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Abstract： In the process of industrialize the buildings including the old and new buildings， more and more architects try to employ the prefabricated external wall because of its unique feature， structure and materials. This paper systematically analyzes three types of the prefabricated external wall with typical examples from the viewpoints of their features， related construction and the scope of application.
Key words： prefabricated external wall， wall-hanging prefabricated external wall， inlaid prefabricated external wall， structural prefabricated external wall
Prefabricated external wall building is the main direction of building industrialization and it is an effective way to embody the harmonious symbiosis between architecture and environment. In recent years， the technology of production and construction in China becomes more and more mature， which lays a foundation for the development of prefabricated exterior wall architecture.
The paper first reviews the development of prefabricated external wall and then combined with typical architectural examples， according to the differences in their construction method， this paper categorizes three kinds of prefabricated external wall： wall-hanging prefabricated external wall， inlaid prefabricated external wall and structural prefabricated external wall， from which this paper systematically analyses their characteristics and expounds their related construction technology and application scope.
1.Prefabricated External Wall and its Advantages and Restrictions
The prefabricated external wall is a special form of the curtain-like wall. The wall components are prefabricated in the factory and then direct transported to the site. According to design， it aims to complete the connection of the external components and the main structure， or after assembly it is directly regarded as part of the building structure， in order to achieve the function of exterior wall enclosure and image requirements.
Prefabricated exterior wall came into being depending on the technological innovation of social industrialization and it rises around 1960s. It experienced several periods indicating the industrialization period， the post-industrialization period and the digital period. On the way of its development， the prefabricated external wall displays its advantages and restrictions.
So far as its advantages are concerned， the prefabricated external wall is helpful to improve the efficiency of project construction because the prefabricated exterior wall panel adopts factory production and the use of the assembly wall for construction can greatly reduce wet work and manual labor. Secondly it saves resources so as to reduce energy consumption and environmental pollution. China is a resource-deficient country. Clay Brick is the traditional wall material in China， and the production of clay bricks is about 1 billion m3 each year， which causes a great deal of occupation and destruction of farmland. At the same time， the production of clay brick coal will result in sulphur dioxide， carbon dioxide discharged into the air and dust pollution to the environment of serious pollution which is also the problem we have to solve. Thirdly the prefabricated external wall offers flexible layout and easy space modification. The construction of a large bay is facilitated by the frame structure of prefabricated walls， and the wall can be disassembled according to needs， which provides a good flexibility for the layout of the building. Lastly， the prefabricated external wall is helpful to improve the construction level and the quality of the project. Prefabricated units are produced in batches in the factory and then transported to the construction site using large machinery for installation. This method of construction reduces the application of formwork and scaffold and also reduces the danger of construction site. About its restrictions， firstly the form of accessory products is single. The application of prefabricated assembled wallboard has strict requirements in the process of controlling construction and construction technology. the proportion of prefabricated buildings in China’s new buildings has been less than 5% so far （Xinhuanet. 2016）， which has made the advantages of assembly-style buildings difficult to put into use. Secondly， on the basis of large-scale industrialization， industrialized production can greatly improve labor efficiency while saving economic cost. As far as the current situation of China’s industrialization is not high， the assembled wallboard is applied to the renovation project， and the one-off input is generally higher than the traditional building. Lastly， the connection between prefabricated external wall components and structural system is one of the key problems in prefabricated fabricated external wall building. In the adaptive reuse of the project， the use of prefabricated assembly wall panel updates the external wall of the old building.
Thus， as Li. B. （2014） pointed， the research and popularization of new prefabricated fabricated external wall materials can undoubtedly provide a richer choice for China’s prefabricated external wall system. More reseachers should consider how to further develop the diversification of prefabricated fabricated exterior wall.
2.Three kinds of prefabricated external wall
At present， the prefabricated wall which can be used in old building renovation projects are divided into three categories： wall-hanging prefabricated external wall， structural prefabricated external wall and inlaid prefabricated external wall which can be analyzed in the following sections.
2.1The wall-hanging prefabricated external wall
（1）Characteristics
The wall-hanging prefabricated external wall means that the precast component of the outer wall is suspended directly from the main structure through the embedded parts， which satisfies the partition space and other functions of the outer wall. The most common form is prefabricated by concrete material. The precast exterior wallboard is usually precast concrete slab on both sides， and the intermediate interlayer is insulation waterproof material. Another common form is a single layer of the prefabricated external wall panel， this kind is thicker， and the outer wall of the insulation layer is typically internal insulation. The inner side of the outer wall plate is the concave and convex method， which is convenient to connect with the insulation material and the panel. （2）Related construction technology
In terms of the structure of the outer wall hanging board， the connection mode of the hanging plate assembling outer wall and the main structure of the building is that the beam is poured into the “T” shape， using the concave and convex of the outer wall plate and the angle steel member to connect （there is also the method of connecting the embedded parts with the main structure through the embedded parts） the external wall board itself with the waterproof insulation layer （sandwich layer insulation or inner layer insulation）. In order to facilitate the construction， the outer wall is usually reserved for 25 to 35mm construction split seam. In the construction， the elastic caulking material is filled and the insulating layer is made outside the main structure of the split seam to avoid the appearance of “Cold Bridge”. The wall-hanging prefabricated external wall can be divided into its subtypes by the size of the prefabricated hanging board and these subtypes have their own features which can be discussed in Table 1.
2.2The structural prefabricated external wall
（1）Characteristics
The structural prefabricated external wall is more dependent on the development of the structural technology. In the structural prefabricated external wall building， the periphery guard system and the structure system are completely unified. The external wall of structural assembly has changed the relationship between the traditional perimeter guard system and the building structure.
（2）Related construction technology
The selection of materials in the structural assembly exterior is important. On the one hand， with the development of diversified structure technology， architects have been building new methods to arouse the rebirth of traditional materials. On the other hand， the exploration of new materials also provides a wider selection of structural prefabricated exterior walls. In general， there are existing three methods of material selection： a） a unique organization of common materials which means architects reorganiz the ordinary materials and present in a new way. This method can be displayed in the example the New Synagogue is a synagogue in Dresden， Germany.b） conversion of unconventional materials which means architects change the unlisted materials into the materials needed in keeping the history and high technology. This method was used in “Cans cubic building” which uses cans as the material for the structual assembly exterior and it is the building beyond the construction of ordinary building materials . c） Exploration of new structure which means with the revival of prefabricated assembly technology， the architect has explored the new structure and manifestation of structural assembly exterior wall with unprecedented enthusiasm and rich imagination. For example， the material for the Mikimoto Ginza which is a fantastic building designed by Toyo Ito （OPENBUILDINGS. n.d.）. To sum up， the appearance of structural prefabricated external wall makes the prefabricated external wall and the structure system combined， which can realize the construction process through the advanced operation method and the construction way.
2.3The inlaid prefabricated external wall
（1）Characteristics
The structure of the inlaid outer wall is the grid structure of the bar， and the precast outer wall plate is connected directly with the rod and the node through the linking components. In the grid structure， each componen is connected with each other to form a structured system that supports the main structure. The inlaid prefabricatd external wall， according to the space dimension of inlay type， is divided into two subtypes： Two-dimensional grid structure of inlaid assembling exterior wall and three-dimensional grid structure of inlaid assembling exterior wall.
The two-dimensional grid structure has the characteristics of high adaptability and high efficiency， and it is suitable for building periphery structure. The Sainsbury Centre for Visual Arts designed （SCVA. n.d.） by the architect Norman Foster is the typical example of this type. The three-dimensional grid structure has a high degree of stability because it is a steel-member frame and the angle and position of each interface tangent are different. For example， the Beijing Swimming Center Program “Water Cube” adopted this three-dimensional grid structure form.
（2）Related construction technology
Grid structure is generated by computer calculation and the bar and enclosure component specifications are accurate， fully exerting the CAD/CAM technology under the digital technology. The traditional GRC plate， recycled paperboard， ETFE materials and alloy materials are more in line with the contemporary ecological concept of synthetic materials， but also more in line with the mosaic-type assembly of the external wall of lightweight removable features.
To sum up， The wall-hanging prefabricated external wall is developed in the traditional structure form， and realize the integration of heat preservation， decoration and enclosure function. The inlaid prefabricated external wall is a kind of assembled external wall mode which is applied to large space truss structure. Because of its simple connection mode， it can realize the renewal and replacement of the external wallboard. Meanwhile， the material selection of the external wall is more flexible and can be better combined with the concept of ecological architecture. The structural prefabricated external wall is an integration of architectural epidermis and structure system， which realizes the adaptability of the prefabricated assembly exterior wall building to the environment change and the users ‘ demand by means of the unique organization of ordinary materials， the transformation of unconventional materials and the exploration of the new structure Through the study of advantages and restrictions of the prefabricated external wall， we understand the prefabricated external wall needs further development in China. And from the discussions on three types of the prefabricated external wall， we understand their different features and usages， from which we believe in the adaptive reuse of old buildings， the prefabricated external wall can be widely used in China’s construction field because it is the direction of architectural industrialization and is an embodiment of building sustainable development.
References
[1]LI. B. （2014） “The present situation and development of prefabricated building in China”， Chinese science and technology information， No. 07， pp 114-115.
[2]XINHUANET （2016） Executive meeting of the State Council to send “Dongfeng” to the development of prefabricated buildings. [Online] XINHUANET. Available from： http：//news.xinhuanet.com/politics/2016-09/14/c_1119567790.htm [Accessed 09/09/17]
[3]Xiao Yongliang. On the Protection of Unique Modern Architecture in China [J] IRA-international Journal of Management and Social Sciences 2019（15）：137-141
Key words： prefabricated external wall， wall-hanging prefabricated external wall， inlaid prefabricated external wall， structural prefabricated external wall
Prefabricated external wall building is the main direction of building industrialization and it is an effective way to embody the harmonious symbiosis between architecture and environment. In recent years， the technology of production and construction in China becomes more and more mature， which lays a foundation for the development of prefabricated exterior wall architecture.
The paper first reviews the development of prefabricated external wall and then combined with typical architectural examples， according to the differences in their construction method， this paper categorizes three kinds of prefabricated external wall： wall-hanging prefabricated external wall， inlaid prefabricated external wall and structural prefabricated external wall， from which this paper systematically analyses their characteristics and expounds their related construction technology and application scope.
1.Prefabricated External Wall and its Advantages and Restrictions
The prefabricated external wall is a special form of the curtain-like wall. The wall components are prefabricated in the factory and then direct transported to the site. According to design， it aims to complete the connection of the external components and the main structure， or after assembly it is directly regarded as part of the building structure， in order to achieve the function of exterior wall enclosure and image requirements.
Prefabricated exterior wall came into being depending on the technological innovation of social industrialization and it rises around 1960s. It experienced several periods indicating the industrialization period， the post-industrialization period and the digital period. On the way of its development， the prefabricated external wall displays its advantages and restrictions.
So far as its advantages are concerned， the prefabricated external wall is helpful to improve the efficiency of project construction because the prefabricated exterior wall panel adopts factory production and the use of the assembly wall for construction can greatly reduce wet work and manual labor. Secondly it saves resources so as to reduce energy consumption and environmental pollution. China is a resource-deficient country. Clay Brick is the traditional wall material in China， and the production of clay bricks is about 1 billion m3 each year， which causes a great deal of occupation and destruction of farmland. At the same time， the production of clay brick coal will result in sulphur dioxide， carbon dioxide discharged into the air and dust pollution to the environment of serious pollution which is also the problem we have to solve. Thirdly the prefabricated external wall offers flexible layout and easy space modification. The construction of a large bay is facilitated by the frame structure of prefabricated walls， and the wall can be disassembled according to needs， which provides a good flexibility for the layout of the building. Lastly， the prefabricated external wall is helpful to improve the construction level and the quality of the project. Prefabricated units are produced in batches in the factory and then transported to the construction site using large machinery for installation. This method of construction reduces the application of formwork and scaffold and also reduces the danger of construction site. About its restrictions， firstly the form of accessory products is single. The application of prefabricated assembled wallboard has strict requirements in the process of controlling construction and construction technology. the proportion of prefabricated buildings in China’s new buildings has been less than 5% so far （Xinhuanet. 2016）， which has made the advantages of assembly-style buildings difficult to put into use. Secondly， on the basis of large-scale industrialization， industrialized production can greatly improve labor efficiency while saving economic cost. As far as the current situation of China’s industrialization is not high， the assembled wallboard is applied to the renovation project， and the one-off input is generally higher than the traditional building. Lastly， the connection between prefabricated external wall components and structural system is one of the key problems in prefabricated fabricated external wall building. In the adaptive reuse of the project， the use of prefabricated assembly wall panel updates the external wall of the old building.
Thus， as Li. B. （2014） pointed， the research and popularization of new prefabricated fabricated external wall materials can undoubtedly provide a richer choice for China’s prefabricated external wall system. More reseachers should consider how to further develop the diversification of prefabricated fabricated exterior wall.
2.Three kinds of prefabricated external wall
At present， the prefabricated wall which can be used in old building renovation projects are divided into three categories： wall-hanging prefabricated external wall， structural prefabricated external wall and inlaid prefabricated external wall which can be analyzed in the following sections.
2.1The wall-hanging prefabricated external wall
（1）Characteristics
The wall-hanging prefabricated external wall means that the precast component of the outer wall is suspended directly from the main structure through the embedded parts， which satisfies the partition space and other functions of the outer wall. The most common form is prefabricated by concrete material. The precast exterior wallboard is usually precast concrete slab on both sides， and the intermediate interlayer is insulation waterproof material. Another common form is a single layer of the prefabricated external wall panel， this kind is thicker， and the outer wall of the insulation layer is typically internal insulation. The inner side of the outer wall plate is the concave and convex method， which is convenient to connect with the insulation material and the panel. （2）Related construction technology
In terms of the structure of the outer wall hanging board， the connection mode of the hanging plate assembling outer wall and the main structure of the building is that the beam is poured into the “T” shape， using the concave and convex of the outer wall plate and the angle steel member to connect （there is also the method of connecting the embedded parts with the main structure through the embedded parts） the external wall board itself with the waterproof insulation layer （sandwich layer insulation or inner layer insulation）. In order to facilitate the construction， the outer wall is usually reserved for 25 to 35mm construction split seam. In the construction， the elastic caulking material is filled and the insulating layer is made outside the main structure of the split seam to avoid the appearance of “Cold Bridge”. The wall-hanging prefabricated external wall can be divided into its subtypes by the size of the prefabricated hanging board and these subtypes have their own features which can be discussed in Table 1.
2.2The structural prefabricated external wall
（1）Characteristics
The structural prefabricated external wall is more dependent on the development of the structural technology. In the structural prefabricated external wall building， the periphery guard system and the structure system are completely unified. The external wall of structural assembly has changed the relationship between the traditional perimeter guard system and the building structure.
（2）Related construction technology
The selection of materials in the structural assembly exterior is important. On the one hand， with the development of diversified structure technology， architects have been building new methods to arouse the rebirth of traditional materials. On the other hand， the exploration of new materials also provides a wider selection of structural prefabricated exterior walls. In general， there are existing three methods of material selection： a） a unique organization of common materials which means architects reorganiz the ordinary materials and present in a new way. This method can be displayed in the example the New Synagogue is a synagogue in Dresden， Germany.b） conversion of unconventional materials which means architects change the unlisted materials into the materials needed in keeping the history and high technology. This method was used in “Cans cubic building” which uses cans as the material for the structual assembly exterior and it is the building beyond the construction of ordinary building materials . c） Exploration of new structure which means with the revival of prefabricated assembly technology， the architect has explored the new structure and manifestation of structural assembly exterior wall with unprecedented enthusiasm and rich imagination. For example， the material for the Mikimoto Ginza which is a fantastic building designed by Toyo Ito （OPENBUILDINGS. n.d.）. To sum up， the appearance of structural prefabricated external wall makes the prefabricated external wall and the structure system combined， which can realize the construction process through the advanced operation method and the construction way.
2.3The inlaid prefabricated external wall
（1）Characteristics
The structure of the inlaid outer wall is the grid structure of the bar， and the precast outer wall plate is connected directly with the rod and the node through the linking components. In the grid structure， each componen is connected with each other to form a structured system that supports the main structure. The inlaid prefabricatd external wall， according to the space dimension of inlay type， is divided into two subtypes： Two-dimensional grid structure of inlaid assembling exterior wall and three-dimensional grid structure of inlaid assembling exterior wall.
The two-dimensional grid structure has the characteristics of high adaptability and high efficiency， and it is suitable for building periphery structure. The Sainsbury Centre for Visual Arts designed （SCVA. n.d.） by the architect Norman Foster is the typical example of this type. The three-dimensional grid structure has a high degree of stability because it is a steel-member frame and the angle and position of each interface tangent are different. For example， the Beijing Swimming Center Program “Water Cube” adopted this three-dimensional grid structure form.
（2）Related construction technology
Grid structure is generated by computer calculation and the bar and enclosure component specifications are accurate， fully exerting the CAD/CAM technology under the digital technology. The traditional GRC plate， recycled paperboard， ETFE materials and alloy materials are more in line with the contemporary ecological concept of synthetic materials， but also more in line with the mosaic-type assembly of the external wall of lightweight removable features.
To sum up， The wall-hanging prefabricated external wall is developed in the traditional structure form， and realize the integration of heat preservation， decoration and enclosure function. The inlaid prefabricated external wall is a kind of assembled external wall mode which is applied to large space truss structure. Because of its simple connection mode， it can realize the renewal and replacement of the external wallboard. Meanwhile， the material selection of the external wall is more flexible and can be better combined with the concept of ecological architecture. The structural prefabricated external wall is an integration of architectural epidermis and structure system， which realizes the adaptability of the prefabricated assembly exterior wall building to the environment change and the users ‘ demand by means of the unique organization of ordinary materials， the transformation of unconventional materials and the exploration of the new structure Through the study of advantages and restrictions of the prefabricated external wall， we understand the prefabricated external wall needs further development in China. And from the discussions on three types of the prefabricated external wall， we understand their different features and usages， from which we believe in the adaptive reuse of old buildings， the prefabricated external wall can be widely used in China’s construction field because it is the direction of architectural industrialization and is an embodiment of building sustainable development.
References
[1]LI. B. （2014） “The present situation and development of prefabricated building in China”， Chinese science and technology information， No. 07， pp 114-115.
[2]XINHUANET （2016） Executive meeting of the State Council to send “Dongfeng” to the development of prefabricated buildings. [Online] XINHUANET. Available from： http：//news.xinhuanet.com/politics/2016-09/14/c_1119567790.htm [Accessed 09/09/17]
[3]Xiao Yongliang. On the Protection of Unique Modern Architecture in China [J] IRA-international Journal of Management and Social Sciences 2019（15）：137-141