Mechanical behavior and permeability properties of sustainable and high-performance anisotropic three-dimensional printable concrete

Abstract

Three-dimensional printable concrete requires further development owing to the challenges encountered, including its brittle behavior, high cement requirement for the buildability of layers, and anisotropic behavior in different directions. The aim of this study is to overcome these challenges. First, three-dimensional printable concrete mixtures were prepared using silica fume, ground blast furnace slag, and metakaolin, instead of cement, to reduce the amount of cement. Subsequently, the rheological and mechanical behaviors of these concretes were investigated. Second, three-dimensional printable concrete mixtures were prepared using 6-mm-long steel and synthetic fibers to eliminate brittleness and determine the effect of those fibers on the anisotropic behavior of the concrete. As a result of this study, it is understood that printable concretes with extremely low permeability and high buildability can be achieved using mineral additives. In addition, results showed that three-dimensional concrete samples containing short steel fibers achieve fracture energies up to 36 times greater than that of plain concrete. Meanwhile, its characteristic length values, as indicators of ductility, are 22 times higher than those of plain concrete. The weakest strength was recorded at the interfaces between layers. The bending and splitting tensile strengths of three-dimensional printed plain concrete samples were 15% and 19% lower than those of casted samples, respectively. However, the addition of fibers improved the mechanical strength of the interfaces significantly.

Graphical abstract

Keywords

three-dimensional concrete printing / rheology / high performance / mineral additives / anisotropy / fiber

1 Introduction

The use of concrete and mineral additives in three-dimensional (3D) printers is prevalent in the fields of architecture and construction. The use of novel methods such as 3D printable concrete technology in the construction industry is expected to revolutionize construction processes, thus imparting significant transformations to this industry in the near future [1,2]. Currently, 3D concrete printing (3DCP) is extensively investigated in both academia the building industry [3,4]. Although the technology is currently used widely by researchers and private organizations, it requires further development. Variants of this production method are continually being developed and improved. The fundamental relationships among the material, design, process, and final product are yet to be elucidated. Nonetheless, this technology is extremely promising to the construction industry [5].

Considering the advantages and optimized design of concrete printed using 3DCP, researchers are constantly attempting to improve its processes. This new method requires concrete design with good extrudability and sufficient early strength. In addition, the rheological properties of fresh concrete must be optimized based on the process parameters, such as the printing pressure and printing speed. A holistic approach is required to test and model the printability of this technique based on concrete rheology, i.e., the pumpability, extrudability, and buildability of cementitious composites [6]. The 3DCP technology presents a few challenges in terms of its processes. In this technology, the entire process is based on the step-by-step addition of concrete layers [7], and the concrete used must exhibit sufficient flowability during this process. Additionally, the pump or transport hose should not be obstructed. However, the lower layers must be able to support the weight of the new upper layer. The rheological behavior parameters, such as the yield stress and plastic viscosity, which determine the extrudability and buildability of 3D printable concrete, must be of certain magnitudes [8]. In 3D printable concrete, the bottom layers must undergo minimal deformation under the weight of the upper layers, and the layers must adequately bond with each other [9].

Identifying and solving the various production constraints of 3DCP technology are critical to its application in the construction industry. The rheological properties of concrete are related to many variables, such as the type and amount of materials, aggregate particle size, and chemical and mineral additives [10,11]. The rheological properties of concrete mixes and behavior of fresh concrete are crucial in 3DCP technology. Therefore, the effects of concrete rheological properties on 3D printable concrete have been investigated extensively [12–17].

The most critical properties of 3D printable concrete are its extrudability and buildability. Extrusion is expressed as the capacity of the concrete to pass through delivery hoses and spray nozzle without causing obstruction. Good extrudability can be achieved by considering both the self-compacting concrete and shotcrete design principles. In 3D printable concrete, the lower layers must support the upper layers continuously without any deformation. Workability and sufficient open time are required to ensure the good extrudability and continuous flow of concrete. To achieve buildability, the concrete must be set promptly to gain strength. However, in this case, the concrete loses its consistency rapidly, the printing speed decreases, and the transmission hose may be obstructed. By contrast, a long opening time improves the extrusion ability and facilitates the bonding between layers. However, excessive deformation might occur in the layers and their buildability might deteriorate [18].

Standardized test methods for determining the rheological properties of 3D printable concrete do not exist. The slump test is the most typically used method for determining the rheological properties of printable concrete as well as conventional concrete. However, the same slump value may correspond to different workabilities owing to factors such as aggregate particle shape, aggregate size, and chemical additive type used in the concrete composition. In addition, slump flow provides a first indication of flowability and extrudability but does not provide any information regarding buildability. Although the slump test is useful for monitoring the stability of concrete mixes, it does not provide sufficient information regarding the properties of printable concrete, such as extrudability, transportation in the delivery hose, and printing from the nozzle. Therefore, researchers have proposed new test methods for 3D printable concrete [19–21].

Although 3DCP is a promising construction technology, the low tensile strength and brittleness of printable concrete pose significant problems. In addition, the early age strength of printable concrete should be sufficient to ensure buildability without plastic collapse and elastic buckling [22]. The requirement for a high amount of cement to achieve adequate strength results in non-economical and non-environmental-friendly consequences. In particular, using Portland cement in 3D printable concrete results in a greater environmental impact compared with using conventional concrete [23]. Another major challenge when using printable concrete is its anisotropic behavior. Different values for concrete properties can be obtained in various directions, particularly in cases where weak bond strengths occur at the contact surface between layers. Continuous efforts have been expended to overcome these challenges [24–27].

In some studies, the mechanical properties, anisotropic behavior, and bond strength of 3D printable concrete have been investigated. The results indicate that printable concrete exhibit extremely anisotropic behavior and that the use of mineral admixtures reduces the degree of anisotropy. In addition, a longer interlayer time between the printed layers may decrease the strength of printable concrete due to dehydration. Studies show that 3D printable concrete is brittle in the plain state. One of the most important issues in the development of 3D printable concrete is the brittleness of concrete [28–30].

Researchers have attempted to improve the mechanical properties of concrete using fibers in 3D printable concrete. Yang et al. [25], Bos et al. [31], and Pham et al. [32] investigated short-cut steel fibers. Hambach et al. [33] and Ding et al. [34] investigated carbon fibers and polyethylene fibers, respectively. van der Putten et al. [35] investigated the effect of fibers on the tensile strength of concrete. These studies show that the tensile and flexural strengths of 3D printable concrete can be improved by incorporating fibers.

Another challenge in the 3DCP technique is the anisotropic behavior of concrete. The anisotropic properties of 3D printable concrete have been indicated in many studies [18,24,31,32,36–39]. Nerella et al. [40] showed that the flexural strengths of concretes can differ by up to 48%, depending on the direction. Some studies showed that bonding risk problems might occur because of cold joints at the interfaces between layers [41,42]. In this context, the lowest mechanical strengths generally occur when loads are applied to the interfaces. However, some studies showed that an increase in the contact area at the interface, the weakest region between layers, or changes in the time gaps for printing between layers can increase the bond strength [43,44].

Nerella et al. [29] investigated the effect of time intervals between layers on bond strength. They discovered that the time intervals and material composition significantly affected the quality of the interfaces. In addition, the printing parameters (i.e., nozzle offset and printing speed) exerted significant effects. In some samples, sufficient interfacial bonding between layers was observed. They hypothesized that the interface defects self-healed with the hydration and/or carbonation products. Ultimately, they predicted that printable compositions with minimal or no loss due to layering can be achieved. Marchment and Sanjayan [45] stated that increasing the contact surface between layers significantly improved the bond strength. Additionally, they mentioned that the bond strength increased when a cement slurry was applied to the surface of the layers.

Another challenge in 3DCP technology is the ineffective use of mineral additives in cement-based printable concrete. During cement production, approximately 1 ton of CO₂ gas is released for 1 ton of cement, and the cement industry accounts for 7% of the greenhouse gas emissions in the world [46–48]. Therefore, mineral additives from various industries are typically used instead of cement to reduce the carbon footprint [49]. However, 3D printable concrete has rheological properties that differ significantly from those of normal concrete. The most basic property that complicates the use of mineral admixtures in 3DCP is the requirement for 3D printable concrete to attain sufficient strength in the early stages of buildability without collapse and elastic buckling. Some properties, such as the workability and pumpability of 3D printable concrete, can be improved by incorporating mineral additives. However, in this case, the early age green strength of printable concrete may not be at the desired level. When sufficient strength is not achieved, the lower layers cannot adequately support the upper layers without collapsing or elastic buckling. Hence, researchers have focused more on geopolymer mortar applications in terms of using pozzolanic mineral additives in 3DCP [50–53]. The use of geopolymer concrete mixtures in the construction industry is scarce compared with that of conventional mixtures. In addition, a high compressive strength is required for most construction applications. In most cases, the strength of 3D printable geopolymer concrete is low, and some methods have been applied to increase the strength [54]. Adhering to these difficult processes is challenging. Therefore, more studies must be conducted to develop environmentally friendly and cost-effective cementitious 3D printable concretes using mineral admixtures. In fact, many studies have been performed to develop more sustainable 3D printable concrete using waste materials instead of aggregates in the concrete composition [55,56].

The main purpose of this study is to develop solutions to address some of the disadvantages of 3D printable concrete. A significant amount of cement is used to ensure the buildability of layers in 3D printable concretes. The first aim of this study is to replace a certain amount of cement with mineral additives and to develop a more sustainable cement-based 3D printable concrete that contains less cement and exhibits lower permeability. Another important aim of this study is to eliminate the brittleness of 3D printable concrete and to improve the low mechanical strength in some directions by incorporating fibers. First, mineral additive concrete mixtures using ground granulated blastfurnace slag (GGBS), silica fume (SF), and metakaolin (MK), as well as a reference concrete mixture without mineral additives for comparison are prepared. Some printability properties (workability, pumpability, open time, and buildability) of these mixtures are investigated via slump tests and using a hopper to squeeze the cement paste. Flexural strength, splitting tensile strength, compressive strength, rapid chloride ion permeability, capillary water absorption, and water absorption tests are performed on the samples. Second, mixtures containing 1% steel fiber by volume and 1% synthetic fiber by volume are prepared based on the reference mixture design. Subsequently, the abovementioned mixtures are printed using a 3D printer. The flexural strength, compressive strength, splitting tensile strength, fracture energy, and characteristic lengths of the printed samples in different directions are investigated. Changes in the capillary water absorption values are examined. The abovementioned properties are similarly investigated for casted samples. By performing these studies, we aim to mitigate the challenges posed by the high cement requirement, anisotropic behavior, and brittleness of 3D printable concretes by incorporating mineral additives and fibers.

2 Experimental study

First, concrete mixtures were prepared in the laboratory by replacing 25% of the cement with GGBS, SF, or MK mineral additives. The rheological properties (workability, pumpability, open time, and buildability) of these mixtures for printable concrete were investigated. In addition, test specimens of hardened concrete were prepared from these mixtures, and splitting tensile, flexural, and compressive strength tests were performed. Additionally, rapid chloride ion permeability, water absorption, and capillary water absorption tests were conducted. Second, a plain concrete mixture and concrete mixtures prepared by adding various fibers to plain concrete were printed. The mechanical strengths, fracture energies, and characteristic length values of all printed concretes were determined for different directions. In addition, the permeability properties of the concrete in different directions were investigated via capillary water absorption tests. The test results of all printed samples were compared with those of the casted samples.

2.1 Materials and mixture designs

2.1.1 Cement, mineral additives, and siliceous aggregates

The cement used was CEM I 52.5 R-type white cement with a Blaine-specific surface of 439 m²/kg and specific gravity of 3.07 g/cm³. Aggregates of 0–0.5 mm silica powder and 0.5–1.5 mm silica sand were used. These aggregates are preferred for high-performance 3DCP because of their superior strength properties. Sieve analysis results of the sand aggregates are presented in Tab.1. The properties of the cement, mineral additive, and siliceous sand aggregates used in this study are listed in Tab.2.

The mixtures were first prepared to determine the effects of mineral additives on the rheological properties of 3D printable concrete in the fresh state and on the mechanical behavior and permeability properties in the hardened state. Additionally, a plain concrete mixture without mineral additives was prepared. We aimed to perform performance-based comparisons among all the mixtures. In this context, mixtures prepared with GGBS are denoted as 3D-GGBS; mixtures prepared with SF are denoted as 3D-SF; and mixtures prepared with MK are denoted as 3D-MK. Meanwhile, the 3D-REF code is used for plain concrete mixtures without mineral additives. The materials used in all concrete mixtures are listed in Tab.3.

Second, the effects of the addition of steel and synthetic fibers to 3D printable concrete were examined. Mixtures were prepared to determine the effects of various fibers on the mechanical behavior and permeability properties of the 3D printable concrete. In addition, the properties of the samples printed in different directions were examined. The results of the samples printed in all directions were compared with those of the casted samples. In this study, mixtures prepared with steel fiber are denoted as 3D-STF, whereas those prepared with polyamide fiber are denoted as 3D-PA. For the plain concrete mixture, the 3D-REF code was re-used because its compositions were the same of those of the plain concrete mixture without mineral additives. The X, Y, and Z notations at the end indicate the axis parallel to which the load is applied in the tests for examining the mechanical behavior of the printed samples.

Concrete has low ductility and tensile strength. Hence, reinforcement must be added to it such that it can resist the strains and stresses in structural concrete elements. Fibers are used in concrete to prevent cracks and increase ductility and fracture energy. The steel fibers used in this study were synthesized based on the European norm (EN) standard, and micro synthetic fibers were synthesized based on the EN 14889-2 Class I standard. The effects of the fibers on the mechanical strength, fracture energy, and ductility of the 3D printable concrete were investigated. The properties of the fibers used are listed in Tab.4.

2.2 Specimen preparation and fresh concrete tests

This study was conducted in two stages. In the first stage, 3D printable concrete mixtures were prepared using mineral admixtures. In the second stage, a plain concrete mixture and two different concrete mixtures were prepared using various fibers. In all mixtures, rheology modifier additives were used to adjust the flowability (initial slump of 200 mm) of the 3D printed concrete and increase the open time (45 min), and viscosity regulatory additives were used to obtain higher shear strength. Regulatory additives were used at a maximum rate of 0.1% by the cement weight. In the first stage, cement, mineral additives, viscosity-regulating additives, rheology regulatory admixtures, and aggregates were blended in dry form for 1 min. One-half of the water was added to the mixture. Finally, the remaining half of the water and superplasticizer were mixed in a container and added gradually in a controlled manner. In the second stage, the cement, viscosity regulator, rheology regulator, and aggregates were first blended dry, and then water and chemical additives were added, similar to the procedure in the first stage. Finally, the fibers were added to the mixture. In both stages, the casted samples were placed in molds via a vibration table for better compaction.

The bending, splitting tensile, and compressive strength tests were conducted on all concrete series prepared in the first stage, in which mineral admixtures were used. Rapid chloride ion permeability, capillary water absorption, and water absorption tests were performed to determine the permeability properties.

In the second stage, three concrete mixtures were prepared: plain concrete mixture, concrete mixture using 1% steel fiber by volume, and concrete mixture using 1% polyamide fiber by volume. The concrete mixtures were printed using a printer. The mechanical strengths, fracture energies, and permeability properties of the 3D printable concrete in various directions were compared with those of the casted samples of the same mixtures. Three-point bending, compressive strength, and splitting tensile strength tests were performed on the casted and printed concrete samples in different printing directions. The width of the printed layers used in this study was 9 cm. Thus, we were able to prepare samples of size 70 mm × 70 mm × 280 mm from the printed samples using a diamond cutting saw. Samples of the same dimensions were obtained by pouring concrete into molds. Thus, comparisons can be performed between the printed and casted samples.

The fracture energies and ductilities of the samples were determined. In addition, capillary water absorption tests were performed to determine the permeability of the printable concrete in different directions. Details regarding the tests in both sections and dimensions of the prepared samples are listed in Tab.5.

The samples casted in the study were demolded after 24 h and transported to the water tank. The printed concrete samples were retrieved from the printing location after 24 h and transported to the same curing area. All samples were stored in a water curing tank at (20 ± 2) °C for 28 d. A minimum of three samples were obtained for each test. The properties of the fresh concrete mixtures are listed in Tab.6.

Disc specimens were obtained from Ø100, h200 mm cylindrical specimens using a diamond stone cutter saw.

In a study pertaining to the rheology of 3D printable concrete, the pumpability and buildability properties of 3D printable concrete with the maximum height of the layers printed before collapse and the pumpability index were evaluated. In this study, these properties were used to define the printable region along with the slump and slump-flow values [57]. The slump test is associated with pumpability and buildability. The printable region based on the slump values is shown in Fig.1; it is the section where pumpability and buildability are provided simultaneously.

Pumpability was determined as “very good” for slump values of 120–200 mm; “good” for 80–120 mm; and “not good” for 50–80 mm. After 45 min of concrete production, the pumpability of the mixtures containing SF and MK was low. Changes in the slump values of the mixtures are shown in Fig.2.

Concretes using all mineral additives in the buildability region were printed in at least 10 layers, and the ability of the layers to support their own weight without plastic collapse and elastic buckling was verified via visual inspection.

2.3 Fracture test procedure

The three-point bending test was performed on casted and printed beam samples measuring 70 mm × 70 mm × 280 mm dimensions, which were obtained from the plain concrete mixture and fiber-reinforced mixtures, as shown in the bending test image in Fig.3. Prior to the tests, notches with a depth measuring 40% of the prism depth were created at the mid-point of the beam span using a diamond cutting saw to allow fracture to occur in all specimens at the desired cross-section. Thus, samples with an effective test area of 70 mm × 42 mm and length of 280 mm were obtained.

In the plain concrete, the displacement velocity was set to 0.01 mm/min to achieve more controlled loading. For the prism-shaped beam samples containing steel fiber and synthetic fiber, a deflection of 0.5 mm at a velocity of 0.0175 mm/min was applied in one case, whereas a deflection of 4 mm at a velocity of 0.1 mm/min was applied in another. The deflections were measured using two linear variable displacement transducers (LVDTs) as the average of two measurements from the mid-span of the prisms. Load–displacement curves were obtained by measuring the load versus deflection values for all prism specimens. The tests were performed in a 200 kN capacity loading device. In this study, the fracture energies of all samples were calculated from the areas under the load–displacement curve using Eq. (1), as recommended in RILEM (TC-50FMC) [58]. The fracture energy test was continued until a final deflection of 10 mm was achieved, and then the fracture energies were determined. However, the fiber-containing samples exhibited extremely low residual strengths after a displacement of 4 mm. In the fracture energy tests, displacements of up to 4 mm were obtained. The cutting and preparation of the 3D printed samples using a saw for the tests are illustrated in Fig.4.

(1)G_{\mathrm{F}}=\frac{W_{\mathrm{o}}+mg{\displaystyle \frac{S}{L}}{\delta }_{\mathrm{s}}}{B(D-a)},

where B is the width, D is the depth, a is the notch depth, S is the span, L is the length, m is the mass, W_o is the area under the load–mid span displacement curve, δ_s is the deflection of the beam (i.e., 4 mm), and g is the gravitational acceleration.

To determine the ductility of the concrete samples, the characteristic length was calculated using Eq. (2), which was proposed by Hillerborg et al. [59] where E, G_F, and f_t represent the modulus of elasticity, fracture energy, and direct tensile strength, respectively. In this paper, the term “direct tensile strength” is replaced with “splitting tensile strength,” which is abbreviated as f_st. The compressive and flexural strength test setups and loading directions are shown in Fig.5.

(2)l_{\mathrm{c}\mathrm{h}}=E\cdot G_{\mathrm{F}}/f_{\mathrm{t}}^2.

3 Tests and results

3.1 Mechanical tests

3.1.1 Compressive strength

Compressive strength tests were conducted on cube samples containing mineral additives measuring 100 mm × 100 mm × 100 mm. Cubic samples measuring 70 mm × 70 mm × 70 mm were prepared from the mixtures containing fiber; subsequently, tests were performed on these samples. All the compressive strength tests were performed in accordance with the EN 12390-3 standard [60].

3.1.2 Flexural strength

The bending strength tests were conducted on beam samples of dimensions 100 mm × 100 mm × 500 mm and 70 mm × 70 mm × 280 mm prepared from concrete mixtures containing mineral additives and fibers, respectively. All flexural strength tests were performed in accordance with RILEM (TC-50FCM) [58].

3.1.3 Splitting tensile strength

The residual from the beams prepared from the concrete mixtures containing mineral additives and fiber were used for splitting tensile tests. The cross-sections of the samples from the former and latter measured 100 mm × 100 mm and 70 mm × 70 mm, respectively. All splitting tensile strength tests were conducted in accordance with EN 12390-6 [61].

3.2 Permeability tests

3.2.1 Water absorption and capillary water absorption

For the mixtures containing mineral additives, capillary water absorption and water absorption tests were conducted on the test samples remaining from the flexural tests. However, only capillary water absorption tests were performed on the concrete containing fibers, and the tests were performed on the beam samples measuring 70 mm × 70 mm × 280 mm. The capillary water absorption and water absorption tests were performed in accordance with ASTM C1585-2020 [62] and EN 12350-6, respectively. The capillary water absorption results were obtained as the weight of water absorbed after 6 h. Images of the permeability tests are shown in Fig.6.

3.2.2 Rapid chloride ion permeability

Chloride permeability tests were performed on Ø100 mm and h50 mm disc samples in the concrete mixtures containing the mineral admixtures. The tests were conducted in accordance with the ASTM C1202-2005 standard [63], and the results were evaluated based on the permeability classes provided in the standard. The test was based on determining the electrical current passing through the samples after 360 min.

3.3 Test results of mineral additive three-dimensional printable concretes

The test results of the mineral additive 3D printable concrete are presented in Tab.7. Based on the results, the values for the mechanical properties of all the mineral-added concrete mixtures were determined to be equal to or higher than those of the plain concrete mixtures. Additionally, the chloride ion permeability, capillary water absorption, and water absorption values of the printable concrete decreased significantly owing to the incorporation of mineral additives. The rapid chloride ion permeability and capillary water absorption of conventional concrete decrease significantly by the use of mineral additives [64]. In this study, the incorporation of mineral additives resulted in a decrease in the capillary water absorption and rapid chloride ion permeability by 31%–66% and 80%–95%, respectively, compared with those of the reference mixture without mineral additives. This indicates that mineral additives effectively reduces the permeability of 3D printable concretes to a level similar to that of conventional concretes.

3.4 Mechanical test results of fiber-reinforced three-dimensional printable concrete

The test results for the mechanical behaviors are listed in Tab.8. As shown, the splitting tensile strength, bending strength, and fracture energy of steel and polyamide fiber-reinforced concrete are higher than those of plain concrete. In addition, the mechanical strength of the mixtures containing steel fibers is significantly higher than that of the mixtures containing polyamide fibers. Flexural test images of the printed samples are shown in Fig.7. The load–displacement curves of the tested concrete series are shown in Fig.8.

The setup for the capillary test was the same as those for the flexural tests. Beam samples measuring 70 mm × 70 mm × 280 mm (as shown in Fig.5) were subjected to capillary water absorption tests, where they were in contact with water via their lower surfaces. The maximum capillarity value was indicated in the x-direction, where the contact area between the interlayer surface with water was the largest.

4 Discussion

Based on this study, it is determined that more environmentally friendly 3D printable concretes with low permeability properties can be synthesized using various mineral additives instead of cement. Printable concretes synthesized using mineral additives exhibit the desired rheological properties (workability, pumpability, open time, and buildability) in the fresh state and provide desirable concrete properties in the hardened state. In this study, the mechanical behavior and permeability properties of plain concrete without mineral additives and concrete formed by adding steel and polyamide fibers were investigated in different directions. With the addition of fiber, all the mechanical properties of the concretes improved (e.g., the compressive strength, splitting tensile strength, and bending strength). The most significant improvement was observed in the fracture energy and ductility of the steel fiber-reinforced 3D printed concrete. The increase in mechanical properties with the addition of steel fiber at the same rate by volume was greater than that with the addition of polyamide fiber.

F represents the applied load; X, Y, and Z represent the directions of the load; C represents the casted samples; and STF, PA, and REF represent the concrete containing steel fibers, polyamide fibers, and non-fibers, respectively. The flexural strengths of both printed plain concrete and fiber-added printable concrete in all printing directions were compared with those of the casted samples. The ratios F_REF-X/F_REF-C, F_REF-Y/F_REF-C, and F_REF-Z/F_REF-C for the plain concrete were 0.85, 0.90, and 0.97, respectively. The ratios F_PA-X/F_PA-C, F_PA-Y/F_PA-C, and F_PA-Z/F_PA-C for the polyamide fiber printable concrete were 0.90, 1.08, and 1.09, respectively. The ratios F_STF-X/F_STF-C, F_STF-Y/F_STF-C, and F_STF-Z/F_STF-C for the steel fiber printable concretes were 0.97, 1.01, and 1.10, respectively.

The splitting tensile strengths of both the printed plain concrete and fiber-added printable concrete in all printing directions were compared with those of the casted samples. The ratios F_REF-X/F_REF-C, F_REF-Y/F_REF-C, and F_REF-Z/F_REF-C for the plain concrete were 0.81, 1.03, and 1.04, respectively. The ratios F_PA-X/F_PA-C, F_PA-Y/F_PA-C, and F_PA-Z/F_PA-C for the polyamide fiber printable concrete were 0.84, 1.02, and 1.01, respectively. The ratios F_STF-X/F_STF-C, F_STF-Y /F_STF-C, and F_STF-Z/F_STF-C for the steel fiber printable concretes were 0.82, 0.99, and 0.94, respectively.

Next, the compressive strengths of both the printed plain concrete and printable concrete with added fiber in all printing directions were compared with those of the casted samples. The ratios F_REF-X/F_REF-C, F_REF-Y/F_REF-C, and F_REF-Z/F_REF-C for the plain concrete were 0.94, 1.05, and 0.96, respectively. The ratios F_PA-X/F_PA-C, F_PA-Y/F_PA-C, and F_PA-Z/F_PA-C for the polyamide fiber printable concrete were 0.93, 0.94, and 0.95, respectively. The ratios F_STF-X/F_STF-C, F_STF-Y/F_STF-C, and F_STF-Z/F_STF-C for the steel fiber printable concretes were 0.92, 0.94, and 0.99, respectively.

Subsequently, the capillary water absorption of both the printed plain concrete and fiber-added printable concrete in all printing directions was compared with those of the casted samples. The ratios k_REF-X/k_REF-C, k_REF-Y/k_REF-C, and k_REF-Z/k_REF-C for the plain concrete were 1.20, 1.15, and 1.06, respectively. The ratios k_PA-X/k_PA-C, k_PA-Y/k_PA-C, and k_PA-Z/k_PA-C for the polyamide fiber printable concrete were 1.17, 1.13, and 1.04, respectively. The ratios k_STF-X/k_STF-C, k_STF-Y/k_STF-C, and k_STF-Z/k_STF-C for the steel fiber printable concretes were 1.20, 1.12, and 1.08, respectively.

The maximum difference in all the mechanical strengths between the printed and casted samples was observed when the load was applied in the x-direction. When the load was applied parallel to the x-axis, the interlayer bonding was the most affected. Similarly, the capillary water absorption values were the highest in the x-direction, where the water established the highest amount of contact between the layers. The results indicate that the weakest region for 3D concrete is the interface between the layers.

5 Conclusions

The results of the study can be summarized as follows.

1) Concrete mixes prepare using GGBS, SF, and MK mineral admixtures instead of 25% cement by weight exhibited the rheological properties required for 3D printable concrete. The buildability of printable concrete was sufficient for all the mixtures containing mineral admixtures. All mixes were evaluated based on the printing parameters and hardened concrete properties, and the results showed that the mixture using GGBS exhibited better rheological properties, such as pumpability and workability. However, the mixtures using SF and MK showed better mechanical behavior and permeability properties. When only the permeability properties were examined, the mixture with MK indicated the best results, i.e., a rapid chloride ion permeability of 170 C and capillary water absorption value of 0.053 mm. Considering the rheological behavior and hardened concrete properties of 3D printable concrete, a concrete mixture using GGBS may be more appropriate for 3D printable concrete owing to its longer open time, higher pumpability, sufficient mechanical strength, and low permeability properties.

2) Fiber-reinforced printable concrete exhibited a significantly higher fracture energy and ductility than plain concrete in all printing directions. The maximum fracture energy of the printable plain concrete in all directions was 77 N/m, and the maximum characteristic length was 109 mm. Meanwhile, the minimum values of these parameters for the printable concrete using steel fibers were 1324 N/m and 1143 mm, respectively. In the printable concrete using polyamide fibers, the minimum values of these parameters were 950 N/m and 1130 mm, respectively. Thus, fiber addition significantly increased the fracture energy and ductility of printable concrete. Furthermore, it significantly increased the compressive, splitting tensile, and flexural strengths of the printable concrete in all directions. The increase in mechanical properties afforded using steel fibers was greater than that using polyamide fibers.

3) The mechanical properties and capillary water absorption values of the 3D printable concrete were evaluated simultaneously in all printing directions. The results showed that the mechanical strength and capillary water absorption values were similar to those of the casted samples in the y- and z-direction. The lowest mechanical strength was obtained in the x-direction, where the bending and splitting loads were applied along a straight line between the layers. This shows that the most critical region of 3D printable concrete is the interface between the layers. The lowest flexural strength values in all printed samples were obtained when the load was applied as a straight line between the layers. However, in this direction, the flexural strengths and capillary water absorption values showed a maximum deviation of 15% and 20% from those of the casted samples, respectively. This suggests that high-performance printable concrete can be obtained without additional processes that can increase the bonding between layers, such as changing the printing gap times between layers, using interlayer bonding agents, and changing the printing speed or path.

4) In this study, the lowest strengths were obtained at the contact surfaces between the layers. Therefore, the most critical region for 3D printable concretes is the interfaces between layers. Compared with the casted samples, the printed samples exhibited a decrease in flexural strength by up to 15%, particularly in the x-direction, where weak bonding existed between the layers. The maximum strength difference between the printed and casted samples was observed in the splitting tensile strength values. The load was applied parallel to the x-axis, and the splitting tensile strength difference was determined to be 19%. Similarly, in the compressive strength tests, the maximum decrease was realized when the load was applied parallel to the x-axis. However, the difference in compressive strength was much less than the change in the bending and splitting tensile strengths, with a maximum difference of 8%.

In this study, the width of the layers was set to 9 cm such that beam samples of dimensions 70 mm × 70 mm × 280 mm could be prepared from the printed single-row concrete layers. This layer width can be regarded as large. When the layer width is 4–8 cm, the strength losses in the printed samples may be higher than those in the casted samples owing to the reduced contact area. The anisotropic behavior of 3D printable concrete should be reevaluated if the layer width is reduced.

The total binder for the 3D printable concrete was determined to be 800 kg/m³ in this study, where 25% of this amount by weight was replaced with mineral additives, which resulted in high-strength 3D printable concretes. Whether normal-strength 3D printable concrete with a total binder amount of approximately 600 kg/m³ can be obtained using mineral additives should be investigated in future studies.

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