# Recycling 3D Printed Concrete Waste for Normal Strength Concrete Production
## 2 Materials and Methods
### Testing Methods  
The testing method employed is the standard compression test, conducted in accordance with EN 12390-3:2001 [20], to determine the compressive strength of cubic specimens. The results of the test are then checked for adherence to the criteria specified in EN 206:2013 [25]. The compressive strength testing utilizes a standardized compression testing machine, specifically the FORMTEST ALPHA 3-3000. Within this controlled testing environment, the specimens undergo a carefully regulated testing process, exposed to a consistent rate of loading set at 11.5 kN/s. As per EN 1992-1-1:2004 [26] standard, the cylindrical mean compressive strength value (\(f_{cm,cyl}\)) in MPa is determined by adding 8 to its cylindrical characteristic compressive strength value (\(f_{ck,cyl}\)). It is important to note that this relationship specifically applies to cylindrical samples measuring 150 mm in diameter, 300 mm in height, aged for 28 days, and cured underwater throughout the entire duration (wet cured) [27]. In practical concrete technology, the conformity of compressive strength is commonly assessed using 150 mm-sized cubes, tested at the age of 28 days, and subjected to mixed curing. For such scenarios, the required minimum mean cubic compressive strength values (\(f_{cm,cube,H}\)) for concrete classes up to C45/55 are provided in the BV-MI 01:2005 (H) [15], specifically for specimens aged 28 days and subjected to mixed curing as per MSZ 4798-1:2004 [23]. To meet the conformity criteria of EN 206:2013 [23] for compressive strength, this value should be less than or equal to the measured average value of its compressive strength (\(f_{cm,cube,test,H}\)) in accordance with BV-MI 01:2005 (H) [15].  
In concrete, the interfacial transition zones (ITZs) serve as a bridge between the mortar matrix and the coarse aggregate particles. Even when the individual components are of high stiffness, the stiffness of the concrete may be low because of breaks in these bridges (i.e., voids and micro-cracks in the interfacial transition zone), which do not permit stress transfer [28]. In recycled aggregate concrete, there are more interfacial transition zones than normal aggregate concrete [29]. Therefore, the interfacial transition zone should be considered when the strength of the concrete is evaluated. Taking this into account, scanning electron microscopy (SEM) testing is conducted to assess the microstructure of the concrete at ITZ. The significance of the interfacial bond between the aggregate and the cementitious matrix has been underscored by various researchers [29; 30; 31]. The ITZ is commonly regarded as the weakest region in concrete, impacting both mechanical properties and durability. Understanding the stress deformation behavior of concrete is crucial, particularly in this zone, as it differs from that of its individual components; namely, hydrated cement paste and aggregate. This test examines the SEM images of the specimens at the interfacial transition zone (ITZ) to assess how the aggregate types influence the nature of aggregate-cement paste interfacial bonding.