Reducing anisotropic behaviour of 3D printed concrete through interlocked filaments

Three-dimensional printed concrete commonly exhibits anisotropic mechanical behaviour due to inherent weaknesses at filament interfaces, creating preferential failure planes. This study evaluates the efficacy of interlayer interlocking geometries in reducing anisotropic behaviour and improving mechanical performance. Three interlocking surface topologies (sinusoidal, square, inclined) were implemented using specially designed extrusion nozzles and compared against a flat reference geometry. Mechanical tests, including direct tensile and uniaxial compression tests, were conducted along three orthotropic loading directions: perpendicular (w), parallel (u), and transverse (v) to the printing orientation. All interlocking topologies substantially improved mechanical performance relative to the flat reference. The sinusoidal pattern consistently demonstrated the greatest improvement, increasing tensile strength in the w-direction by 213% and compressive strength by 45.1%. In the v-direction, the inclined pattern achieved the highest compressive gain (83.9%). Furthermore, the sinusoidal pattern exhibited near-isotropic behaviour, reducing the tensile anisotropy index from 2.29 to 0.16 and the compressive index from 0.37 to 0.05. While the inclined and square patterns also reduced anisotropy, their influence was comparatively moderate. Overall, the findings demonstrate that mechanical interlocking provides a viable, geometry-based strategy for mitigating anisotropy and enhancing the structural performance of 3D printed concrete, thereby supporting its broader adoption in structural applications.