Extended finite element multiscale modelling for crack propagation in 3D-printed fibre-reinforced concrete

Fracture toughness provides a quantifiable means of evaluating the ability of brittle materials, such as concrete, to withstand crack propagation. This is particularly relevant in the emerging field of 3D concrete printing. In this study, the influence of various pitch angles ranging from 0 degrees to 90 degrees and the number of layers (10 and 15 layers) on the fracture behaviour of the Bouligand beams is numerically investigated. A multi-scale modelling method has been employed to consider the anisotropy of the material caused by the presence of fibres. The model's accuracy has been confirmed by comparing the bending load-deflection curves and failure modes with experimental data. In addition, X-ray micro-computed tomography analysis has been conducted to provide further insight into the slight deviation between the finite element analysis and experimental results. This study reveals that the fracture toughness can be significantly improved by introducing a slight deviation angle (between 5 degrees and 15 degrees) to the helically twisted sequence in the Bouligand architecture. Moreover, as the number of layers increases in 3D-printed fibre-reinforced concrete beams, the crack pattern becomes more intricate, forming a complex spiral pattern prior to failure. The findings from this study may potentially guide the selection of an appropriate range of deviation angles for utilising the Bouligand architecture in 3D concrete printing to achieve desirable fracture toughness.