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Concrete 3D printing is a sustainable solution for manufacturing efficient designs and creating less waste, and selecting the optimal materials to use can amplify the advantages of this technology. In this study, we explore printing lightweight concrete by replacing normal weight aggregate with lightweight aggregates such as cenospheres, perlite, and foam beads. We adopt a systematic approach to investigate mixtures using different formulation methods such as the specific gravity and packing factor methods to improve the printing and mechanical performances of the mixtures. The rheological results showed significant improvement in the flow characteristics of the different mixtures using both the specific gravity method and the packing factor method to formulate the mixtures. Furthermore, a statistical tool was used to achieve optimal performance of the mixtures in terms of high specific compressive strength, high flow characteristics, and good shape retention capability by maximizing the specific compressive strength ratio, slump flow, and the static yield stress, while minimizing the slump, dynamic yield stress, and plastic viscosity. With the above design objectives, the optimal percentages of the aggregate replacements (cenosphere, perlite, and EPS foam beads) were 42%, 68%, and 44%, respectively. Finally, the optimized results also showed that the mixture with cenosphere aggregate replacement had the highest specific strength.

The disposal of post-consumer glass has been a major issue due to its inert properties that may cause environmental effects, while recycling of these glasses is only feasible if the waste glass recovered is sorted into its different colour to prevent chemical incompatibility in the manufacturing process. 3D printing in building and construction has gain increasing attention in the past decade and provides a potential to sustainably utilize the recycled unsorted glasses. This paper examines the use of recycled glass as the fine aggregates for 3D concrete printing applications. Despite the several studies done on the use of recycled glass in concrete, there is a lack of focus on the rheology of the material which is essential to the performance of the material in 3D concrete printing. Although results have shown that the mechanical strength for the recycled glass concrete is lower than the sand aggregates concrete, yet the flow properties of the former is better than the latter. Nonetheless, a balance between the mechanical strength and flowability of the mix design should be studied. The future work revolves around the optimization of the mix design using a combination of sand and recycled glass, adjusting the gradation of recycled glass particles and addition of accelerators to improve its buildability and mechanical strengths. Accepted version

3D concrete printing (3DCP) is gaining much attention due to the absence of formwork, allowing more freedom in the architectural design, and reducing construction costs significantly. The lack of formwork configuration in the 3DCP technique requires the printable concrete to possess self-supporting properties. Furthermore, the yield stress evolution over time caused by the structuration build-up of the cement also allows a larger buildable height of the print. However, before the structuration in the material occurs, the concrete needs to possess high initial yield stress to maintain its shape upon extrusion during the early stage of the printing process. Although it is advantageous to have higher yield stress for the buildability of the concrete, the extrudability of the concrete should also be considered. When the yield stress of the concrete is too high, the pumping effort to cause flow in the concrete becomes too high and the extruded filament displays defects on the surface of the filament. This study aims to study the effects of various material parameters on the concrete extrudability and buildability to determine the extrudable region of the concrete mix design using a graphical approach. The material parameters that are being studied in this research includes aggregates fraction, attapulgite nano-clay content, and aggregates fineness. The results shows that the mix designs at the extrudable limit surface is most optimal for 3D concrete printing as it has the maximum buildability with the extrudable region. National Research Foundation (NRF) This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme, Singapore Centre for 3D Printing, National Additive Manufacturing Innovation Cluster, Enviro Sand Pty Ltd, Chip Eng Seng Corporation and Berkeley Education Alliance for Research in Singapore (BEARS) for the Singapore-Berkeley Building Efficiency and Sustainability in the Tropics (SinBerBEST) Program.

The structural capacity of construction joints in concrete bridges, deck and pavements mainly depends on the bond strength between the old substrate and new overlaid concrete. Sometimes, a mismatch in the properties of old and new concrete may lead to early age failure and shortened service life. Since in 3D concrete printing (3DCP), the whole object is made by layer by layer, bond strength is considered as one of the key parameters to ensure stability in the structure. For understanding bond mechanism, it is essential to measure bond strength at the interface between new and old layer and investigate significant parameters affecting this property. In this direction, our current work targets to analyse tensile bond strength of 3D printed geopolymer mortar with respect to printing time gap between layers, nozzle speed and nozzle standoff distance. A novel formulation of fly ash based geopolymer was made and printed using four-axis automated gantry system. Experimental findings reveal that the bond strength is a function of state of interface material between two nearby layers which can be influenced by material strength development rate and 3D printing parameters. NRF (Natl Research Foundation, S’pore) Accepted version

When conducting corner printing with rotational rectangular nozzle, a greater amount of material is deposited inside the filament and hence tearing and skewing will occur on the surface of the printed filament. With the aim of maintaining the surface finish and mechanical properties of the printed filament, a 3D numerical model is developed to study the flow mechanism at a corner under various conditions during the extrusion and deposition processes with rotational nozzle. After experimental validation, the numerical model is employed to study the material flow mechanism under various conditions. The results indicate that the rheological properties have little effect on the mass distribution ratio. However, a high relative nozzle travel speed, larger corner radii and lower nozzle aspect ratio is a promising route in obtaining a uniform material distribution ratio. The interlinking of process parameters affects the material distribution ratio significantly as well. Furthermore, the importance of the factors that affect the mass distribution was determined quantitatively. National Research Foundation (NRF) Accepted version This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme, Sembcorp Design & Construction Pte Ltd, and Sembcorp Architects & Engineers Pte Ltd.

This paper studies the effects of recycled glass cullets as fine aggregates for the material in construction 3D printing. A reference material with natural river sand as fine aggregate was used as the control and recycled glass cullet of similar grading was used to replace the natural river sand at 25, 50,75 and 100% by mass. The effects of incorporating the recycled glass on the rheological properties such as the static yield stress, dynamic yield stress and plastic viscosity were studied. 3D printing investigation was also conducted on the mixtures to evaluate the materials printability (pumpability and buildability). Mechanical properties such as compressive strength and interlayer bond strength by splitting tensile were also examined. Results show that the static yield stress decreases while the dynamic yield stress and plastic viscosity increases as the recycled glass cullets content increases. The change in rheological properties also affected the printability of the mixtures. In the mechanical properties test, the increase in recycled glass cullets content also reduces the compressive strength and interlayer bond strength of the specimens. National Research Foundation (NRF) Accepted version This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme, National Additive Manufacturing Innovation Cluster, Singapore Centre for 3D Printing, Enviro Sand Pty Ltd, and Chip Eng Seng Corporation Ltd.

Rheological studies are important for successful 3D concrete printing. The main challenge for successful 3D concrete printing is the complex characteristic the materials should possess. It should be flowable enough to be pumped and extruded through the hose, as well as gaining sufficient strength and stiffness for buildability after the layer by layer deposition. Existing literature has various mixtures proposed for successful 3D concrete printing. Most of these studies used rheometers to measure the dynamic yield stress and plastic viscosity. As the measurement with rheometer is sensitive to the protocols and is controlled by the rheologists, as well as data processing if non-standardized measuring geometries are used, results could vary significantly. This study used standardized field-friendly protocols to measure the slump and slump-flow of the mortars. The pumpability and buildability are evaluated in terms of the pumpability index and maximum height printed before collapsing. These result together with the slump and slump-flow values are used to define the printable region. Accepted version

3D printing is getting significant attention in the construction industry. This technology, which has been talked for years, is now delivering tangible results, however it is not yet ready for mass production in mainstream construction. Thixotropy material, 3D printer together with 3-dimensional computer model are the key elements required for robust concrete printing. For a freshly printed material, the increase in structural buildup at rest, prior to the placement of a successive layer can result poor interlayer bond strength and therefore, in this paper, we aimed to investigate the structuration effect of a nano-clay modified geopolymer, used for 3D concrete printing. Different structuration rate was achieved by changing molar ratio of activator and experimental results conclude that, there exists an optimum printing zone for different molar ratios, beyond which interlayer bond strength will be very weak.

Scalability is a problem common to most existing 3D printing processes, where the size of the design is strictly constrained by the chamber volume of the 3D printer. This issue is more pronounced in the building and construction industry, where it is impractical to have printers that are larger than actual buildings. One workaround consists in printing smaller pieces, which can then be assembled on-site. This workaround generates however additional design and process complexities, as well as creates potential weaknesses at the assembly interfaces. In this paper, we propose a 3D printing system that employs multiple mobile robots printing concurrently a large, single-piece, structure. We present our system in detail, and report simulation and experimental results. To our knowledge, this is the first physical demonstration of large-scale, concurrent, 3D printing of a concrete structure by multiple mobile robots. Accepted version

An advancing technology that combines the concrete extrusion with a motion control to create structures with complex geometrical shapes without the need for formwork is known as 3D concrete printing. Since this technique prints layer by layer, the time taken to reach the same position in the subsequent layer is important as it will create an anisotropic property that has a weaker tensile strength at the bond interface of the two printed filaments. Through rheological measurement, which reveals the material deformation and flow behaviour, it is possible to examine the material structural build-up due to time-gap effect by measuring at different time delay. This paper focuses on investigating the time-gap effect on the printed filament with rheological and observation at macroscopic-scale to understand the material behaviour of the initial and subsequent printed layer during its fresh phase. Rheological experiment findings reveal that the tensile strength of the printed specimen is correlated to the material modulus at the initial layer. NRF (Natl Research Foundation, S’pore) Accepted version

The main advantage of 3D concrete printing (3DCP) is that it can manufacture complex, non-standard geometries and details rapidly using a printer integrated with a pump, hosepipe and nozzle. Sufficient speed is required for efficient and fast construction. The selected printing speed is a function of the size and geometrical complexity of the element to be printed, linked to the pump speed and quality of the extruded concrete material. Since the printing process requires a continuous, high degree of control of the material during printing, high performance building materials are preferred. Also, as no supporting formwork is used for 3DCP, traditional concrete cannot be directly used. From the above discussion, it is postulated that in 3DCP, the fresh properties of the material, printing direction and printing time may have significant effect on the overall load bearing capacity of the printed objects. The layered concrete may create weak joints in the specimens and reduce the load bearing capacity under compressive, tensile and flexural action that requires stress transfer across or along these joints. In this research, the 3D printed specimens are collected in different orientations from large 3DCP objects and tested for mechanical properties. For the materials tested, it is found that the mechanical properties such as compressive and flexural strength of 3D printed specimen are governed by its printing directions. Accepted version

This paper evaluates the potential of fly ash based geopolymer cement for large scale additive manufacturing (AM) of construction elements. Geopolymer is considered as a green construction material and its use in AM may contribute towards sustainable environment since in AM process material is only deposited whereby it is necessary. As part of this research, an industrial robot was employed to print geopolymer mortar in layer-by-layer manner directly from 3D computer-aided design (CAD) model. The characteristic of raw materials and fresh properties were examined by rheology, x-ray diffraction (XRD), and scanning electron microscopy (SEM). Mechanical tests such as compression, flexural and tensile bond strength were conducted on the printed geopolymer in different printing directions and their performance was compared with casted samples. It was found, from the experimental results, that the mechanical properties of 3D printed geopolymer are mostly dependent of loading directions due to anisotropic nature of the printing process and retains intrinsic performance of the material. NRF (Natl Research Foundation, S’pore) Accepted version