Precast concrete members are precast concrete members in the factory or on site.
Assembled concrete structure is a concrete structure assembled by precast concrete members through reliable connection. That is, pure PC structure.
Assembled monolithic concrete structure is a kind of assembled concrete structure which is reliably connected by precast concrete members and forms a whole with on-site post-pouring concrete and cement-based grouting materials. That is, the structure of PC and cast-in-place * * *.
The main types of PC components are: external wall panels, internal wall panels, laminated panels, balconies, air conditioning panels, stairs, precast beams and precast columns.
Prefabrication rate refers to the volume ratio of the material consumption of prefabricated components in the main structure and envelope above the outdoor terrace of prefabricated concrete buildings to the total material consumption of corresponding components. Prefabrication rate is the prefabrication index of single building. For example, the precast rate of a building is 15%, that is, the volume of precast members is 150 cubic meters, accounting for 1 000 cubic meters of the total concrete.
Assembly rate refers to the ratio of the number (or area) of prefabricated components and building parts to the total number (or area) of similar components or parts in prefabricated buildings. Assembly rate generally refers to the proportion of prefabricated buildings in engineering projects. For example, a project * * * has 10 buildings, and five buildings are PC structures, that is, the assembly rate is 50%.
Second, the history of PC structure
As early as 1950s, China introduced precast concrete building technology from the former Soviet Union and vigorously developed various buildings based on PC. By the 1980s, a complete PC technology system was basically formed. Products involve industrial and civil buildings, municipal facilities, large-scale infrastructure and so on.
Later, there was a split. In the field of architecture, PC applications gradually shrink, quit or even disappear, and turn to cast-in-place mode. In the field of municipal engineering, prefabrication technology has always occupied a dominant position. Freeways, bridges, ports, urban viaducts, underground pipelines, subway shield segments and prefabricated public tunnels are all based on PC. In the field of railway construction, the application of PC has reached the extreme.
The reason is that building products are dominated by the diversified needs of consumers, and the diversified and personalized design concept is not conducive to product standardization, so it is difficult to prefabricate complex and staggered components in factories. Even for municipal projects with high PC rate, complex overpasses can only be poured in situ, and cannot be generalized. At the same time, with the development of commercial concrete, large formwork and other technologies, the difficulty and cost of on-site cast-in-place construction are reduced; At that time, some defects of PC buildings (such as poor earthquake resistance, waterproof and sound insulation) have not been continuously improved. Compared with traditional cast-in-place buildings, prestressed concrete buildings have high cost and poor quality, or even both.
Assembly has limitations. Not all parts of all buildings can be PC-based, such as foundation and basement bottom reinforcement.
Third, the PC structure has attracted attention in recent years.
To explore the road of building industrialization, we have tried a large number of pilot projects, and the technical level, material selection, construction technology and quality standards are not the same as those of 30 years ago.
Vanke and Zhongnan tried PC first, but it was not widely used in subsequent projects. Why? It all makes sense. Perhaps one or several advantages of PC are not enough to completely replace cast-in-place concrete, or the comprehensive advantages of PC are not as good as cast-in-place concrete. The advantages of cast-in-place concrete are integrity, plasticity, operability and low cost, which are precisely the shortcomings of prefabricated buildings.
Precast concrete assembled monolithic structure has the advantages of fast construction speed, easy quality control, material saving, good appearance quality of components, good durability, less wet work on site and environmental protection.
In order to further strengthen the concept of low-carbon, green and environmental protection and promote the development of prefabricated buildings, the Ministry of Housing and Urban-Rural Development and local governments have repeatedly issued policies to encourage and force the use of prefabricated buildings.
Three-year Action Plan for Green Building Development in Shanghai (20 14-20 16)[ Hu Fu Fa [2014] No.32]
"No less than 25% of the newly-built prefabricated buildings in 20 14; Not less than 50% in 20 15; In principle, all new civil buildings that meet the requirements within the outer ring road in 20 16 years adopt prefabricated buildings. "
During the policy period, the municipal finance will give 60 yuan/square meter financial subsidies to the assembly-type construction projects that meet the demonstration requirements (the maximum subsidy for a single project is 6 million yuan), and priority will be given to the awards under the same conditions.
Implementation Opinions on Promoting the Development of Prefabricated Buildings in this Municipality [Shanghai Construction Management Committee (2014) No.901] and all newly-built civil buildings within the outer ring road of 20 16 meet the requirements in principle. If the prefabricated assembly rate reaches 40% or above, the subsidy per square meter is 100 yuan, and the maximum subsidy for a single project is 100 million yuan. The prefabricated part of the prefabricated external wall or laminated external wall of this project may not be included in the construction area, but it shall not exceed 3% of the construction area of 0.00 or more prefabricated residential buildings.
Notice of Shanghai Municipality on Further Promoting the Development of Green Buildings and Improving Building Performance [Shanghai Construction Management Association (2015) No.417] document (released on June 6, 20 16).
After many years, PC architecture has once again attracted attention in China. This seems to be a historical cycle, but it is not a simple repetition. PC technology and construction methods have undergone fundamental changes.
Fourth, the advantages and disadvantages of PC structure
Standing on the historical coordinate system, a large number of PC buildings in the 1970s and 1980s proved to be "short-lived", leaving many sequelae, many of which became dangerous buildings and needed to be reinforced, rebuilt and demolished. Some PC structures in Changzhou have been worn out, the external wall panels have cracked and fallen off, the stairs and walls have cracked, the concrete protective layer of balcony beams and slabs has fallen off, and the railings have broken.
In terms of quality, the common problems of PC buildings in China, such as "poor connection, joint leakage and low thermal insulation", cannot be overcome.
But these are not the fault of the PC itself, but the function of other external factors, such as the lack of technology and shoddy construction. There is basically no such problem in PC technology in Japan, Germany and France. It can be said that PC technology is quite mature in the world.
To do a good job in PC architecture, we need to innovate and improve the business model, technical route and management means.
The PC structure has the following advantages:
1. High seismic performance.
The calculation of PC structure is mainly based on the bearing capacity of each component itself, and connected into a whole by appropriate means. Direct transfer of joint and joint pressure through post-pouring concrete or grouting or grouting; The tensile force is transmitted through the welding part of the connecting rib and the embedded part. When the bond strength of precast concrete joint interface is higher than the tensile and shear strength of concrete in the component itself, it can be regarded as equivalent to cast-in-place concrete.
According to the direction and size of deformation, the connection part can be made to slide, hinge or fix (it is difficult to make a rigid connection in assembly). When earthquake and other disasters occur, PC structure mainly eliminates stress through the strain at the nodes, so as to prevent the stress from continuing to transfer inside the structure and prevent the structure from collapsing continuously.
2. Factory production
Due to the industrial production of PC components, hard concrete, extrusion molding, high-frequency vibration, high-temperature curing, centrifugal molding and other technologies can be used, and the compressive strength of concrete can easily reach above 80MPa. However, the site cast-in-place structure is limited by conditions and is difficult. At the same time, due to different technologies, it is easy to make "fair-faced concrete" and "decorative concrete" without increasing the cost, thus reducing the cost of subsequent painting and decoration. Due to the large-scale production of PC components in auto production line, the production cost of finished products is decreasing.
3. productization of 3.PC components
Components in some industrial workshops and some civil buildings are highly standardized products, which can be continuously produced in batches according to the manufacturing mode, forming industrial products inventory for procurement and sales. Customization of orders will inevitably increase production costs.
The advantage of PC is what cast-in-place concrete can't have.
5. Is the 5.PC structure expensive?
At present, the biggest problem encountered by the industry in promoting PC construction is "high cost", which is a fact or a false proposition.
The composition of PC building cost is definitely different from the cast-in-place structure, and its technology is essentially different from the traditional cast-in-place technology. Different construction techniques will lead to different building performance and quality, and their "costs" are not comparable. There are two ultimate goals: improving building performance at the same cost, or reducing costs at the same cost. If a new technology can reduce the cost, it will be better to improve the quality of the building, but it should not be too idealistic.
Developers are sensitive to "cost". Considering the cost, in order to achieve the goal of cost control and the mentality of "low cost" of new technology, developers rarely take the initiative to adopt PC buildings when making decisions. This is completely understandable. I suggest that the technology is immature and the cost is high at this stage. As long as the government-mandated PC index goes offline, there is no need to be so active for the time being.
In a more macro thinking dimension, cost is just a factor, a point to consider. Considering the overall situation, the whole and the long term, it is acceptable to increase the cost appropriately in order to be green, environmentally friendly and low-carbon, improve the building quality in the future, and with the continuous progress of PC technology, the cost will gradually decrease.
PC standardization, design and production of standardized components, can be used to assemble buildings, building industrialization will first complete the "modular coordination principle standard". Only in this way can the PC cost be reduced. The non-standardization of PC parts in China is an obstacle to the popularization of prefabricated buildings.
6. How to optimize the design and construction of PC?
The improvement of PC structure is still through technical improvement and other means, with cost reduction as the breakthrough.
Optimize the design of PC building, adopt the design scheme suitable for prefabrication, improve the added value of PC building and reduce the cost. It should meet the requirements of building function, modulus and standardization, adopt the method of combining cast-in-place and prefabrication, and constantly optimize the design.
1. Take the following design optimization measures.
1) The fabricated structure adopts high-strength concrete and high-strength steel bars.
2) Adopt the integrated technology of main structure, decoration and equipment pipeline assembly. Equipment and pipelines should be designed comprehensively to reduce plane crossing and adopt drainage design at the same floor. The plane dimensions of the kitchen and bathroom meet the requirements of standardized integrated cabinets and integrated toilets.
3) Building envelope, stairs, balconies, partition walls, air conditioning panels, pipe wells and other supporting components and indoor decoration materials should adopt industrialized and standardized products. Windows and doors adopt standardized components.
4) Exterior wall facing shall be made of durable and pollution-free materials. Exterior wall decoration material formed by reverse stamping at one time. External wall insulation is changed to internal insulation, and styrofoam is sprayed.
5) Increasing the PC rate appropriately can reduce the cost.
2. Optimization measures in construction
1) Improve the manufacturing process of PC components and reduce the amortization expenses of factory measures;
2) Improve the installation and construction technology to reduce the mechanical and labor consumption;
3) Indoor decoration reduces the wet work on the construction site;
4) For prefabricated buildings, connection is the difficulty and focus of construction, one is the connection between prefabricated components, and the other is the connection between prefabricated components and newly poured concrete. The technical and efficiency problems of connection are solved, so the application bottleneck of assembly is solved.
3. Optimization measures in cost management.
1) For two houses with the same rooms, the cost index of prefabricated buildings is about 20% higher than that of cast-in-place structures, and the steel bar content index of prefabricated buildings is about 15% higher than that of ordinary cast-in-place concrete structures; However, the increase in the cost of prefabricated buildings is not entirely due to the increase in the content of steel bars and concrete, but is offset by other aspects, such as the increase in PC prices, but the decrease in the content of masonry and the reduction in the cost of measures.
2)PC A provides that if the general taxpayer chooses the manufacturer to pay taxes, the input tax can be deducted from the cost;
3) Identify suppliers, and the market price is generally 2200~3000 yuan/m3, so as to strive for more favorable prices;
4) Select the contractor and report the comprehensive unit price of PC installation fee. According to different types of components, the installation cost is generally 400~ 1000 yuan/cubic meter, so as to get a more competitive price.
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