Shaping mechanical properties of concrete elements by 3D‐printed plastic gyroid reinforcing elements

Traditional steel rebar reinforcement in concrete structures often results in suboptimal spacing and underutilization of its tensile and shear strengths. The advent of 3D printing has opened up exciting possibilities for creating innovative and efficient concrete reinforcement. Previous research has highlighted the potential of using 3D-printed plastic elements as a viable alternative to steel rebar, particularly in applications like stay-in-place formwork and spatial reinforcement.1 While plastic possesses lower mechanical properties compared to steel, the unique geometries achievable through 3D printing offer significant advantages in terms of reinforcement spacing and design flexibility. One promising geometry for future plastic reinforcement is the gyroid structure, a design that leverages these advantages. This intricate, triply periodic minimal surface, discovered by Alan Schoen in 1970, offers several unique benefits.2 Its interconnected network and lack of planar symmetries create two congruent spaces, potentially enhancing the structural behavior of concrete elements. The gyroid was selected as the base triple periodic minimal surface geometries (TPMS) cell due to prior research conducted by members of the research team on such a structure fabricated using selective laser melting.3 The current research program investigates the use of 3D-printed gyroid lattice structures as concrete reinforcement (see Figure 1) for compressed elements. By varying the geometry and volume of plastic used, the study aims to understand the influence of these factors on the mechanical properties of the reinforced concrete. The potential to replace steel with 3D-printed plastic reinforcement offers significant environmental benefits, promoting a more sustainable construction industry. Moreover, the use of recycled plastics for 3D printing further enhances the eco-friendliness of this approach. This research represents a significant step toward developing innovative and sustainable solutions for concrete reinforcement. For the creation of spatial plastic reinforcing elements, a commercially available 3D printer was used. Based on previous experience,4, 5 the acrylonitrile butadiene styrene (ABS) plastic was chosen as a filament for the creation of the reinforcing elements. Key properties of the ABS filament are as follows: flexural strength, 70.5 MPa; tensile stress at break, 33.9 MPa; tensile modulus, 1.7 GPa; flexural modulus, 2.1 GPa. The 3D-printed spatial ABS reinforcing elements (diameter 100 mm, height 200 mm), ready for use, are presented in Figure 2. The mass of the printed elements ranged from 3240 to 3300 g. The printing process was conducted using an infill density of 15% and a grid infill pattern, consistent with the previous research programs.4, 5 All parameters and details of the 3D printing technique were identical to those in the previous research programs. The standardized mortar described in EN 196-1 was adopted as the cement matrix representing concrete. The mortar was modified with two commercially available admixtures: a viscosity-modifying agent and a superplasticizer. The mortar was successfully utilized by the research team in previous research programs regarding 3D-printed plastic reinforcement for concrete.4, 5 The composition of the cement matrix is presented in Table 1. The consistency of the fresh mix was tested using a flow table and measured 285 mm. The hardened cement matrix, after 28 days of curing, was characterized by an apparent density, compressive strength, and flexural strength of 2.0 g/cm3, 22.7 MPa, and 4.3 MPa respectively, tested using standard 40 mm × 40 mm × 40 mm cubes and 40 mm × 40 mm × 160 mm beams used for ordinary cement mortars. Load–strain relations of the initial concrete-plastic specimens during compression are presented in Figure 3. Compressive strengths (calculated for max. load) are as follows: 13.0 MPa, 11.2 MPa, and 12.1 MPa for Kac1, Kac2, and Kac3, respectively, with an average of 12.1 ± 0.9 MPa. The difference among the specimens was ±7.4%. An important part of the initial analysis was to determine the strain at which the maximum load was achieved for each type of reinforcement: 1.03%, 1.93%, and 2.69% for Kac1, Kac2, and Kac3, respectively. More notably, the areas under the load–strain curves, which represent the dissipated energy, are very similar, with a difference of ±0.5%. The load–strain relations of the concrete-plastic specimens are similar to the behavior of steel fiber reinforced concrete (SFRC) with a high fiber volume (e.g., over 1.5%) or ferrocement. The maximum volume of steel fiber typically added during the in situ casting of full-scale concrete elements is generally capped at 2%, with common usage ranging between 0.5% and 1.0% of fiber. However, for 3D-printed plastic reinforcement, the optimal maximum volume within concrete elements remains undetermined in terms of technological feasibility and mechanical benefits. The gyroid elements utilized in the research did not appear to achieve the maximum possible volume of plastic reinforcement. Additionally, the design of the 3D plastic reinforcement's geometry plays a crucial role in determining the best structural outcomes. In Figure 4, images of specimens during the compressive test are presented. One image was taken at 5% strain, and another was taken at 10% strain. When examining the images, it is crucial to recognize the substantial contrast in the modulus of elasticity between cement mortar and the ABS reinforcement. The failure mechanisms for the reinforcements shown and for 3D printed plastic reinforcements in general are still not completely explored, modeled, or documented. This exploration is a primary objective of the current research project. Compared to traditional reinforcements like steel rebar, ferrocement, or fiber, the failure dynamics of 3D printed plastic reinforcements are more complex and influenced by numerous variables. Besides the properties of the plastics used in 3D printing filaments, several aspects of the 3D printing process affect the characteristics of the resulting elements. The design of the printed structure, the spacing of the printed layers relative to the direction of applied force, and the finish of the external surface all play roles in determining how the plastic reinforcement behaves. The current research initiative is dedicated to exploring the use of 3D printed plastic gyroids as reinforcements in concrete columns. Initial phases of the research have demonstrated the viability of this concept, but additional studies are necessary to comprehensively understand how these reinforcements interact with the cement matrix. A detailed examination of these properties involves testing with various specimen sizes and shapes, employing digital image correlation (DIC) analysis, experimenting with different gyroid designs, and evaluating 3D printing techniques with various plastics. Future applications for plastic-concrete elements might include scenarios where structures could face accidental loads from impacts, shockwaves, water waves, vibrations, earthquakes, or explosions. Beyond opening up new architectural and engineering possibilities, 3D-printed plastic reinforcements could also have a positive environmental impact on the concrete industry by allowing the manufacture of 3D printer filaments from recycled plastics. This paper was created as part of the project No. CZ.02.01.01/00/22_008/0004631 “Materials and technologies for sustainable development” within the OP JAK program financed by the European Union and from the State Budget of the Czech Republic. Jacek Katzer, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Olsztyn, Poland. Email: [email protected]. Aneta Skoratko, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Olsztyn, Poland. Email: [email protected]. Tomasz Szatkiewicz, Faculty of Mechanical and Power Engineering, Koszalin University of Technology, Koszalin, Poland. Email: [email protected]. Oldřich Sucharda, Faculty of Civil Engineering, VSB – Technical University of Ostrava, Ostrava, Czech Republic. Email: [email protected]. The data that support the findings of this study are available from the corresponding author upon reasonable request.