Characterization of Cold Joint Formation in Digitally Printed Mortar During the Dormant Phase (2025-06)¶

Extrusion-based 3D printing of concrete, despite being an advancing technology, faces the challenge of cold joint formation at the interface of two layers. This has been reported to weaken the integrity of the structure. Despite previous attempts to characterize the hardened state of the interlayer bond strength, the lack of uniform testing standards and the large number of variables affecting the bond strength emphasizes the need for further investigation. This investigation analyses the influence of bond strength in fresh state concrete, considering the effects of interlayer time intervals, aging y duration, and the self-weight of the superimposed layers. These variables were experimentally evaluated utilizing an automated direct shear testing apparatus, equipped with a specifically designed proctor. The findings indicate that with an increase in the interlayer time gap, the ultimate shear strength reduces approximately to 80% after 150 minutes of aging in 20 minutes of interlayer time gap, and the influence of the load exerted by preceding layers appears to be minimal on the bond strength evolution attributable to the continuous hydration at the interface, which renders the concrete more resistant to compression. Finally, an analytical equation proposed in this study serves as a predictive tool for bond strength development and helps in numerical simulation of the 3D printable fresh state mortar. This study could be a precursor in understanding the bond strength in hardened concrete; however, further studies are necessary to validate it.