On-line and in-line quality assessment across all scale levels of 3D concrete printing

3D Concrete printing requires much more elaborate quality control procedures compared to conventional concrete processing. Due to the various process steps, and the corresponding variation in material behaviour, time-, and length-scales, a single quality indicator and measurement technique (similar to the 'slump test' for traditional construction) cannot be selected. Instead, three families of quality indicators have been established: homogeneity during material production and deposition (quality variations), material evolution during printing (transient material behaviour), and macroscopic features and geometric conformity during printing and of the final object (geometry). For each family, quality assessment techniques which have been proven in other fields or for different applications, have been successfully transferred and adapted to the 3DCP process. In some cases, completely new methods have been developed. This paper aims to provide the state-of-the-art in such quality assessment methods, indicating high potential methods and research gaps across all scale levels of 3D concrete printing processes.