristics of the resulting construction material. Figure 1: Aggregates: (**a**) Quartz gravel; (**b**) RA of 3D-printed concrete waste. Figure 2: Grading curve of river sand. In Figure 3, the data reveals that 68% of the recycled aggregate, based on mass, constitutes a coarse aggregate. Especially, its grading curve deviates from the specified boundaries of curves A and C, contravening the stipulations outlined in MSZ 4798-1 [23] regarding grading distribution. To rectify this discrepancy, a crucial step involves blending the recycled aggregate with sand in carefully determined proportions. The key objective is to establish a specific grading for the combined fine and coarse aggregates, ensuring an optimal distribution that maximizes the aggregate content while minimizing void spaces. This approach serves to reduce reliance on paste (water and cementitious material) in concrete formulations, consequently enhancing dimensional stability and overall durability. Adhering to this guiding principle, the integration of recycled aggregates with sand has been executed with a specific approach, taking into account both the maximum level of replacement required to achieve a Class I and Class II aggregate size distribution [15; 23]. Illustrated in Figures 4 and 5 are the replacement levels that successfully meet these criteria, represented by recycled aggregate to sand mass ratios of 2:1 (67% replacement) and 1:1 (50% replacement). This visual representation serves as a clear demonstration of the effectiveness of these specific ratios in achieving the desired aggregate classifications. Figure 4: Grading curve for 67% replacement ratio (2:1). Figure 3: Grading curve of crushed 3D-printed concrete. By strategically incorporating these replacement levels, the aim is to strike a balance between sustainable practices and meeting the specified standards for aggregate distribution. The selection of these replacement levels is informed by careful consideration of the desired concrete properties and the need to optimize the use of recycled materials. The concrete selected for this study belongs to the strength classes of C30/37 and C40/50, both of which fall within the category of normal strength concrete, as they are under the C50/60 threshold [24]. In summary, Figure 6 illustrates the grading curve of the aggregate, representing the combined fine and coarse aggregate particle size distribution, for both strength classes of concrete when there is 0% (i.e., no replacement level), 50% and 67% replacements. However, when we examine the scenarios with 50% and 67% replacement levels, irrespective of the concrete's strength class, a notable observation emerges.