Development of high-performance fibre-reinforced concrete for 3D construction printing

<p>This PhD aims to develop high-performance concrete, that achieves high printing quality, noticeable buildability, and high strength, for the 3D printing technique. Additionally, it aims to develop an understanding of the correlation between fresh properties and printing characteristics of printable concrete. Furthermore, this work focuses on reinforcing the concrete with different fibres and using different printing patterns to arrange the concrete layers in order to improve the mechanical properties of hardened concrete.</p> <p>The main objectives of the research are to develop an understanding of the effect of admixtures, including retarder and clay, on printable concrete in the fresh state with the purpose of controlling fresh properties and improving printing performance, which would increase flexibility in designing printable concrete and meeting diverse applications; to propose an acceptable approach for predicting failure time and buildability of fresh concrete; to understand the capacity of enhancing mechanical properties of printed concrete by using different types and volume fractions of fibres (steel, polyvinyl alcohol (PVA) and polypropylene (PP)); to investigate the relationship of different printing patterns (unidirectional, cross-ply, quasi-isotropic and helicoidal layups) with the mechanical performance of printed concrete, especially interface bond strength; to examine the internal structures of hardened concrete in terms of porosity and fibre orientation by performing X-ray micro-computed tomography (micro-CT) measurement.</p> <p>Based on a literature review, experimental methods are selected to characterise the critical properties of printable concrete in both fresh and hardened states. For fresh properties, flow and green strength tests are conducted after mixing for two hours to identify the evolution of flowability, green strength and Young's modulus of printable concrete. The obtained properties are then correlated with buildability, which shows the capacity to carry consecutive layers in a specific condition, referring to the shape and size of the printed model, printing strategies (e.g. printing speed, extrusion pressure) and the printer. Buildability and open time of fresh concrete are determined through practical use of the printer. Based on existing theoretical frameworks, the research proposes methods for predicting the buildability of printable concrete with and without clay. Compressive and flexural strength tests are undertaken to characterise the mechanical properties, where flexural strength could reflect interface bond performance between layers or strips.</p> <p>The results demonstrate that the addition of retarder leads to an increase in flowability, therefore, extending open time, which facilitates the printing process. However, due to the effect of slowing down the hydration process, mixtures with a higher amount of retarder have lower green strength and Young's modulus at any time after mixing, within the testing period of two hours. Therefore, an overdose of retarder can be detrimental to the capacity to stack layers. A small amount of clay significantly improves the buildability of the concrete mixture. Considerable enhancement in buildability resulting from the use of clay could potentially be used for producing complex structures. Understanding the effect of retarder and clay on fresh properties of printable concrete is important to increase flexibility in design of different printing projects. Two prediction approaches proposed based on existing analytical frameworks are acceptable for estimating the buildability of printable concrete with and without clay. Besides, it is found that, in general, the presence of steel fibres of any length and volume fraction could moderately-to-significantly enhance the compressive strength of printed concrete, with some exceptions. Meanwhile, the flexural performance of printed concrete in terms of strength, deflection capacity and failure mode could be improved when there is sufficient length and content of fibres, along with their alignment in the preferred direction. Furthermore, unconventional printing patterns enhance the compressive and flexural strengths of printed concrete without fibres compared to the unidirectional layup. Arrangement of steel fibres into cross-ply, quasi-isotropic and small-angle helicoidal patterns could prevent catastrophic failure in two directions instead of only one direction in the unidirectional pattern.</p>