Keywords

1 Introduction

In recent years, 3D printing in the construction industry has generated a considerable interest, both in academic and industry, owing to significant benefits in terms of higher quality and productivity, faster construction processes, higher geometrical freedom, and cost-efficiency. Concrete 3D printing, as an essential part of digital construction, has been successfully applied in infrastructure due to its flexibility, speed, and economy, which can significantly save the material and time costs of engineering projects. Due to the principle of additive manufacturing strategy, this concrete extrusion technique creates great opportunities for designing freeform geometries for surface decoration since this material has a promising performance of high compressive strength, low deformation, and excellent durability.

In the era of mass customization, the manufacturing strategies for prefabricated free form architecture elements become popular and urgent. In the process of traditional complex concrete elements, to achieve high precision, it is necessary to make temporary timber formworks before construction. This traditional formwork installation system and structure reinforcement strategies have advantages for labor costs but often cause crucial material waste.

The erection of molds and the placement of reinforcement still requires physically demanding labor, when bespoke geometries are particularly needed. This results in personal health issues of construction workers that should be avoided as much as possible, particularly with an aging work force as in many developed countries [1]. The method of applying permanent 3D printing concrete formwork to build reinforced concrete prefabricated components, combining 3D printing with traditional construction techniques is a new field needed to be explored. In this paper, we investigate the possibility of introducing this technology in the real engineering project. Taking the geometry deformation, surface crack performance, curing time, usage of steel bars, printing time, weight, and accessibility of installation into consideration, two group of contrast experiments, using different types of steel rebars and fiber additive agent are represented for comprehensive results and analysis.

2 Background

2.1 Transition to Robotic Fabrication in the Digital Era

The construction industry is the pillar industry of the economy in China. The proportion of value reached 7.01%. As a labor-intensive industry, labor cost is one of the essential costs in the construction industry. With the continuous intensification of labor shortages, backward production technology, waste of production resources, and soft skills of workers in the industry in recent years, robotic fabrication as the core has become an important opportunity for upgrading the construction industry.

As one of the core parts of robotic fabrication, building robotics is a digital design and construction tool with multi-degree of freedom, high precision, and high efficiency. It surpasses the processing limitation of traditional technology and can more efficiently complete the customized processing production of large quantities of building components. At the same time, the robot can break through the limitations of arm span and load-bearing and enhance the realization of the limits that cannot be reached by manual operation, to complete large or complex multi-scale and multi-function construction tasks.

2.2 3D Printing Concrete as a New Normal

Due to the economy of concrete materials and its excellent structural performance, concrete has become the most significant amount of construction material today. 3D printing technology brings freedom of form and automated process to concrete construction that is unavailable in traditional construction modes. In the era of mass customized building components, the construction technology of 3D printing concrete has a great application prospect. However, due to the weak layer bonding, it is difficult to transmit shear and tensile forces, and can only be used for purely compressed structures [2]. The function of 3D printing concrete is still limited and restricted by structure performance. In fact, 3D printing permanent formwork technology is promising for promoting the application of 3DCP in the field of structural engineering because the reinforcement cage can be designed and fabricated according to existing design specifications [3].

2.3 Previous Attempts of Reinforcement Strategies in 3D Printing Concrete

Many previous attempts have been made to enhance the structural performance of 3D printing concrete. Due to lack of interlayer bonding and printing defects, the performance of 3D printed concrete is slightly lower than the mold cast concrete [5]. It is suggested that adding 13 mm PVA Fiber and combining it with primary concrete material is a practical way to avoid surface cracks and modify the bending strength of printed elements [4]. In this paper, we also use a fiber-reinforced high-strength cementitious mortar as printed material, and the large components of the mortar are shown in the following table. However, only adding fiber is not enough to ensure the quality, especially when large components are required in mass customization projects. Thus, it is still necessary to start a further investigation for optimal reinforcement placement, configuration, and reinforcement material (Fig. 1 and Table 1).

Fig. 1.
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Material and mixture design

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Table 1. Relationship between time cost and reinforcement strategies
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In fact, the existing reinforced solutions have many disadvantages and can hardly be used in the project. For instance, Bos uses 6 mm steel fiber to prepare 3D-printed concrete. The addition of fiber can significantly improve the flexural strength of concrete but cannot enhance the strain-softening behavior of concrete material. ETH Zurich has proposed a 3D printed concrete material reinforced with steel mesh, using a robotic arm system to build steel mesh and formwork simultaneously. This method can effectively improve the mechanical properties of 3D-printed concrete structures, but it is difficult to match the concrete printing process due to the local high temperature generated during the steel welding process. The toughening method of 3D printed reinforcement materials is a method that uses 3D printing technology to print auxiliary materials and toughen concrete. It can significantly modify the shear and bending strength, but the time cost is massive, and it can barely work with concrete printing at the same time. In this way, only using robotic fabrication in construction is not a mature technology. In the current situation integrating necessary labor production and prefabricated reinforced steel bars is vital in engineering projects [5].

3 Methodology

We developed a new control group experiment program according to the existing research. Since the structural behavior and bond durability of the 3D printing concrete formwork are significant, this experiment aims to achieve a permanent use of external concrete printing formwork.

Firstly, the structural strength, including the compressive, shear strength, and the bonding strength between the printed formwork and the internal casting material, can be obtained by changing the fiber proportion in the gap between different layers of formwork. Secondly, by changing the shape of the steel reinforcement bar during the robotic fabrication, the effect of varying placement of reinforcement on the strength of the printed piece can be obtained. The experimental results have a decisive impact on the subsequent large-scale printing experiment.

The permanent concrete printed outer formwork and its own structural strength determines whether it can be used in specific engineering conditions. Due to the technical characteristics of 3D printing concrete formwork technology, the printing path design in the fabricating process is looped laminated printing. The internal cavity is bound to be generated, while the strength of the printed formwork produced by such a cavity printing mode is weaker than the printed mode without a cavity. Therefore, in the experiment, it is hoped to obtain the best material proportion that can improve the strength of the printed formwork by casting the cavity with different material proportions. At the same time, in the experiment, the internal casting material with different material ratios will also affect the bond strength between the casting concrete and the printed concrete. Therefore, the following control group was set up in the experimental process. The result was to explore the influence of different fiber adding ratios of casting concrete on the strength of the printing formworks under the condition of the same printing speed and material extrusion width.

Group A test plan:

Test the concrete structure strength of different material ratio results.

Group B test plan:

Test the bond strength of concrete with different material ratio.

Group C test plan:

Use different (L-shaped and C-shaped) steel bars to reinforce the externa formwork of 3D-printed concrete.

By combining the above three test plans, the experiment produced ten blocks of 3D printing concrete. (its size is 1000 * 500 * 100 mm) As shown in the figure, the processing of L-shaped steel bars is placed outwards with sharp corners. Each layer is set with 5 bars and 20 layers apart. In this way, the steel bars effectively reinforce the structure between the printing layers of concrete, and the gap formed can be used as an external formwork to add structural steel bars. However, the measurement results show that this method of placement is less efficient.

The sequence of steel bars in type C is staggered between 2 layers. Six steel bars are placed in the two adjacent layers and placed at an interval of 20 layers. In the end, the geometry deformation, surface crack performance, curing time, usage of steel bars, printing time, weight, and accessibility of installation (Figs. 2 and 3).

Fig. 2
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L-shaped steel bars

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Fig. 3.
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C-shaped steel bars

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4 Integrated Human and Robotic Fabrication

4.1 FU Robot as a Platform for 3D Printing Process

The digital design software of the toolpath generation and the stop point control are built in the Rhino/Grasshopper environment. After curvature and contour analysis of printing components, the toolpath is generated. Previous experience suggested that the foundation printing speed should be set to 0.1 m/s while the body part should be set around 0.2 m/s so that the extrusion width could be controlled at approximately 30 mm. Before traditional printing, the experiment is needed to check whether the toolpath has any problem, avoiding failure in the later printing process. The grasshopper parametric design program and complete digital analysis and design workflow are shown in Fig. 4.

Fig. 4.
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Grasshopper and FU robot platform

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4.2 Additive Concrete Formwork Manufacturing Process

In the construction of architectural concrete, concrete formwork and mold have played a critical role because of the construction characteristics of concrete. Concrete formwork and mold refer to a complete set of the structural system of formwork, mold, and supporting mold plate formed by pouring concrete. The process of formwork directly affects the final quality of concrete. In this concrete formwork manufacturing process, we first generate three layers of extrusions, serving as the foundation for a self-horizontal plane followed by a stopping point. Then, since each experimental material is one meter high and we have two different steel rebar types, one of the experimental groups has five more stop points. In contrast, the other group has ten more stop points averagely allocated in each layer for reinforced steel rebars. After setting the endpoint location of the whole printing file, the script is then generated by the entire environment and platforms so that the program can be copied into the KUKA demonstrator to start the fabrication process. When the printing process is finished, curing A specific flowchart of this design to the construction process is shown in Fig. 5.

Fig. 5.
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Digital analysis and design workflow

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4.3 Additive Concrete Formwork Manufacturing Process

Besides the robotic fabrication process, labor also costs a key role in determining the appearance and quality of printed concrete. In the entire process, mixing concrete with PVA fiber, placing reinforced steel rebars, and casting formworks with different mixed materials occupy most labor costs. Compared with traditional construction methods, it has enormous advantages, such as cost, safety, and construction speed. Moreover, when sustainable development is increasingly ethically significant, this approach can make construction operations environmentally friendly.

5 Result and Discussion

5.1 Time and Material Cost

The total processing time of concrete 3d printing outer formwork can be estimated using Eqs. (1–3) shown below. The total 3D-printing time depends on the volume of the concrete formwork, the height of the concrete printed layers, the horizontal cross-sectional area, the speed of the motor, the time it takes to manually place the steel rebar and the time of initial curing of printed concrete. The total time shown here is an estimate, however, the experimental time in real environment is often different, because the environmental factors (like temperature and humidity) also affect the time of 3D printing. However, the experimental time in the natural environment is often different because environmental factors (like temperature and humidity) also affect the time of 3D printing [6]

$$ T_{total} = \frac{{V_{Con} }}{{V_{t} \times N}}t_{1} + t_{2} $$
(1)
$$ V_{t} = S_{{con}} \times H_{{con}} $$
(2)
$$ t_{2} = t_{3} + t_{4} $$
(3)

Ttotal–the estimated total time for 3D printing process, \(V_{Con}\)–the total volume of the design Brep, \(V_{t}\)–Concrete extrusion volume per unit time, \(N\)–the speed of the motor, \(S_{con}\)–The sectional area of the concrete printing formwork, \(H_{con}\)–the height of one-layer printed concrete, \(t_{3}\)–the estimated time for curing time of 3D printed concrete, \(t_{4}\)–the estimated time for manually place the steel rebar.

5.2 Fabrication Deviation

The current study shows that the proposed integrated reinforced methods have different structure behaviors and surface appearances. To conclude, the larger the components are, the more deformation and cracks will exist. Type C reinforced rebars have better deformation performance while it costs more labor in the printing process, optimizing bending strength significantly. However, compared with the Type L rebars, it can result in more flexural cracks (Figs. 6, 7 and Table 2).

Fig. 6.
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Flowchart of design to construction process

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Fig. 7.
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Printing process

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Table 2. Surface and deformation performance
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5.3 Different Types of Additive Agent

From the perspective of the additive agent, in the following table, it is essential to note that the time cost of preparing steel fiber takes twice as long as PVA fiber since we use the same proportion of PVA fiber in the casting part and printing part. Specifically, the steel fiber is much harder and stronger than PVA fiber so it is not allowed to be blended in the pump, which means another blender is required before printing. Furthermore, steel fiber will add additional weight in the formwork itself, occupying two percent of the volume fraction of casting concrete. The casting process and finished prototypes are shown in Figs. 8, 9 and Table 3.

Fig. 8.
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Casting process

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Fig. 9.
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Finished surface

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Table 3. Different additive agent and labor cost
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6 Conclusion and Future Work

In this paper, we first compared different reinforcement strategies for 3D concrete printing and then proposed a new method that significantly improves the bending strength, toughness, and flexibility of permanent 3D concrete printing formworks. Integrating prefabricated and standardized rebar 3D printed concrete elements not only opens new function possibilities but also enhances the reliability of robotic fabrication. In addition, this system can be adopted for the construction of mass customization elements, where traditional concrete fabrication could cause more carbon footprint through the process of fabricating timber formworks and casting concrete. It is a potential approach that efficiently achieves complex 3D geometries through parametric design tools, promoting the specification related to 3D printing architecture integrity. However, more scientific experiments like four-point bending tests and toughness and ductility tests still need to be done to get specific bending strength and flexural characteristics of 3D printing concrete. Additionally, we can get more promising results and possibilities in engineering projects with further modifications to the printing environment referring to temperature and humidity, more types of addition agents in the formworks, and more rational curing methods. Thus, more non-uniform geometries of large, prefabricated formwork components could be achieved.