Rheology and pumpability of mix suitable for extrusion-based concrete 3D printing – A review

3D printing of concrete is a rapidly growing additive manufacturing technology, bringing advantages like freedom of geometry, formwork-free construction, reduced construction time, and wastage, contributing towards a sustainable built environment. The technology also has the potential to mimic bio-inspired material architectures with enhanced performance. Among the available additive manufacturing techniques, the scalability and the speed of construction motivate the adaptation of extrusion-based concrete 3D printing technology. For conventional concrete pumping, the rheological behaviour and the mix design have been well established. However, for 3D printing, the pumpability of the mix which is governed by its rheology and the flow mechanism has to be viewed in conjunction with the extrudability, shape-retention, and buildability. This study provides a review of significant rheological properties, the effect of various material mixes, and printer configurations on the pumping of conventional as well as printable concrete, and a general comparison between them. The requirements of a pumpable concrete mix based on rheology and the optimal hardened properties for the 3D printing application are discussed. The significant rheological properties affecting the printability of concrete are plastic viscosity, static yield stress, thixotropy, and open time. A detailed review of various factors affecting the pumpability of a printable concrete mix is presented. The key constituents of a concrete mix affecting pumpability identified here are the type of binder(s), chemical admixture, the inclusion of nano-fillers, aggregate shape, size and grading, and aggregate-to-cement ratio. The range of yield stress and plastic viscosity are discussed for the 3D printable mixes. Recommendations are provided to improve the process and quality of 3D concrete printing. State-of-the-art applications and the potential of this technology to manufacture nature-inspired material architectures with superior toughness is highlighted.