Characterizing buildability development of early-age 3D printed concrete using guided waves

Recent advancements in 3D printing of concrete (3DPC) have highlighted significant potential for formwork-free construction, geometric freedom, and reduced labor demand. However, ensuring buildability during layer-by-layer fabrication requires real-time control of fresh-state concrete properties, yet parameters that relate material behavior to structural buildability in 3DPC are still lacking. To address the need, this study explores an ultrasonic guided-wave technique for monitoring green strength development in 3DPC during fabrication across different mixture types. Ultrasonic and green strength tests were performed over the first 120 min after specimen fabrication. External waveguides, acrylic and aluminum plates equipped with two bonded PZT sensors, were mounted on one side of the specimens to monitor the leakage of ultrasonic energy from the plates into the concrete. Of the two waveguide materials, acrylic was more effective for monitoring green strength development at this very early age. As hydration-induced solidification progressed, the guided-wave parameter (normalized S0 guided-mode peak amplitude) decreased owing to increasing energy leakage into concrete, while the corresponding attenuation parameter increased, quantifying leakage-related energy loss. Strong correlations (R 2 higher than 0.93) between the ultrasonic parameters and green strength indicate that the guided-wave parameters can reliably capture early-age mechanical evolution relevant to buildability in 3DPC. Furthermore, the ultrasonic measurements demonstrated sensitivity to differences in early-age material development among the mixtures. An approach to use the developed ultrasonic parameters to estimate green strength development is proposed.