Preliminary Assessment of Ultrasonic Pulse Velocity for Quality Control of Shotcrete 3D-Printed Concrete

Digital fabrication with concrete enables manufacturing of geometrically complex and individualized elements, increasing the need for scalable non-destructive quality-control methods. This study is a preliminary investigation of the ultrasonic pulse velocity (UPV) use for shotcrete 3D-printed (SC3DP) concrete, with a particular focus on material variability caused by changes in the key manufacturing parameters, i.e., nozzle traverse speed and distance, as well as air volume flow and concrete pump speed. A series of small SC3DP specimens was produced using different manufacturing parameters settings. UPV was first measured transversely, across a single layer. Cores were then extracted, and UPV was measured axially through multiple layers before compression testing. The results showed a good correlation between UPV and bulk density but only a weak relationship between UPV and compressive strength. Pulse velocities measured across a single layer were lower than those measured through multiple layers, indicating direction-dependent wave propagation different to what is known from extrusion-based concrete 3D printing. Subsequently, UPV measurements were performed on two full-scale wall elements. The wall elements exhibited very low spatial variability in UPV, although their absolute velocities were lower than those measured for the small specimens. Overall, UPV appears promising for relative quality and homogeneity assessment of SC3DP elements, but further systematic research is required to clarify its relationships with microstructure and mechanical properties imposed by variation of manufacturing parameters.