Mechanical anisotropy, rheology and carbon footprint of 3D printable concrete: A review

3D printing (3DP) technology has gained more recognition in the construction industry in recent years. This technique puts forward stringent requirements for the performance of cementitious materials such as extrudability and buildability. However, due to the layer-by-layer printing process, 3D printing concrete (3DPC) shows characteristically different properties from conventional mold-casting concrete. This study critically reviews the mechanical behaviors and rheological properties of 3DPC, focusing on how important fibers, aggregates, printing parameters and environmental conditions impact mechanical anisotropy, and how well they could be described by different rheological models. The review found that fibers and aggregates significantly affect the anisotropic behavior of 3DPC. In particular, the interlayer bonding strength of 3DPC can be enhanced by introducing fibers and aggregates, but it can be diminished by some solid waste aggregates. Environmental conditions are found to have minor to some influences over the mechanical strength and anisotropy of 3DPC. In addition, this review has evaluated the carbon footprint of different 3DPC recipes reported in the literature and found that the sustainability of the 3DP construction technique is questionable from the mix design perspective, while additional research is urgently needed on this aspect of the technology.