Fracture in 3D-Printed Concrete Beams: Deflection and Penetration of Impinging Cracks at Layer Interfaces

Structural assemblies produced using three-dimensional (3D) concrete printing consist of multiple layers of extruded material deposited along precise trajectories. The stress response of the printed assembly relies on the stress transfer between the individual layers. The interface tensile bond strength formed between extruded layers is less than the tensile strength of the extruded parent material in the printing direction. The interface tensile bond strength decreases with an increase in the time gap between layers, which ranges from a few seconds to tens of minutes. Crack propagation was evaluated in a beam made of multiple printed layers using two-dimensional (2D) digital image correlation. The crack propagates by penetrating the interface between layers printed with smaller time intervals. Crack deflection occurs at the interface before the crack emerges into the next layer because the bond between the layers weakens with an increasing time gap. Decreasing bond strength between layers results in significant crack propagation along the interface and even a doubly deflected crack at the interface. A linear elastic fracture mechanics (LEFM)-based formulation of a crack impinging normally on a bimaterial interface was used to provide insights into crack propagation at a layer interface in a printed assembly. The crack deflection at the interface is interpreted as a decrease in the critical interface energy release rate ($G_{\mathrm{intc}}$) relative to the critical fracture energy release rate for penetration ($G_{\mathrm{mc}}$). The reduction in the $G_{\mathrm{intc}}$ to values lower than a threshold value of $G_{\mathrm{mc}}$ produces a deflection in the crack path at the interface. Crack propagation along the interface results in a mixed-mode fracture condition, and $G_{\mathrm{intc}}$ contains contributions from Modes 1 and 2. The continued decrease of $G_{\mathrm{intc}}$ relative to $G_{\mathrm{mc}}$ produces a doubly deflected crack at the interface between layers. The crack deflection into the interface provides a rational reference for identifying strong and weak interfaces between the layers. The reduced capacity of the interface for identifying a weak interface leading to a cold joint can be identified using a fracture-based evaluation of crack deflection.