Esposito, Laura (2022) Innovative structures obtained with digital fabrication technologies. [Tesi di dottorato]

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Digital Fabrication with Concrete is now emerging in the civil engineering field since it enables the construction of complex shape structures with a high level of digitalisation. The potentialities of digital fabrication employment in the construction field are many, and, to fully exploit them, it is necessary to expand the technical "know-how”. The technological progress in this sector has recently grown, resulting in the design and realisation of numerous structural projects. However, different challenges must be solved concerning the cementitious material requirements and characterisation, code compliance, and definition of a structural design approach. The present work is collocated within this scenario to explore and address critical issues related to the most widespread additive manufacturing technique: 3D Concrete Printing (3DCP). This technology consists of a continuous extrusion of concrete filament through a nozzle attached to a robotic system, and it is based on early-age material requirements satisfaction (pumpability, extrudability and buildability). Therefore, the first topic addressed in this thesis work is the physical-mechanical characterisation of the fresh printable mortars. Specific testing procedures and experimental methods were developed to determine the main material properties (such as strength and stiffness under axial compressive load) and their evolution with the cement hardening process and build up progressing. These studies were performed in collaboration with the industrial partner Italcementi Heidelberg Cement Group. Experimental results were used to define an appropriate constitutive model that took into account the complex visco-elastic response of such innovative mortars. Moving to the hardened state of the printed structures, the reinforcement strategy was investigated since the cementitious material is brittle and has low tensile strength. In this framework, an interlaboratory study with a foreign institute (ETH Zürich) was conducted to investigate the steel bar reinforcement effectiveness (in terms of bond behaviour), focusing on the influence of the material/printing setup. The comparison between printed and cast pull-out specimens, made with a different type of printable mortars and realised with two different printing setups, allowed understanding the main influencing factors of steel reinforcement inclusion. As a final step of the thesis work, the mechanical response of 3D printed structural elements was investigated through flexural tests carried out on topology optimised beams. For such beams, the effect on the resistance mechanism of the previously mentioned reinforcement strategy was also examined. The thesis work addresses different but strictly correlated topics of the 3DCP technology concerning the design, production and testing of the structural element obtained. The presented research activities and the experimental results could be helpful for the standardisation of the testing procedures (for the material characterisation) and design approaches in the 3DCP field.

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