Interlayer bond deterioration of 3D-printed ultra-high performance concrete under freeze-thaw cycles: damage evolution and governing mechanisms

The freeze-thaw (F-T) durability of 3D-printed ultra-high-performance concrete (3DP-UHPC) is influenced not only by the dense UHPC matrix but also by weak interlayer interfaces created during layer-by-layer deposition. This study investigates interlayer bond deterioration under different printing intervals, interface treatments, and laboratory F-T exposure levels. The results show that interfacial deterioration progresses through four stages: initial defect formation, pore expansion, damage coalescence, and interface instability. Increasing the printing interval aggravates initial defects and increases F-T sensitivity. Polymer emulsion treatment more effectively mitigates delayed-printing-induced weakness during the early and intermediate stages of F-T exposure, whereas cement paste treatment exhibits better stability after the extended exposure of 900 F-T cycles. A semi-empirical interlayer bond strength degradation model incorporating interfacial pore characteristics and damage-induced roughening was developed. The calculated values showed acceptable agreement with the measured results within the present dataset, with an average absolute relative error of 8.48%. The model indicates that interlayer bond deterioration is jointly associated with effective load-bearing area reduction caused by local pore damage and force-transfer degradation induced by roughening-related stress concentration. These findings provide experimental and mechanistic support for interfacial design and F-T durability assessment of 3DP-UHPC under the investigated laboratory conditions.