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1.Introduction

The climatic characteristics of cold areas will cause a lot of quality and safety problems to the construction in winter and also exert a serious impact on the construction project benefit. At the same time, due to the disappearing demographic dividend and the change of building technologies, the construction industry gradually turns to an intensive and refined mode. In response to the relevant requirements of "low-carbon economy, green building" and the promotion of prefabricated building, this study combines the advantages of prefabricated building and software engineering to meet the serious challenge in the development of the construction industry in cold areas. This paper analyzes the prefabricated buildings in cold areas from four engineering aspects: quality, schedule, cost and safety. Moreover, an improved system of Quality Management and Safety Evaluation for prefabricated buildings in cold areas is developed.

Quality control software is often used to monitor the quality of product processing procedures, and to analyze the statistical process and abnormal quality problems through informatics technology. By analyzing the process, the system will consider the mean value, standard deviation, and process capability index of the cold area characteristics so that conclusions are generated by the system for quality management and safety evaluation.

This research focuses on quality and safety management aspects particularly. Cold area classifications and their characteristics are formulated. The quality assessment multi-criteria approach for the Prefabricated Buildings is proposed, which can be applied both within the quality management system and in integrated management systems. The safety assessment method is proposed based on the developed multi-criteria approach and information analysis method.

2.The Characteristics of Cold Areas

Influenced by the latitude, topography and other conditions, the climate in different regions varies greatly. In the construction of the construction project, the surrounding environment, climatic conditions and geological conditions will greatly affect the progress of the construction, and increase the difficulty of construction. Either the operation of mechanical equipment or the activities of the construction personnel will be influenced. Therefore, for diverse climatic conditions, there are different ways concerning the design and construction of buildings in different places.

2.1.The Temperature Range and Climatic Characteristics of Cold Areas

In China, the cold area refers to the region where the average temperature of the coldest month ranges from -10℃ to 0℃, and the average daily temperature is ≤ 5℃ for 90-145 days. The severe cold area refers to the region where the average temperature of the coldest month is ≤ -10℃, and the number of days with the average daily temperature ≤ 5℃ is ≥ 145 days. In China, Beijing, Tianjin, Hebei, Shandong, Liaoning, Gansu, Xinjiang, and Tibet are all cold areas, while Jilin and Heilongjiang in the northeast are severe cold areas. The winter in cold areas is long and accompanied by snowfall, freezing, cold and dryness. Besides, the construction period of large-scale projects is long and affected by the natural climate, so it is inevitable to carry out winter construction. According to China's Construction Engineering Winter Construction Regulations which writes "if the average daily outdoor temperature is constantly below 5℃ for 5 consecutive days, it will enter winter construction", meaning that applying winter construction technology is necessary [1]. In cold areas, the construction time in winter can last 3-6 months, and the construction technology in winter is difficult and costly.

2.2.The Characteristics of Winter Construction in Cold Areas

The winter construction of cold areas is quite different from the normal construction. Many aspects are influenced by the cold weather.

2.2.1.The Impact on Project Quality

Negative temperature conditions will have a greater impact on project quality, mainly including aspects of concrete strength, masonry mortar strength, steel bar welding quality, and component connection.

Under the harsh climate conditions, the hydration of cement continues to weaken due to the lower temperature, which results in a particularly slow increase in the strength of newly poured concrete. When water freezes, the hydration of concrete stops, which leads to concrete volume expansion and loose structure, resulting in varying degrees of loss of concrete strength. In severe cases, the concrete strength cannot meet engineering requirements. The shrinkage stress caused by the temperature difference makes the concrete more prone to cracks, which affects the strength of the structure to some extent and makes the durability continuously decline. In addition to the low temperature when concrete is poured, the strength of the structure will be reduced. If the temperature drops suddenly, a series of problems will also occur in the newly finished product. Before concrete curing is completed, if the temperature drops, the strength of the product will also be affected, leading to early freezing damage of the concrete [2].

The mortar and the lime paste are susceptible to freezing. Under the condition of negative temperatures, the mortar strength increases slowly and the drying time is prolonged, which will destroy its original quality. Due to the change of water form, the consistency of masonry mortar is also affected, leading to a decrease in mortar strength.

There is a big difference between negative temperature welding and normal temperature welding. In the test welding, the welding process should be adjusted according to the temperature condition. Flux and electrode are highly susceptible to moisture during the transportation and preservation in winter. The use of wet flux or electrode will cause the gas mixed in the weld pool and stay in the welding bead, causing the pores to affect the quality of the welded joint [3].

The connection of some components is also affected by the ambient temperature. For example, in the connection of high-strength bolts, the torque coefficient is the calibration value at normal temperature; while in winter construction, the torque coefficient should be re-regulated. Otherwise the values given in the manual, the tensile stress generated by the bolt may be insufficient or the bolts are over tightened, which affects the safety of the structure [4].

2.2.2.The Impact on Project Schedule

Negative temperature conditions will greatly influence project schedule.

Winter insulation measures take up the construction period. Due to the special nature of the environment, preparation work should be done before construction, such as preparing the winter construction plan or training the construction personnel, specifying strict winter construction management regulations, determining the requirements of construction environment and temperature during construction, making construction objectives and preventive measures for different parts of equipment, materials and construction, keeping track of daily, weekly and monthly temperature changes, preparing a reservoir to raise the temperature [5].

Strength increases slowly at low temperatures. When the outdoor temperature is lower than 0℃, the duration of concrete curing is relatively long, and only when the strength of concrete reach a certain standard the next process can be carried out. When the concrete reaches its own frost critical strength and suffers low temperature freezing damage, the final standard strength of 28d will not be significantly reduced compared to concrete that has not suffered frost damage. Therefore, the time to reach the critical strength is a key issue and the shortest curing of concrete must be longer than this time. The increase of maintenance time will have a certain impact on the entire construction period [6].

Difficulties occur in coordinating multiple jobs. During the construction, attention should be paid to the coordination among the various professions. The negative temperature conditions will change the coordination between the construction schedule and other majors, which will affect the overall progress of the project.

2.2.3.The Impact on Project Cost

Negative temperature conditions will increase the project cost.

Winter construction measures increase the cost of the project. Be equipped with necessary safety protection equipment before construction such as raw material heating equipment, heat preservation and maintenance facilities, water supply and firefighting pipelines, etc. It is also necessary to equip the construction personnel with relevant heating and warming equipment, which will result in an additional cost.

The material cost is also increased. In order to accelerate the hardening of the concrete, admixtures need to be added to adjust the mix proportion of concrete, and the amount of the thermal insulation covering material according to the characteristics and methods of the construction needs to be determined. All these jobs will greatly increase the cost of materials.

2.2.4.The Impact on Project Safety

Negative temperature conditions will make a greater impact on project safety.

The cold weather causes safety hazards on the work surface. Construction work surface and road are prone to ice and slip, resulting in accidents of machinery and vehicles.

There will be safety hazard at the technical level. Construction personnel wearing thick during the winter construction, which makes the movement insensitive, leading to engineering accidents.

There will be safety hazard in equipment. Construction workers use high-power heating equipment at the construction site, which leads to the occurrence of fire.

The difficulties of site construction in cold areas in winter are mainly caused by the climate. Some difficulties can be prevented and prepared in advance, but there are still many sudden risks that we cannot guard against. Even if the prevention and preparation in advance can solve some problems like schedule, safety, and cost from the perspective of engineering quality, there are still many hidden dangers in winter construction. With the development of China's economy and technology, the production mode of the construction industry has changed from the extensive to the refined and intensive. The application of new construction methods and management mode can better solve the problem of winter construction in cold areas and is more suitable for the development of construction industry in cold areas.

3.EPC Mode Combined with Prefabricated Buildings

In response to the industrialization and refinement requirements of construction industry, prefabricated buildings have been vigorously promoted. The technical problems of prefabricated buildings have been solved step by step, making prefabricated buildings a trend of development in future. In order to promote the development of prefabricated buildings, people should not only make breakthroughs in technology and solve technical difficulties, but also innovate in management and adopt the management mode that best meets the characteristics of prefabricated buildings. The EPC general contracting mode is applied to the management of prefabricated construction projects, and the EPC general contracting management mode made from the perspective of overall project management in combination with BIM information technology. From the perspective of the entire life cycle of the prefabricated building and the entire industrial chain, the effective application of the EPC general contracting mode in prefabricated construction projects will be realized. The advantages of prefabricated building will be fully utilized [7].

3.3.Prefabricated Buildings

3.3.5.The Concept of Prefabricated Buildings

A building assembled from prefabricated parts on construction site is called the prefabricated building. According to the form and construction method of prefabricated components, it can be divided into five types: block building, plate building, box building, skeleton plate building and riser lifting structure [8].

In the design stage, the design of prefabricated building runs through all aspects of the project. It is not only necessary to meet the needs of the owner under the requirements of the basic design specifications, but also required to consider the split and transportation of the components in the production process, as well as problems of lifting and connection during the construction process. BIM technology is used to deepen the design from the design stage. The force situation of the component needs to be analyzed, and the difficulty of component production and the utilization rate of the mold should be considered so as to reasonably split the components. Finally make a budget, plans, detailed drawings of components, connection diagrams of each node, etc. Combined with information technology for visual simulation, the relevant role of each component in the structural space will be considered. Therefore, in the design stage of the prefabricated building, the owner, the constructor, the manufacturer and the designer should make negotiation together.

In the production stage, in order to ensure and improve the quality of the building, the factory needs to produce the components strictly and precisely according to the design requirements, and produce the connection nodes and reserved holes more accurately.

In the procurement stage, the prefabricated building mainly purchases the processed components; in the transportation stage, transportation distance is from the factory to the construction site. As China's prefabricated buildings are in the initial stage, the corresponding PC factories in various places are still not perfect. This results in high transportation costs, high PC costs, and high degree of wear and tear on components during transportation.

In the construction stage, prefabricated building transports the finished components produced on the factory assembly line to the construction site for installation and connection. The vertical and horizontal lifting of the machine and the connection of the nodes should be considered during construction. At present, there are three common joint connections: sleeve grouting connection, slurry-anchor lap connection with spiral stirrups and slurry-anchor lap connection of metal bellows.

3.3.6.The Advantages of Prefabricated Building

The first advantage is speeding up construction. The design runs through the whole process of the prefabricated building, which requires the cooperation of various professionals and multi-teamwork. Multidisciplinary collaborative work helps speed up construction and reduce construction period.

The second advantage is the refinement of the construction process and the limitation by climatic conditions. The components processed at the factory are usually maintained by steam or pool, which improves the precision of the components to some extent. At the same time, since most of the components are processed in the factory, the construction project is less affected by the weather, which reduces the risk to a certain extent and improves the project quality.

The third advantage is the standardization and industrialization of buildings, the improvement of efficiency and the reduction of costs. In order to increase profits while reducing costs and the difficulty of construction, modular design and mass production are required for these components produced on the factory assembly line [9].

The fourth advantage is safety management and civilized construction. Prefabricated buildings shorten the construction time and reduce many uncertainties. The number of ordinary workers required for on-site construction is greatly reduced while the number of skilled workers is increased, which is more convenient for management, speeds up the schedule, and promotes safe production. The factorization of component production also makes the site construction assembly.

The fifth advantage is saving resources, protecting the environment and implementing green buildings. The comparison of dust monitoring between prefabricated construction site and traditional construction site shows that the prefabricated building has more advantages in urban environment and ecological civilization. Prefabricated buildings drastically reduce construction waste on site, and the construction period of prefabricated buildings is short with few workers. Most of the components are produced in factories, which save both natural resources and human resources.

3.3.7.The Problems in Prefabricated Construction Project Management

Most of the current prefabricated buildings adopt the traditional management mode (Design-Bid-Build mode). The prefabricated building in this management mode has the following problems.

The degree of systematic organizational management is low. The traditional management mode makes the design, production, construction and other stages of the prefabricated building disconnect from each other, which affects the ability to coordinate work [10]. There is no whole-cycle system planning that leads to a disconnect between management and technology. The integration advantages of design, production and construction of prefabricated building have not been realized.

The industrial chain is incomplete. Prefabricated building requires deep cross-coordination among different stages (design, production, construction, etc.) and specialties. However, in the traditional management mode, the owner signs contracts with the design, production, construction and other units respectively. This requires the owner to be responsible for communication and coordination of various major in each stage, resulting in disconnection among the various stages. The entire process does not form a complete industrial chain, and is unable to integrate resources and give play to the integration advantages of the whole industrial chain [10].

The communication efficiency and degree of information are low. The prefabricated building is an integrated structure of design, procurement and construction. It requires systematic design of the general project, and involves many professions and complicated problems. Therefore, strengthening mutual communication between majors is the key to resolve problems. However, the traditional management method mainly relies on the owners' communication and there is no face-to-face communication between the professionals, which leads to inefficient communication. In addition, the application of information technology in prefabricated engineering project is insufficient, and the low degree of management information hinders the development of prefabricated buildings.

The prefabricated construction project is characterized by integrated design, factory production and assembly construction. The current management mode cannot adapt to the development of prefabricated buildings, so it is necessary to seek an appropriate management mode.

3.4.EPC Management Mode and Advantages

3.4.8.Management Modes Commonly Used in Engineering Projects

The traditional project management mode (DBB mode) is the Design-Bid-Build mode. In this mode, the owner signs contracts with the designer, supplier, and construction company respectively. The project is carried out in the order of design-production-construction. This mode is restricted by the owner's knowledge system, and all departments need to communicate with the owner.

The project management contracting mode (PMC mode) is the Project-Management-Contracting mode. The owner hires a consultant or a management contractor with comprehensive management capabilities to manage the project on behalf of the owner [10]. In this mode, the participation of the owner is low, and the cost control of the owner is more favorable. But, the coordination is difficult, resulting in a serious disconnect between management and construction.

The design and construction general contracting mode (DB mode) is the Design-Build mode. The owner hands over the design and construction work to the same contractor. Although this mode largely avoids the design change problems caused by the disconnection between design and construction, it is easy to have problems with unclear rights and responsibilities among various parties [11].

3.4.9.EPC General Contract Management Mode

The Engineering Procurement Construction is a management mode that integrates design, procurement and construction. Under the constraints of the engineering contract, the general contracting enterprise undertakes all the work of the whole project in the design, procurement, construction and other stages. The enterprise is fully responsible for the project [12]. The general contractor is responsible for the whole life cycle management, has great freedom and is at the core of the project. He/she coordinates communication and coordination among various professions, and integrates design into each stages of various professions, which can effectively solve the problem of design and construction disconnection.

3.4.10.Advantages of EPC General Contract Management Mode

EPC General Contract Management Mode offers effective control over the total cost of the whole construction period. It enables design, procurement and construction to operate in a professional manner while overlapping in depth and rationally, which not only guarantees the quality and quantity of the work as required by the contract, but also effectively shortens the construction period and reduce costs.

It makes the project holistic and systematic. According to the project requirements, the general contractor builds a project department that meets the project scale and characteristics, which reduces the burden of multiparty management and communication for the owners. The contractor also reduces the contradictions arising from the collision of various majors during the construction, such as design changes and other sudden problems in the construction process. The general contractor can optimize the resource allocation in the whole project, and carry out reasonable design, procurement and construction to ensure all links can be properly coordinated.

EPC General Contract Management Mode gives a full play to the leading role of design. The general contract management breaks the traditional mode of design, bidding and construction. This is good for the optimization of engineering projects.

It makes full use of the BIM technology. It realizes the information management of the entire construction of the prefabricated building from design, procurement and construction, thereby maximizing the advantages of prefabricated buildings and promoting the stable and healthy development of fabricated buildings.

3.5.The Prefabricated Building Combined with EPC Mode

The advantages of combining the EPC mode with the prefabricated building are shown as follows. It forms an integrated building management mode, reduces costs by exerting the leading advantages of design, and shortens the construction period by achieving a highly organized project.

First, it makes full use of the integrated building management mode. The general contractor develops into an integrated enterprise of design, procurement and construction. Designers, PC component manufacturers and construction builders form a complete industrial chain, which is conducive to optimizing the industrial structure and resource allocation. Figure 1 shows the organization of the project in the assembled EPC mode. In this mode, the owner gives the project to the EPC general contractor, who is responsible for the management of design, construction, safety, capital costs, and comprehensive development. This mode has clear rights and responsibilities, perfect organizational structure, and can better cope with problems in the project.

Second, it takes full use of the leading role of design. Prefabricated buildings are designed through various disciplines and stages of the building process to systematically optimize costs. The concept of pre-design is conducive to avoiding risks and reducing costs.

Third, it can help achieve a highly organized construction project. The EPC management mode of the prefabricated building realizes the reasonable connection of various professions, overcomes the difficulty of cross-coordination, and solves the problem of separation between management and technology. Through the high organization of the project, the period of construction can be optimized to the maximum extended and then shortened.

Fourth, it makes full use of construction project informatization. Under the EPC general contracting mode, BIM technology is applied to establish an integrated information management platform for design, production and construction to realize data sharing. This is beneficial to integrate industrial chain advantages, reduce design changes, optimize the total amount of engineering, reduce costs and improve efficiency [13].

Compared with traditional concrete buildings, prefabricated buildings exhibit a series of advantages, such as high degree of refinement, shorter construction period, less environmental impact and higher degree of mechanization. The combination of prefabricated buildings and EPC mode is an advantage of prefabricated mode.

4.Analysis of EPC Mode for Promoting Prefabricated Buildings in Cold Areas

Due to the influence of climatic conditions, there will be many difficulties in the construction process in cold areas. The EPC management mode of the prefabricated building can better solve the construction obstacles caused by the cold.

4.6.Construction Quality

The prefabricated components (PC components) of the prefabricated buildings are completed in the component processing factory, which are not affected by climatic conditions and can avoid quality problems caused by the winter construction in the cold areas. At the same time, production process automation, refinement and industrialization make it easier to improve component quality. Figure 2 shows the production process of PC components.

The production of prefabricated components can be uniquely identified by two-dimensional code or RFID technology. The traceability information management can be carried out for the whole life cycle of raw materials, production quality, production assembly, transportation logistics, post-operation and maintenance, etc. to further ensure the construction quality. The installation of walls, beams, boards, columns and stairs of prefabricated buildings, the connection between components (such as sleeve grouting) and other construction details, and the construction operations in complex or dangerous areas have become the complex processes and key nodes of the construction industry. Informatization enables the transformation of the construction industry while virtual simulation methods are used to simulate the complex construction processes and key nodes mentioned above. The difficulties and key points in the production process are previewed by means of construction animation to eliminate possible accidents during the production process [14].

Under the EPC general contract management mode, the general contractor must carry out strict quality management work in the design stage, production stage and construction stage, establish corresponding quality management mechanism, and carry out strict quality inspection on all stages of work. This ensures that the overall quality of the prefabricated building meets the expected standards.

4.7.Construction Schedule

The prefabricated building is restricted by weather conditions. BIM technology is used to establish a shared platform for planning coordination and intelligent management to achieve dynamic planning adjustment, coordinate the construction schedule plan, collaborative production plan and delivery plan in time, link the daily construction tasks with the progress mode, carry out detailed management on the site, and push the task to the relevant personnel to complete. Figure 3 shows the management process of applying BIM technology to the construction progress.

In terms of design, BIM technology standardization design and integrated information platform are conducive to efficient communication among various disciplines and saving time. Collision inspection using BIM technology can identify problems in the design in advance, optimize the design scheme and avoid delays caused by design changes.

In terms of production, component manufacturers can use the integrated information platform to obtain the required information of prefabricated components. The contractor can get timely feedback of component production schedule on the information platform, so as to control the progress of the project more accurately.

In terms of construction, through the integration of BIM technology and the construction schedule, space and time are integrated to predict construction progress. The problems that may occur during the construction phase are modified, corresponding countermeasures are developed in advance to ensure the implementation of the project, and the construction plan is formulated.

According to the publicity material diagram of a construction management, under the same construction project, the traditional management mode and the EPC management mode duration are compared as Figure 4.

Design 30 days Purchase 20 days Design deeply 15 days Machining 20 days Main body 40 days Decoration 40 days

Traditional

EPC

Design 20 days Purchase front 10 days Main body front 10 days Decoration interspersed 30 days Deepening at the same time 10 days Machining 10 days

Compared with the same building application projects mentioned above, the traditional management period can reach 165 days, while the EPC general contracting mode can shorten the period by 100 days and the efficiency is increased by 60%. Finally, it can be completed in only 65 days. It can be seen that the use of prefabricated EPC management mode in cold areas is especially important for the optimization of the construction period.

4.3.Construction Cost

Since the prefabricated components are produced in the factory, the cost of keeping warm during field construction in cold areas in winter is greatly reduced. EPC general contracting mode is adopted for prefabricated building management and overall planning for the whole process cost control of prefabricated construction projects, which can further reduce the project cost.

At the design stage, construction drawing design, split design and deepening design are the main contents. By optimizing the design, the types and specifications of components and their molds are reduced, which reduces cost.

At the production stage, through control, the cost increase caused by the cold can be avoided. Also, the increased material cost, construction management cost, and personnel cost due to winter construction can all be reduced.

At the construction stage, visual guidance through BIM technology ensures the accuracy and coordination of components, equipment and other parts during the installation process. This can reduce construction errors, risks and control costs.

4.4.Construction Safety

The production of components in the factory can avoid construction accidents caused by insensitive response and slow response, and avoid safety problems such as fires caused by frequent use of high-power heating equipment on site. At the scene of prefabricated construction, most use large mechanical lifting, which can ensure the construction safety to the greatest extent. Under the EPC general contract management mode, the general contractor establishes corresponding safety management mechanism during the construction process to strictly control construction safety.

At the design stage, safety design is required. We can use EPC management mode combined with BIM technology to provide a visual collaborative platform. Through the simulation of the construction, the potential safety hazards are identified as much as possible, the construction simulation of the temporary construction and construction general layout are carried out, and the design is safer and more reasonable to reduce the occurrence of accidents.

In the construction process, we can integrate BIM technology with the cloud positioning system. Then locate the workers in real time in the cloud, monitor the violations of workers, and assist the on-site safety monitoring and safety warning through BIM technology.

The EPC mode of the prefabricated building not only solves the safety problems caused by climatic conditions in cold areas, but also better avoids the safety hazards caused by the construction under normal conditions.

5.Conclusion

In summary, promoting EPC mode of prefabricated building in cold areas can obtain greater advantages in terms of quality, schedule, cost, and safety, such as effectively shortening construction period, improving quality, green environmental protection, reducing cost and maximizing the target benefit. The introduction of BIM technology in the EPC mode can reduce errors, improve communication and management efficiency, and optimize engineering quantities. In the end, this study forms a powerful, theoretical basis for the application and promotion of EPC management mode of prefabricated buildings in cold areas.

The authors have declared that no competing interests exist.

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