Air cushion formwork construction method for precast floating slab wet joint

Abstract

This invention relates to the field of urban rail transit track construction technology, and provides a construction method for air cushion formwork for wet joints of precast floating slabs. The method includes: laying precast unit slabs on a base and welding pre-reserved longitudinal reinforcing bars; inserting and fixing a hollow pressure pad under the wet joint, laying an isolation membrane, and installing side formwork to form a split-type air cushion formwork system; injecting a pressurizing medium into the hollow pressure pad to cause it to expand, and the isolation membrane changes from a relaxed state to a taut and flat state, forming a flat concrete surface; monitoring and maintaining internal pressure stability in real time; pouring concrete for the wet joint to dynamically balance the concrete load; and slowly depressurizing after the concrete reaches 75% of its design strength, allowing the isolation membrane to detach naturally, removing the pressure pad, and dismantling the side formwork. This invention uses a split structure to decouple the flexibility of the bottom formwork from the rigidity of the side formwork, and maintains the bottom surface elevation accuracy through dynamic pressure maintenance. This invention can effectively improve construction efficiency.

Description

Technical Field

[0001] This invention relates to the field of urban rail transit track construction technology, and in particular to a method for constructing a wet joint of a precast floating slab using an air cushion template. Background Technology

[0002] Floating slab track is a preferred structural form for special vibration reduction sections of urban rail transit. Its basic principle is to set elastic supports between the track bed and the foundation to form a "mass-spring" vibration isolation system, thereby reducing vibration and noise by lowering the system's natural frequency. The construction methods of floating slab track are mainly divided into two categories: one is the continuous cast-in-place method, which involves tying steel bars, erecting formwork, and pouring concrete on-site in the tunnel to form a long floating slab; the other is the prefabrication and assembly method, which involves prefabricating short unit slabs in the factory, transporting them to the site for laying, and then connecting them with wet joints to form a long floating slab.

[0003] Continuous cast-in-place methods have inherent drawbacks such as long construction periods, high on-site labor intensity, and significant susceptibility to environmental factors in terms of quality. Precast assembly methods transfer the main processes to standardized factory production, requiring only assembly and connection on-site, effectively improving construction efficiency, ensuring track accuracy, and shortening the construction period. However, the length of precast unit slabs is typically limited by transportation conditions, tunnel construction boundaries, and hoisting equipment capabilities, usually only 3.6m–6m, far shorter than the continuous length of cast-in-place floating slabs (which can reach over 25m). Therefore, after the precast unit slabs are laid, adjacent short slabs must be connected into longer slabs through wet joints to ensure the smoothness of train operation and the integrity of the track structure.

[0004] Wet joints are a critical step in the construction of precast floating slab tracks. The essence of this technology lies in the secondary pouring of concrete within the gaps between adjacent precast unit slabs, transforming the separated unit slabs into a continuous, load-bearing structure. The quality of wet joint construction directly affects the structural continuity, vibration damping performance, durability, and track geometry retention capabilities of long floating slab tracks.

[0005] Wet joint construction faces three major technical constraints: First, spatial constraints. Precast unit panels are laid on the substrate, and the gap between the bottom of the panel and the substrate is usually only tens of millimeters. The wet joint is located at the end gap of adjacent panels, and its bottom space is narrow and has limited height, making the erection and dismantling of traditional formwork systems extremely difficult.

[0006] Second, there are constraints on the quality of the molding process. The bottom surface of the wet joint concrete is the bottom surface of the long floating slab. Its flatness, density, and elevation accuracy directly affect the contact state between the floating slab and the elastic support, and thus affect the mechanical performance and vibration reduction effect of the vibration isolation system. If the bottom surface is uneven, has honeycomb-like pits, or has elevation deviations, it will lead to uneven stress on the floating slab and local stress concentration, which may cause support failure or slab cracking under long-term operation.

[0007] Third, there are constraints on construction efficiency. Urban rail transit track laying is usually located on critical lines, with significant time pressure. As the final step in track structure forming, the construction efficiency of wet joints directly affects the overall track laying progress. Traditional formwork systems involve cumbersome procedures for erection, reinforcement, dismantling, and cleaning, making it difficult to meet the demands of rapid construction.

[0008] To address the aforementioned triple constraints, traditional solutions have developed two types of template solutions, but both have insurmountable drawbacks.

[0009] First type of solution: Rigid formwork system Rigid formwork systems use steel or wooden formwork as the bottom and side formwork for wet joints. The formwork is fixed in the design position by means of support frames, tie bolts or adhesive fixing, and is removed after the concrete is poured and the required strength is reached.

[0010] The technical flaw of this solution is: (1) Poor spatial adaptability. The space at the bottom of the wet joint is narrow, the space for the support frame is insufficient, and the formwork is difficult to reinforce. For curved sections or uneven base sections, it is difficult to ensure the fit between the formwork and the base, and quality defects such as grout leakage and root rot are prone to occur.

[0011] (2) The dismantling process is complicated. The bottom formwork needs to be sent in from the end of the wet joint or the gap at the bottom of the slab. The support frame needs to be installed piece by piece. After the concrete hardens, the support frame is difficult to dismantle and often requires destructive dismantling. The turnover rate of the formwork is low.

[0012] (3) The quality of the bottom surface is uncontrollable. Although the surface of the steel formwork is smooth, the joints are prone to leakage of grout, forming burrs, which need to be removed manually after demolding. The wooden formwork absorbs water and deforms, has poor bottom flatness, and has strong adhesion to concrete, which can easily cause damage to the bottom surface during demolding, and usually requires secondary roughening treatment.

[0013] (4) The elevation accuracy depends on manual labor. The elevation of the formwork is adjusted by the support frame. The adjustment accuracy is affected by the operator's skill level. In addition, the support frame may settle during the concrete pouring process, resulting in deviation of the bottom elevation.

[0014] Second option: "U"-shaped integrated air cushion template To overcome the spatial adaptability and construction efficiency problems of rigid formwork, a "U"-shaped integrated air cushion formwork solution has been proposed. This solution uses flexible airtight material to make an integrated "U"-shaped bladder. After inflation, the bottom mold and the two side molds expand simultaneously to form a formwork cavity that wraps around the bottom and sides of the wet joint. After the concrete is poured, the air is released and the formwork is removed, achieving rapid erection and dismantling.

[0015] While this solution alleviates the problems of space constraints and construction efficiency to some extent, it introduces new technical drawbacks: (1) Insufficient structural adaptability. The internal contour of the "U"-shaped template is fixed after inflation and must be precisely matched with the cross-sectional contour of the precast unit slab. However, there are various cross-sectional types of precast floating slabs, and the slab thickness and slab gap width vary due to different design parameters. A single specification of "U"-shaped template can only be applied to a specific cross-section, resulting in poor versatility. If universality is forced, there will be gaps or compression between the template and the precast unit slab after inflation, leading to deviations in forming dimensions or damage to the template.

[0016] (2) The quality of the side molding is uncontrollable. The side of the "U" shaped template is formed by the expansion of a flexible bladder, and its shape and rigidity depend entirely on the internal gas pressure. When the concrete is poured, the lateral pressure acts on the flexible side, causing the side to bulge or dent, making it difficult to ensure the verticality and flatness of the wet joint side. Especially under high-flowability concrete or vibration, the side deformation is more significant, affecting the splicing accuracy of adjacent slabs.

[0017] (3) The accuracy of the bottom elevation is difficult to guarantee. The "U"-shaped formwork uses a gas medium, which is compressible. When the concrete self-weight load and the pouring impact load are applied to the bottom surface of the formwork, the gas medium is compressed, the formwork as a whole sinks, and the bottom elevation of the wet joint is lower than the design value. Moreover, this compression deformation is an irreversible plastic accumulation. As the pouring progresses, the bottom surface continues to sink, and the elevation deviation continues to expand. Although the traditional scheme specifies the initial inflation pressure (e.g., greater than 10 kPa), it does not solve the problem of dynamic pressure loss compensation during the pouring process, and the accuracy of the bottom elevation cannot be controlled.

[0018] (4) Poor quality of the interface between the formwork and the concrete. The inner surface of the "U"-shaped formwork is made of a flexible, airtight material with a high surface roughness and strong adhesion to the concrete. When the formwork is removed, concrete residue is easily adhered, which damages the bottom surface of the concrete and contaminates the formwork, affecting its future use. Although the traditional solution lays an isolation membrane inside the formwork, the isolation membrane is laid before inflation. During inflation, the gas between the membrane and the inner surface of the formwork cannot be completely expelled, forming air bubbles and wrinkles, resulting in pitting and ripples on the bottom surface of the concrete, with poor flatness, which still requires roughening treatment.

[0019] (5) Restricted demolding operation. The "U"-shaped template covers the bottom and two sides of the wet joint. After venting, it needs to be pulled out from the end of the wet joint or the gap between the plates. For long wet joints or small gaps between the plates, the pulling resistance is large, the operation is difficult, and the template may even be damaged due to friction.

[0020] Based on the above analysis, traditional wet joint template technology has not yet solved the following core problems: Question 1: How to achieve rapid erection and dismantling of the formwork system in a narrow space, while ensuring the adaptability of the formwork to different cross-sectional shapes and joint sizes.

[0021] The contradiction between the spatial adaptability and construction efficiency of rigid formwork is prominent. Although "U"-shaped air cushion formwork improves spatial adaptability, its one-piece molding structure sacrifices cross-sectional adaptability. A new formwork system that combines both spatial and structural adaptability is needed.

[0022] Question 2: How to maintain the stability of the formwork support force throughout the concrete pouring process and prevent the bottom elevation deviation caused by formwork compression deformation.

[0023] Traditional air cushion formwork only focuses on the initial inflation pressure, neglecting the dynamic changes in concrete load. A formwork mechanism is needed that can balance concrete load in real time and maintain stable support.

[0024] Question 3: How to ensure the forming quality of the concrete bottom surface, so as to achieve a flat, smooth, and rough surface that does not require roughening, thereby reducing subsequent processing steps.

[0025] Traditional separator membrane laying processes suffer from defects such as air bubbles and wrinkles, and the quality of the bottom surface formation is uncontrollable. A separator membrane process that can automatically eliminate wrinkles and create a smooth surface is needed.

[0026] Question 4: How to achieve functional separation between the bottom mold and the side mold, so that the bottom mold can focus on elevation accuracy control and the side mold can focus on side forming quality control?

[0027] The "U"-shaped integrated template binds the bottom mold and side mold functions together, causing them to restrict each other. A separate template system is needed where the bottom mold and side mold functions are independent and can be optimized separately. Summary of the Invention

[0028] The purpose of this invention is to solve at least one technical problem in the background art and to provide a construction method for an air cushion template for wet joints of prefabricated floating slabs.

[0029] To achieve the above objectives, the present invention provides a method for constructing an air cushion formwork for wet joints of precast floating slabs, comprising: S1. Precast unit slab laying and rebar connection: Multiple precast unit slabs are laid on the base according to the design position, and the reserved longitudinal rebars of adjacent precast unit slabs are butted and welded to form a continuous long floating slab structure. S2. Air Cushion Formwork System Installation: A hollow pressure pad is inserted below the wet joint between adjacent precast unit panels as a bottom mold. The hollow pressure pad is centered and fixed along the length of the wet joint. An isolation film is laid on the upper surface of the hollow pressure pad. The isolation film is a flexible and smooth material used to isolate the concrete from the formwork and to serve as the forming surface of the concrete bottom. Auxiliary side formwork is installed on both sides of the hollow pressure pad, so that the upper surface of the hollow pressure pad and the side formwork together enclose a formwork cavity with an open top. S3. Air cushion expansion and sealing formation: Pressurizing medium is injected into the hollow pressure pad to expand the hollow pressure pad and tightly adhere it to the isolation membrane on the bottom surface of the wet joint. The expansion pressure is used to flatten the isolation membrane and eliminate wrinkles, forming a sealed template cavity with a flat support surface. The internal pressure of the hollow pressure pad is monitored. When the pressure reaches the design pressure and remains stable, it is determined that the template cavity is ready for concrete pouring. S4. Wet joint concrete pouring: Wet joint concrete is poured into the cavity of the template. The wet joint concrete fills the cavity of the template from bottom to top. Its bottom surface is formed by the expansion support surface of the hollow pressure pad, and its sides are formed by the side template. During the pouring process, the internal pressure of the hollow pressure pad is kept stable to resist the lateral pressure and gravity load of the concrete. S5. Template Removal and Recycling: After the concrete of the wet joint has hardened to the demolding strength, the pressurizing medium in the hollow pressure pad is slowly released to make it shrink evenly. The isolation membrane naturally detaches from the concrete surface as the hollow pressure pad shrinks. The shrunken hollow pressure pad is pulled out from the bottom of the wet joint, the side template is removed, and the wet joint construction is completed.

[0030] According to one aspect of the present invention, the hollow pressure pad is a flexible sealed bladder, the cross-sectional height of which is less than the design bottom mold height of the wet joint when not pressurized, and the cross-sectional height after pressurization and expansion matches the design bottom mold height of the wet joint; the expansion height of the hollow pressure pad is adjusted by controlling the injection amount of the pressurizing medium.

[0031] According to one aspect of the present invention, the isolation membrane is made of a smooth, flexible material and is laid on the upper surface of the hollow pressure pad to isolate the concrete from the contact surface of the formwork. During laying, the isolation membrane naturally covers the surface of the hollow pressure pad. When the hollow pressure pad expands, the expansion pressure pushes the isolation membrane evenly towards the bottom surface of the wet joint, causing the isolation membrane to change from a relaxed state to a taut and flat state, eliminating wrinkles generated during laying, and forming a smooth concrete forming surface.

[0032] According to one aspect of the present invention, the pressurizing medium is a liquid or a gas; when a liquid pressurizing medium is used, the incompressibility of the liquid is used to make the hollow pressure pad form a rigid support surface, thereby enhancing its resistance to concrete loads; when a gas pressurizing medium is used, the compressibility of the gas is used to make the hollow pressure pad undergo adaptive deformation during the concrete pouring process, thereby buffering the impact load on the concrete.

[0033] According to one aspect of the present invention, the method for determining the stability of the pressure is as follows: after the hollow pressure pad is pressurized to the design pressure, it is left to stand for a preset period of time for observation. If the pressure drop does not exceed a preset proportion of the initial pressure, the sealing performance is deemed qualified; if the pressure drop exceeds the preset proportion, the leak point is investigated and repaired, and then the pressure is repressurized until the pressure stabilizes.

[0034] According to one aspect of the invention, during the concrete pouring process, the internal pressure of the hollow pressure pad is maintained at more than or equal to 1.2 times the sum of the lateral pressure and buoyancy of the concrete; at the same time, the pressure is controlled to be less than or equal to the bearing capacity of the hollow pressure pad.

[0035] According to one aspect of the invention, the demolding strength is 75% of the design strength of the wet joint concrete; the rate of slow release of the pressurized medium is controlled such that the pressure drop per unit time is less than or equal to 10% of the initial pressure.

[0036] According to one aspect of the present invention, the welding of the reserved longitudinal steel bars is carried out by splice welding, and the length of the splice bar meets the following requirements: greater than or equal to 6 times the diameter of the steel bar when welding on both sides, and greater than or equal to 12 times the diameter of the steel bar when welding on one side; after welding, at least two sets of transverse steel bars are tied in each wet joint to form a steel bar skeleton in the wet joint area.

[0037] According to one aspect of the present invention, the hollow pressure pad is fixed by setting limiting members on the bottom surface or base of the prefabricated unit plate at both ends of the wet joint, and locking the two ends or the two side edges of the hollow pressure pad in the limiting members to prevent the hollow pressure pad from sliding or shifting laterally along the length direction of the wet joint during the pressurization and expansion process.

[0038] According to one aspect of the invention, the isolation membrane is laid continuously as a whole or laid in sections with overlapping; when sections are overlapped, the overlap length between adjacent isolation membrane sections is greater than or equal to 20 cm, and the overlap is sealed with adhesive.

[0039] According to one aspect of the present invention, the design pressure is determined based on the following: the area of ​​the support surface of the hollow pressure pad after expansion multiplied by the design pressure is equal to the sum of the self-weight load of the wet joint concrete, the construction live load, and the safety factor; the design pressure ensures that the expansion deformation of the hollow pressure pad is within its own elastic range.

[0040] According to the present invention, an air cushion formwork construction method for wet joints of prefabricated floating slabs is provided. Through the synergistic effect of the split air cushion formwork system, dynamic pressure maintenance process and automatic flattening technology of the isolation membrane, the core technical problems of poor formwork adaptability, insufficient bottom elevation accuracy and low construction efficiency in the prior art are solved, resulting in significant technical effects.

[0041] Regarding the template structure, this invention employs a split structure combining a hollow pressure pad as the bottom mold and independent rigid side molds, achieving functional decoupling between the flexibility of the bottom mold and the rigidity of the side molds. The hollow pressure pad, when unpressurized, has a small thickness, allowing it to be inserted into narrow gaps between plates. After pressurization, its expansion height is adjusted by the injection volume to accommodate different plate thicknesses and gap widths. One template system can cover various cross-sectional shapes, overcoming the shortcomings of fixed outlines in "U"-shaped integrated templates and the limitation that one specification can only match specific cross-sections. The independent rigid side molds ensure the verticality and flatness of the sides, avoiding a decrease in splicing accuracy caused by the bulging and deformation of the flexible sides.

[0042] Regarding the quality of the bottom surface forming, this invention utilizes a two-stage process of relaxed laying and expansion flattening of the release liner, transforming the difficult manual operation into an adaptive process. During laying, the release liner is allowed to naturally relax and cover; during expansion, the elastic deformation of the bladder wall generates uniform pushing pressure, ensuring uniform pressure transmission. The release liner transitions from a relaxed state to a taut and flattened state, automatically eliminating wrinkles and air bubbles, forming a smooth concrete surface. After hardening, the release liner naturally detaches as the pressure pad contracts, resulting in a flat and dense bottom surface that requires no roughening and can be directly used as the bottom surface of a floating slab or a leveling layer base.

[0043] Regarding the accuracy of the bottom elevation, this invention establishes a dynamic pressure maintenance process. Throughout the concrete pouring process, the pressurizing medium is monitored and replenished in real time to maintain an internal pressure greater than or equal to 1.2 times the concrete load. This ensures that the supporting force is always dynamically balanced against the external load, preventing bottom subsidence. The design pressure is precisely calculated based on the mechanical equilibrium equation, transforming empirical operations into a quantifiable and controllable technical process, guaranteeing the overall smoothness of the long floating slab.

[0044] In terms of construction safety and recycling, this invention adopts a slow pressure relief process to control the pressure drop per unit time to not exceed 10% of the initial pressure, so that the pressure pad shrinks evenly and the isolation membrane peels off gradually, avoiding early brittle damage to the concrete; after pressure relief, the bladder completely shrinks within the elastic range, which is convenient for extraction and recycling, reducing material consumption and construction costs. Attached Figure Description

[0045] Figure 1 The flowchart schematically illustrates a method for constructing an air cushion template for a wet joint of a prefabricated floating slab according to an embodiment of the present invention. Detailed Implementation

[0046] The invention will now be discussed with reference to exemplary embodiments. It should be understood that the described embodiments are merely intended to enable those skilled in the art to better understand and thus implement the invention, and are not intended to imply any limitation on the scope of the invention.

[0047] As used herein, the term "comprising" and its variations are to be interpreted as open-ended terms meaning "including but not limited to". The term "based on" is to be interpreted as "at least partially based on". The terms "one embodiment" and "an embodiment" are to be interpreted as "at least one embodiment".

[0048] Figure 1 This is a flowchart illustrating a method for constructing an air cushion formwork for a wet joint of a precast floating slab according to an embodiment of the present invention. Figure 1 As shown, in this embodiment, the construction method of the air cushion formwork for the wet joint of the precast floating slab includes: S1. Precast unit slab laying and reinforcement connection: Multiple precast unit slabs (precast floating slabs) are laid on the base according to the design position, and the reserved longitudinal reinforcement of adjacent precast unit slabs are butted and welded to form a continuous long floating slab structure. S2. Installation of the air cushion formwork system: Insert a hollow pressure pad as a bottom mold under the wet joint between adjacent precast unit panels. Arrange and fix the hollow pressure pad in the center along the length of the wet joint. Lay an isolation film on the upper surface of the hollow pressure pad. The isolation film is a flexible and smooth material used to isolate the concrete from the contact surface of the formwork and to serve as the forming surface of the bottom of the concrete. Install auxiliary side formwork on both sides of the hollow pressure pad so that the upper surface of the hollow pressure pad and the side formwork together enclose the formwork cavity with the top open. S3. Air cushion expansion and sealing formation: Pressurized medium is injected into the hollow pressure pad, causing the hollow pressure pad to expand and tightly adhere to the isolation membrane on the bottom surface of the wet joint. The expansion pressure is used to flatten the isolation membrane and eliminate wrinkles, forming a sealed template cavity with a flat support surface. The internal pressure of the hollow pressure pad is monitored. When the pressure reaches the design pressure and remains stable, it is determined that the template cavity is ready for concrete pouring. S4. Wet joint concrete pouring: Wet joint concrete is poured into the formwork cavity. The wet joint concrete fills the formwork cavity from bottom to top. Its bottom surface is formed by the expansion support surface of the hollow pressure pad, and its sides are formed by the side formwork. During the pouring process, the internal pressure of the hollow pressure pad is kept stable to resist the lateral pressure of the concrete and the gravity load. S5. Template Removal and Recycling: After the concrete of the wet joint has hardened to the demolding strength, slowly release the pressurizing medium in the hollow pressure pad to make it shrink evenly. The isolation membrane will naturally detach from the concrete surface as the hollow pressure pad shrinks. Pull out the shrunken hollow pressure pad from the bottom of the wet joint, remove the side formwork, and complete the construction of the wet joint.

[0049] In this embodiment, the present invention employs a split-type air cushion template system, separating the functions of the bottom formwork and the side formwork. The hollow pressure pad serves only as the bottom formwork, providing support, while the independent side templates handle the side forming function. These functions are decoupled and optimized independently. The hollow pressure pad, when unpressurized, is relatively thin and can be inserted into the bottom of the precast unit slab from the end or side gap. After insertion, it unfolds along the length of the wet joint, eliminating the need for simultaneous alignment of the bottom and two sides as required by the "U"-shaped integrated template. This provides greater adaptability to construction space, particularly in curved sections or tunnel sections with varying cross-sections. The expansion height after pressurization can be adjusted by the injection volume, matching the actual joint height regardless of changes in slab thickness or joint width. The independent side templates can be individually selected based on the wet joint width, unaffected by the expansion state of the bottom formwork. One template system can adapt to various cross-sectional forms, overcoming the shortcomings of the "U"-shaped integrated template, which has a fixed outline and can only match specific cross-sections with a single specification.

[0050] The release liner employs a two-stage process: relaxed laying and expansion flattening. During the laying stage, the release liner is allowed to naturally cover the surface of the hollow pressure pad, resulting in localized relaxation and wrinkles. At this stage, gas between the release liner and the pressure pad surface can freely escape, eliminating any closed air cavities. This process is extremely simple and requires no manual tightening. In the expansion stage, after injecting pressurizing medium into the hollow pressure pad, the pad expands, and the upper surface rises uniformly, propelling the release liner upwards. Because the upper surface of the release liner is constrained by the boundaries of the template cavity, the side templates limit its lateral displacement, and the cavity height limits its longitudinal displacement, the release liner transitions from a relaxed state to a taut and flattened state. The uniformity of expansion pressure and the uniformity of the release liner's flattening are ensured, and wrinkles are automatically eliminated during the expansion process. This process transforms the difficult manual tightening and flattening operations of traditional construction into an adaptive process of automatic expansion and flattening, simplifying the operation and improving the quality of the bottom surface formation. After the release membrane is flattened, it forms a smooth shaped surface. The concrete bottom surface replicates this surface morphology. After hardening, it naturally detaches as the pressure pad shrinks, without the need for manual prying. The bottom surface is flat and dense, and can be used directly as the bottom surface of the floating slab or as the base layer of the subsequent leveling layer, eliminating the need for the roughening process in traditional construction.

[0051] The dynamic pressure maintenance process (automatic adjustment via a pressure-stabilizing valve or manual periodic monitoring and replenishment) transforms the formwork system from passive load-bearing to active balancing. During the pouring of wet-joint concrete, the load acting on the bottom surface of the formwork changes dynamically with time and space. In the initial stage of pouring, the concrete is in a flowing state, and the load is distributed in the form of hydrostatic pressure. In the later stage of pouring, early shrinkage stress is generated, and temperature deformation accumulates continuously during hardening. Vibration operations generate impact loads, and aggregate settling and slurry floating lead to load redistribution. The "U"-shaped integrated air cushion formwork adopts a mode of maintaining pressure after initial inflation. The compressibility of the gas medium causes the formwork to sink when the concrete load increases, and this compression deformation is an irreversible plastic accumulation. Even if subsequent inflation is added, the resulting bottom elevation deviation cannot be recovered. This invention monitors the internal pressure of the hollow pressure cushion in real time throughout the entire concrete pouring process. When a pressure drop is detected, pressurizing medium is added through the injection interface to restore the design pressure, maintain the support force greater than the concrete load, prevent the expansion height from decreasing, and keep the bottom elevation stable. The design pressure is based on the principle that the supporting force is equal to the pressure multiplied by the supporting surface area. This supporting force is not less than the sum of the concrete self-weight, lateral pressure and live load multiplied by the safety factor. This establishes a quantitative relationship between pressure and load, transforming formwork support from an empirical operation into a calculable and controllable technical process.

[0052] The slow pressure relief removal process is optimized for the early mechanical properties of concrete. When wet joint concrete reaches demolding strength, its elastic modulus is low but its brittleness is high, making it sensitive to uneven deformation. The cross-sectional strength distribution is uneven, with surface strength higher than the interior, stress concentration at corners, and microscopic physical adsorption at the concrete-to-membrane interface. If rapid pressure relief is used, the inconsistent shrinkage rate of the pressure pad leads to asynchronous loss of support at different points on the concrete bottom surface, generating uneven stress. Stress concentration points are prone to cracking. Rapid separation of the membrane from the concrete interface generates peeling impact, potentially damaging the concrete surface. Localized negative pressure exacerbates the peeling force, leading to surface spalling. This invention controls the pressure drop per unit time, ensuring uniform shrinkage of the pressure pad and synchronous loss of support at all points on the concrete bottom surface, resulting in uniform stress distribution and avoiding stress concentration. The slow separation rate of the membrane from the concrete interface ensures that the peeling force is always less than the concrete surface strength, achieving non-destructive separation. Slow pressure relief ensures that the hollow pressure pad retracts within its elastic range, completely restoring its original shape, facilitating removal and reuse, and enabling formwork recycling.

[0053] Furthermore, according to one embodiment of the present invention, the hollow pressure pad is a flexible sealed bladder, the cross-sectional height of which is less than the design bottom mold height of the wet joint when not pressurized, and the cross-sectional height after pressurization and expansion matches the design bottom mold height of the wet joint; the expansion height of the hollow pressure pad is adjusted by controlling the injection amount of the pressurizing medium.

[0054] In this embodiment, the hollow pressure pad is a flexible, sealed bladder. Its unpressurized cross-sectional height is less than the design bottom formwork height of the wet joint. This structural feature determines its ability to be inserted into narrow spaces. The wet joint is located at the end gap of adjacent precast unit slabs. The gap between the bottom of the slab and the base is usually only tens of millimeters. If the initial thickness of the hollow pressure pad is equal to or greater than the design bottom formwork height, it cannot be inserted into the bottom of the slab from the side or end gap. It must be inserted from above the wet joint or pushed in from the tunnel end. The former is limited by the fact that the top of the wet joint is covered by the precast unit slab and cannot be implemented. The latter is only applicable to the tunnel end construction and not to the middle section. The initial thickness is less than the design bottom formwork height, which allows the hollow pressure pad to pass through the narrow channel in a flat state. After reaching the bottom of the wet joint, it expands under pressure to reach the working height. This realizes the spatial adaptation logic of first inserting and then forming, and solves the spatial contradiction of "forming size is greater than channel size" faced by both rigid templates and "U"-shaped integrated templates.

[0055] The height of the cross-section after pressurization and expansion matches the height of the bottom mold in the wet joint design. This matching relationship is achieved by controlling the injection volume of the pressurizing medium, rather than relying on the fixed shape of the bladder itself. From a mechanical perspective, the expansion process of the flexible sealed bladder follows the law of volume conservation: the volume of the injected medium equals the increase in internal volume after the bladder expands. For hollow pressure pads with an approximately rectangular cross-section, there is a calculable correspondence between the expansion height and the injection volume. Given a fixed bladder material, width, and length, the injection volume is positively correlated with the expansion height, and the expansion height can be controlled by measuring the injection volume. This controllability allows hollow pressure pads of the same specification to adapt to different bottom mold heights, requiring only adjustment of the injection volume, unlike "U"-shaped integrated templates which require custom-made templates for different plate thicknesses. For example, for wet joints with a plate thickness of 250mm and 450mm, the same hollow pressure pad can expand to the corresponding heights with different injection volumes, while "U"-shaped templates require prefabrication of bladders with two different internal contours.

[0056] The expansion height adjustment mechanism also brings about the technical effect of adaptive construction error. The actual joint height of precast unit panels after laying is affected by multiple factors such as the flatness of the base, the manufacturing error of the panels, and the positioning error of the laying, resulting in deviations from the design value. Traditional rigid templates need to be prefabricated according to the design height. If the height is found to be inconsistent on site, it is necessary to raise or lower them, which is difficult to adjust and has poor accuracy. The inflatable outline of the "U"-shaped integrated template is fixed and cannot adapt to the actual height deviation. The hollow pressure pad of this invention can determine the target expansion height on site according to the measured joint height. By precisely adjusting the injection volume, the actual height after expansion matches the measured value, rather than the design value. This realizes the construction logic of adaptive construction based on actual construction conditions rather than rigid execution according to design drawings, reduces the sensitivity of the precast floating panel laying accuracy to the construction quality of wet joints, and improves the controllability of the overall construction quality.

[0057] From a materials mechanics perspective, when the expansion process of a flexible, sealed capsule remains within its elastic deformation range, it can completely retract to its initial state after pressure release. This is the physical basis for achieving recycling. If the expansion deformation exceeds the elastic range, residual plastic deformation occurs, preventing complete retraction after pressure release. This leads to difficulties in extraction or an increase in initial thickness and decreased delivery performance during subsequent use. Controlling the injection volume to ensure the expansion height precisely matches the designed bottom mold height, rather than excessive expansion, is the key to keeping deformation within the elastic range. Excessive expansion not only causes plastic deformation but may also thin the capsule wall, reduce strength, and pose a risk of bursting. Insufficient expansion results in an inadequate bottom mold height, with the concrete bottom elevation lower than the design value. Therefore, controlling the injection volume is not only a height adjustment method but also a core process parameter for ensuring material mechanical properties, achieving safe and reliable construction, and enabling recycling.

[0058] In this embodiment, the hollow pressure pad can be made of materials with high airtightness, high elasticity, and fatigue resistance, such as rubber or thermoplastic polyurethane elastomer. Taking rubber as an example, natural rubber or nitrile rubber can be used as the base material, and it is formed into a flat bladder structure through a vulcanization process. The bladder wall thickness is usually 2mm to 5mm to ensure sufficient pressure bearing capacity and tear resistance. The bladder has a sealed hollow chamber inside, and a medium injection port and a pressure monitoring port are configured on the outside. The injection port is connected to the pressurized medium supply pipeline, and the monitoring port is connected to a pressure gauge or pressure sensor. When not pressurized, the bladder is in a flat state, and the cross-sectional height is usually 20mm to 50mm. This height is much smaller than the bottom mold height of the wet joint design (usually 80mm to 150mm), allowing it to be smoothly inserted through the narrow gap at the bottom of the plate. After being pressurized, the bladder expands elastically, increasing its cross-sectional height. The maximum expansion height is determined by the elastic limit of the bladder material and the structural design, and can usually reach 3 to 5 times the height when unpressurized, which is sufficient to cover the design bottom mold height range of common wet joints.

[0059] The specific structural form of the capsule can be a single-layer flat capsule or a multi-layer composite structure. A single-layer flat capsule consists of two rubber diaphragms sealed along their edges by vulcanization, forming a single internal chamber. This structure is simple, inexpensive, and suitable for applications with relatively uniform gap widths. A multi-layer composite structure incorporates internal ribs or reinforcing layers, resulting in a smoother upper surface after expansion and reducing localized bulging deformation. This is suitable for applications requiring high flatness of the bottom surface. Rigid frames or lugs can be provided along the capsule's edges for use with limiting components to achieve fixation and prevent slippage or displacement during pressurized expansion.

[0060] The relationship between the injection volume of the pressurizing medium and the expansion height can be determined through pre-calibration. The calibration method is as follows: place the hollow pressure pad on a standard test platform, gradually inject the pressurizing medium, record the injection volume and the corresponding expansion height each time, and plot the injection volume-expansion height curve. This curve shows an approximately linear relationship within the elastic deformation range. During on-site construction, the corresponding injection volume can be found according to the target expansion height, or calculated through linear interpolation. Due to the creep characteristics of the bladder material, i.e., the expansion height increases slowly over time under constant pressure, the loading time needs to be recorded during calibration. During on-site construction, the pressurization rhythm should be controlled according to the calibration time, or a feedback control method that monitors the expansion height in real time can be used to ensure that the final expansion height accurately matches the design value.

[0061] Furthermore, according to one embodiment of the present invention, the isolation membrane is made of a smooth, flexible material and is laid on the upper surface of the hollow pressure pad to isolate the concrete from the contact surface of the formwork. During laying, the isolation membrane naturally covers the surface of the hollow pressure pad. When the hollow pressure pad expands, the expansion pressure pushes the isolation membrane evenly towards the bottom surface of the wet joint, so that the isolation membrane changes from a relaxed state to a taut and flat state, eliminating the wrinkles generated during laying and forming a smooth concrete forming surface.

[0062] In this embodiment, the laying of the release membrane in the existing precast floating slab wet joint construction faces the technical challenge of extremely limited operating space: the gap at the bottom of the wet joint is usually only tens of millimeters, making it impossible for operators to enter the bottom of the slab for fine operations such as tightening and leveling. If it is forcibly inserted from the top and manually tightened, not only is the construction efficiency low, but the residual air bubbles and wrinkles between the membrane and the template are also difficult to completely eliminate, resulting in pitting, ripples, or even honeycomb defects on the bottom surface after the concrete hardens, which seriously affects the contact quality between the bottom surface of the floating slab and the elastic support. This invention adopts a natural relaxation covering process, which lays the release membrane in a relaxed state on the surface of the hollow pressure pad, allowing for local sagging and wrinkles in the membrane. This relaxation of the initial state fundamentally eliminates the need for fine operations in a narrow space, simplifying the laying process to two actions: feeding and unfolding, significantly reducing the construction difficulty and labor intensity.

[0063] The release membrane is made of a smooth, flexible material, such as polyvinyl chloride (PVC), polyethylene, or polypropylene film. It has a low coefficient of friction, strong alkali resistance, and does not chemically bond with the concrete slurry. The membrane thickness is typically 0.5mm to 2mm, providing sufficient tear strength to withstand the lateral pressure of the concrete while maintaining adequate flexibility to accommodate deformation of the pressure pad surface. During installation, the membrane is pre-cut according to the length and width of the wet joint. The membrane is naturally inserted from the end of the wet joint or the gap between precast unit panels, covering the upper surface of the hollow pressure pad. The edges of the membrane extend to the outer wall of the side formwork and are fixed, requiring no manual tightening or fixing. The edges of the membrane can naturally overlap the end face of the precast unit panel or the inner side of the side formwork, forming a preliminary covering layer.

[0064] When the hollow pressure pad expands, its internal pressure is evenly transmitted to the upper surface of the bladder. The elastic deformation of the bladder wall generates a uniform pushing force, so the pushing force of the expansion pressure on the bottom surface of the separator membrane is evenly distributed throughout the entire length of the wet joint. As the thickness of the pressure pad increases, its upper surface rises uniformly, pushing the separator membrane upwards synchronously. Due to the constraint of the template cavity boundary, the separator membrane is laterally limited by the ends of the prefabricated unit panels or side templates, and longitudinally limited by the height of the top of the cavity. During the upward displacement, the membrane material is forced to change from a relaxed drooping state to a taut flat state. In this process, the residual air between the membrane material and the surface of the pressure pad is pushed towards the end of the wet joint and discharged by the expansion pressure, and the air bubbles between the membrane material and the bottom surface are automatically eliminated. At the same time, the original wrinkles of the membrane material are gradually straightened and flattened under the action of uniform tension, finally forming a smooth and flat bonding state without wrinkles or air bubbles.

[0065] After the release liner is flattened, its smooth upper surface directly serves as the forming interface for the bottom surface of the wet joint concrete. After the concrete is poured, the cement paste comes into contact with the smooth surface of the release liner. During the hardening process, there is only weak physical adsorption and no chemical bonding between the two. Therefore, when demolding, the release liner can naturally detach with the contraction of the hollow pressure pad, without the need for manual chiseling or prying. More importantly, the concrete bottom surface accurately replicates the smooth surface morphology of the flattened release liner. The surface flatness is significantly better than the rough interface formed by traditional formwork. The bottom surface is dense, without honeycomb or burrs, and can be used directly as the bottom surface of the floating slab or as the base layer for the subsequent mortar leveling layer, eliminating the necessary roughening process in traditional construction. This technology not only improves the geometric quality of the bottom surface of the floating slab but also avoids damage to the concrete surface and secondary pollution such as dust and noise caused by roughening operations.

[0066] This invention transforms the installation of the release liner from manual, active tensioning to passive, pressure-driven flattening: utilizing the expansion of a hollow pressure pad—originally a process for establishing support—to simultaneously shape and vent the release liner. This transformation not only solves the engineering challenge of manual operation being impractical in confined spaces but also replaces the uneven tension of manual operation with uniform pressure from fluid dynamics. This allows the flattening quality of the release liner to be controlled by quantifiable pressure parameters, rather than relying solely on the operator's experience, thus improving the stability and repeatability of the construction quality.

[0067] Furthermore, according to one embodiment of the present invention, the pressurizing medium is a liquid or a gas; when a liquid pressurizing medium is used, the incompressibility of the liquid is used to make the hollow pressure pad form a rigid support surface, thereby enhancing its resistance to concrete loads; when a gas pressurizing medium is used, the compressibility of the gas is used to make the hollow pressure pad undergo adaptive deformation during the concrete pouring process, thereby buffering the impact load on the concrete.

[0068] In this embodiment, the choice of pressurizing medium directly determines the mechanical working characteristics of the hollow pressure pad, thereby affecting the formwork system's response to concrete loads and the forming quality. The liquid and gaseous media in this invention are not simply substitutes, but rather correspond to two distinct working modes, suitable for different construction conditions and technical requirements. This selectivity enables the formwork system to adapt to different working conditions.

[0069] When a liquid pressurization medium is used, the bulk modulus of the liquid (usually water) is approximately 2.1 × 10⁻⁶. 9 Pa, in an engineering sense, can be considered an incompressible fluid. After water is injected into the hollow pressure pad, the volume of the water hardly changes with pressure. Therefore, the expansion height of the pressure pad is mainly determined by the amount of water injected. Once the target height is reached and the injection port is sealed, even if the external concrete load increases, as long as the water does not leak, the volume and expansion height of the pressure pad remain essentially unchanged. This incompressible characteristic makes the hollow pressure pad form a near-rigid support surface: the self-weight load of the concrete and the lateral pressure are transferred to the upper surface of the pressure pad after acting on the isolation membrane. The incompressibility of the water resists the tendency of the pressure pad to compress, resulting in stable elevation of the support surface and no significant subsidence. This working mode is suitable for situations requiring strict accuracy of the bottom surface elevation, such as the starting and ending points of long floating slab connection sections, and transition sections connecting to stations or turnouts. Elevation deviations in these areas directly affect track smoothness and train operation safety. In practice, clean tap water or an aqueous solution mixed with rust inhibitor can be used as the medium. It can be injected by a manual or electric pump. A one-way valve is installed at the injection port to prevent backflow. A regular pressure gauge can be used for pressure monitoring. Since the liquid pressure is stable, the monitoring frequency can be appropriately reduced.

[0070] When using a gaseous pressurizing medium, the bulk modulus of elasticity of the gas (usually compressed air or nitrogen) is much smaller than that of the liquid, and it follows the ideal gas law PV=nRT, where pressure is inversely proportional to volume. When a hollow pressure pad is inflated, if the external concrete load increases, the gas is compressed, the pressure pad volume decreases, and the expansion height drops. However, the internal pressure increases accordingly, automatically generating a greater supporting force to balance the increased load. This compressibility gives the hollow pressure pad an adaptive buffering capacity: when impact loads during concrete pouring (such as concrete dumping or vibrator vibration) act on the supporting surface, the elastic deformation of the gas absorbs the impact energy, avoiding stress concentration and membrane tearing that might occur on a rigid supporting surface; fluctuations in concrete load (such as changes in local stacking height) are also smoothed out by the elastic regulation of the gas, and the supporting force adapts to the load rather than rigidly resisting it. This working mode is suitable for situations where the concrete pouring impact is large, or where the unevenness of the substrate leads to uneven load distribution. In practice, an air compressor commonly available at the construction site can be used to supply air. The inflation pressure is controlled by a pressure reducing valve and a pressure stabilizing valve. Since gas pressure is easily affected by temperature, pressure monitoring and automatic air replenishment devices need to be installed to maintain the pressure within the design range.

[0071] The choice between the two media can be determined from the perspective of engineering requirements: if the accuracy of the bottom elevation is a priority, a liquid medium should be preferred; if ease of construction and impact buffering are prioritized, a gaseous medium should be preferred. Within the same project, different sections can be flexibly switched according to changing working conditions. For example, a liquid medium can be used to ensure elevation accuracy on straight sections, while a gaseous medium can be used on curved superelevation sections to adapt to uneven loads caused by uneven base surfaces. This selectivity breaks through the limitation of the single mode of using only a gaseous medium in "U"-shaped integrated air cushion formwork, allowing the mechanical properties of the formwork system to be actively designed according to engineering requirements, rather than passively accepting the inherent properties of the material, thus enhancing the engineering adaptability and depth of the technical contribution.

[0072] Furthermore, according to one embodiment of the present invention, the method for determining the stability of pressure is as follows: after the hollow pressure pad is pressurized to the design pressure, it is left to stand for a preset time. If the pressure drop does not exceed a preset proportion of the initial pressure, the sealing performance is deemed qualified; if the pressure drop exceeds a preset proportion, the leak point is investigated and repaired, and then the pressure is repressurized until the pressure stabilizes.

[0073] In this embodiment, the determination of the pressure stability is a key process to ensure the sealing performance of the template cavity and thus guarantee the quality of concrete pouring. As a flexible airtight capsule, the sealing performance of the hollow pressure pad is affected by multiple factors: there may be microscopic pores or manufacturing defects in the capsule material itself, there are mechanical fit gaps at the pipe connections of the injection interface and the monitoring interface, and there are gaps at the overlapping boundaries between the isolation film and the surface of the pressure pad. These potential leakage points will form a medium leakage channel under the pressurized state, resulting in the decay of the internal pressure over time. If the concrete is poured when the sealing performance is unqualified, the pressure will continue to drop during the pouring process, and the supporting force will not be sufficient to resist the concrete load, which will cause deformation of the template cavity, sinking of the bottom surface, and even slurry leakage, resulting in quality defects in the wet joint and being difficult to repair afterwards. Therefore, the determination of pressure stability before concrete pouring is the core link of preventive quality control.

[0074] The determination method adopts the static observation method: after pressurizing the hollow pressure pad to the design pressure, close the injection port valve to isolate the system from the outside world and statically observe for a preset duration. The setting of this duration needs to consider both the determination reliability and the construction efficiency, usually ranging from 10 minutes to 30 minutes, and is specifically adjusted according to the project scale, environmental temperature, and medium type; when the temperature is high, the thermal expansion effect of the gas medium is significant, and the observation duration needs to be appropriately extended to eliminate the interference of temperature fluctuations; the liquid medium is less affected by temperature, and the observation duration can be appropriately shortened. During the observation period, continuously record the pressure value through a pressure gauge or a pressure sensor, and calculate the ratio of the pressure drop value to the initial pressure, that is, the pressure drop rate. The setting basis of the preset ratio is the allowable pressure loss threshold of the project, usually taking 5% to 10%; this threshold reflects the acceptable level of the leakage rate. If the pressure drop rate is lower than the threshold, it indicates that the leakage channel is small or does not exist, and the pressure decay is controllable within the limited duration of subsequent concrete pouring; if the pressure drop rate exceeds the threshold, it indicates that there is an obvious leakage, and it is necessary to check and repair.

[0075] When checking for leakage points, the segmented isolation method or the sensory detection method can be used. The segmented isolation method is applicable to occasions with a complex pipeline system. By closing the valves of different sections and observing the pressure drop situation section by section, the pipeline section where the leakage is located can be located; the sensory detection method is applicable to gas media. Apply soap solution to the suspected leakage part and observe whether bubbles are generated. The point where bubbles are generated is the leakage point. This method is simple and intuitive and does not require special equipment. For liquid media, the leakage location can be judged by observing whether there are traces of liquid leakage at the pipeline connection. After repairing the leakage point, re-pressurize and conduct static observation again until the pressure drop rate meets the requirements of the preset ratio, and then the sealing performance can be determined to be qualified and the concrete pouring process can be entered.

[0076] This assessment method transforms the sealing performance of the formwork system from experience-based judgment to quantitative testing. Traditional formwork systems rely on visual inspection of joint gaps or tactile assessment of contact tightness by construction workers, resulting in high subjectivity and poor reliability. This invention establishes an objective standard for sealing performance through the quantifiable parameter of pressure attenuation, providing repeatable and verifiable quality control points for formwork system preparation. Furthermore, this assessment method aligns perfectly with the mechanical characteristics of hollow pressure pads: the sealing chamber of the pressure pad itself serves as a natural carrier for pressure monitoring, eliminating the need for additional testing devices. Simply connecting a pressure gauge to the existing injection port or monitoring interface achieves the desired result, without increasing system complexity or cost. This design organically integrates quality control methods with the construction process, rather than treating them as an additional, independent step, demonstrating the integrated and practical nature of the technical solution.

[0077] Furthermore, according to one embodiment of the present invention, during the concrete pouring process, the internal pressure of the hollow pressure pad is maintained to be greater than or equal to 1.2 times the sum of the lateral pressure and buoyancy of the concrete; at the same time, the pressure is controlled to be less than or equal to the bearing capacity limit of the hollow pressure pad.

[0078] In this embodiment, during concrete pouring, the internal pressure of the hollow pressure pad is maintained at more than 1.2 times the sum of the lateral pressure and buoyancy of the concrete. This lower limit control is a mechanical guarantee to prevent grout leakage and deformation of the formwork cavity. During concrete pouring, the concrete grout in the wet joint generates lateral pressure on the bottom and sides of the formwork cavity. This pressure is determined by the fluidity of the concrete, the pouring height, and the vibration effect, and can be estimated according to the hydrostatic pressure model, that is, the lateral pressure is equal to the concrete unit weight multiplied by the pouring height multiplied by the lateral pressure coefficient. At the same time, the concrete grout generates an upward buoyancy on the bottom surface. This buoyancy is equal to the volume of the concrete grout displaced multiplied by the concrete unit weight. If there is a gap or residual gas between the isolation membrane and the hollow pressure pad, the buoyancy effect is more significant. If the internal pressure of the hollow pressure pad is insufficient to resist the sum of lateral pressure and buoyancy, the concrete slurry will break through the constraint of the isolation membrane and seep out from the bottom or side gaps of the formwork cavity, resulting in slurry leakage. This leakage not only causes concrete material loss and surface defects, but more seriously, it leads to incomplete concrete filling within the formwork cavity, resulting in voids or looseness in the wet joints, severely weakening the structural continuity of the long floating slab. A safety factor of 1.2 is a conventional value used in engineering design to balance safety and economy. It considers the uncertainty of concrete load calculations, errors in pressure monitoring, and dynamic disturbances during construction, ensuring that even under the most unfavorable conditions, the internal pressure remains greater than the external load, maintaining the geometric stability of the formwork cavity.

[0079] Simultaneously, controlling the pressure to be less than or equal to the pressure-bearing limit of the hollow pressure pad is a safety guarantee to prevent bladder damage and excessive wet joint dimensions. As a flexible sealed bladder, the pressure-bearing capacity of the hollow pressure pad is limited by the material strength, wall thickness, and structural form: for rubber pressure pads, the pressure-bearing limit is usually controlled by the circumferential tensile stress of the bladder wall. According to the tensile stress formula for thin-walled cylinders, the circumferential tensile stress equals the pressure multiplied by the radius divided by the wall thickness. When the circumferential tensile stress reaches the tensile strength of the material, the bladder ruptures; for thermoplastic polyurethane pressure pads, the pressure-bearing limit may also be affected by material creep and fatigue properties. Under long-term high pressure, the material gradually deteriorates, and the short-term pressure-bearing limit is higher than the long-term safe pressure-bearing value. If the internal pressure exceeds the bearing capacity limit, the pressure chamber may burst, causing instantaneous leakage of the pressurized medium and concrete slurry, resulting in construction safety accidents and quality problems. Even if bursting does not occur, excessive expansion will cause the cross-sectional height of the hollow pressure pad to exceed the design bottom formwork height, leading to an increase in the elevation of the bottom surface of the wet joint and an increase in the width of the wet joint, affecting the splicing accuracy of adjacent precast unit panels and the geometry of the track. The bearing capacity limit needs to be determined in conjunction with product specifications or type test data. For self-made pressure pads, it can be determined through a hydrostatic burst test, and one-third to one-half of the burst pressure can be taken as the bearing capacity limit for engineering applications, taking into account both safety margin and material durability.

[0080] The coordinated control of the lower and upper limits ensures that the hollow pressure pad remains within a safe operating range throughout the entire concrete pouring process: excessively low pressure leads to grout leakage and deformation, while excessively high pressure causes bursting and exceeding safety limits. Only by maintaining the pressure within a reasonable range can construction safety, molding quality, and structural accuracy be guaranteed simultaneously. In practice, a dual protection system of a pressure-reducing valve and a safety valve can be installed at the injection port: the pressure-reducing valve regulates the supply pressure to the design range to prevent overpressure injection; the safety valve automatically opens to release pressure in case of abnormal overpressure, serving as a final safety barrier. Pressure monitoring uses real-time readings from a pressure gauge, and the manual or automatic control system adjusts the injection volume based on the monitored values ​​to stabilize the pressure within the target range. For example, in a working condition with a slab thickness of 350mm and a wet joint width of 300mm, the estimated lateral pressure of the concrete is 0.015MPa, and the estimated buoyancy is 0.008MPa, with a sum of 0.023MPa. After multiplying by a safety factor of 1.2, the lower limit is 0.028MPa. If a rubber bladder pressure pad with a wall thickness of 3mm and a width of 500mm is used, its bearing capacity is approximately 0.5MPa. Therefore, the pressure control range is 0.028MPa to 0.5MPa. In actual operation, 0.03MPa to 0.3MPa is taken as the working pressure range, which is far from the lower limit to ensure safety and far from the upper limit to extend service life.

[0081] Furthermore, according to one embodiment of the present invention, the demolding strength is 75% of the design strength of the wet joint concrete; the speed of slowly releasing the pressurized medium is controlled such that the pressure drop per unit time is less than or equal to 10% of the initial pressure.

[0082] In this embodiment, the demolding strength is set at 75% of the design strength of the wet joint concrete. This value stems from the balance between the early mechanical properties of concrete and structural safety during formwork removal. Concrete strength development follows the timeline of hydration reaction. Initial strength increases rapidly after pouring; the 3-day strength reaches 40% to 50% of the design strength, the 7-day strength reaches 60% to 70%, and the 14-day strength approaches 80% to 90% of the design strength. A demolding strength of 75% falls within the 7-day to 14-day strength range. At this point, the concrete has sufficient load-bearing capacity to support its own weight and the upper construction loads, and early shrinkage deformation is largely complete, leading to a stable structural form. If the demolding strength is too low, such as 50% or 60%, the hydration products within the concrete have not yet formed a sufficient skeletal structure. During demolding, the constraints of the formwork system are suddenly released, causing the concrete to undergo instantaneous elastic rebound and plastic deformation under its own weight. Settlement cracks or flexural deformation may appear on the bottom surface of the wet joint, and this deformation cannot be recovered during subsequent hardening, resulting in excessive overall smoothness of the long floating slab. If the demolding strength is too high, such as 90% or 100%, the demolding time will be significantly delayed, the formwork system's usage period will be extended, construction efficiency will be reduced, and the interfacial bond between the concrete and the release membrane will strengthen over time, increasing the difficulty of demolding and thus increasing the risk of surface damage. The 75% strength threshold is an empirically optimized value in engineering practice, balancing the dual goals of structural safety and construction efficiency. This value can be determined through compressive strength tests on specimens cured under the same conditions. The specimens are cured in the same environment as the wet joint concrete, and the test results directly reflect the actual structural condition.

[0083] The rate of slow release of the pressurizing medium is controlled so that the pressure drop per unit time is less than or equal to 10% of the initial pressure. This rate limit stems from the brittle mechanical properties and interfacial debonding dynamics of early-stage concrete. Concrete reaching 75% demolding strength has an internal structure composed of cement hydration products, unhydrated cement particles, and aggregates. The hydration products interlock to form a porous skeleton, but the porosity of this skeleton is higher than that of later-hardened concrete, and the bond strength in the interfacial transition zone is not yet fully developed. Macroscopically, this manifests as a lower elastic modulus, higher brittleness, and sensitivity to uneven deformation. The hollow pressure pad serves as the bottom formwork support, covering the entire area of ​​the wet joint bottom surface. The supporting force is uniformly transferred to the concrete bottom surface through the isolation membrane. If the pressurizing medium is released rapidly, the contraction rate at different points on the pressure pad is inconsistent. Areas near the vent experience faster pressure drop and earlier contraction, while areas farther from the vent experience slower pressure drop and later contraction. This results in asynchronous loss of support at different points on the concrete bottom surface. Areas that contract first are partially suspended, while areas that contract later remain supported, creating an uneven stress field. The concrete in the suspended area undergoes downward bending deformation under its own weight, while the adjacent supporting area constrains this deformation, resulting in stress concentration. The brittle skeleton is prone to cracking at the stress concentration point, and the cracks extend along the interface transition zone, forming hairline cracks on the bottom surface or corner chipping.

[0084] From the perspective of interfacial peeling kinetics, there is a microscopic physical adsorption force between the release liner and the concrete substrate. This adsorption force originates from intermolecular van der Waals forces and capillary action at the interface, and its magnitude is positively correlated with the contact area, surface roughness, and contact time. The peeling process of the release liner during demolding can be regarded as a crack propagation process: the peeling front starts from a certain point and gradually expands along the interface. If the peeling speed is too fast, the expansion speed of the peeling front exceeds the stress relaxation speed of the concrete matrix, generating dynamic tensile stress at the interface. This stress may exceed the surface tensile strength of the concrete, causing the concrete surface layer to be torn and peeled off, forming a pitted surface or exposed aggregate. Controlling the pressure drop rate, i.e., controlling the contraction speed of the pressure pad, and thus controlling the peeling speed of the release liner, ensures that the expansion of the peeling front is always in a quasi-static process. The interfacial stress has sufficient time to relax through the viscoelastic deformation of the concrete matrix, and the peeling force is always less than the surface strength of the concrete, achieving non-destructive detachment. The pressure drop per unit time does not exceed 10% of the initial pressure, which means that the depressurization process lasts at least 10 time units. For example, if the initial pressure is 0.3 MPa, then the pressure drop per unit time (e.g., 1 minute) does not exceed 0.03 MPa, and the entire depressurization process takes at least 10 minutes. This duration is sufficient to ensure stress relaxation of the concrete matrix and quasi-static delamination of the interface.

[0085] In practice, the pressure relief rate can be controlled by adjusting the opening of the relief valve. For gaseous media, a needle valve or fine-tuning valve is used. The opening is gradually increased by rotating the handle, and the pressure gauge reading is observed to ensure that the pressure drop rate meets the requirements. For liquid media, the effect of liquid gravity must be considered during the relief process. A throttling orifice plate or capillary tube can be installed in the relief pipeline to limit the relief flow rate through flow resistance, thereby achieving uniform pressure relief. For example, if the initial pressure is 0.2 MPa and the target is to complete the pressure relief within 15 minutes, the allowable total pressure drop is 0.2 MPa, and the upper limit of the pressure drop per unit time (1 minute) is 0.02 MPa, which is 10% of the initial pressure. In actual operation, the pressure drop is controlled at 0.02 MPa / minute for the first 5 minutes, 0.015 MPa / minute for the middle 5 minutes, and 0.01 MPa / minute for the last 5 minutes, gradually slowing down but still not exceeding the upper limit, to adapt to the changes in the stress relaxation characteristics of concrete over time. The initial viscoelastic response of concrete is fast and can withstand a slightly faster pressure relief, while the response slows down in the later stages, requiring a slower pressure relief.

[0086] Furthermore, according to one embodiment of the present invention, the welding of the reserved longitudinal steel bars adopts the method of splicing, and the length of the splice bar meets the following requirements: greater than or equal to 6 times the diameter of the steel bar when welding on both sides, and greater than or equal to 12 times the diameter of the steel bar when welding on one side; after welding, at least two sets of transverse steel bars are tied in each wet joint to form a steel bar skeleton in the wet joint area.

[0087] In this embodiment, the welding of the reserved longitudinal reinforcement bars adopts splice welding. This welding method is chosen due to the extremely limited construction space of wet joints. The wet joint is located at the end gap of adjacent precast unit slabs, and the gap width is usually only about 300mm. Operators perform reinforcement connection work above the slab joint, making it impossible to perform contact welding or bevel welding, which require multi-directional operation, from the side or bottom. Splice welding only requires welding on one or both sides of the reinforcement lap section. The welder can complete all welding actions from above the wet joint, and its spatial adaptability is significantly better than other welding methods. The length of the splice is set based on the force transmission mechanism of reinforcement welding: the welded joint transmits force from one reinforcement bar to the splice bar through the weld, and then from the splice bar to another reinforcement bar. There are two abrupt changes in the cross-section of the force transmission path, and the stress concentration at each abrupt change is directly related to the weld length. When welding on both sides, the weld is distributed on both sides of the reinforcing bar, resulting in symmetrical force transmission and a smaller stress concentration factor. A splice length of 6 times the reinforcing bar diameter is sufficient to ensure that the weld bearing capacity is not less than the yield bearing capacity of the reinforcing bar base material. However, when welding on one side, the weld is distributed only on one side of the reinforcing bar, leading to eccentric force transmission and a larger stress concentration factor. Therefore, the splice length needs to be doubled to 12 times the reinforcing bar diameter to compensate for the adverse effects of eccentric force. This length parameter is derived from the "Code for Welding and Acceptance of Reinforcing Bars" (JGJ 18) and is a minimum structural requirement proven effective in engineering practice. A splice weld joint meeting this length has a tensile bearing capacity not less than the standard value of the tensile strength of the reinforcing bar base material, satisfying the force transmission requirements of the wet joint area as a load-bearing part of the long floating slab.

[0088] After welding, at least two sets of transverse reinforcing bars are tied in each wet joint to form a reinforcing skeleton for the wet joint area. The technical purpose of this structural measure is to inhibit crack development in the wet joint area and ensure the integrity of the cross-section. After the longitudinal reinforcing bars are welded, the longitudinal stress continuity of the wet joint area is restored. However, during the concrete pouring and hardening process, cement hydration shrinkage and temperature drop shrinkage generate tensile stress in the longitudinal direction. If only longitudinal reinforcing bars are configured without transverse restraint, the shrinkage tensile stress will cause through cracks to form in the wet joint concrete along the longitudinal direction. The crack width increases with shrinkage, weakening the integrity and durability of the long floating slab. The transverse reinforcing bars are arranged perpendicular to the longitudinal reinforcing bars and tied with them to form a spatial skeleton, providing lateral restraint to the longitudinal reinforcing bars. This allows the longitudinal reinforcing bars and concrete to work together, dispersing the shrinkage tensile stress into multiple small cracks rather than a single through crack. The crack width is controlled below 0.2 mm, meeting the requirements for waterproofing and durability. At the same time, the transverse reinforcing bars themselves bear the transverse tensile stress in the wet joint area. This stress originates from the Poisson effect under train load and the warping deformation caused by temperature gradient. Although the reinforcement ratio of the transverse reinforcing bars is lower than that of the longitudinal reinforcing bars, it is sufficient to prevent the development of transverse cracks. Each group of transverse reinforcement typically consists of two bars parallel to the width of the slab, located at the top and bottom of the wet joint section, respectively. The intersections with the longitudinal reinforcement are securely tied with binding wire to form a stable reinforcement mesh. At least two groups of transverse reinforcement are evenly distributed along the length of the wet joint to ensure that the longitudinal reinforcement is transversely restrained throughout its entire length, avoiding blind spots. For example, in a slab thickness of 350mm and a wet joint width of 300mm, the longitudinal reinforcement has a diameter of 20mm. When welding the splice, the double-sided welded splice is 120mm long (6 times the diameter), and the single-sided welded splice is 240mm long (12 times the diameter). The transverse reinforcement has a diameter of 12mm, with two bars per group, arranged along the length of the wet joint at a spacing of 150mm. After being tied with the longitudinal reinforcement, a 200mm × 150mm reinforcement mesh is formed, resulting in a reasonably reinforced wet joint section after concrete pouring.

[0089] Furthermore, according to one embodiment of the present invention, the hollow pressure pad is fixed by setting limiting members on the bottom surface or base of the prefabricated unit plate at both ends of the wet joint, and locking the two ends or the two sides of the hollow pressure pad in the limiting members to prevent the hollow pressure pad from sliding or shifting laterally along the length of the wet joint during the pressurization and expansion process.

[0090] In this embodiment, the hollow pressure pad acts as a flexible, sealed bladder. Its pressurization and expansion process is accompanied by significant changes in morphology and mechanical behavior. Without proper restraint, uncontrollable displacement will occur, leading to displacement of the template cavity and deviations in the dimensions of the wet joint. When unpressurized, the hollow pressure pad is flat, and its contact with the substrate or the bottom surface of the precast unit plate is surface contact with minimal friction. After pressurization, the internal pressure of the bladder increases, generating circumferential tension in the bladder wall. This tension generates a component force in the tangential direction between the bladder and the contact surface, pushing the pressure pad to slide along the length of the wet joint or to shift laterally. The driving forces for sliding and shifting originate from two aspects: first, the flow impact force when the pressurizing medium is injected. The medium enters from the injection port, flows along the internal cavity of the bladder, and exerts dynamic pressure on the bladder wall, creating a thrust on the opposite side of the injection port; second, the elastic rebound force of the bladder wall during expansion. When the bladder changes from a flat state to a bulging state, the material fibers undergo stretching and bending deformation. The tendency to recover from deformation is converted into sliding kinetic energy when there is insufficient restraint.

[0091] The limiting components are designed to provide a balancing reaction force for the aforementioned driving force, locking the spatial position of the hollow pressure pad within the design range. These limiting components are positioned on the bottom surface of the precast unit slabs or the base at both ends of the wet joint. This location utilizes the available space of the existing structural surface, eliminating the need for additional excavation or support, thus simplifying the fixing process. The limiting components can be rigid members such as angle steel, flat steel, or rebar ends, fixed to the bottom concrete of the precast unit slab or the base concrete using expansion bolts or chemical anchors. The fixing points are spaced along the length of the wet joint to ensure uniform distribution of the limiting force. The height of the limiting components is slightly higher than the thickness of the hollow pressure pad when unpressurized, allowing the pressure pad to naturally fall into the groove or slot formed by the limiting components after insertion. The width of the limiting components is determined based on the allowable lateral offset of the pressure pad, typically 1.1 to 1.2 times the width of the pressure pad, providing the necessary lateral space for the pressure pad's expansion while limiting its lateral offset within the allowable range.

[0092] The hollow pressure pad's two ends or side edges are secured within limiting members. The technical advantages of this securing method are: the securing position is located in the edge area of ​​the pressure pad, away from the expanded central support surface, preventing the limiting members from interfering with the bottom surface molding quality; the securing force acts in the thickness direction or lateral direction of the pressure pad, without damaging the integrity of the bladder wall, preventing media leakage caused by puncture or tearing. For slippage along the length of the wet joint, the limiting members form stops at both ends. The slippage driving force must overcome the friction between the pressure pad and the substrate, as well as the end reaction force of the limiting members. The end reaction force is provided by the anchoring strength of the limiting members and is usually much greater than the slippage driving force, thus completely constraining the slippage. For lateral offset, the limiting members form guide grooves on both sides. When the pressure pad expands, lateral expansion is limited by the groove walls, and the offset is controlled within ±5% of the designed joint width, ensuring that the bottom surface of the wet joint is centered between adjacent precast unit slabs, avoiding localized grout shortages or uneven joint widths caused by misalignment.

[0093] In this embodiment, for example, the wet joint is 4.5m long and 300mm wide, the hollow pressure pad is 500mm wide, and the unpressurized thickness is 30mm. A set of limiting components is installed at each end of the wet joint. Each set consists of two L50×5 angle steels, each with a 50mm long leg, fixed to the bottom surface of the precast unit slab with M10 expansion bolts at 200mm intervals. The length of the angle steel is equal to the width of the slab bottom. The shorter leg of the angle steel faces downwards to form a vertical retaining edge, while the longer leg rests against the slab bottom to form an anchoring surface. The net distance between the two angle steels is 520mm, slightly greater than the 500mm width of the pressure pad. After the hollow pressure pad is inserted, its two edges fall between the angle steel retaining edges, and its ends abut against the ends of the angle steels. After pressurization and expansion, the width of the pressure pad increases to approximately 480mm, and its thickness increases to 80mm. The lateral offset is limited to within 10mm by the limiting components, and the end slippage is limited to zero by the reaction force at the end of the angle steel. After the concrete is poured, the pressure pad thickness returns to 30mm after the pressure relief and shrinkage, and the lateral gap increases, allowing it to be easily pulled out from between the limiting components. The limiting components are retained at the bottom of the plate as a permanent structure or can be recycled after removal.

[0094] Furthermore, according to one embodiment of the present invention, the isolation membrane is laid continuously as a whole or laid in sections with overlapping; when sections are overlapped, the overlap length of adjacent isolation membrane sections is greater than or equal to 20cm, and the overlap is sealed with adhesive.

[0095] In this embodiment, the laying method of the release liner must balance construction convenience and sealing reliability. The length of the wet joint is usually an integer multiple of the length of the precast unit panel. However, the width of the release liner roll is limited. When the length of the wet joint exceeds the length of a single roll of membrane, segmented overlapping laying is required. The overlap length is greater than or equal to 20cm. This value is derived from engineering experience regarding the penetration depth of concrete grout: the particle size of cement grout is usually in the micrometer range. Under pressure differential, it can penetrate several centimeters to more than ten centimeters along the gap. The 20cm overlap length provides sufficient attenuation distance for the penetration path, so that the grout stops penetrating before reaching the far end of the overlap due to increased flow resistance, ensuring the sealing effectiveness of the overlap. The overlap is sealed with an adhesive, such as epoxy resin, polyurethane sealant, or rubber asphalt, which are alkali-resistant and elastic. The adhesive is applied to the upper surface of the lower isolation membrane section's end, and then the upper isolation membrane section is pressed together to form a continuous sealing interface. This prevents concrete slurry from seeping into the gap between the isolation membrane and the pressure pad, avoiding the formation of slag inclusions or adhesion after hardening, which would affect formwork removal and the quality of the bottom surface. When the wet joint length is short and the membrane material specifications allow, continuous, whole-piece laying is preferred to eliminate overlaps, simplify construction, and improve sealing reliability. Segmented overlaps serve as a supplementary method, providing dual protection through overlap length and adhesive sealing, while still meeting the isolation function requirements.

[0096] Furthermore, according to one embodiment of the present invention, the design pressure is determined based on the following: the area of ​​the support surface after the hollow pressure pad expands multiplied by the design pressure is equal to the sum of the self-weight load of the wet joint concrete, the construction live load, and the safety factor; the design pressure ensures that the expansion deformation of the hollow pressure pad is within its own elastic range.

[0097] In this embodiment, the determination of the design pressure is based on establishing a quantitative balance between the formwork support force and the external load, transforming the working state of the hollow pressure pad from an empirical operation to a calculable and controllable technical process. The total support force that the formwork system can provide is the area of ​​the expanded support surface of the hollow pressure pad multiplied by the design pressure. This support force must be greater than or equal to the sum of the self-weight load of the wet joint concrete, the construction live load, and the safety factor, ensuring that the support force is always sufficient to resist external loads throughout the concrete pouring process, preventing compression deformation or grout leakage of the formwork cavity. The self-weight load of the wet joint concrete is calculated by multiplying the concrete density by the wet joint volume. The construction live load includes the impact of concrete pouring, the vibration of the vibrator, and the operating load of construction personnel. The safety factor is taken as 1.2 to 1.5, comprehensively considering the uncertainty of load calculation and dynamic disturbances during construction. The technical significance of this inequality lies in transforming the originally vague empirical practice of "applying pressure to a certain level" into a clear mechanical equilibrium equation. The design pressure can be accurately calculated based on specific engineering parameters, rather than relying on empirical estimation, thus improving the scientific nature and reliability of the construction plan.

[0098] The design pressure ensures that the expansion and deformation of the hollow pressure pad are within its elastic range. This limitation forms the physical basis for the recycling of templates. As a flexible, sealed bladder, the hollow pressure pad's material (such as rubber or thermoplastic polyurethane) has a defined elastic limit. Within this range, the expansion and deformation are approximately linearly related to the internal pressure. After pressure release, the deformation can be fully recovered, and the bladder shrinks back to its original thickness, facilitating extraction from the bottom of the wet joint and reuse in the next construction section. If the pressure exceeds the elastic limit, the material undergoes residual plastic deformation. After pressure release, the bladder cannot fully shrink back, increasing its thickness and making insertion difficult. Furthermore, the strength decreases at locally thinned areas, posing a risk of bursting. The elastic range can be determined using the material's stress-strain curve. Typically, 80% of the yield strength is taken as the upper limit of the working pressure, or one-third of the burst pressure is taken as the safety limit.

[0099] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in this application.

[0100] It should be understood that the sequence number of each step in the invention and its embodiments does not absolutely imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

Claims

1. A construction method for air cushion formwork at wet joints of precast floating slabs, characterized in that, include: S1. Precast unit slab laying and rebar connection: Multiple precast unit slabs are laid on the base according to the design position, and the reserved longitudinal rebars of adjacent precast unit slabs are butted and welded to form a continuous long floating slab structure. S2. Air Cushion Formwork System Installation: A hollow pressure pad is inserted below the wet joint between adjacent precast unit panels as a bottom mold. The hollow pressure pad is centered and fixed along the length of the wet joint. An isolation film is laid on the upper surface of the hollow pressure pad. The isolation film is a flexible and smooth material used to isolate the concrete from the formwork and to serve as the forming surface of the concrete bottom. Auxiliary side formwork is installed on both sides of the hollow pressure pad, so that the upper surface of the hollow pressure pad and the side formwork together enclose a formwork cavity with an open top. S3. Air cushion expansion and sealing formation: Pressurizing medium is injected into the hollow pressure pad to expand the hollow pressure pad and tightly adhere it to the isolation membrane on the bottom surface of the wet joint. The expansion pressure is used to flatten the isolation membrane and eliminate wrinkles, forming a sealed template cavity with a flat support surface. The internal pressure of the hollow pressure pad is monitored. When the pressure reaches the design pressure and remains stable, it is determined that the template cavity is ready for concrete pouring. S4. Wet joint concrete pouring: Wet joint concrete is poured into the template cavity. The wet joint concrete fills the template cavity from bottom to top. Its bottom surface is formed by the expansion support surface of the hollow pressure pad, and the side templates surround the cavity. During the pouring process, the internal pressure of the hollow pressure pad is kept stable to resist the lateral pressure and gravity load of the concrete. S5. Template Removal and Recycling: After the concrete of the wet joint has hardened to the demolding strength, the pressurizing medium in the hollow pressure pad is slowly released to make it shrink evenly. The isolation membrane naturally detaches from the concrete surface as the hollow pressure pad shrinks. The shrunken hollow pressure pad is pulled out from the bottom of the wet joint, the side template is removed, and the wet joint construction is completed.

2. The construction method of air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The hollow pressure pad is a flexible, sealed bladder. Its cross-sectional height when unpressurized is less than the design bottom mold height of the wet joint, and its cross-sectional height after pressurization and expansion matches the design bottom mold height of the wet joint. The expansion height of the hollow pressure pad is adjusted by controlling the injection volume of the pressurizing medium.

3. The construction method of air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The isolation membrane is made of a smooth, flexible material and is laid on the upper surface of the hollow pressure pad to isolate the concrete from the formwork. During installation, the isolation membrane naturally covers the surface of the hollow pressure pad. When the hollow pressure pad expands, the expansion pressure pushes the isolation membrane evenly towards the bottom of the wet joint, causing the isolation membrane to change from a relaxed state to a taut and flat state, eliminating wrinkles generated during installation and forming a smooth concrete surface.

4. The construction method of air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The pressurizing medium can be liquid or gas. When liquid pressurizing medium is used, the incompressibility of the liquid makes the hollow pressure pad form a rigid support surface, enhancing its resistance to concrete loads. When gas pressurizing medium is used, the compressibility of the gas makes the hollow pressure pad undergo adaptive deformation during concrete pouring, buffering the impact load on the concrete.

5. The construction method of air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The method for determining the stability of the pressure is as follows: after the hollow pressure pad is pressurized to the design pressure, it is left to stand for a preset time. If the pressure drop does not exceed the preset proportion of the initial pressure, the sealing performance is deemed qualified; if the pressure drop exceeds the preset proportion, the leak point is investigated and repaired, and then the pressure is repressurized until the pressure stabilizes.

6. The construction method of air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, During the concrete pouring process, the internal pressure of the hollow pressure pad is maintained at more than 1.2 times the sum of the lateral pressure and buoyancy of the concrete; at the same time, the pressure is controlled to be less than or equal to the bearing capacity of the hollow pressure pad.

7. The method for constructing air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The demolding strength is 75% of the design strength of the wet joint concrete; the speed control of slowly releasing the pressurized medium is: the pressure drop per unit time is less than or equal to 10% of the initial pressure.

8. The method for constructing air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The reserved longitudinal steel bars are welded using splice welding, and the length of the splice bars meets the following requirements: greater than or equal to 6 times the diameter of the steel bar when welding on both sides, and greater than or equal to 12 times the diameter of the steel bar when welding on one side. After welding, at least two sets of transverse steel bars are tied in each wet joint to form a steel bar skeleton in the wet joint area.

9. The method for constructing an air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The hollow pressure pad is fixed by setting limiting components on the bottom surface or base of the prefabricated unit plate at both ends of the wet joint, and locking the two ends or sides of the hollow pressure pad in the limiting components to prevent the hollow pressure pad from sliding or shifting laterally along the length of the wet joint during the pressurization and expansion process.

10. The construction method of air cushion formwork for wet joints of precast floating slabs according to claim 1, characterized in that, The isolation membrane is laid continuously as a whole or laid in sections with overlapping sections; when sections are laid with overlapping sections, the overlap length between adjacent isolation membrane sections is greater than or equal to 20cm, and the overlap is sealed with adhesive.

11. The method for constructing air cushion formwork for wet joints of precast floating slabs according to any one of claims 1-10, characterized in that, The design pressure is determined by multiplying the support surface area of ​​the hollow pressure pad after expansion by the design pressure, which equals the sum of the self-weight load of the wet joint concrete, the construction live load, and the safety factor; the design pressure ensures that the expansion deformation of the hollow pressure pad is within its own elastic range.

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