Visualisation of Inter-filament Voids and Observation of Compressive Failure in 3D Concrete Printing Using Elastic Wave Tomography

While non-destructive testing (NDT) is applied for process control during 3D concrete printing (3DCP), systematic quality assessment of hardened printed structures is still underdeveloped. The layered structure of 3DCP can introduce anisotropic defects, such as inter-filament voids, which critically affect the mechanical performance and structural integrity. Therefore, a reliable NDT method for internal quality assurance is essential. This study investigates the applicability of elastic wave tomography (EWT) for these purposes. First, specimens with controlled inter-filament void structure were fabricated by modifying printing parameters. The influence of input wave frequency on internal void detection was systematically examined by applying impacts with steel balls of different diameters. Furthermore, the progressive fracture behaviour under compression was monitored sequentially, with EWT performed at pre-peak load steps and digital image correlation (DIC) capturing the surface strain field. Results demonstrated that EWT can effectively visualise the presence and distribution of internal voids. However, detection sensitivity and spatial resolution were found to be strongly dependent on the input frequency. Lower frequencies excelled at identifying widespread, low-quality zones by averaging material properties, while higher frequencies offered greater potential for localising discrete voids. In compressive testing, EWT captured crack propagation and visualised fracture progress. The detected area showed good agreement with the strain field obtained from DIC, demonstrating that EWT is capable of monitoring the fracture progression.