Sustainable Uses of 3D Printing Applied to Concrete Structures ^†

Abstract

This study investigates the application of honeycomb-patterned PLA as a reinforcement in concrete structures. The research focused on identifying the optimal 3D printing layout for this reinforcement and examining how the orientation of 3D-printed PLA affects the mechanical properties of the concrete. The study compares the performance of concrete reinforced with 3D-printed PLA to both unreinforced concrete and concrete reinforced with recycled amorphous aggregate from printing waste. The results demonstrate how printing orientation influences concrete strength and the potential for using recycled PLA to enhance sustainability in construction.

1. Introduction

Concrete is a fundamental material in construction due to its outstanding mechanical strength, durability, and cost-effectiveness. However, “pure” concrete has limitations, particularly its brittleness and lower tensile and shear strength, which restrict its application in stress-dominated and thin structures. Numerous studies [1,2,3] have explored methods to enhance the tensile strength and ductility of concrete [4] using discontinuous fibers and various lattice structures to address these limitations. Currently, different reinforcing materials, such as metal, polymer, basalt, and carbon fibers, are available [5]. However, there are no standardized guidelines specifying the material, shape, or orientation of these reinforcing elements. As a result, a significant amount of research focuses on these variables, examining different materials and printing orientations [6,7].

One notable study by Santana et al. [8] proposes using geopolymer meshes as reinforcing elements, integrating textile concrete technology with 3D printing. In this approach, objects are constructed layer by layer using materials like PET (polyethylene terephthalate), PLA (polylactic acid), or ABS (acrylonitrile butadiene styrene), enabling the creation of complex structures. Their research demonstrated that both homogeneous (4.75% polymer by volume) and graded (3.34% polymer by volume) reinforcement resulted in 47- and 52-fold increases in toughness, respectively, without compromising the composites’ load-bearing capacity or critical stress factor.

However, the size of 3D-printed products is limited by the dimensions of the printing bed and volume, which restricts the ability to produce larger specimens, such as fiber reinforcement for railway cross beams. As a result, many studies focus on proportionally scaled-down elements [9,10,11]. In addition to lattice structures, honeycomb designs with a hexagonal base are also preferred due to their superior mechanical properties. Peng et al. [12] conducted research comparing traditional square-base lattice structures with a new honeycomb structure made of PLA, demonstrating improved compression properties and energy absorption. Their findings suggest that these modified structures have significant research potential and broad application prospects. This led to the idea of using a 3D-printed honeycomb frame as a fiber reinforcement element in concrete structures.

This research addresses two key challenges: using 3D-printed elements with specific geometries and recycling amorphous printing waste. With the European Union producing 16.13 million tons of plastic waste in 2021, a figure expected to grow, proper disposal is crucial due to plastics’ non-biodegradability. Recycling plays a vital role in reducing the environmental impact of incineration, landfilling, and dumping. Given the waste generated by 3D printing, this study investigates the potential for recycling PLA waste to enhance sustainability.

Additionally, Panda et al. [13] examined geopolymer concrete (PSGPC) enhanced with waste plastic for sustainability. Their research indicated that incorporating plastic fibers in PSGPC improved various durability properties. They recommended using granular and flaked plastics instead of strips for better workability. Another study by Akçaözoğlu et al. [14] explored using crushed PET bottle granules as lightweight additives in mortar. With a water-binder ratio of 0.45 and a PET-binder ratio of 0.50, using granules sized 0–4 mm, they found that these modified mortars reduced the weight of concrete due to their lower specific mass, classifying them as structural lightweight concrete.

The primary goals of this research are twofold. First, it aims to identify a PLA reinforcement method for concrete elements (e.g., a reinforced concrete railway sleeper in a ballast bed [15]) that enhances mechanical properties. Second, it seeks to determine the optimal printing parameters and orientation for the honeycomb grid structure and compare the mechanical properties of PLA honeycomb reinforcement with those of amorphous fine-grained printing waste. The study explores sustainability from two angles since PLA was used in various forms—both as a 3D-printed mesh and as recycled amorphous waste for reinforcement. Material sustainability is considered through the reusability of PLA, while the research also addresses the sustainability of new applications for PLA as waste from 3D printing. Therefore, this study uses 3D printing waste in experiments to propose future uses, potential applications, and recycling methods.

2. Materials and Methods

2.1. About PLA

Basic ECO PLA was used in the experiments. The recommended and applied printing parameters and properties of the PLA used are summarized in

Table 1. Basic PLA recommended and applied printing parameters and properties.

Printing Parameters	Recommended	Applied
Nozzle temperature	195–225 °C	210 °C
Nozzle size	0.2–1.2 mm	0.4 mm
Bed temperature	max 70 °C	70 °C
Bed condition	kapton, glass tape, or glue	glass
Cooling fan	up to 100%	100%
Layer height	0.4–0.8 mm	0.8 mm
Print speed	20–80 mm/s, optimal, max. 250 mm/s	50 mm/s
Properties	-	Test method
Tensile strength	32 ± 2 MPa	ISO 527 [17]
Tensile modulus	1.8 ± 0.1 GPa	ISO 527 [17]
Tensile elongation	2–4%	ISO 527 [17]
Notched Izod impact	4.95 kJ/m2	ISO 180 [16]

[16,17].

2.2. Applied Printers, Printing Setups, and Structures

In this study, all test specimens were printed with a 20% infill size on both printers using the parameters detailed in Table 1. The same setup parameters were applied across all specimens to identify the most favorable print orientation for use as a fiber-reinforcing element and to enhance the mechanical properties of the concrete during three-point bending tests. The design of the printed structures was created using SolidWorks 2019, and the resulting STL files were converted into printable instructions through UltiMaker Cura 5.3.0.

The lattice structures are designed around a honeycomb pattern consisting of regular hexagons featuring maximum internal angles of 120°. The unit cell has a thread diameter of 1.25 mm and side lengths of 11.88 mm. The hexagonal cells have a depth of 45 mm and a length of 246 mm, with each unit cell occupying a volume of 3240.96 mm³ and weighing 3.24 g. Figure 1 illustrates the experimental setup for the test specimens.

Figure 1 illustrates the experimental setup for the honeycomb structure. In the diagram, the dashed line represents the supports, which are spaced 160 mm apart, while the thick solid line through the center of the specimen indicates the direction of the applied load at a distance of 125 mm.

Two different printers, each with a distinct print orientation, were used to produce the 45 × 246 × 45 mm specimens, specifically designed to fit the ZD40-VEB 40-ton tensile (VEB, Leipzig, Germany) and compression testing machine. The two orientations were as follows:

• Laid (L) Orientation: Printed parallel to the specimen table and laid on the longer side of the specimen. These grids were produced using a Creality CR-S10 PRO printer and were printed with white PLA.
• Upright (U) Orientation: Printed perpendicular to the specimen table and positioned on the smaller side of the specimen. These grids were printed on a Creality Ender-5 printer using red PLA.

Scrap pieces from 3D printing were also utilized to prepare the test specimens. These scraps were ground into small particles (0–5 mm) using an F. ILLI VIRGINIO knife hammer grinder before being incorporated into the experiments. Labels were assigned for clarity: “R” for the reference specimens, “A” for those made from amorphous grinding, “U” for the upright printing position, and “L” for the laid printing position.

2.3. About Test Specimen Molding

Concrete was used as the binder for the specimens, and its composition is detailed in

Table 2. Concrete receipt.

Material	Manufacturer (Brand)	Amount/Quantity (grams)
Water	-	2250.00
Cement	CEM-II-AS-42.5	6420.00
Limestone powder	Lafarge	1538.00
Additive	OH0/1	6975.00
Fluxing agent	VC 5 NEW	41.25

. The mix used for each specimen totaled 7.5 L.

The mold used for casting the specimens was a 70 × 70 × 250 mm cast iron mold. To achieve a high-quality surface finish, Sika Separol AR-2 ECO release agent was applied to the inside of the mold before the casting process began. Once the mold was prepared, the concrete elements were cast, including those reinforced with 3D-printed hexagons and amorphous PLA waste, as well as reference elements without any reinforcements. The mixing ratio used for the concrete mixed with amorphous PLA was 138.5 g of PLA to 3.75 L of concrete.

After casting, the specimens were compacted using a Matest C278 vibro table to remove any trapped air. The specimens were left to solidify in the mold under water for seven days after this process. This underwater curing was essential to ensure the surface quality of the concrete elements, preventing cracking that might occur with strengthening/curing on air.

2.4. Measurement Setup

In the experiments, the concrete elements were reinforced with two types of reinforcement made from a 3D-printed basic PLA grid. The first type of reinforcement was a printed honeycomb grid structure, while the second was an amorphous aggregate recycled from PLA waste. Additionally, reference elements without any reinforcement were tested. The concrete specimens underwent fracture testing using the ZD40-VEB 40-ton tensile(Matest S.p.A., Treviolo, Italy) and compression machine, where they were subjected to increasing loads until failure. The machine was also used to measure the displacement of the machine’s “bridge”, comparing the downward displacement of the specimens to their initial condition.

Observations were made on the surface area of the specimen fractures during the first week of the 28-day strengthening (curing) cycle. It is important to note that this phase represents the initial stage of the experiments, with observations limited to the first week due to the constraints of the conference paper. Nevertheless, even at this early stage, significant changes in the mechanical properties of the specimens were already apparent (see Section 3).

3. Results and Discussion

During the 3D printing process, the laid orientation generated more print waste due to the larger surface area involved. Additionally, the nozzle had to travel a greater distance, resulting in a longer printing time of 18 h and 45 min. In contrast, the upright orientation required 6 h less printing time.

After the test specimens were cast, the force-vertical deformation diagrams for the specimens were plotted following the bending tests, as illustrated in Figure 2.

Figure 2 demonstrates that the concrete specimens reinforced with the honeycomb grid sustained the highest loads during testing. Among these, the upright (U) orientation specimen withstood the highest load at 5.414 kN, followed by the laid orientation specimen at 5.172 kN. The amorphous reinforcement specimen sustained a load of 4.707 kN, while the reference specimens, which lacked reinforcement, bore the lowest load at 2.667 kN. The corresponding fractures of these specimens are depicted in Figure 3.

Figure 3 reveals that the amorphous PLA waste tends to migrate toward the concrete surface during the curing process, resulting in an uneven distribution within the concrete. Additionally, precipitation is observed on the concrete surface above the amorphous binder areas. The surface condition of the honeycomb structures is also critical to the research findings. While the areas surrounding the hexagonal reinforcements have dried, the interior sections of the hexagons remain undried. Analysis of the fractured surfaces indicates that the ECO PLA has begun to dissolve into the concrete, which could pose problems in the future, mainly if the PLA fully dissolves.

4. Conclusions

In conclusion, the honeycomb structures effectively identified the optimal printing orientation, enhancing concrete’s mechanical properties and supporting sustainability by reducing printing waste. However, the PLA grain liner made from waste did not meet the study’s casting conditions, as shown in the blocks’ plots (see Figure 3) and mechanical loading results. The low density of the PLA caused particles, particularly amorphous ones (A), to rise during casting due to the significant density difference and grain size. Future solutions might involve using higher-density or coarser PLA particles, or PLA spheres measuring approximately 1 cm in size, produced by the Bambu Lab A1 mini printer, which could reduce weight variation.

Three key observations emerged from the fragment diagram (see Figure 3). First, the issue with PLA particles was noted. Second, the surface of specimen A showed a crystallized lime layer, possibly due to a reaction between amorphous PLA particles and concrete or the high pH of the concrete (around 11). Third, the inner parts of hexagon-reinforced elements formed closed cavities trapping water, which could raise long-term durability concerns given PLA’s low thermal stability, hydrolytic degradation susceptibility, and potential for thermal expansion.

Figure 2 shows that PLA reinforcement increases concrete’s toughness and mechanical properties. However, since these experiments only cover the initial week of the 28-day curing cycle, PLA’s tendency for hydrolytic degradation could lead to deterioration over time, which is a focus for future research. Given the significant potential in this area, further studies are needed, especially considering the importance of sustainability.