Keywords

1 Research Background

In the past 30 years, China's labor-intensive industries such as manufacturing and construction have made great progress due to the large labor supply. However, with China's aging population becoming more and more serious, China's labor-intensive industries began to appear insufficient labor supply phenomenon. The aging of the effective labor force will become the biggest obstacle to the future development of China's manufacturing and construction industries.

As a new digital construction technology, building 3D printing technology may become one of the solutions to the above problems. Building 3D printing technology integrates computer, numerical control, material forming and other technologies, using the principle of layered superposition of materials, the shape, size and other relevant information of the three-dimensional building model is obtained by the computer, and it is processed to a certain extent, according to a certain direction (usually Z-direction) the model is decomposed into a layer file with a certain thickness, and the numerical control program is generated. Finally, the mechanical device is controlled by the CNC system, and the automatic Construction of the building or structure is realized according to the specified path movement, which is called “Additive Construction”, as shown in Fig. 1 and Fig. 2.

Fig. 1.
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3D printed buildings

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Fig. 2.
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Building 3D printing process

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2 Research Status of Architectural 3D Printing

Researchers at home and abroad carry out research on architectural 3D printing from two different research perspectives. One is to study key issues such as machinery and materials of digital construction technology from an engineering perspective, such as Ding Lieyun from Huazhong University of Science and Technology and Feng Peng from Tsinghua University [1]. The other is to explore the new logic, theory and working mode brought by digital construction technology in architectural design. The business community is inclined to explore the industrial application of 3D printing digital construction technology in architecture.

There are still a lot of technical problems in building 3D printing from the full application of the real industry. First, there is the issue of printing materials. As mentioned above, due to the lack of mechanical properties of materials, the existing building 3D printing is still in the contour printing stage. To date, none of the materials used for 3D construction of buildings (cement-based materials, carbon fiber, nylon fiber, and steel fiber) can achieve the ductility of steel; Secondly, the structural weakening caused by construction stratification has not been solved.

Strictly speaking, “concrete layered spray and extruding superposition” is only the outer outline printing of the building, as shown in Fig. 3 and Fig. 4, and is not a 3D printing of the building in the true sense.

Fig. 3.
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The order of 3D printing

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Fig. 4.
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3D printed outline of the reinforcement

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2.1 The Printing Material Lacks Sufficient Strength and Ductility

According to the Code for Seismic Design of Buildings GB5011–2010, “The measured value of the total elongation of the steel bar under the maximum tension should not be less than 9%, and the elongation should not be less than 20%”. The excellent tensile ductility of the steel bar, coupled with the correct structural design and construction, the traditional reinforced concrete structure has enough ductility. However, at this stage, metal 3D printing mostly uses “laser sintering”, that is, the technology of sintering powder compact with laser as a heat source. The high cost and energy consumption of this technology make it difficult to apply to practical projects. In contrast, concrete is made of hydraulic gels, sand, stone and industrial wastes (such as fly ash and mineral powder), which is inexpensive and has excellent room temperature plasticity, making it a natural 3D printing material. However, because of its low tensile strength and poor tensile ductility, concrete cannot be used for structural construction alone.

To improve the mechanical properties of concrete, the researchers reinforced the 3D-printed mortar with fibers such as glass fiber, steel fiber, and polyvinyl alcohol (PVA). According to the existing literature, the tensile strength of the most ductile PVA fiber reinforced cement-based composite is about 3 MPa – 7 MPa, and the corresponding tensile strain is 2%– 4%, which is still lower than the level of construction steel. Due to the lack of mechanical properties, ordinary fiber reinforced concrete still can not be used as a single structural material [2]. The lack of mechanical properties of materials is one of the main problems that architectural 3D printing stays at the contour printing level. Therefore, to achieve true 3D printing of buildings, we must first break through the material barrier.

2.2 Mechanical Problems Caused by Layered Printing

  • Architectural 3D printing mostly uses “extrusion hardening molding technology”, which extrudes materials through the nozzle and lays them layer by layer, and the materials between layers are discontinuous. Even if the mechanical properties of the material meet the requirements, the interlayer interface will still be a weak link. Under the action of different forms of loads, the beam, wall and column members printed in layers will be separated or slipped, thus reducing the bearing capacity and stiffness of the members. The material layering causes the mechanical properties of the components to weaken, which is another reason why the 3D printing of buildings is still in the “contour printing” stage at this stage [3].

3 Research on New Materials for Building 3D Printing

At present, the materials commonly used in this field are fiber-reinforced cement-based composite materials and shell mother-of-pearl bionic structures. The following will highlight the application of two materials:

3.1 Research and Application of Fiber-Reinforced Cement-Based Composites

(1) Foreign studies

Over the past 30 years, researchers around the world have tried to apply ECC materials to structural engineering. Billington [4] pointed out that in addition to collapse resistance, the structure using ECC also has high damage resistance, and the residual crack width after damage is small, which can greatly reduce the repair cost. As for the application of reinforced ECC in the field of construction, foreign researchers have conducted a large number of experimental studies, and the test objects are mostly structural members under low cyclic load, including beams, columns, beam-column connecting members [6], filled walls, frames, piers [5], and connecting beams. Damping memberet al. These studies have proved that ECC has good seismic performance and minimal post-earthquake repair cost. It is worth noting that reinforced ECC also exhibits high ductility behavior under shear stress, high energy absorption behavior, stable hysteresis ring under large lateral displacement, and structural integrity. The most important characteristic of ECC is its tensile ductility. Even when the steel bar reaches plastic yield, ECC can still coordinate deformation with the steel bar. In addition, the impact resistance of ECC has also passed the test. Tests by Maalej et al. confirmed that ECC plates subjected to high-speed projectile impact had little damage, good integrity, multi-slit distribution cracking, and strong energy dissipation.

(2) Domestic research

Gao Danying and Zhu Haitang et al. [7] of Zhengzhou University conducted a series of experiments and theoretical studies on steel fiber and hybrid fiber concrete. Zhang Jun et al. from Tsinghua University conducted research on cement-based materials reinforced by steel fiber, polyvinyl alcohol fiber and wood fiber. Zhang Zhigang team of Chongqing University proposed an effective method to reduce shrinkage of high-toughness fiber-reinforced cement-based composites, which solved the difficult problem in this field.

Deng Zongcai et al. [8] from Beijing University of Technology conducted material property tests on a variety of fiber concrete, including cellulose fiber, polypropylene fiber, modified acrylic fiber, alkali-resistant glass fiber, PVA fiber, etc. Bu Liangtao et al. from Hunan University conducted a study on the interface between various fiber reinforced mortar and concrete, proposed a method for on-site detection of fiber reinforced mortar, and studied the application of fiber reinforced mortar in the reinforcement of existing concrete structures.

In recent years, Guo Liping et al. successfully produced ecological and high ductility cement-based composites (ECC) using domestic PVA fibers [9]. The tensile properties of ECC material prepared by domestic PVA fiber reach the world's advanced level [10], but the fiber cost is only 1/5 of the imported fiber from Japan.

3.2 Research Status of Biomimetic Structure of Shell Mother-of-Pearl

The typical characteristic of mother-of-pearl structure is “brick-mud” structure. “Brick” mainly refers to micron-scale wafers such as bioactive ceramics, calcium carbonate, and “mud” mainly refers to proteins. To mimic this “brick-mud” structure, many synthetic two-dimensional structures at the nano–or micron scale are used as “bricks” and polymers as “mud”. Luz et al. have made a comprehensive review of biomimetic materials imitating mother-of-pearl structures. Mother of Pearl is the innermost layer of the shell and provides important strength and toughness to the shell.

The excellent mechanical properties of Mother of Pearl are due to its multi-scale-layered micro–and nano-structure form. The first is the micron-scale “brick-mud” multi-layer aragonite structure, with approximately hexagonal aragonite sheets layered on top of each other, and the interface is bonded by organic matter. Secondly, there are more subtle secondary structures on the surface of the aragonite sheet: nanoscale rough bulges and mineral Bridges. The rough protrusion makes the adjacent layers interlock with each other, forming part of “self-locking”. In addition, there are inorganic mineral Bridges running through the arvinite pieces, forming a unique “brick-bridge-mud” multi-level structure model.

4 New Material Research of UHTCC

Regarding the problem of “printing materials lacking sufficient strength and ductility”, the research team has a preliminary solution. This project is supported by the research team of Associate Professor Zhang Zhigang, School of Civil Engineering, Chongqing University. The lead researcher of this team learned the preparation method of Engineered cementitious composites (ECC, also known as UHTCC) for special fiber concrete during his study abroad. In the following years, the research team changed the original design method to prepare a cement-based composite with higher strength and ductility. The tensile strength of the material is up to 20 MPa, which is equivalent to the compressive strength of ordinary concrete. The highest compressive strength reaches 150 MPa, and the corresponding uniaxial tensile strain reaches more than 8% – 12%, which has the ductility level of conventional steel. The bending test of beams shows that the bearing capacity of unreinforced beams cast with this material is equivalent to that of ordinary reinforced concrete beams with reinforcement ratio of 1.5%. The team then improved the material further. When the compressive strength is not higher than 35 MPa, the material shows the characteristics of compression strengthening, and the corresponding limit uniaxial tensile strain exceeds 6%, becoming a cement-based material that may be strengthened under both tension and pressure. Due to its excellent deformation ability, the material is named Ultra-high ductile cementitious composites, or UHDCC for short. The emergence of this material makes it possible to use 3D printing technology to build buildings without reinforcement.

Preliminary tests have been carried out in this project, and the toughening effect of stratified beams has been preliminatively proved through the trial test. Figure 5 shows the four-point flexural load-displacement curves of the full-cast, 5-story and 10-story beams. The layered UDHCC substrate achieves the crack deflection between layers during loading. However, due to the use of hydrophobic dielectric layer (polyethylene film), the interlayer adhesion is too small, and the interlayer slip occurs prematurely, resulting in a decrease in stiffness and strength. Compared with the whole cast beam, the strength of the layered cast beam has decreased, but the ductility has been significantly improved, and its energy dissipation capacity has increased by more than 1 times. According to the existing test results, after further improvement, it is entirely possible to give consideration to the bending capacity and ductility of the members cast by layers, and the toughening effect is remarkable, which provides a reliable test basis for the smooth development of the project.

Fig. 5.
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Tentative test of layered assembly beams

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5 Conclusion

  1. (1)

    Commonly used mineral fibers (such as glass fiber, carbon fiber or basalt fiber, etc.) or biological fibers can improve the cracking resistance of concrete, but the effect against tensile strength and ductility is limited; Steel fiber can obviously improve the tensile strength of concrete, but in the best case, the tensile ductility is 0.5%-1.0%. The tensile strength of the specially designed polyvinyl alcohol fiber reinforced cement based composite (PVA-ECC) is about 3 MPa – 7 MPa. Its tensile limit strain is about 2% to 4%, which is still lower than the ductility level of construction steel. Due to the lack of mechanical properties, ordinary high performance cement-based composite materials can not be used as structural materials alone. The compressive strength and tensile strength of the materials used by the project team range from 30 MPa to 150 MPa, and from 5 MPa to 20 MPa, with an average tensile strain of 8% and a maximum tensile strain of more than 12%, which is close to the level of construction steel. The emergence of this material makes it possible to 3D print buildings without ribs.

  2. (2)

    According to the existing research results, there are two main methods for artificial synthesis of “brick-mud” structure, one is to self-assemble the structural framework with inorganic nanomaterials, and then fill the polymer into the gap of the nanoframe. Another method is to form the mother-of-pearl structure by self-assembling the polymer with the two-dimensional inorganic assembly unit. However, due to the different construction materials and construction scales, these micro-level methods are difficult to apply in the field of civil engineering. So far, in the field of civil engineering, there are few reports about the results of mother-of-pearl structure bionics. For the research object of this application, it is a very challenging research task to use what materials and how to carry out this super-structure bionics.

  3. (3)

    The research team's novel material layered UDHCC substrate achieves crack deflection between layers. The ductility has been significantly improved, and its energy consumption capacity has been increased by more than 1 times. According to the existing test results, it is possible to give consideration to both flexural capacity and ductility, and the toughening effect is remarkable.