Understanding Effects of Nano and Micro Scale Additives on Rheological, Buildability, and Mechanical Characteristics of 3D Printed Concrete

3D Concrete Printing (3DCP) is an emerging construction technology where the design, fabrication, and assembly process is digitally controlled. The cementitious mixtures used in 3DCP need to satisfy various rheological properties at different stages of 3DCP process such as pumping, extruding, and building. A workable mixture with low plastic viscosity and dynamic yield stress is needed during pumping and extrusion processes, while a mixture with high static yield stress and high thixotropy is needed to ensure shape retention and buildability. Due to these contradicting requirements, the research on the rheological properties of 3D printable cementitious mixtures is important for the development of novel mixtures for 3DCP. While contemporary mix design for 3DCP relies on a trial-and-error procedure, a scientific understanding on the effects of different additives on rheological properties of fresh cementitious mixtures can guide the mix design for 3DCP. As fresh state properties of printable mixtures are important to ensure smooth fabrication of 3D printed concrete structures, achieving good hardened-state properties of printed mixtures is essential to truly leverage the benefits offered by this technology. Incorporating nano and micro-scale fibers into printable mixtures can improve performance of 3D printed concrete structures.

This research aims to develop an understanding on the role of different additives on rheological characteristics and printability of cementitious mixtures as well as mechanical performance of 3D printed samples. First, the influence of a starch-based viscosity modifying agent (VMA) and attapulgite nanoclay (ANC) on rheo-viscoelastic and printability characteristics of cementitious mixtures is evaluated. Then, the effect of cellulose nanofibers (CNF) on rheological and printability characteristics of the cementitious mixtures is evaluated. Next, the development of high-performance 3D printable mixtures using graphene nanoplatelets (GNPs) or polyethylene (PE) fibers is explored. The rheology of the GNP-reinforced and PE fiber reinforced cementitious mixtures is thoroughly evaluated using a shear rheometer. Tensile, flexural and compressive test specimens are fabricated using a 3D concrete printer with a screw type pumping mechanism. The mechanical tests are conducted in two directions, parallel and perpendicular to the printing direction, to evaluate the effect of anisotropy in 3D printed specimens. The findings of these investigations indicates that ANC can effectively alter rheo-viscoelastic properties of mortar composites for 3DCP, while the GNPs and PE fibers significantly enhance mechanical properties of 3D printed samples. In addition, tensile and flexural behavior of 3D printed samples shows considerable anisotropy.