Real-time extrusion quality monitoring techniques for construction 3D printing

•Four techniques for real-time extrusion quality monitoring are evaluated.•Responsiveness of these techniques to material variations is studied.•Computer vision seems to be the most reliable extrusion monitoring technique.•The advantages, disadvantages, and potential of each technique is discussed. Process quality control is a critical aspect of construction 3D printing as an automated construction system. Four techniques for inline real-time extrusion quality monitoring during construction 3D printing are proposed in this study. These techniques include power consumption measurements for the agitator motor, extrusion pressure measurements, electrical resistivity measurements, and computer vision. These techniques are implemented, discussed and compared in terms of repeatability and responsiveness to six mixture variation levels. The results indicate that among the four approaches, computer vision is the most reliable and accurate technique for instant detection of variations in the printing material. On the other hand, electrical resistivity measurements seem to be the least effective technique for detecting material variations in this study. Advantages, disadvantages, and potential of the proposed techniques for quality monitoring purposes are also discussed in this paper.