Critical Factors that Impact the Mechanical Properties of Robot Assisted 3D-Printed Cement Based Structures

This research was motivated by pressing challenges in the construction industry. These challenges include, but are not limited to, high-cost of materials, labour shortages, safety risks and environmental impact. Despite the significant contribution of the building and construction sector to the global economy, the sector lags in technological adoption, leading to inefficiencies and waste. Robot-assisted 3D printing, particularly for cement-based structures, offers a transformative solution by automating production, reducing material waste and enabling complex designs. This approach eliminates the need for traditional formwork and minimises labour demands, aligning construction practices with sustainability goals through lower CO₂ emissions. This PhD research, therefore, aimed to investigate the critical factors in cement-based Additive Manufacturing (AM) processes that impact the mechanical properties of 3D-printed cement-based structures. Given these benefits, the study investigated the critical factors in cement-based additive manufacturing processes that impact the mechanical properties of 3D-printed cement-based structures. Through an experimental approach, the research examined layer bonding, print speed, layer height and material composition, focusing on optimising the process parameters to enhance structural integrity. However, despite advancements in printable concrete technology, maintaining high-quality printing remained a challenge. Quality control was closely linked to both geometric and material aspects of the printer nozzle design, especially for small-scale printing applications suited to Small and Medium-sized Enterprises (SMEs). This research also explored the design and development of a robot nozzle system that was optimised for small-scale 3D printing of cement-based structures. The nozzle design considered key factors, such as weight, nozzle diameter and shape, material compatibility, flow control, mixing mechanisms, temperature resistance, cost-effectiveness, adaptability, safety and ease of maintenance. Iterative designs were developed, focusing on stress concentration mitigation and material flow optimisation. These considerations informed the adoption of an on-demand accelerator spraying system, which overcame challenges associated with integrating mixing mechanisms directly into the nozzle. This method used a micro-peristaltic pump connected to an accelerator tank to spray the accelerator onto the surface of deposited material as the robot moved along its programmed path, thus enhancing layer stability and print quality. The mechanical performance and microstructural characteristics of 3D-printed cement structures were also examined, focusing on compressive and flexural strengths alongside SEM analysis of interlayer bonding. Finite Element Analysis (FEA) revealed high-stress concentrations at edges under compression and at mid-span under flexural loads, correlating with observed crack patterns. SEM analysis highlighted the effects of aluminium sulphate on interlayer bond strength, with early ettringite formation improving initial adhesion but excessive sulphate leading to expansive crystal growth, reduced bond strength, and premature cracking over time. From the experimental results, mechanical testing demonstrated that 3D-printed structures gained strength over time, with compressive strength increasing from 2 MPa on day 1 to 22 MPa on day 28, although this remained lower than the 39.4 MPa achieved by traditional monolithic structures. Early flexural strength in 3D-printed samples benefited from the use of aluminium sulphate additive that was introduced as an accelerator, reaching 0.25 MPa on day 28, nearly matching the 0.27 MPa of monolithic samples, though initial benefits diminished with curing. Split tensile testing showed that interlayer bond strength improved, with tensile strength increasing from 0.35 MPa on day 1 to 3.65 MPa on day 28. However, excessive aluminium sulphate was observed to reduce bond strength due to the formation of ettringite, highlighting the importance of controlled accelerator integration. Importantly, the optimal printing parameters emerged as crucial to achieving buildable structures, with speeds up to 20 mm/s supporting up to 10 stable layers with a thickness of 40 mm. Higher speeds, such as 80 mm/s, reduced layer thickness to 13 mm, but compromised interlayer bonding, resulting in early structural collapse. An optimal layer height of 10 mm proved effective, enabling stability for up to 10 layers, whereas larger layer heights led to reduced adhesion. Aluminium sulphate significantly enhanced early setting and enabled up to 14 stable layers at a 45% concentration, although higher concentrations were detrimental to long-term tensile strength. This research provides valuable insights into additive manufacturing in construction, showcasing that robotic 3D printing can produce sustainable, viable structures by optimising printing parameters, thereby addressing environmental and labour challenges. The study also highlights the role of aluminium sulphate in accelerating setting time and early strength development, though initial integration faced challenges. Early attempts with direct mixing led to some issues, including backpressure and mixing inefficiencies, which were later mitigated by using a peristaltic pump and auger, though these modifications introduced new challenges, such as peristaltic pump nozzle blockages. This can be attributed to insufficient dissolution of powder solute/accelerator in the solvent/distilled water. Ultimately, surface spraying of the accelerator on each layer proved effective, significantly improving layer stability and surface finish.