Microstructure and mechanical properties of interlayer regions in extrusion-based 3D printed concrete: A critical review

3D printed concrete structures often exhibit considerable degree of anisotropy and inhomogeneity because of layered material deposition and inferior properties of the interlayer regions between concrete filaments. These interlayer regions, which will be called interfaces hereafter for better readability, can significantly affect overall mechanical performance of printed concrete structures. Therefore, this paper provides a critical review of recent research advances on interfaces in 3D printed concrete. Firstly, the bond between printed layers is discussed from the microscopic point of view. Thereafter, test methods suitable to evaluate interfacial bond strength within 3D printed concrete are summarized. The effects of printing parameters, material composition and other factors on the interfacial bond strength are discussed, and the corresponding modification methods are addressed. Furthermore, the existing analytical models for 3D printed concrete interfaces are compared to offer a base for structural finite element analysis. Finally, some ideas on the future research are put forward.