Optimal casting window defined by electrical resistivity for enhancing interfacial bond strength in 3D printed-cast concrete composites

Integrating 3D printed concrete with conventionally cast concrete offers a hybrid pathway for fabricating large-scale composite structures. The structural reliability of such systems depends critically on the mechanics of the printed-cast concrete interface. This study investigates the effect of casting time on interfacial bonding and proposes electrical resistivity monitoring to identify the optimal casting window (OCW). The practical OCW is considered as the time interval extending from the point identified by electrical resistivity to approximately one hour thereafter. A non-destructive monitoring system was developed to track the early-age hydration and setting behavior of printed concrete formworks, with post-cast concrete placed at selected intervals. Bi-surface shear tests revealed a clear transition in failure modes with increasing delay: samples cast at the OCW exhibited the highest shear strength (up to 3.49 MPa), cohesive fracture within the post-cast concrete, while delayed casting (72 h) resulted in adhesive failure. Scanning electron microscopy further confirmed widening and porosity of the interfacial zone with prolonged delay. These findings demonstrate that electrical resistivity monitoring not only provides a non-destructive basis for casting decisions but also establishes quantitative correlations between material state evolution, interfacial mechanics, and fracture behavior. • Electrical resistivity was used to define an optimal casting window (OCW), which occurs later than the setting times and better captures the onset of critical microstructural evolution. • Samples cast at the OCW achieved the highest interfacial bond strength (up to 3.49 MPa), whereas 72 h casting reduced the strength to 1.69 MPa and shifted failure modes from cohesive to adhesive. • A dual-criterion framework was proposed that provides a quantitative basis for casting decisions in hybrid 3D printed-cast concrete systems