Cristina Zanotti

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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A significant portion of concrete infrastructure in North America is deteriorating and will require repair or rehabilitation action in the near future. An effective repair can be jeopardized by a bond which is unable to withstand the subjected loading and durability conditions. Fatigue loads are a cause of structural concrete deterioration and cracking with respect to service load conditions such as vehicular traffic as well as high amplitude loading events such as earthquakes and storms. Currently the response of cementitious repair interfaces tested under fatigue loading is not comprehensively examined in existing literature. In this study, the modified slant shear cylinder test with different bond plane inclinations was used to experimentally determine the bond strength of composite substrate-repair specimens subjected to monotonic and fatigue loading protocols. The effect of both roughened and near-smooth interfacial profiles is considered, as well as the use of steel fiber reinforcement in the repair material. The associated bond parameters are derived from experimental test results using previously identified predictive models and failure envelopes for concrete interfaces. 2D Digital Image Correlation (DIC) is additionally used to examine strain distributions on the surface of slant shear specimens prior to failure. A discussion on the degradation of the adhesive bonding mechanism is presented and the resulting implications on bond strength and bond parameters are examined. The interfacial bond investigation is complemented with a discussion on the limitations of the employed predictive models as well as a review of relevant code provisions and research guidelines pertaining to interfacial shear bond subject to cyclic and fatigue loads.

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Recycling waste materials as replacement for natural resources in building products could help in lowering the environmental impact of the construction industry, provided that their application and processing is properly engineered. Wood waste is one of the chief construction wastes in Canada and many other countries worldwide. In this study, a novel bio-composite panel made with recycled wood chips is presented and assessed.The bio-composite binder was a natural hydrated lime with addition of metakaolin. The inclusion of various additives such as sodium hydroxide, recycled wood ash, and magnesium-rich powder recycled from the cutting waste in the production of magnesium oxide boards, was also explored. The effects of different binder compositions and proportions on the microstructure of the binder, its compatibility with the wood chips, and the engineering properties of the final bio-composite product were investigated. Finally, the effect of different curing conditions is considered, including accelerated carbonation by CO₂ incubation.Following ASTM specifications, 3 replicates of 550 x 550 x 70 mm panels for each bio-composite formulation and curing condition were cast. Specimens were cut from the panel to test the mechanical and thermal properties. Extent of carbonation within the panel was analyzed by standard colouring observation after addition of phenolphthalein and compared with control specimens cured without CO₂ incubation. These analyses were supported by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) of the binders and the bio-composite to monitor the various reactions occurring in the binder, including hydraulic and pozzolanic reactions, carbonation and additional reactions.An optimised binder composition of lime and metakaolin was successfully designed based on the strength data obtained on binder pastes. The bio-composite produced by combining wood chipsand the optimised binder possessed good mechanical and thermal properties. While it met the minimum requirements of compressive strength and thermal conductivity as specified by ASTM standards, future work will need to focus on improvement of the flexural behavior. It was also found that carbonation curing of specimens helped in rapid strength gain and improvement of the matrix. Further, microstructure analysis of the bio-composite helped identify and characterise the reaction products that enhanced its properties.

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