Development of mix design and evaluation methodology for 3D printing concrete
Is Version Of
Civil, Construction, and Environmental Engineering
3D printing concrete (3DPC) is an emerging additive manufacturing technology in construction fields, and it has been spotlighted due to its unique construction method, which is a layer-by-layer manner. As compared to conventional concrete casting, 3DPC does not necessarily use formworks to construct buildings, bridges, and structural components. The absence of formworks leads to several benefits, including reduced labor costs and wastes, prompted construction time, and increased design flexibility. However, application of 3DPC for constructions has been limited since there are no standards, specifications, and guidelines on mix proportioning and evaluation methods for 3DPC yet. This dissertation includes four studies to (1) propose a mix design methodology for 3D printing concrete using Taguchi incorporated with Grey relational analysis (Taguchi-GRA) method; (2) develop a printable zone by flow table tests for determining printability of printable mixtures; (3) evaluate fresh/hardened, printable, and microstructural properties of 3DPC containing a cementitious grout; (4) develop a quantitative evaluation method on printing quality of 3D printed objects using 3D structured light scanning system (3D-SLSS). Three papers are published in peer-reviewed journals, and one paper will be submitted to a journal. Due to the lack of standards on the mix proportioning and a set of conflicting properties of 3DPC, various mix design factors have been exploited by researchers using a trial-and-error method, which is a time-consuming and labor-intensive process. In the first paper, a new mix design methodology using the Taguchi-GRA method is proposed to effectively develop 3D printable mixtures. Four mix design factors (w/b, s/b, VMA%, and Grout%) and three responses (flowability, buildability, and extrudability) were determined to optimize the mix proportion of 3DPC. Finally, an optimized mix was developed and verified by experiments. In the second paper, a printable zone was identified based on flowability measured by the flow table test, which is a simple and convenient test method. This zone is a coordinate system with initial (D0) and final (D25) diameters and can be used to easily determine the printability of printable mixtures. A mixture with D0=105-110 mm and D25=165-185mm while being printed can warrant good extrudability and buildability. In the third paper, a cementitious grout was used to facilitate the mix proportioning of 3DPC, and its effects on fresh and printable properties were evaluated. Pore/micro-structures of printed layers and their bond were also examined to understand the interlayer properties. The results reported a better understanding of the anisotropic behavior of 3D printed mortar. In addition, using a cementitious grout can facilitate the mix proportioning of 3DPC since it is not necessary to find a balance among different types of admixtures. A quantitative evaluation method on the printing quality of 3D printed objects was developed by using 3D-SLSS in the fourth paper. Three objects with different printing parameters (printing speed and extrusion flow rate) were scanned by 3D-SLSS, and 3D reconstructed images were obtained. Geometries, distortion, and surface roughness of objects were then computed. Based on the obtained results, an integrated diagnosis plot was developed. The results revealed that the combination of printing parameters significantly affected the printing qualities.