Influence of interface orientation on the flexural behaviour of 3D-printed concrete beams: A finite element study

3D Concrete Printing (3DCP) is an emerging construction technique that enables automated, layer-by-layer fabrication of structural components. The layered deposition process produces horizontal interlayers and vertical interfilaments, resulting in directional mechanical behaviour. Most experimental studies on anisotropy, however, assess flexural response using a limited range of loading directions. Furthermore, numerical studies often assume identical properties for interlayers and interfilaments, potentially overlooking sources of anisotropy. This study presents a comprehensive Finite Element framework that explicitly addresses these gaps by modelling six possible flexural loading configurations, including those rarely examined in previous research, and by independently representing the mechanical behaviour of interlayers and interfilaments. The framework integrates a cohesive-zone model for interfaces with a Concrete Damage Plasticity model for the bulk material. Validation against published experimental results demonstrates the model’s ability to accurately capture flexural response and failure mechanisms in beams containing both interlayers and interfilaments. The FE model is then extended to assess flexural anisotropy under three interface conditions (equal interfaces, weaker interlayer, weaker interfilament) and three nozzle geometries (10 ×10 mm, 10 ×20 mm, and 10 ×30 mm). Results show that two rarely studied configurations consistently govern the minimum flexural capacity. Unequal interface properties, modelled by assuming one interface type is 50% weaker than the other, nearly doubled the anisotropy parameter and reduced flexural strength by an average of 44% in the weakest orientations. Rectangular nozzles amplified directional sensitivity, although the 10 × 30 mm nozzle produced an interface arrangement that interacted asymmetrically with the applied loading direction, increasing strength by up to 22% in these critical directions. Within the investigated parameter range, variations in interlayer and interfilament properties produced a more pronounced influence on anisotropy than the nozzle geometries considered. The results highlight the importance of explicitly accounting for differences between interface types when evaluating the structural response of 3D-printed concrete beams. While explicit modelling of every interface is impractical in design, the proposed framework provides mechanistic insight into the role of interface orientation, interface properties, and filament geometry on the flexural behaviour. The framework may support future investigations of larger structural systems, although further validation is required before direct application to structural design.