3D Printed High-Performance Concrete for Structures Resisting Large Loads
Concrete additive manufacturing, also known as concrete 3D printing, opens tremendous opportunities in the construction industry, architectural design, defense and space exploration. Unlike conventional cast concrete which requires formworks, concrete 3D printing relies on a layer-by-layer deposition process automated through a robotic arm or gantry printer. The automated construction of concrete components or structures can significantly reduce labor and material costs, remove cumbersome formworks, accelerate the manufacturing process, and allow construction in extremely harsh and less accessible environments. It also enables possibilities for more complex and versatile designs of structural geometries, such as by mimicking designs in nature to achieve unprecedented mechanical properties.To enable such additive manufacturing processes, fundamental knowledge is needed as to how the material early-age rheology affects the microstructure development and the late-age mechanical behavior of 3D printed concrete. The knowledge is important for creating new material design strategies to achieve the desired time-dependent developments of rheology, hydration chemistry, microstructure and improved mechanical properties of 3D printed concrete. Also, in order to rationally analyze and reliably design structures made of 3D printed concrete that can resist large loads, it is critical to understand how the new materials and additive manufacturing processes, such as the introduction of printing interlayers and inhomogeneities and the modular assembly method, affect the structural behavior of 3D printed concrete components at larger scales. This dissertation aims to address these research needs at the interfaces of 3D printed concrete materials science and engineering, additive manufacturing, and structural engineering, with the goal of realizing 3D printed concrete structures that can reliably resist large loads. To achieve this overarching goal, the research objectives are to (1) understand how the material designs, rheological properties and additive manufacturing process affect the fracture behavior of 3D printed concrete; (2) enable new material designs for 3D printed concrete with significantly higher fracture resistance and strength; (3) link materials and manufacturing to structural performance, by enabling novel large-scale additive manufacturing of reinforced concrete columns and by understanding the column behavior under seismic loading through experiments and numerical analysis. This research elucidated the relationships among the early-age development of material rheology, the additive manufacturing parameters, the material microstructure especially the pores, flaws and imperfections in the interlayer region, and the fracture characteristics of 3D printed concrete. It explained the fundamental mechanisms responsible for such relationships. It revealed that different from conventionally manufactured concrete, the microstructure and the resulting mechanical properties of additively manufactured concrete strongly depends on how the material is “born”, e.g., its early-age rheology throughout the additive manufacturing process as a function of time. Based on the obtained knowledge, this research created new material designs of fiber-reinforced 3D printed concrete featuring ultra-high fracture energy as well as the desired rheology for additive manufacturing. The research also explored a novel self-healing functionality after mechanical damage was induced in the material. Furthermore, this research enabled the linkage from small-scale to large-scale concrete additive manufacturing through advancements in materials, rheology and robotics. For the first time, it demonstrated the large-scale additive manufacturing and modular assembly process of a reinforced concrete column, as a representative structural component for ultra-tall wind turbine towers, or bridges and buildings. A large-scale experiment and finite element analyses on this 3D printed reinforced concrete column generated key understandings of the structural performance under seismic loading, examined the 3D printed concrete behavior at different scales, and paved the way for future design and analysis of novel 3D printed concrete structures.