By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Sep 10 2024A recent article published in Applied Sciences proposed a sustainable and removable support material for overhang sections in three-dimensional (3D)-printed concrete structures. The efficacy of the printable recycled glass-based support material was demonstrated by printing a structure with a free-form opening and overhang section.
Product achieved by D-shape printing method [9]. Image Credit: https://www.mdpi.com/2076-3417/14/17/7800
Background
The 3D concrete printing technology can easily fabricate free-form structures for practical applications. However, the printing of openings for windows and doors is limited due to overhanging sections, which do not have any support or surface to rest upon. Consequently, the fresh concrete filaments collapse due to gravitational force.
Most 3D concrete printing technologies use temporary supports, such as wooden planks and corbelling, for handling overhangs, which restrict the design flexibility and slope angles. Alternatively, some 3D printing technologies use support materials similar to the primary materials but printed at different parameters. This weakly bonded support structure can be easily removed after completion of the printing process.
The printability is generally optimized by introducing modifications to the concrete in small batches or applying set-on-demand interventions near the print head. The latter is more effective in improving buildability. Thus, this study developed a material for removable support with sufficient printability and effective sustainability.
Methods
The support material paste was formulated using different proportions of fly ash (FA; 60% to 30%) and finely ground recycled glass (RG; 40% to 70%). This soft material, devoid of Portland cement, could ease removing the support from the main overhang structure after curing.
The water/paste ratio was maintained at 0.3 for all the mixtures. Additionally, natural sand (0.8 weight ratio) was added to the paste to achieve higher yield stress and shape retention of the mixture
The prepared material’s printability was examined through pumpability and buildability tests. The formed was assessed through volumetric flow rate measurements through the printer nozzle under a constant pump speed. Alternatively, cylindrical specimens were printed to test the material’s buildability from the maximum number of layers printed without excess deformation of the filaments.
A four-axis gantry system was used for printing experiments. The material’s compression strength was measured to evaluate its resistance to deformation in the fresh state. Additionally, a flow table test using conical-shaped molds was conducted to determine the flowability of the mixtures.
The mix design was optimized using Minitab statistical software by comprehensively evaluating a single variable (mixture ratio) with two responses (compression load and flowability). This helped determine the impact of variable combinations on optimizing a single response, which aimed to maximize the support material’s compression load and flowability.
Results and Discussion
The designed material’s volumetric flow decreased with increasing RG content, attributed to the RG particles’ angular shape and sharp edges. Alternatively, the spherical FA particles enhanced the flow properties of the suspension material by reducing friction among the angular glass particles through the ‘ball bearing effect’.
A higher proportion of RG (over 50%) significantly impaired the material’s pumpability. Notably, the mixtures with over 60% RG were non-pumpable due to clogging in the pump.
However, the material’s buildability improved with increasing RG content due to the reduced filament deformation at a higher glass-to-binder ratio. Thus, the mixture with 60% RG could build more layers with lower deformation than 40% RG, which deformed rapidly with subsequent layers.
The compression load required to deform the specimens increased with increasing RG content but peaked at 65% RG. This increase was attributed to the angular shape of the RG particles, which resisted the material deformation. Consequently, the compression load required to deform the material increased, and it could attain a higher buildability.
The optimized mixture was used to print a free-form overhang section, exhibiting its effectiveness as a temporary support material. A slot opening was printed and later removed after the concrete hardened. Thus, the support material could be 100% recovered from the structure's post-processing and reused in other printing projects due to its non-curing nature and non-reactivity of the raw materials.
Conclusion
Overall, the researchers successfully designed a sustainable temporary support material for overhang sections during 3D concrete printing. The proposed temporary support could be easily removed once the primary concrete hardened.
Non-reactive components like fly ash, recycled glass, and natural sand facilitated this, ensuring that the material remained in paste form until the concrete hardened.
At 60% RG content, a balance between the material’s flowability and buildability was achieved. This optimized material was successfully used to print a free-form opening and overhang section. However, this study was limited to the immediate response of the support material to incremental stresses.
The researchers suggest incorporating time-dependent measurements of creep and compressive strength to better understand material performance. This will help evaluate the material’s long-term stability and deformation behavior under constant loads, thereby enhancing the reliability of 3D concrete printing technologies in advanced architectural applications.
Journal Reference
Ting, A., Wei, Y., Noel, K., Tan, M. J., & Wong, T. N. (2024). Sustainable Support Material for Overhang Printing in 3D Concrete Printing Technology. Applied Sciences, 14(17), 7800. DOI: 10.3390/app14177800, https://www.mdpi.com/2076-3417/14/17/7800
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