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UBC Theses and Dissertations

Fresh-state assessment of 3D printable concrete mixtures Som, Debadri

Abstract

In spite of recent advances in construction digitization and concrete 3D printing, engineering a printable concrete mixture for extrudability and buildability still presents technical challenges. Nano-additives such as nano-silica and nano attapulgite clay are gaining increasing interest for their attributes as rheological modifiers. Recent studies showed some of the potential rheological benefits of these materials, although there is a lack of comprehensive studies in this domain. In the current study, nano-silica, nano attapulgite clay and cellulose ether viscosity modifying admixture (VMA) were used as rheological modifiers in varying proportions. Using flow diameter recommendations for printable materials, concrete mixture designs were selected for further assessment of rheological and hardened properties. A time-based flowability protocol was used to assess the change in flow diameter up to 30 minutes after mixing. The percentage change in flow diameter between two resting times was used as an in-situ quantification technique of material thixotropy. Further, uniaxial compressive load was applied on fresh cylindrical concrete specimens and apparent stress-vertical strain curves were studied through post-peak behavior analysis, analysis of apparent green strength and apparent elastic modulus. In addition to these, rheometric assessments in the form of stress growth tests and flow curve tests were performed in a rotational vane-shear rheometer. The stress growth test applied a constant shear rate to fresh concrete specimens up to 60 minutes after mixing. Based on static yield stress data at different resting times and Roussel’s model fits, thixotropic analysis was performed. The flow curve test applied increasing shear rate in steps to fresh concrete specimens. Through shear stress vs shear rate plots and corresponding Bingham model fits, yield stress and plastic viscosity of the mixtures were analyzed. In addition to the testing of 3D printable concrete mixtures in their fresh-state, 28-day compressive and split tensile strength tests were performed in order to study the effects of the rheological modifiers on the hardened state of 3D printable concrete mixtures. Finally, statistical correlations were developed and analyzed based on the rheological data obtained in the current study. The analysis covered the comparability between different fresh-state assessment techniques used in the current study.

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