Production process control of prestressed square piles
First, the concrete mixing process
The square pile concrete mixing process (process) is the key process (process) in the production process of square piles - the quality control point. How to properly implement this quality control point to ensure the quality of this critical process is controlled, should start from the following points.
1. Stability of the weighing system
(1) Weighing system. It is mainly composed of a scale stand, a scale bucket, a sensor, a display instrument, a connection line (signal cable) and the like. The stability and precision of the weighing system directly affect the quality of the concrete.
(2) The reliability of the scale stand. Most weighing platforms (seats) are made up on the floor of the mixing floor, either with crane scales or with cantilever scales. Therefore, the weighing platform supporting the sensor must be firm and rigid. After weighing, the weighing platform bracket (seat) will not be deformed or shaken, and the stirring floor frame should have sufficient rigidity to avoid the swaying deformation of the weighing platform when the mixer is running.
(3) The freedom of the scales. As a storage container for the concrete raw material ratio, the weighing bucket must be fully flexible and free in weighing and weighing. Except for the fulcrum of the sensor, it can not be interfered by other connectors, otherwise it will affect the accuracy of the weighing. To ensure that the gap between the weighing scale and the surrounding frame is ≥ 2cm, the feeding port and the discharging port of each weighing bucket should use soft connection, and the cement and mineral powder should be softly connected by filter cloth to play the exhausting effect. The water scale and the admixture scale are made of soft PVC double-sided glue tape, which is soft and impervious. Do not pull too tightly when fixing the soft joint to ensure that the scale has a certain amount of shaking. The pipe inlet of the water scale and admixture scale should be fixed separately. It should not be placed directly on the scale bucket. The pneumatic butterfly valve should be installed at the pipe outlet to avoid affecting the accuracy of weighing.
(4) Sensor selection and installation. The weighing range of the sensor should generally be 3~5 times of the weighing weight and the total weight of the weighing bucket. It is necessary to consider the impact of the material when weighing, and the accuracy should be considered when the amount is too large. The structure of the sensor is suspended (suspended) and cantilever. The hanging type is suitable for weighing less. Generally, it is like water scale, admixture scale, cantilever type is suitable for stone scale, sand scale, cement scale, blending. Material scales, etc. However, in the ground weighing system of many mixing stations, there are also a large number of hanging sensors, which are directly suspended under the storage bin, and are weighed together with the horizontal belt or a separate weighing scale and then placed in a transverse belt. The cantilever sensor should be installed according to the direction of the arrow indicated by the sensor, and should not be reversed. After weighing, you should also pay attention to whether the bottom surface of the cantilever end of the sensor and the fixed bracket have a clearance of more than 5 mm. During use, it is also necessary to check whether the gap is blocked by cement agglomerates, other gravel, sand and other debris. Heavy precision. In addition, when installing or maintaining the weighing system, the sensor cannot be used as the connecting conductor for electric welding. Any large current generated during ignition and welding will inevitably cause sensor failure distortion. In the production process, the system should be inspected and maintained regularly with a symmetrical quantity system, and the calibration should be calibrated regularly.
2. Advanced, reliable and stable control system
(1) The composition of the control system mainly includes the electrical control system, the microcomputer control system and the television monitoring system. There are two kinds of control methods that can independently complete the production: one is the manual mode, that is, the manual operation of each button to complete the production by means of the industrial computer; the other is the automatic mode, that is, the automatic production of the concrete by the industrial computer And management. Two control modes can be converted to each other through the transfer switch.
(2) The electrical control system consists of two parts: the electrical cabinet and the operating console. Electrical components should be made of high quality and reliable products, and the quality of the products conforms to the relevant international standards (IEC-947). The electrical components in the electrical cabinet are installed by standard rails, and the wiring in front of the board brings great convenience to maintenance and installation. The system has two large and small operating tables (or operating areas). The large operating table mainly controls and monitors the production of mixing equipment. The small operating table is used to control and monitor the aggregate bins and various aggregates. Material condition, powder silo loading control. The buttons of the two consoles are arranged according to the production process, and are marked in Chinese, which makes the operation more clear and clear, and the operator is easy to grasp and easy to use. The input power of the control system should be stable. It is recommended to install a voltage regulator. The console should be grounded reliably to prevent static electricity. Due to the large dust in the mixing building, the components should be dusted and dusted regularly to prevent short circuit trips, crashes, malfunctions, and burnt components.
(3) The microcomputer control system adopts MK2000 square pile production special management system software. The software system is mainly composed of two major functional modules: automatic production control system and production management system. The automated production control system uses the standard IPC industrial control machine to measure, display and control the control signals of the concrete mixing plant production system through industrial standard sensors, hardware circuits and I/O interface boards. Automatic compensation of drop, sandstone moisture content compensation, etc., and the ability to input, recall and modify the set values and recipe numbers of each material accurately and conveniently, to make mistakes in the production process, to achieve automation of concrete production. The production management system uses computer network control to realize the comprehensive networking of the business departments such as the dispatching room, the laboratory, the control room, the transportation department, and the general manager's office. The production task list, construction ratio, raw material feeding and consumption, and commodity 砼Production and transportation records (small ticket records), production reports and other important production data for comprehensive digital management, greatly facilitate the management and decision-making of enterprises, and improve the competitiveness of enterprises.
The system has perfect self-locking and interlocking functions to ensure accurate and reliable operation of the system, and has common fault detection and alarm functions.
(4) The TV monitoring system is mainly composed of a color camera, an auto iris lens, a color multi-picture processor, and a color industrial monitor. External monitoring can be easily observed through TV monitoring to prevent accidents. The system has multiple sets of matching libraries, which can meet the needs of many technical departments, such as construction ratio and theoretical ratio. All technical reports are automatically generated, can be printed at one time, and various report users can adjust themselves like WORD. The technical department receives production task information through the network, from the matching ratio to the factory data, whether it is working time or workload, it is a qualitative leap before using the system.
(5) At present, many enterprises still use the unilateral consumption of production mix ratio multiplied by the production quantity to calculate the total raw material usage of each raw material, but the proportioning materials and actual consumption materials ( There is a certain error in the material scale). After the automatic collection of consumables, the system will calculate the amount of material consumed per plate of concrete, compare it with the design amount and the actual amount, and display the two types of consumption and error rate, and record in detail the production of each concrete. Information such as time, volume, feeding number, and square pile model specifications provide accurate and safe basis for multi-party management analysis such as quality tracking of concrete, proofing of position of mixing system, and actual consumption of raw materials.
3. The mixing function inside the mixer is reasonable and evenly stirred.
The mixer has two types: vertical shaft planetary mixer and double horizontal shaft forced mixer. These two mixers have their own advantages and are widely used.
(1) The principle of mixing the vertical axis planetary mixer is revolution and self-rotation. It has a complicated mixing curve and can complete the mixing work in a short time. It is a complete forced mixer with long-term dry stirring ability. Compared with the horizontal shaft mixer, it has the characteristics of no shaft end seal and no slurry leakage; compared with the vertical shaft mixer, it has the advantages of small motor power, no jamming, and low wear of wear parts.
(2) The double-shaft forced-type mixer agitator shaft adopts the shaft-end new conical spiral bevel gear-planetary gear reducer transmission, and the horizontal cylinder body is equipped with a double-axis rotary reversed blade, and the blade is oriented at a certain angle to the material. Rolling in the axial and radial directions allows the material to mix quickly and evenly. It is also a forced mixer, and the intense mixing movement shortens the mixing time and is faster and more efficient. Since the drive reduction gearbox is outside the cavity, maintenance and repair is easier.
(3) Stirring is to use the force of the stirring blade and the gravity of the material to uniformly mix various materials such as coarse aggregate, fine aggregate, powdery cementing material, water and water reducing agent to form a glue. Condensed flowable concrete process. The mixing process is divided into two stages. The first stage is dry mixing, generally 5~30s. This stage has a great influence on the uniformity of concrete mixing. The latter stage is wet mixing, generally controlled between 70~120s. Mix all the materials together to make them a new gelatinous concrete material. If the agitation is uneven, the concrete will be isolated, so the strength of the coarse material is high, and the strength of the fine material is relatively low. Therefore, the design angle of the mixing blade and the blade size should be suitable for the production requirements of the pipe pile concrete, and the raw material blanking procedure should also be rationally designed.
(4) The order of material feeding and the water consumption of concrete (water-to-binder ratio) control play a key role in the workability and fluidity of concrete. At present, there are the following feeding methods: 1 one-time feeding method, one-time injection of cement, admixture, sand and stone, dry mixing, then water and wet mixing, and finally adding water-reducing agent to stir; 2 sequential feeding method, according to certain The order, such as sand, cement, admixture, gravel, sequential feeding, dry mixing, followed by wet mixing, and finally add water reducing agent; 3 mixed feeding method, first put a part of the same material, and then continue Invest, such as sand, cement, admixture, dry mix, and then add most or all of the water, dry for a short time, then cast gravel to continue stirring, add the remaining water and then stir. The water reducing agent is more evenly weighed if added to the water.
The mixing process of the square pile concrete should adopt the mixed feeding method, which can make full use of the effective time and comprehensively balance according to the characteristics of the materials. When the sand is cut, the mixture is stirred while being opened. After the gravel is finished, the cement and the admixture are put down. The dry mixing time is short, 60% water and water reducing agent are added, and finally the remaining 40% water is stirred. Of course, the square pile manufacturers can also adjust and test the specific feeding sequence of aggregates and powders in combination with the actual conditions. It is necessary to make the cementing materials play a role in improving the strength, and also to consider the performance characteristics of the equipment of the mixer itself. Reasonable feeding sequence is of great help to improve concrete strength and reduce cement consumption.
Second, the prestressed tension process
The prestressing effect is mainly: pre-compressive stress is applied to the steel bars in the tensioned pile before the pile is subjected to the load, the rigidity and bending resistance of the member are improved, the time of occurrence of the crack is postponed, and the durability of the member is increased. Therefore, in order to ensure the safe and reliable safety of the strip pile, such as demoulding lifting, loading into the kettle, storage stacking, delivery and loading, site unloading, pile foundation construction, pile foundation bearing and durability, Process) It is necessary to determine the key processes (processes) and quality control points of the quality system. After the concrete fabric and the mold is clamped, the prestressed steel bars are tensioned before the centrifugal forming and steaming. The prestressed tensioning adopts the external one-piece tensioning process of the steel mold, and has two kinds of central tensioning and outer tensioning. Generally, the large-diameter square pile with a diameter of 800 or more adopts an outer tensioning method. The tensioning process adopts double control of prestressing (oil pressure gauge display value) and elongation value, mainly based on oil pressure, and the control elongation value is auxiliary. Both data should be recorded for reference.
1. Calibration calibration of the tensioner
The calibration of the prestressed tension jack is a very important part of the prestressed structural engineering. The accuracy of the correction will directly affect the prestress value and the performance of the pipe pile built on the pipe pile member, especially at the small prestress value. In the construction of square pile members, the check of the jack should be paid more attention to. In addition, the oil pressure gauge must also be calibrated at the same time. The above verification work should be carried out by the local metrology department on a regular basis and an inspection report issued.
2. Processing, assembly precision and tension specification for tensioning tooling parts (five sets)
Tensile tooling parts (tensioning rods) generally consist of five sets of tensioning screws, tensioning nuts, anchoring plates, fixing plates and tensioning plates, also called five-piece sets. The end plates of the tension plate, the fixed plate and the square pile are assembled, and the concentricity, the outer circle dimensional precision and the like are matched with the inner hole size of the steel mold, which determines whether there is a loss of the prestress during tensioning. Similarly, the tension screw The concentricity of the inner hole of the anchoring plate and the small friction of the gap may also cause loss of prestress. After the tension is pulled, the pressure should be kept for 10~20s, and the tools such as the board and the pawl should be quickly locked with the tensioned large nut or the large spiral ring (outer shaft tension) to minimize the prestress loss. The old square pile factory that has been put into production for several years, due to the damage of the tensioned screw thread, the tension will often use the parts such as the pad screw, the sleeve, the gasket, etc., this method should be avoided as much as possible because the tensioned large nut is not easy to lock. tight.
3. Clean and lubricate the mold
Generally, in the steel mold cylinder of the corresponding position of the tension plate and the fixed plate, the machine oil should be scrapped. If there is residual concrete and cement slurry, it must be cleaned so that the frictional resistance is small and the prestress loss is small. After feeding and before clamping, it is also necessary to check whether concrete debris has fallen into this position and must be cleaned and then pulled.
The steel cage is automatically spot welded by the welding machine. If the welding is not strong, or the cage is folded during assembly and feeding, the ring ribs fall, and the outer diameter of the steel cage becomes smaller at the time of prestressing tension. The prestressed steel bars will not be in a straight line, and the prestress will decompose a force in the vertical direction, causing partial loss of the prestress.
In summary, there are various prestress losses in the prestressed tensioning process. Therefore, although the process requires that the prestress value is 70% of the tensile strength of the standard prestressed steel bar, the loss of the prestress should be considered in the actual operation, and the oil pressure gauge value should be controlled between 70% and 75%. The limits are appropriate, especially for long piles.
Third, the centrifugal molding process
Centrifugal molding refers to the process of ordering the moving concrete in the steel mold by using the centrifugal force of the material during the rotation to form a dense concrete layer process (process). The denser the concrete, the higher the strength, and vice versa. The rationality of the design of the centrifugal process system directly determines the strength of the concrete and the appearance quality of the pipe pile.
1. Precision of operation of centrifuges and steel molds
(1) The level and parallelism of the centrifuge must be corrected regularly. The screws and connecting screws of the various places are not loosened. The centrifuge does not vibrate when it is running. The distance between the rollers of the centrifuge is accurate and the position is slipped. The belt tension is not slipped. .
(2) The steel mold has good concentricity, the running wheel wears consistently, the straightness is good, and it is not deformed. It is well aligned with the steel mold running wheel, and the tube mold does not jump when centrifugal.
(3) Regularly use the tachometer to verify that the rotational speed of the steel mold is consistent with the setting of the centrifugal process speed.
2. Determination of the centrifugal process system
(1) Selection of centrifugal speed: The centrifugal speed determines the magnitude of the centrifugal force. Under certain slump conditions, it is critical to choose the optimum centrifugal speed. The centrifugation process is generally divided into four stages: low speed, low medium speed, medium speed, and high speed. The low speed stage, also known as the cloth speed stage, aims to make the concrete in the steel mold evenly distributed along the longitudinal direction of the steel mold, and has some mixing and cooperation. The medium speed and low medium speed are the transition speed stages, which play the role of uniformity, mixing and speed conversion, so that the concrete has a gradual continuous process; the high speed stage is the concrete compact speed stage, which increases the centrifugal force and makes the concrete mixture Fully dense, precipitated slurry.
(2) Selection of centrifugation time: According to the actual situation of the concrete mixture and the operation condition of the equipment and the steel mold, the centrifugal time of four stages is reasonably selected. Low speed should not be too short or too long. If it is too short, the concrete adjustment will be uneven.
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