Control of the production process of pre-stressed square piles

In order to strictly control the quality of the products, it is necessary to strengthen the control of the four key processes of mixing, tensioning, centrifugation and maintenance in order to meet the requirements of customers and improve the internal management level.
I. Concrete Mixing Process
The square pile concrete mixing process (process) is the key process (process) in the square pile production process - the quality control point. How to implement this quality control point correctly and ensure that the quality of this key process is under control should start from the following points.
1. Stability of the weighing system
(1) Weighing system. It is mainly composed of weighing platform frame, weighing hopper, sensor, display instrument, connection line (signal cable) and so on. The stability and precision of the weighing system directly affects the manufacturing quality of concrete.
(2) the rigidity of the scale platform reliability. Most of the scale pedestal (seat) are made on the floor frame of the mixing building installed, or with a crane scale, or with a cantilever scale. Therefore support the sensor scale platform frame must be solid, good rigidity, weighing after the scale stand (seat) will not be deformed, shaking, the mixing floor frame should also have sufficient rigidity to avoid the scale platform frame shaking deformation when the mixer is running.
(3) The freedom of the weighing hopper. Weighing hopper as a concrete raw material ratio of the storage container, in weighing measurement must be fully flexible and free, in addition to the sensor pivot point, it can not be affected by the interference of other connections, otherwise it will affect the accuracy of weighing. To keep the weight of the weighing hopper and the surrounding frame clearance ≥ 2cm, each weighing hopper inlet, outlet should use soft connection, cement, mineral powder using filter cloth to do soft connection, in order to play the role of exhaust. Water scales, admixture scales are used soft PVC double-sided glue tape, both soft connection and impermeable. Do not pull too tightly when fixing the soft connection, to ensure that the scale bucket has a certain amount of wobble. Water scales, admixture scales should be additionally fixed at the input of the pipe, not directly resting on the scale bucket, the pipe outlet installed pneumatic butterfly valve, so as not to affect the accuracy of weighing.
(4) the selection and installation of sensors. Sensor weighing range should generally be weighed and the total weight of the weighing hopper 3 to 5 times appropriate, both to consider the impact of weighing materials, but also to consider the value of too large when the accuracy is reduced. The structure of the sensor form hanging (hanging) and cantilever two, hanging suitable for weighing small cases, such as water scales, admixture scales, cantilever suitable for stone scales, sand scales, cement scales, admixture scales, etc.. However, in many mixing plants there is also a large number of ground weighing systems using suspended sensors, which are suspended directly under the storage bin, together with the horizontal belt weighing or a separate weighing hopper scale before being placed into the horizontal belt. The cantilevered sensor should be installed according to the direction of the arrow indicated by the sensor and should not be installed in the opposite direction. After weighing should also pay attention to the sensor cantilever end of the bottom surface and fixed bracket whether there is more than 5mm gap, use should also be frequently checked whether the gap part is cement lumps, other debris, sand and other debris blocked and affect the weighing accuracy. In addition in the installation or maintenance of the weighing system, the sensor can not be used as a welding connection conductor, arbitrary fire, welding in the high current will inevitably cause sensor failure distortion. In the production process, the weighing system should also be frequently checked, maintenance, regular calibration of the scale.
2. advanced, reliable and stable control system
(1) the composition of the control system mainly includes the electrical control system, microcomputer control system and television monitoring system. There are two types of control systems that can be used to produce concrete independently: one is manual, i.e. the production of concrete is completed by manually operating the relevant buttons with the aid of the industrial control machine; the other is automatic, i.e. the automatic production and management of concrete is achieved entirely by the industrial control machine. The two control methods can be interchanged by means of a changeover switch.
(2) The electrical control system consists of two parts: the electrical cabinet and the operating console. The electrical components shall be of high quality and reliable products, and the quality of the products shall conform to the relevant international (IEC-947) standards and regulations. The electrical components in the electrical cabinet are installed with standard card tracks and wired in front of the board, which brings great convenience to maintenance and installation. The system has two operating consoles (or operating areas): the large console mainly controls and monitors the production of the mixing equipment, while the small console is used to control and monitor the loading of the aggregate bin divider and various aggregates, and the loading control of the powder bin. The buttons of the two operating consoles are arranged in accordance with the production process and are marked in Chinese to make the operation clearer and easier for the operator to grasp and use. The input power supply to the control system should be stable and it is recommended that a voltage regulator be installed. Due to the large dust in the mixing building, the components should be blown and dusted regularly to prevent the accumulation of dust from causing short-circuit tripping, deadlock, misoperation and burning of components.
(3) The microcomputer control system adopts the MK2000 square pile production management system software, which is mainly composed of two functional modules: automatic production control system and production management system, and is powerful. The automatic production control system uses standard IPC industrial control machine to measure, display and control the control signals of the concrete mixing plant production system in real time through industrial standard sensors, hardware circuits and I/O interface boards, with functions such as ratio storage, automatic compensation of dropout, compensation of moisture content of sand and gravel, etc. It can also accurately and conveniently input, call out and modify the set values and formula numbers of each material. It can also make hints for errors in the production process and realise the automation of concrete production. The production management system is controlled by a computer network, which enables comprehensive networking of the dispatching office, test room, control room, transport department, general manager's office and other business departments. It provides comprehensive digital management of important production data such as production task orders, construction ratios, raw material intake and consumption, commercial concrete production and delivery records (concrete mini-ticket records) and various production reports, which greatly facilitates the management and decision-making of the enterprise and improves its competitiveness. This greatly facilitates management and decision-making and improves the competitiveness of the enterprise.
The system has perfect self-locking and interlocking functions to ensure accurate and reliable operation, as well as common fault detection and alarm functions.
(4) The TV monitoring system is mainly composed of colour cameras, auto iris lenses, colour multi-screen processors and colour industrial monitors. External production can be easily observed through TV monitoring in order to prevent unexpected situations. The system has several sets of proportioning libraries, which can meet the needs of the technical department in many ways, such as construction proportioning and theoretical proportioning. All technical reports are automatically generated and can be printed at once, and various reports can be adjusted by the user like WORD. The technical department receives information on production tasks through the network, from the matching ratios to the factory information, both in terms of working time and workload, which is a qualitative leap forward from before the use of the system.
(5) At present, many enterprises in the statistics of the number of raw materials consumed, still use the production ratio of a single party consumption multiplied by the amount of production to calculate the overall amount of raw materials for each raw material, but the proportion of materials used and the actual consumption of materials (material scale) is a certain error. The system automatically collects and transfers the amount of material consumed for each tray of concrete produced, compares the design and actual amounts, displays both amounts of material consumed and the error rate, and records in detail the time, volume, feed truck number and pile type and specification of each tray of concrete produced, providing an accurate and secure basis for multiple management analyses such as concrete quality tracking, batching system bin calibration and actual consumption of raw materials. 3. the design of the mixer's internal mixing function
3. The mixer's internal mixing function is well designed and uniform
There are two types of mixers: vertical shaft planetary mixers and double horizontal shaft forced mixers. These two types of mixers have their own characteristics and are widely used.
(1) vertical shaft planetary mixer mixing principle for the public rotation plus rotation, there is a complex mixing curve, can be completed in a very short period of time mixing work, is a complete forced mixer, with long-term dry mixing capacity. 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 material jamming and small wear-resistant parts.
(2) double horizontal shaft forced mixer mixing shaft using a new shaft-end conical spiral bevel gear - planetary gear reducer drive, horizontal cylinder body installed with a double shaft rotating reverse paddle, paddle into a certain angle of the material to the axial, radial tumbling, so that the material quickly and evenly mixed. It is also a compulsory mixer, the intense mixing movement shortens the time of one mixing, more rapid and efficient. As the transmission gearbox outside the cavity, maintenance maintenance is easier.
(3) Mixing is the use of mixing leaves of the compulsory force, the gravity of the material, so that a variety of materials such as coarse aggregate, fine aggregate, powdered cementitious materials, water, water reducing agent, etc. evenly mixed together to form a gel-like flowable concrete process. The mixing process is divided into two stages, the first stage is dry mixing, generally 5 to 30s, this stage has a great impact on the homogeneity of the concrete mix; the latter stage is wet mixing, generally controlled between 70 to 120s, all the materials will be mixed together, so that it becomes a new gelatinous concrete material. If the mixing is not homogeneous, the concrete will segregate, then the strength will be higher for more coarse materials and relatively lower for more fine materials. Therefore, the design angle of the mixing blade, blade size should be suitable for the production requirements of pipe pile concrete, in addition to the raw material feeding procedure should also be reasonably designed.
(4) The material feeding sequence and the water consumption of concrete (water-cement ratio) control plays a very critical role in the concrete's compatibility and flowability. At present, there are the following feeding methods: ① a feeding method, the cement, admixture, sand, stone, a one-time input, dry mix and then throw water wet mix, and finally add water-reducing agent mixing; ② sequential feeding method, according to a certain order, such as sand, cement, admixture, gravel sequential feeding, first dry mix, then add water wet mix, and finally add water-reducing agent mixing; ③ mixed feeding method, first into a part of the same material, and then successively Input, such as sand, cement, admixture, dry mixing, and then cast most or all of the water, wet mixing for a short time, and then cast the gravel to continue mixing, plus the remaining water and then mixing. Water reducing agents are weighed more evenly if they are added to the water together.
The mixing process for square pile concrete is preferable to the mixed feeding method, which makes full use of the effective time and is balanced according to the characteristics of the material. In the sand and gravel under the material on the side of the open mixing, under the sand and gravel and then under the cement and admixtures, dry mixing time to be short, add 60% water and water reducer wet mix, and finally add the remaining 40% water mixing. Of course, square pile manufacturers can also aggregate, powder, the specific feeding order combined with the actual adjustment, test, both to make the cementitious materials to improve the strength of the role, but also to take into account the performance characteristics of the mixer itself equipment. Reasonable feeding order to improve the strength of concrete, reduce the amount of cement is of great help.
Second, the prestressing tensioning process
The role of prestressing is mainly: to apply precompressive stress to the steel reinforcement in the tensioned square pile before the pile is subjected to load, to improve the stiffness and flexural properties of the member, to delay the appearance of cracks and to increase the durability of the member. Therefore, in order to ensure the safety and reliability of the square piles in terms of demoulding and lifting, loading into the cauldron, storage and stacking, shipping and loading, site unloading, pile construction, pile bearing and durability, it is necessary to identify the prestressing tensioning process (process) as a key process (process) and quality control point of the quality system. The prestressing reinforcement is tensioned after the concrete is placed and moulded, before centrifugal forming and steaming. The prestressing tensioning process is carried out in a single step on the outside of the mould, with both central tensioning and external tensioning, generally for large diameter square piles above 800. The tensioning process is controlled by both the prestressing force (shown by the oil pressure gauge) and the elongation value, with the oil pressure being the main control and the elongation value being the secondary control, both data being recorded for inspection.
1. Calibration of the tensioning machine
The calibration of the prestressing tensioning jack is a very critical part of the prestressing structure project. The accuracy of its calibration will directly affect the prestressing value and the performance of the pile on which it is built, especially in the construction of square pile elements with small prestressing values. In addition, the oil pressure gauge should also be calibrated at the same time. The above calibration work should be carried out regularly by the local metrology department and an inspection report should be issued.
2. Tensioning tooling components (five sets) of processing, assembly accuracy, tensioning specifications
Tensioning tooling components (tensioning device) generally have tensioning screw, tensioning nut, anchor plate, fixed plate, tensioning plate five pieces, also called tensioning five pieces set. The concentricity of the tensioning plate, the fixing plate and the end plate of the square pile, their concentricity, the accuracy of the outer circle size and the clearance of the inner hole of the steel mould determine whether there is a loss of prestress during tensioning, similarly, the concentricity of the tensioning screw and the inner hole of the anchor plate, the small clearance and friction will also lead to a loss of prestress. After tensioning, the pressure should be maintained for 10-20s, and the tensioning nut or ring should be quickly locked with plates, pawls and other tools to minimise the loss of prestress. In old square pile plants that have been in operation for several years, tensioning often uses parts such as pad screws, sleeves and spacers due to damage to the fillet teeth of the tensioning screw, which should be avoided as far as possible because it is not easy to lock the large tensioning nut.
3. Cleaning and lubrication of the die
Generally in the tensioning plate, fixed plate assembly corresponding to the position of the steel die cylinder inner circle surface to brush waste machine oil, if there is residual concrete, cement paste must be cleaned up, so that tensioning friction resistance is small, less prestress loss. After feeding and before closing the mould, it must also be checked that no concrete debris has fallen into this position and must be cleaned before tensioning.
4. Ring reinforcement
The cage is automatically spot welded by a roll welder, if the welding is not secure, or if the cage falls apart during assembly and feeding, the ring reinforcement falls down and the outer diameter of the cage becomes smaller, the prestressing bars will not be in a straight line during prestressing tensioning and the prestressing force will decompose a force in the vertical direction, resulting in a partial loss of prestressing force.
In summary, there are a variety of prestress losses in the prestressing tensioning process. Therefore, although the process regulations require the prestressing value to be 70% of the tensile strength of the standard prestressing reinforcement, in practice the loss of prestressing should be considered and the oil pressure gauge value should be controlled between 70% and 75%, taking the upper limit as appropriate, especially for long piles.
III. Centrifugal forming process
Centrifugal forming is the process of using the centrifugal force of the rotating material to orderly arrange the swimming concrete in the steel mould to form a dense concrete layer (process). The denser the concrete, the higher the strength and vice versa. The design of the centrifugal process system directly determines the strength of the concrete and the quality of the appearance of the pile.
1. The precision of the operation of the centrifuge and the steel mould
(1) The level and parallelism of the centrifuge must be regularly corrected, the foot screws and connection screws are tight and not loose, there is no vibration when the centrifuge is running, the spacing of the centrifuge pallets is accurate and not slippery, and the transmission belt is tight and not slippery.
(2) Good concentricity of the steel mould, consistent wear of the running wheel, good straightness and no deformation, no misalignment with the steel mould running wheel, no jumping of the tube mould during centrifugation.
(3) Periodically use the tachometer to check whether the rotational speed of the steel mould is consistent with the set centrifugal process speed.
2. Determination of the centrifugal process system
(1) Choice of centrifugal speed: The centrifugal speed determines the size of the centrifugal force. Under certain conditions of collapse, it is crucial to choose the best centrifugal speed. The centrifugal process is generally divided into four stages: low speed, low to medium speed, medium speed and high speed. The low speed stage, also known as the fabric speed stage, is designed to make the concrete in the mould evenly distributed along the longitudinal direction of the mould and to have a partial mixing effect. Medium speed, low medium speed for the transition speed stage, play a uniform, mixing, speed conversion role, so that the concrete forming a gradual and continuous process; high-speed stage for the concrete dense speed stage, it increases the centrifugal force, so that the concrete mix fully dense, precipitation slurry water.

(2) Selection of centrifugal time: According to the actual situation of the concrete mix and the running condition of the equipment and steel mould, the centrifugal time of the four stages should be reasonably selected. The low speed time should not be too short and too long, too short then the concrete adjustment is not uniform, too long will intensify the friction between the inner wall of the steel mould and the concrete, destroying the isolation effect, thus leading to the outer wall of the square pile sticky skin, pockmarked, spalling, difficult to take off the mould. The total centrifugation time is controlled within 9-15min to improve the efficiency of the centrifuge and to save electricity. The four stages of centrifugation time is about 2~4min for slow speed, 1~2min for low to medium speed, 1~2min for medium speed and 4~8min for high speed, for reference only.