Keywords

1 Introduction

The construction industry, as a pillar industry of the national economy, has provided strong support for the sustainable and healthy development of China’s economy. Since the issuance and implementation of the Guiding Opinions of the General Office of the State Council on Vigorously Developing Prefabricated Building [3] No. 71, the new construction industrialization represented by prefabricated buildings has been rapidly promoted, and the construction level and building quality have been significantly improved.

Prefabricated buildings have come a long way in China, which, however, are limited by the unsatisfactory performance of intelligent equipment, leading to the fact that tying reinforcement in prefabricated component factories still depends on manual work, far from industrialization. On 28 July 2020, the Ministry of Housing and Construction and thirteen other departments jointly issued the “Guidance on Promoting the Coordinated Issuance of Intelligent Construction and Building Industrialization” Jian Shi [8] No. 60. On 28 August 2020, the Ministry of Housing and Construction and nine other departments jointly issued “Several Opinions on Accelerating the Development of New Building Industrialization” Jianbiao Gui [9] No. 8.

Prefabricated component manufacturers need to produce a large number of steel skeletons for wall panels and staircases, but the steel skeletons in three-dimensional form cannot be intelligently tied and have to rely on manual work [1, 5, 7]. This accounts for low production efficiency and hence affects the transformation and upgrading of the construction industry [17].

At present, the main domestic prefabricated component factories, such as Beijing Building Industrialization Group Corp., Ltd., Beijing Yugou Group Co., Ltd., Changsha Broad Homes Industrial Group Co., Ltd., Sany Construction Technology Co., Ltd., China Construction Science & Technology Group Co., Ltd., Beijing Everest Green Building Technology Co., Ltd., My Home Real Estate Group and the like, still rely on manual tying for reinforcement skeletons of shear walls, which is not automated [2]. Among them, Beijing Yantong Architecture Component Co. Ltd. has demonstrated the process of tying steel skeletons with a mechanical arm, but the equipment lacks the functions like horizontal and vertical positioning of reinforcement, installment and welding of tension bars, with problems such as high equipment cost and low work efficiency, which can only be used for demonstration instead of actual production [19].

Domestic rebar line manufacturers including Sany Construction Technology Co., Ltd. and TJK Machinery (Tianjin) Co, Ltd can produce automatic equipment for straightening and bending reinforcement as well as positioning and tying of two-dimensional meshes [12]. However, there is a lack of positioning and tying equipment related to 3D rebar skeletons [13, 16].

There are already departments abroad doing related research and development, but no mature products have been developed [15]. Founded in 2016, US-based Toggle Construction is a manufacturer of industrial automation equipment that develops and introduces industrial robotics and automation into the construction sector to create prefabricated steel skeletons for columns, beams, walls and slabs. According to the information, Toggle’s R&D technology focuses on lifting, positioning, strapping and welding, without considering rebar feeding, arrangements of tension bars, and the relationship between the rebar skeletons and the molds [6, 11, 14].

2 Number and Volume Ratio of Three Types of Shear Walls

Precast shear walls can be divided into opening-less shear walls, single-opening shear walls and double-opening shear walls according to the situation of openings [10]. According to the study of 18 projects concerning prefab shear walls, a resettlement housing project in Daxing, Beijing, has a total of 611 prefabricated shear walls with a volume of 923 1m3, of which the number of shear walls without openings accounts for 53.19%, with its volume representing 50.26%; the number of single-opening shear walls accounts for 34.04%, with its volume representing 50.26%; the number of double-opening shear walls accounts for 12.77%, with its square volume representing 13.49%, as detailed in Fig. 1.

Fig. 1
A grouped column chart of percentage versus the type of sheer wall plots the data in decreasing trend. The bars indicate volume and number with the highest values at 53.19% and 50.26% for no openings.

Resettlement housing project in Daxing district, Beijing

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A residential project in Shunyi District, Beijing, has a total of 1164 precast shear walls and a volume of 821.0 m3, of which the number of shear walls without openings accounts for 52.75%, with its volume of 49.63%; the number of shear walls with single openings accounts for 47.25%, with its volume of 50.37%, and there are no shear walls with double openings; see Fig. 2 for details.

Fig. 2
A grouped column chart of percentage versus the type of sheer wall plots the data in decreasing and fluctuating trends for volume and number. The highest values are 52.75% and 49.63% for no openings.

Residential project in Shunyi district, Beijing

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A residential project, 1#2#building, in Fengtai District, Beijing, has a total of 760 blocks of precast shear walls and a volume of 956.2 m3, of which the number of the shear wall without opening accounts for 100.0%, with its volume of 100.0%, and there are no single-opening shear wall and double-opening shear wall; see Fig. 3 for details.

Fig. 3
A grouped column chart plots the percentage versus the type of sheer wall. The bars indicate the number and volume at 100% each for no openings and 0% for single and double openings.

Residential project (#1#2# Building) in Fengtai District, Beijing

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A total of 19 precast shear wall projects have been counted, and the quantity percentages of shear walls without openings are distributed between 32–100%, depending on specific projects, and the percentages of volumes are between 23.69–100%, mainly concentrated in 50–70%. See Fig. 4.

Fig. 4
2 dot plots plot graphs with 19 plots each. They plot percentage versus frequency with plot values ranging between 32% and 100%, and 23.69% and 100%.

Number of shear walls without openings and percentage of volume

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3 External Profile Dimensions of Precast Shear Walls

A total of 19 precast shear wall projects and 9922 shear walls without openings have been studied, with a discovery of a height distribution between 2.5–2.95 m and a width distribution between 0.8–4.0 m, as shown in Fig. 5.

Fig. 5
2 dot plots the distribution of heights and widths for sheer walls without opening. The plots concentrate between 2.5 and 2.95 meters and 0.8 and 4 meters, respectively.

Distribution of heights and widths of shear walls without openings

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A total of 19 precast shear wall projects and 5050 single-opening shear walls have been studied, with a discovery of a height distribution between 2.58–2.95 m and a width distribution between 1.59–5.0 m, as shown in Fig. 6.

Fig. 6
2 dot plots the distribution of heights and widths for single-opening sheer walls. The plots concentrate between 2.58 and 2.95 meters and 1.59 and 5 meters, respectively.

Distribution of heights and widths of single-opening shear walls

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A total of 19 precast shear wall projects and 994 single-opening shear walls have been studied, with a discovery of a height distribution between 2.61–2.95 m and a width distribution between 3–5.9 m, as shown in Fig. 7.

Fig. 7
2 dot plots the distribution of heights and widths for double-opening sheer walls. The plots concentrate between 2.61 and 2.95 meters and 3 and 5.9 meters, respectively.

Distribution of heights and widths of double-opening shear walls

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A total of 19 projects and 16,036 walls have been studied, with the results shown in Table 1.

Table 1 Summary table of shear wall heights and widths
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The equipment of steel skeletons for precast shear walls should be able to produce wall panels with a height of 2.5–2.95 m and a width of 0.8–5.9 m.

4 Types of Prefabricated Reinforcement

The connection method for common precast shear walls is sleeve joint, whose horizontal distribution reinforcement consists of 2 types: closed stirrups and open stirrups [4, 18]. The closed stirrups can guarantee the thickness of the reinforcement skeleton in the production process, but the vertical reinforcement needs to be put from the top to the bottom during construction, which is a large workload on site, so the open stirrups are in high demand; see Fig. 8.

Fig. 8
A set of 2 illustrations of the closed and open stirrups in horizontal and vertical reinforcements are presented inside a circular frame.

Illustration of closed and open stirrups

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There are 9 types of precast shear wall reinforcement for closed stirrups and 8 types of reinforcement for open stirrups; see Fig. 9.

Fig. 9
A set of 2 tables comprise 7 columns each with 9 and 8 rows. The column headers are number, type, diameter, shape, length, spacing, and quantity.

Reinforcement types of closed and open stirrups

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According to the study, the following results could be obtained:

  1. (1)

    There are 6–9 reinforcement types for shear walls without openings and 25 reinforcement types for shear walls with openings.

  2. (2)

    Common reinforcement diameters are 6–25 mm.

  3. (3)

    Rebar spacing is diversified, including 50, 75, 117, 140, 150, 170, 200, and 300 mm.

5 Thickness of Steel Skeletons

The thickness of a common load-bearing shear wall is 200 mm, with the minimum thickness of the concrete protection layer being 15–20 mm and the outer diameter of the grout sleeve being 32–50 mm, and hence the thickness of the steel skeleton is 150–170 mm; see Fig. 10.

Fig. 10
An illustration of the closed and open stirrups reinforcement type with values 24, 152, and 24 labeled on it.

Layout and thickness of reinforcement frame

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6 Production Methods

A steel skeleton has to be threaded into an opening groove in the edge form, and the edge form of the shear wall weighs about 500–800 kg. If the steel skeleton is produced with the edge form, it is necessary to address the following problems:

  1. (1)

    Penetration of the reinforcement via visual identification. (b) The possibility of failing to identify exactly and the low efficiency of the work might occur due to the ease of rusting and bonding of the steel edge forms to the concrete and thus the small color difference with the mold table.

  2. (2)

    The deformation of bars is large. According to the study, if the bar is 8 mm in diameter and 3000 mm long, when only 2 ends are clamped, the middle of the bar will sag 150 mm.

  3. (3)

    The length and straightness adjustment of the reinforcement deviated from theories. If the theoretical length of the bar is 3000 mm, whose actual length is 2990–3010 mm after being cut, with an error of 10 mm.

  4. (4)

    Due to the large total weight, the finished steel skeletons are inconvenient to be transported between different production lines, different workshops and even different sites.

7 Conclusions

It has been investigated that if automatic tying equipment for steel skeletons of precast shear walls is to be developed, the following requirements should be satisfied:

  1. (1)

    The focus needs to be on non-opening and single-opening shear walls, while double-opening shear walls can be ignored.

  2. (2)

    The equipment should be capable of producing wall panels with a height of 2.5–2.95 m and a width of 0.8–5.9 m.

  3. (3)

    The equipment shall be capable of producing 6–25 reinforcement types, whose diameters shall be ranging from 6 to 25 mm.

  4. (4)

    The equipment shall be capable of producing steel skeletons with a minimum spacing of 5 mm.

  5. (5)

    The equipment shall be capable of producing shear wall skeletons with a thickness of 150–170 mm.

  6. (6)

    When the equipment produces steel skeletons, if producing those with edge forms is facing more problems, they should be produced without edge forms.