Mixture design and processing of novel spray-based cementitious materials for 3D printing

• Main Author: Lu, Bing
• Other Authors: Leong Kah Fai
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/137749
• Institution: Nanyang Technological University

With new manufacturing technology innovations currently in Industry 4.0, there is a rising trend in the number of research studies and engineering applications of 3D printing. Recently, remarkable progress of 3D concrete printing has been achieved, where the printable cementitious materials are deposited layer-atop-layer to build the desired structures. It further facilitates automation in the construction industry, which saves the labour and improves the overall efficiency compared with the conventional construction methods. In addition, 3D concrete printing generates less waste and contributes to green and sustainable production. However, commonly adopted extrusion-based 3D concrete printing has certain limitations when printing in in-situ vertical/overhead structures, e.g. decorative profile on the external wall or ceiling structures. The vertical constraints of extrusion-based 3D concrete printing bring about a bottleneck to the overall automation in construction. As the materials cannot be deposited layer-atop-layer in these applications, a new method of 3D printing and corresponding materials are required. Based on the similarities between the conventional spray concrete technology (also known as shotcrete) and 3D concrete printing process, a spray-based 3D printing process of cementitious materials was proposed as a possible approach. Compared to extrusion-based 3D printing, spray-based 3D printing utilizes the compressed air to project the tailor-designed mixture onto the substrate at high speed. The substrate could be at any arbitrary orientations, and the sprayed material can adhere to the substrate to form a desired profile in layer-by-layer manner. In this regard, the need for design of suitable spray-based 3D printable cementitious materials is both urgent and significant. This is the primary motivation for this work as in this research study. The research study mainly focuses on the design of suitable mixtures and the determination of influence of printing process on material spray-based printing performance. Based on a comprehensive literature review of 3D printable cementitious materials and sprayable cementitious materials, the key properties of the desired mixtures were established and specified. Meanwhile, the limitations in the previous studies were also identified. To tackle the two major research tasks, specific strategies were adopted in mixture design and printing process investigations. Rheological tests and supplementary experiments were applied to evaluate the overall spray-based printing performance and select the optimal mixture. On the other hand, the printing parameters were investigated for their effects on thickness distribution of sprayed material. The research of mixture design yields two different cementitious materials for spray-based 3D printing. With the introduction of fly ash cenosphere and air-entraining agent, the first recipe is lightweight cementitious material for spray-based 3D printing. Based on the overall evaluation in delivery and deposition phases, the optimal mixture is achieved. The selection criteria for this mixture are also proposed. With the elimination of cement usage, the second recipe provides a more sustainable recipe of MgO-activated slag for spray-based 3D printing. The experiment results suggest that slag could be effectively activated by MgO, and the rheological properties could be tailored with the addition of fly ash cenosphere. The research on the printing process has identified the effects of four important printing parameters (i.e., pumping rate, air inject pressure, nozzle travel speed and nozzle standoff distance) on the thickness distribution of sprayed material. An empirical model has been constructed to describe and predict the material distribution. This research helps understand the correlation between the input printing parameters and final spray-print. Finally, possible future research directions are raised. Feasibility of utilizing foam concrete is also proposed, followed by the suggestion of integration with feedback control to realize a feedback-oriented spray-based 3D printing system. Furthermore, the structural performance of hybrid structure by spray-based 3D printing is briefly discussed.