The production of concrete prefabricated components can be said to be the factoryization of the building to a certain extent. Although there is a certain improvement compared with the previous technical methods, the quality is not improved, and it depends on the management of the component production process. Let us discuss the production and management of precast concrete components.
Precast reinforced concrete member
Reinforced concrete structures include cast-in-place monolithic reinforced concrete structures and prefabricated integrated reinforced concrete structures. The prefabricated monolithic structure is a structural form in which various reinforced concrete prefabricated members are mechanically installed and assembled according to design requirements. The manufacturing process of prefabricated components includes the fabrication and installation of templates, the fabrication and installation of steel bars, the preparation and transportation of concrete, the vibration and maintenance of components, the release and stacking.
1. Characteristics of precast concrete members
(1) Achieving batch industrial production, saving materials and reducing construction costs;
(2) It has mature construction technology, which is beneficial to ensure the quality of components, especially the production of standard shaped components. The construction conditions of prefabricated components (field) are stable, the construction procedures are standardized, and the quality is easier to ensure than cast-in-place components;
(3) It is possible to prepare for the construction of the project in advance, and the prefabricated components that reach the strength will be installed during the construction, which can speed up the progress of the project and reduce the labor intensity of the workers.
2. Component manufacturing process
According to the different characteristics of the forming and curing of the tissue components in the production process, the prefabricated component manufacturing process can be divided into three types: the pedestal method, the unit flow method and the conveyor belt method. At present, prefabricated external walls, prefabricated stairs, prefabricated balconies, etc. are still mainly produced by the pedestal method, and some of the prefabricated internal partition wall slabs that have been standardized and produced have realized the unit flow method or the conveyor belt method.
(1) The pedestal method The pedestal is a concrete floor, tire mold or concrete trough with a smooth surface and a steel structure. The production process of forming, curing, demoulding, etc. of the components is carried out on the pedestal.
(2) Unit flow method The unit flow method is divided into several sections in the workshop according to the requirements of the production process. Each section is equipped with corresponding workers and machine equipment, and the forming, curing and demoulding of the components are produced. The process is completed sequentially in the relevant sections.
(III) Conveyor flow method The formwork is moved on a conveyor belt that is closed in a ring shape, and each production process is carried out in each work area which is sequentially distributed along the conveyor belt.
3. Forming of prefabricated components
Common methods of vibrating are vibration method, extrusion method, centrifugal method, etc., mainly based on vibration method.
Vibration method The member was fabricated by the pedestal method and vibrated using a plug-in vibrator and a surface vibrator. When the plug-in vibrator is vibrated, it should be inserted in a plum blossom shape, and the spacing should not exceed 300mm. If the prefabricated component requires a clear concrete surface, the insert vibrator cannot cling to the mold surface, otherwise a wand will remain. The method of vibrating the surface vibrator is divided into a static vibrating method and a dynamic vibrating method. The former is fixed on the mold by an attached vibrator, and the latter is provided with a vibrator vibrating on the pressure plate, and is suitable for a flat concrete member not exceeding 200 mm.
Extrusion method Extrusion method is often used for continuous production of hollow slabs, especially when prefabricated lightweight internal partition walls.
Centrifugal method is to put the concrete-filled template on the centrifuge, so that the template rotates around its longitudinal axis at a certain speed. The concrete in the template is away from the longitudinal axis due to the centrifugal force, evenly distributed on the inner wall of the template, and the part in the concrete. The water is squeezed out to make the concrete dense. Centrifugation is commonly used in the production of large-diameter concrete prefabricated drains.
4. Prefabricated component maintenance
(1) The maintenance methods of prefabricated components include natural conservation, steam curing, hot-mixed concrete hot mold maintenance, solar energy conservation, and far-infrared curing, etc., mainly in natural conservation and steam maintenance.
(2) The cost of natural conservation is low, simple and easy, but the maintenance time is long, the template turnover rate is low, and the occupied space is large. The pedestal production in southern China is mostly used for natural conservation.
(3) Steam curing can shorten the curing time, the template turnover rate is correspondingly increased, and the occupied space is greatly reduced.
(iv) Steam curing is to place the components in a curing room (or kiln) with saturated steam or a mixture of steam and air, and to maintain it in a higher temperature and humidity environment to accelerate the hardening of the concrete to make it shorter. The specified intensity standard value is reached within the time.
(5) The steam curing effect is related to the steam curing system. It includes the standing time before curing, the heating and cooling speed, the curing temperature, the constant temperature curing time, and the relative humidity.
(6) The process of steam curing can be divided into four stages: static stop, warming, constant temperature and cooling. When steam curing, the maximum temperature of concrete surface should not be higher than 65 °C, and the temperature rise should not be higher than 20 °C/h, otherwise the concrete surface Fine cracks should be produced.
Third, concrete prefabricated components management measures
(1) Familiar with design drawings and pre-planning requirements
The technical personnel and the main person in charge of the project department shall determine the production sequence and delivery plan of the prefabricated components according to the prefabricated parts demand plan of the site and the stock of the prefabricated parts; be familiar with the construction drawings in time, understand the prefabricated intentions of the use units in time, and understand the prefabrication The steel bar of the component, the size and form of the formwork, the amount of concrete pouring work and the basic pouring method, in order to achieve high quality, high efficiency and economical purpose in the construction.
(2) Staffing and management
The prefabricated components are various in variety and different in structure. They should be arranged according to the workload and construction level of the construction personnel. The construction technical requirements and the emergency situation of the prefabricated components and the mitigation of the construction personnel should be appropriately allocated to the construction workers to participate in the steel bars and formwork. Concrete pouring. It is necessary to educate all employees on the importance of product quality, cost and progress, so that construction workers must have a clear and strict post responsibility system. There must be strict incentives and penalties.
(3) Site layout design
In order to meet the requirements of prefabricated components, convenient transportation, uniform classification and continuity of production of prefabricated components, site leveling and prefabricated site layout planning are particularly important. The height of the production workshop should fully consider the factors such as the height of the prefabricated components, the height of the mold, the lifting limit of the lifting equipment, the weight of the components, etc., and should avoid the problems of overloading of the equipment in the production process of the prefabricated components and the fact that the components are too high and cannot be lifted normally.
2. Influence and control of raw materials on concrete prefabricated components
Raw materials mainly include cement, fine aggregates, and coarse aggregates. Only high-quality raw materials can produce high-quality concrete components that meet technical requirements.
The cement used for the preparation of concrete is usually made up of five major varieties of Portland cement, ordinary cement, slag cement, volcanic ash cement and fly ash cement. Generally, concrete mixes of ordinary Portland cement work better than slag cement and pozzolan cement. The slag cement mixture has large fluidity, but the cohesiveness is poor, and the water is easily separated. The volcanic ash cement has small fluidity, but the cohesiveness is the best. Precast concrete small components with slag or volcanic ash cement are easy to cause uneven initial moisture, and the color is uneven after dismantling. The slag or volcanic ash incorporated on the concrete surface is easy to form uneven flower bands and black lines, which affects the appearance quality of the components. Therefore, when precast concrete members, ordinary Portland cement should be used as much as possible.
The cement marking should be adapted to the concrete strength of the component to be formulated. If the cement label is too high, the amount of cement in the concrete is too low, which affects the workability and durability of the concrete, resulting in rough and dull components. If the cement label is too low, the cement in the concrete is too large, which is not economical. The purpose is to reduce the technical quality of the concrete components, so that the concrete shrinkage rate increases and the component cracks are severe. Generally, when concrete is formulated, the strength of the cement is 1.5 to 2.0 times the strength of the concrete.
Fine aggregates should be well graded, hard texture, clean particles, and particle size less than 5mm. Sand containing 3% mud. The sand after entering the site shall be inspected and accepted, and unqualified sand shall not be admitted. The inspection frequency is 1 time / 100 cubic meters. Coarse aggregates require a hard, slip-resistant, wear-resistant, clean and compliant grade. The stone strength is not less than 3, the needle-like content ≤ 25%, sulfide and sulfate content