*3.2. Methods*

In this stage, 3D printed concrete paving stones were produced in accordance with TS 2824 EN 1338 [15]. To observe the pumpability and printability characteristics of a cementbased mortar, experiments were carried out in the Construction Materials Laboratory of the Faculty of Engineering of Zonguldak Bulent Ecevit University. The initial and final setting times of the fresh mixture were measured through the Vicat Needle test.

In this study, a new 3D printer was designed and produced (Figure 1). The printer chassis was formed with a 30 × 30 aluminum sigma profile. Plastic parts produced from FDM (fused deposition modeling), stainless steel bolts, and nuts were used. Flexible plastic elevations were attached to the printer's feet to prevent mechanical vibration. The movement in the axes was provided by three Nema 17 step motors utilizing the delta arm design. A double extruder was used to mix cement-based mortar and accelerator admixture during the printing process. Flow settings in each extruder were made using mechanical and software calibrations. Cement-based mortar and accelerating admixture were extruded using a stepper motor and archimedean screws. In the calibration phase, the open source computer supported Repetier Host program was used. Stages such as step settings of the axes, speed, and acceleration were performed through the Repetier Host program. The stl (standard triangle language) file format of the paving stones was sliced with the open source G-code (program language of CNC machines) generator, and then was saved to the SD card. Production was started via the control panel by inserting the SD card into the printer.

**Figure 1.** Three-dimensional printer designed for this study.

Three-dimensional printed paving stone samples were cured using a standard water curing protocol. The compressive strengths of the hardened concrete were measured to elucidate the development of its strength over time in accordance with TS EN 12390-3 [16]. After that, they were compared with those for reference paving stones as per the standard TS 2824 EN 1338.

The machines and equipment required to carry out the above mentioned tests are available in the Construction Materials Laboratory of the Faculty of Engineering of Zonguldak Bulent Ecevit University.
