## 2 System and Process Analysis
### Concrete Printing System  
In extrusion-based concrete printing, concrete is mixed and transported to a printhead, which is attached to a printing system (e.g., gantry and robot). Concrete is extruded through a nozzle located at the tip of the printhead, and it is deposited in place to build a component from a digital 3D model. The elements in concrete printing systems are identified, using a robot concrete printer as an illustrative example. As shown in Figure 1, the robot concrete printer comprises (i) a system command and a controller (control unit), (ii) a robotic arm and a printhead (printing system), and (iii) a mixer and pump system (material transportation system). The components are detailed as follows:  
* **Control unit:** In the control unit, the system command prepares the data necessary for printing, such as user-defined inputs and generating machine instructions. The system command communicates with the controller via human-machine interfaces. The controller handles the kinematics and electromechanical aspects of the robotic arm and printhead. The controller also processes the machine instructions that define toolpaths and process parameters, controls the multiple axes executing motions of the robotic arm and the printhead, and monitors the performance of the concrete printer using sensing technologies. Depending on the complexity of the printing system, the controller may also handle the material transportation system.
* **Printing system:** In the printing system, the robotic arm facilitates the deposition of concrete at desired locations with desired speeds under desired angles. The printhead, as the end effector of the robotic arm, is an element used to extrude concrete, and it consists of an extruding mechanism and a nozzle. The extruding mechanism is a series of parts of the printhead that pushes the concrete through the nozzle. The nozzle, the end part of the printhead, is a hollow element that gives shape to the concrete layer as it is deposited in place to build up a component.
* **Material transportation system:** In the material transportation system, the mixer mixes the raw materials to obtain concrete, and the pump transports the concrete from the mixing unit to the printhead, avoiding segregation and bleeding.  
The system elements, as shown in Figure 1, have a direct effect on the manufacturing process, the printing material, and the quality of the printed component. Concrete should be extrudable and buildable, where each concrete layer, once in place, is capable of retaining shape and adhering to and carrying the load of subsequent layers. Key rheological properties provide the characteristics necessary for concrete to be printable, defining the evolution of viscosity and yield stress of the material over time. A detailed review of the manufacturing process in extrusion-based concrete printing is presented in [16], including the effects of material parameters (e.g., yield stress) and machine settings (e.g., nozzle height and material flow) in pumping, extrusion, and deposition of concrete layers. In the following subsection, the concrete printing process is described and analyzed.