A method and device for in-situ repair of damaged steel pipe joint based on stress field reconstruction

Abstract

The application discloses a damaged steel pipe joint in-situ repairing method and device based on stress field reconstruction, and belongs to the field of steel structure reinforcement. The method establishes a damage model through three-dimensional scanning, assembles a detachable outer sleeve and a positive and negative wire adjustable rod outside a main pipe, forms a temporary stress shunting device, quantitatively transfers a branch pipe load, and makes a damage area in a low stress state; adopts electric arc additive manufacturing to repair defects and open a pipe wall hole; a double-face sand grouting bag is arranged in the main pipe, and an expanded steel fiber concrete is grouted to form a pipe in-situ reinforcing core; the steel pipe and the steel fiber eddy current heating are excited through an outer wall spiral coil to realize rapid maintenance, and then additive sealing, staged unloading and corrosion protection are carried out. The matched device comprises an outer sleeve, an adjustable rod, a composite grouting bag and an induction coil. The application can realize low stress, minimally invasive and internal and external collaborative in-situ repairing, reduces a peak stress of a joint in a repairing stage by 31%, and improves a bearing capacity after reinforcement by 55%, and is suitable for building, bridge and offshore platform steel pipe trusses.

Description

Technical Field

[0001] This invention relates to the field of steel structure repair and reinforcement technology, specifically to a method and apparatus for in-situ repair of damaged steel pipe nodes based on stress field reconstruction, which is particularly suitable for in-situ repair and reinforcement of K-type nodes in steel pipe truss structures that suffer from compressive concave damage and tensile cracking damage. Background Technology

[0002] Steel pipe truss structures, with their advantages of light weight, high load-bearing capacity, and aesthetically pleasing design, have been widely used in large-span public buildings, bridge engineering, and offshore platforms. The K-type joint, the most common connection type in steel pipe trusses, is formed by welding a main pipe to two obliquely intersecting branch pipes and serves as the core hub for force transmission in the entire structure. During long-term service, K-type joints are highly susceptible to two typical types of damage due to static overload, fatigue load, and environmental corrosion: localized inward deformation of the main pipe wall under the action of the compressed branch pipes, and fatigue cracking at the welds of the tension branch pipes. These damages significantly reduce the joint's load-bearing capacity and stiffness, and in severe cases, may lead to the overall collapse of the structure.

[0003] Currently, the technologies for repairing damage to steel pipe joints can be mainly divided into the following four categories: External reinforcement with carbon fiber composite materials: Carbon fiber cloth is bonded to the surface of the joint using epoxy resin adhesive. This method is simple to construct, but it is a passive reinforcement that cannot restore the deformed geometric shape of the joint. Furthermore, the adhesive is prone to aging under high temperature, high humidity, and ultraviolet radiation, making it difficult to guarantee long-term durability, and it is especially unsuitable for marine environments.

[0004] Arc welding additive manufacturing repair method: This method involves layering metal material through welding to fill the damaged area. While this method can accurately restore the node's geometry, the damaged area still bears the original load during the repair process. The combined effect of welding heat input and structural load exacerbates crack propagation. Furthermore, it can only repair surface damage and cannot address the insufficient internal stiffness of the node.

[0005] Steel plate welding reinforcement method: Reinforcing steel plates are welded to the outside of the joint. This method can improve the load-bearing capacity of the joint, but it will significantly increase the self-weight of the structure. The residual stress generated by welding will worsen the fatigue performance of the joint, and at the same time, it will seriously affect the appearance of the structure.

[0006] Concrete-filled pipe reinforcement method: Concrete is poured into the entire length of a steel pipe to form a steel-concrete composite structure. This method is mostly used for new structures. However, for existing damaged joints, it has problems such as the inability to precisely pour concrete locally, difficulties in pouring, and long curing periods. In addition, pouring concrete into the entire pipe will result in a large amount of material waste.

[0007] In summary, existing technologies generally suffer from the following core defects: they cannot effectively control the stress state of the damaged area during the repair process, resulting in repair work being carried out under high stress, which poses safety hazards; they cannot simultaneously achieve precise restoration of the external geometry of the joint and effective reinforcement of the internal structure; the concrete curing cycle is long, and construction efficiency is low; and the improvement in the durability and fatigue performance of the repaired joint is limited. Therefore, developing an in-situ repair technology for steel pipe joints that integrates stress control, geometric repair, internal reinforcement, and rapid curing is of significant engineering importance. Summary of the Invention

[0008] To address the aforementioned issues, this invention aims to propose an in-situ repair method and apparatus for damaged steel pipe joints based on stress field reconstruction. This method can transfer some of the stress in the damaged area during the repair process using a temporary stress diversion device, creating safe, low-stress repair conditions. It utilizes 3D scanning and additive manufacturing technology to accurately restore the geometric shape of the damaged area. Furthermore, by locally injecting expandable steel fiber reinforced concrete into the joint, supplemented by electromagnetic induction heating to accelerate curing, it achieves integrated and coordinated repair and internal reinforcement of concave and cracked damage in the joint, significantly improving the load-bearing capacity and safety of the repaired joint while shortening the construction cycle.

[0009] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for in-situ repair of damaged steel pipe joints based on stress field reconstruction includes the following steps: S1. Three-dimensional digital modeling of damage: Three-dimensional scanning is performed on the concave and cracked damage areas of steel pipe nodes to collect geometric data, construct a three-dimensional damage model, and plan the subsequent additive manufacturing repair path. S2. Temporary stress diversion and control: Install detachable outer sleeves on the intact main pipe sections on both sides of the damaged node, and set up telescopic temporary rods between the two detachable outer sleeves to form a temporary stress diversion device; by adjusting the telescopic temporary rods, change the force on the branch pipe, transfer part of the load of the branch pipe to the intact section of the main pipe, so that the damaged area maintains a low stress repair condition. S3. Pretreatment of damaged substrate: Clean the anti-corrosion coating and oxide rust layer on the surface of the recessed and cracked damaged areas of the nodes, and perform crack arrest and beveling treatment on the cracks. S4. Additive manufacturing restoration of metal defects: Based on the 3D model, additive manufacturing process is used to fill the recessed area of ​​the main pipe in layers, repair the cracks of the branch pipe, and restore the original shape of the node; pipe wall openings are made in the repaired section of the main pipe wall as a channel to connect the inside and outside of the pipe. S5. Built-in double-sided sand-attached grouting installation: The foldable double-sided sand-attached grouting bag is sent into the inner cavity of the main pipe through the opening in the pipe wall. The bag body is axially unfolded, tensioned and positioned by the metal pull rings at both ends of the bag body. The bag body is inflated so that the outer wall of the bag body fits the inner wall of the main pipe. S6. Local composite grouting inside the pipe: Inject expandable steel fiber concrete into the bag, and after the vent hole overflows with continuous grout, seal the grouting port to form an internal concrete reinforcement section inside the main pipe. S7. Electromagnetic induction co-curing: A spiral coil is installed in the area of ​​the internal concrete reinforcement section corresponding to the outer wall of the main pipe. An alternating current is passed through it. Eddy current heat is generated by the steel pipe wall and the steel fibers inside the concrete, which accelerates the hydration and expansion reaction of the concrete. After the concrete reaches the set strength, the exposed bag accessories are cut off and the pipe wall opening is sealed by additive manufacturing process. S8. Graded stress release and temporary device removal: Gradually release the preload of the temporary stress diversion device in stages. After each stage of unloading, monitor the node strain and displacement. Continue unloading only after confirming that the node is in an elastic state. Remove all temporary diversion components after the load is completely reset. S9. Surface anti-corrosion and fireproof protection treatment: Grind and smooth all areas of additive repair and pipe wall welding, and spray anti-corrosion coating and fireproof coating in sequence to complete node protection.

[0010] Furthermore, the retractable temporary rod in S2 is a forward and reverse threaded adjustment rod, including a left-hand screw section, a right-hand screw section, and a matching forward and reverse threaded sleeve; rotating the forward and reverse threaded sleeve can simultaneously extend and retract the two screw sections, applying tension to the compressed branch pipe and pressure to the tensioned branch pipe; and the extension and retraction of the rod is precisely controlled by finite element pre-calibration and torque wrench quantitative control to achieve quantitative stress redistribution.

[0011] Furthermore, the base material of the double-sided sand-attached grouting bag in S5 is double-sided sand-attached cloth; the double-sided sand-attached cloth uses polyester woven fabric as the middle substrate, and both sides of the substrate are coated with a flexible epoxy resin layer and a bonding fine sand layer in sequence; the flexible epoxy resin layer forms a closed water barrier to isolate the pipe wall corrosion and lock in the concrete moisture to achieve self-curing, and the fine sand layer forms a mechanical interlocking force transmission structure with the inner wall of the steel pipe and the concrete respectively.

[0012] Furthermore, the S5 double-sided sand-attached grouting bag integrates metal pull rings at both ends, and a metal ring is embedded in the grouting port. The metal pull ring and the embedded metal ring are integrally formed and used for bag traction, tensioning and positioning.

[0013] Furthermore, the raw materials of S6 expansive steel fiber concrete include cement, fine aggregate, steel fiber, expansive agent, water-reducing agent and water; the steel fiber is uniformly dispersed in the concrete, with a steel fiber volume content of 1% to 2% and an expansive agent content of 8% to 12% of the cement mass. The steel fiber simultaneously achieves the dual functions of matrix toughening and electromagnetic heating medium; during the hardening process, the expansive agent generates radial expansion extrusion pressure, which improves the bonding strength between the concrete and the main pipe and the double-sided sand-attached grouting bag.

[0014] Furthermore, the spiral coil in S7 is a copper spiral coil with a coil turn spacing of 5-10mm, which is wound around the outer wall of the main pipe corresponding to the internal concrete reinforcement section; the heating temperature is constantly controlled at 50℃-60℃, and the heating is carried out for 3-12 hours, and then naturally cooled after reaching the preset time; the alternating magnetic field synchronously excites the main pipe wall and internal steel fibers to generate eddy current heat, so as to achieve uniform heating of the concrete inside and out.

[0015] Furthermore, the S8 stress release is divided into three levels, releasing 30%, 30%, and 40% of the total preload in sequence. After each level of stress release, the system is left to stand for 10 minutes, and the deformation and stress data of the nodes are collected in real time using strain gauges and displacement meters. Only after no abnormalities are found can the next level of unloading operation be carried out.

[0016] Furthermore, in S1, the 3D scanning uses a handheld laser scanner, which is then imported into reverse engineering software to generate a defect model; the S3 pre-processing process includes angle grinder grinding to remove rust and acetone cleaning; the S4 additive manufacturing process uses an electric arc wire melting process, and after every 3 layers are printed, interlayer hammering is performed to eliminate residual stress, with the interlayer temperature controlled at ≤150℃; the pipe wall opening in S4 is a narrow pipe wall opening extending along the pipe axis, and the edges of the opening are smoothly ground to avoid scratching the double-sided sand-coated grouting bag.

[0017] To achieve the above objectives, the present invention also provides a double-sided sand-attached grouting bag, applicable to the above-mentioned repair method, for local injection of expandable steel fiber reinforced concrete into damaged steel pipe joints for reinforcement. The bag includes a sealed bag body, a grouting port, and an vent hole. The bag body is made of double-sided sand-attached fabric, which is composite-formed from a middle polyester woven matrix, two sides of flexible epoxy resin layers, and an outer fine sand layer. Integrated metal pull rings are provided at both ends of the bag body, and a metal ring is embedded in the grouting port. The metal pull rings are used for inserting the bag body through the pipe and for positioning during tensioning and unfolding.

[0018] To achieve the above objectives, the present invention also provides an in-situ repair device for damaged steel pipe nodes based on stress field reconstruction, used to implement the above repair method, comprising: The detachable outer sleeve is installed in pairs on both sides of the intact main pipe section of the damaged node. It is spliced ​​by two semi-arc steel sleeves and locked with anchor bolts. A rubber pad layer is set on the inner side of the sleeve. A retractable temporary rod connects to detachable outer sleeves on both sides to form a temporary stress diversion device; Double-sided sand-lined grouting bags are installed inside the repair section of the main pipe; a spiral coil is wound around the outer wall of the main pipe and connected to a high-frequency induction heating power supply for electromagnetic co-curing of expansion-type steel fiber reinforced concrete.

[0019] Beneficial effects: This invention uses a temporary stress diversion device to actively regulate the stress in the damaged area, creating safe in-situ repair conditions; it uses additive manufacturing technology to accurately restore the geometric shape of the node, and forms a concrete reinforcement section inside the node through double-sided sand-filled grouting bags, combined with electromagnetic induction heating to accelerate curing, achieving synergistic repair of accurate shape restoration and internal reinforcement under low stress working conditions, significantly shortening the construction cycle and improving the load-bearing capacity and durability of the repaired node; Expandable steel fiber reinforced concrete is injected into the joint. The radial expansion of the expansion agent enhances the compression bond between the concrete and the inner wall of the steel pipe. Double-sided sand-attached grouting bags ensure effective force transmission between the concrete and the pipe wall, forming an internal reinforcement body, which significantly improves the load-bearing capacity and stiffness of the joint. Electromagnetic induction heating technology is used to accelerate the curing of steel fiber reinforced concrete. The steel fibers and the pipe wall act as heating elements at the same time, achieving uniform internal heating of the concrete, promoting hydration and expansion reactions, and significantly shortening the concrete strength development time, thereby shortening the overall repair cycle. The flexible epoxy resin layer of the double-sided sand-coated grouting bag not only serves as an adhesive substrate for the fine sand but also constitutes a continuous waterproof barrier. During concrete pouring, it effectively isolates moisture in the concrete, significantly reducing the risk of corrosion inside the steel pipe repair area. During the concrete curing stage, this waterproof layer prevents moisture from escaping from the concrete, creating a self-curing, water-retaining environment that allows the expansive steel fiber reinforced concrete to continuously hydrate and steadily develop its strength.

[0020] This invention achieves the organic synergy between additive manufacturing geometric repair and internal concrete reinforcement: externally, the geometric shape and force transmission path of the structure are restored, while internally, continuous reinforcement is provided by expansive steel fiber concrete. The waterproof and water-retaining function of double-sided sand-filled grouting bags reduces the risk of corrosion and promotes continuous hydration of concrete. Multiple measures work together to ensure the safety and durability of the repaired joint. Attached Figure Description

[0021] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a flowchart of the overall repair method of the present invention; Figure 2 A schematic diagram of the K-type node structure of the damaged steel pipe; Figure 3 A schematic diagram showing the installation status of the temporary stress diversion device; Figure 4 A schematic diagram of the node's shape after additive manufacturing repair; Figure 5 This is a schematic diagram of the detachable outer sleeve structure; Figure 6This is a schematic diagram of a retractable temporary rod structure; Figure 7 A schematic diagram of the installation of double-sided sand-coated grouting bags through pipes; Figure 8 This is a schematic diagram of grouting inside the pipe; Figure 9 This is a structural diagram of a double-sided sand-lined grouting bag. Figure 10 This is a detailed structural diagram of a double-sided layered structure with abrasive cloth applied to both sides. Figure 11 Schematic diagram of electromagnetic maintenance of the outer wall spiral coil; Figure 12 This is a schematic diagram of the node after grouting reinforcement is completed; Figure 13 A schematic diagram of the overall temporary stress diversion device for the truss; Figure 14 This is a schematic diagram of the stress testing points for the truss. Figure 15 This is a schematic diagram showing the relationship between the expansion and contraction of a retractable temporary member and the stress variation at the nodes. Figure 16 This is a schematic diagram of a local concrete infill node; Figure 17 A comparison chart of peak loads for different fill lengths.

[0022] In the diagram, 1-lower chord main pipe, 2-compression branch pipe, 3-tension branch pipe, 4-recessed area of ​​main pipe, 5-cracked area of ​​branch pipe, 6-upper chord main pipe, 7-upper chord outer sleeve, 8-lower chord outer sleeve, 9-anchor bolt, 10-retractable temporary rod, 11-additive manufacturing repair area, 12-upper chord arc-shaped steel sleeve, 13-rubber pad, 14-lower chord arc-shaped steel sleeve, 15-threaded rod, 16-positive and negative threaded sleeve, 17-pipe wall opening, 18-double-sided sand-attached grouting bag, 19-expandable steel fiber reinforced concrete, 20-double-sided sand-attached cloth, 21-vent hole, 22-grouting port, 23-metal pull ring, 24-embedded metal ring, 25-polyester woven fabric, 26-flexible epoxy resin layer, 27-fine sand layer, 28-spiral coil, 29-additive manufacturing repair opening area. Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0024] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0025] Example 1 See Figure 1-12 This embodiment takes a K-type circular steel pipe node as an example to illustrate the overall process flow of the method for repairing damaged nodes based on a combination of additive manufacturing and internal grouting reinforcement.

[0026] S1. Three-dimensional digital modeling of damage: Three-dimensional scanning is performed on the concave and cracked damage areas of steel pipe nodes to collect geometric data, construct a three-dimensional damage model, and plan the subsequent additive manufacturing repair path. In the specific implementation, a handheld 3D laser scanner is used to perform a holistic scan of the damaged area. The acquired point cloud data is imported into reverse engineering software and Boolean operations are performed with the reference model of the undamaged area to accurately obtain the geometric information and geometric model of the main pipe recessed area 4 and the branch pipe cracked area 5. Based on the defect model, the additive printing path is planned: the main pipe recessed area 4 adopts a curved surface printing path arranged along the circumference of the main pipe; the branch pipe cracked area 5 adopts a layer-by-layer filling path along the crack direction.

[0027] S2. Temporary stress diversion and control: Install detachable outer sleeves on the intact main pipe sections on both sides of the damaged node, and set up telescopic temporary rods between the two detachable outer sleeves to form a temporary stress diversion device; by adjusting the telescopic temporary rods, change the force on the branch pipe, transfer part of the load of the branch pipe to the intact section of the main pipe, so that the damaged area maintains a low stress repair condition. In the specific implementation, see Figure 3 Removable outer sleeves are installed on the intact sections of the main pipes on both sides of the damaged node; the removable outer sleeves on both sides are respectively installed between the upper chord main pipe 6 and the lower chord main pipe 1 to form a temporary stress diversion for the damaged node area; as shown below. Figure 5 As shown, the upper chord outer sleeve 7 consists of an upper chord arc-shaped steel sleeve 12 and a rubber pad 13, and the lower chord outer sleeve 8 consists of a lower chord arc-shaped steel sleeve 14 and a rubber pad 13, both fixed to the main pipe by anchor bolts 9. The function of the rubber pad 13 is to protect the surface of the main pipe and provide friction.

[0028] A retractable temporary rod 10 is installed between the upper chord outer sleeve 7 and the lower chord outer sleeve 8. For example... Figure 6 As shown, the retractable temporary rod 10 consists of a threaded sleeve 16 (with and without threads) and two threaded rod sections 15. The first threaded rod section is a left-handed screw, and the second threaded rod section is a right-handed screw. The threaded sleeve 16 has a hexagonal prism shape to facilitate tightening with a torque wrench.

[0029] Before construction, a finite element numerical model is first established based on the actual stress state, geometric dimensions, and material parameters of the node. The correspondence between the expansion and contraction of the expandable temporary member 10 and the stress regulation amount in the damaged area is determined through calculation and analysis. Specifically, different elongation or contraction amounts of the expandable temporary member are applied in the finite element model, and the stress values ​​at key locations in the damaged area are extracted. An expansion-stress regulation curve is plotted to obtain the theoretical expansion amount required to achieve the target stress reduction level. Then, the correspondence between the tightening torque of the adjusting threaded sleeve 16 and the expansion and contraction of the expandable temporary member 10 is established through laboratory calibration. During construction, the required theoretical expansion amount is found based on the target stress regulation amount, and the corresponding tightening torque is determined according to the calibration curve. A torque wrench is used to apply a predetermined torque to the adjusting threaded sleeve 16, thereby quantitatively controlling the degree of local stress regulation at the node.

[0030] In specific operation, for the pressure-bearing branch pipe 2, the forward and reverse threaded sleeve 16 is rotated to extend the retractable temporary rod 10, applying tension to the adjacent node; for the tension-bearing branch pipe 3, the forward and reverse threaded sleeve 16 is rotated in the opposite direction to shorten the retractable temporary rod 10 to apply pressure. During tightening, loading is applied in stages, with each stage increment being 20% ​​of the target torque value, until the predetermined torque is reached. Through the above adjustment method, pressure or tension is applied to the branch pipe, controlling part of the stress in the node area to the intact section of the main pipe, allowing the damaged area to enter a low-stress working state. The entire repair process is within a low-stress elastic range, preventing crack propagation during welding.

[0031] S3. Pretreatment of damaged substrate: Clean the anti-corrosion coating and oxide rust layer on the surface of the recessed and cracked damaged areas of the nodes, and perform crack arrest and beveling treatment on the cracks. In practice, an angle grinder is used to remove the anti-corrosion coating and oxide layer from the recessed area 4 of the main pipe and the cracked area 5 of the branch pipe. A circular anti-crack hole is drilled at the tip of the branch pipe crack, and then a V-shaped bevel is milled along the crack direction. The surface of the repair area is cleaned with acetone to remove rust and impurities, providing an additive metallurgical bonding surface. The bevel and anti-crack hole inhibit the continuous propagation of the crack during the repair process, achieving the cleanliness and roughness required for additive manufacturing.

[0032] S4. Additive manufacturing restoration of metal defects: Based on the 3D model, additive manufacturing process is used to fill the recessed area of ​​the main pipe in layers, repair the cracks of the branch pipe, and restore the original shape of the node; pipe wall openings are made in the repaired section of the main pipe wall as a channel to connect the inside and outside of the pipe. In practice, the repair is carried out using arc welding wire additive manufacturing equipment, and appropriate welding wire is selected based on the principle of equal strength.

[0033] First, repair the cracked area 5 of the branch pipe: After setting the current, voltage, wire feed speed and travel speed, first print the root filling layer of the V-shaped bevel, and then fill layer by layer until it is flush with the original surface of the branch pipe. After every 3 layers of printing, perform interlayer hammering to reduce residual stress.

[0034] Subsequently, the recessed area 4 of the main tube is filled and repaired: based on the set printing parameters and the curved conformal path planned in step S1, the layers are stacked one by one to fill the recessed area 4 of the main tube until it is flush with the original outer surface of the main tube, forming the additive manufacturing repair area 11.

[0035] During the printing process, the temperature of the molten pool is monitored using an infrared thermal imager to control the interlayer temperature to not exceed 150℃. After naturally cooling to room temperature, the additive surface is polished using an angle grinder to ensure a smooth transition with the original surface of the main tube.

[0036] In this step, according to the pre-planned design, two narrow pipe wall openings 17 are cut along the pipe axis on the pipe wall of the main pipe area after additive repair. The two openings are located at the longitudinal ends of the repaired section, and their size should ensure that the folded double-sided sand-coated grouting bag 18 can be just right inserted into the lower chord main pipe 1. The edges of the openings are smoothly polished to avoid scratching the double-sided sand-coated grouting bag 18.

[0037] S5. Built-in double-sided sand-attached grouting installation: The foldable double-sided sand-attached grouting bag is sent into the inner cavity of the main pipe through the opening in the pipe wall. The bag body is axially unfolded, tensioned and positioned by the metal pull rings at both ends of the bag body. The bag body is inflated so that the outer wall of the bag body fits the inner wall of the main pipe. In its specific implementation, the double-sided sand-lined grouting bag 18 is made of double-sided sand-lined cloth 20, a metal pull ring 23, and an embedded metal ring 24. For example... Figure 9 and Figure 10 As shown, the double-sided sand-coated cloth 20 includes a middle polyester woven fabric 25, a flexible epoxy resin layer 26 coated on both sides of the polyester woven fabric 25, and a fine sand layer 27 adhered to both sides of the polyester woven fabric 25 through the flexible epoxy resin layer 26. After curing, the flexible epoxy resin layer 26 forms a continuous and dense waterproof sealing layer, which can not only isolate the moisture in the concrete from contacting the inner wall of the main pipe, reducing the risk of corrosion inside the steel pipe repair area, but also prevent the moisture inside the concrete from escaping outward, forming a self-curing water-retaining environment, allowing the expansive steel fiber concrete 19 to continuously hydrate and steadily develop strength. The fine sand layers 27 on both sides are embedded in the concrete and pressed tightly against the inner wall of the steel pipe, respectively, ensuring reliable adhesion and shear force transfer between the grouting bag and the interfaces on both sides.

[0038] The double-sided sand-lined grouting bag 18 is equipped with a grouting port 22 and a vent 21. An embedded metal ring 24 and a metal pull ring 23 are used for bag forming and temporary fixation of the grouting bag position. Initially, the grouting bag is in a folded state, and its overall dimensions allow it to pass smoothly through the pipe wall opening 17.

[0039] During installation, the folded double-sided sand-coated grouting bag 18 is inserted into the main pipe through the opening 17 on one side of the pipe wall and pushed towards the other opening. The grouting bag is pushed so that only the metal pull ring 23 and the grouting port 22 protrude from the pipe wall opening on the current side, while the metal pull ring 23 at the other end protrudes from the opening 17 on the other side of the pipe wall. The protruding metal pull ring 23 is rotated, causing the bag to rotate and unfold inside the main pipe to a predetermined posture, thus unfolding the originally laterally folded bag along the axial direction of the main pipe. Then, an external tensioning device is used to hook the two metal pull rings 23 and apply appropriate tension to initially tension the grouting bag. Next, low-pressure air is injected into the bag through the grouting port 22, causing the bag to fully expand and unfold until the fine sand layer 27 on its outer surface is tightly adhered to the inner wall of the main pipe, completing the installation and positioning. At this point, the vent 21 should be located at the top of the bag.

[0040] S6. Local composite grouting inside the pipe: Inject expandable steel fiber concrete into the bag, and after the vent hole overflows with continuous grout, seal the grouting port to form an internal concrete reinforcement section inside the main pipe. In specific implementations, such as Figure 8 As shown, expandable steel fiber reinforced concrete 19 is injected into the double-sided sand-coated grouting bag 18 through the grouting port 22. The components of the expandable steel fiber reinforced concrete 19 include cement, fine aggregate, steel fibers, expanding agent, water-reducing agent, and water. Among them, the steel fibers are uniformly dispersed in the concrete, with a volume dosage of 1%-2%, which not only improves the tensile strength and toughness of the concrete, but also serves as the heat transfer medium for subsequent electromagnetic induction heating; the expanding agent dosage is 8%-12% of the cement mass, which generates volume expansion during the concrete hardening process, causing the concrete to exert radial extrusion force on the inner wall of the main pipe and the grouting bag, thereby enhancing the interfacial bonding.

[0041] During the grouting process, the grouting pressure is controlled so that the grout gradually fills the internal space of the grouting bag from bottom to top. When grout overflows evenly from the upper vent 21, it indicates that the grouting bag is full, grouting is stopped, and the grouting port 22 is sealed.

[0042] S7. Electromagnetic induction co-curing: A spiral coil is installed in the area of ​​the internal concrete reinforcement section corresponding to the outer wall of the main pipe. An alternating current is passed through it. Eddy current heat is generated by the steel pipe wall and the steel fibers inside the concrete, which accelerates the hydration and expansion reaction of the concrete. After the concrete reaches the set strength, the exposed bag accessories are cut off and the pipe wall opening is sealed by additive manufacturing process. Electromagnetic heating curing should be carried out immediately after grouting is completed. For example... Figure 11 As shown, a spiral coil 28 is wound around the outer wall of the main pipe at the position corresponding to the internal concrete reinforcement section. The spiral coil 28 is wound with copper wire, with a turn spacing of 5mm-10mm, and the two ends of the coil are connected to a high-frequency induction heating power supply. For structurally complex parts such as intersecting nodes, the coil can be wound in sections or the turn density can be adjusted as needed to ensure heating uniformity.

[0043] After being energized, the high-frequency alternating current generates an alternating magnetic field through the spiral coil 28, inducing eddy currents in the steel fibers within the main pipe wall and concrete, thus heating the concrete using the Joule heating effect. The steel fibers are evenly distributed within the concrete, forming an internal three-dimensional heating network that ensures uniform heating of the concrete. Simultaneously, the main pipe wall also heats up due to the induced eddy currents, resulting in synergistic heating from both inside and outside, promoting the hydration and hardening of the concrete and the hydration reaction of the expansion agent.

[0044] The heating temperature is controlled between 50℃ and 60℃, and the heating duration is 3 to 12 hours, preferably 3 to 6 hours; when the concrete strength grade or ambient temperature is low, the duration can be extended to 6 to 12 hours. Under these temperature conditions, the hydration reactions of cement and the expanding agent are significantly accelerated, and the expanding agent rapidly expands in volume, causing radial extrusion force on the inner wall of the main pipe and the grouting bag, thus rapidly increasing the bond strength between the concrete and the pipe wall. After heating, the mixture is allowed to cool naturally to room temperature.

[0045] During concrete curing, the flexible epoxy resin layer 26 of the double-sided sand-attached grouting bag 18 has excellent water retention properties, which can effectively prevent the internal moisture of the concrete from escaping outward, forming a self-curing water-retaining environment, so that the expansive steel fiber concrete can continue to hydrate and its strength can develop steadily.

[0046] To further illustrate the promoting effect of electromagnetic induction-assisted curing on the early hydration of expansive steel fiber reinforced concrete, the Arrhenius-type equivalent age relationship in maturity theory can be used for estimation:

[0047] In the formula, t e To convert to the equivalent age under standard curing conditions at 20℃, T ref This refers to the absolute temperature corresponding to 20℃. T i Let Δ be the measured absolute temperature inside the concrete during the i-th time period. t i For the corresponding duration, E a The apparent activation energy of concrete hydration. R This is the gas constant. The apparent activation energy of concrete is approximately 32.0–45.7 kJ / mol, corresponding to… E a / R Approximately 3849–5497 K; therefore, in engineering estimation, we can take... E a / R The value is 4000-5000K, and the specific value can be determined based on experimental calibration.

[0048] Using 20℃ standard curing as a reference, when the temperature of the internal concrete reinforcement section is stably controlled at 50℃~60℃ and cured continuously for 6 hours, it can be converted to approximately 21.3~46.5 hours of 20℃ standard curing according to the above relationship; continuous curing for 12 hours can be converted to approximately 42.6~93.0 hours of 20℃ standard curing. This shows that the synergistic eddy current heating of the steel pipe wall and steel fiber can significantly improve the early equivalent age of concrete, promote cement hydration and expansion agent reaction, and enable the internal concrete reinforcement section to obtain an early strength foundation for subsequent staged unloading within a shorter construction time.

[0049] After the concrete has cured for 3-7 days, use a cutting tool to remove the metal pull ring 23, grouting port 22, and excess bag material protruding from the pipe wall opening 17, restoring the pipe wall opening 17 to a smooth surface. Then, use an arc-wire additive manufacturing system to repair the two pipe wall openings 17, filling them with metal layer by layer until flush with the original surface of the main pipe, forming the additive manufacturing repair opening area 29. Use welding materials that match or have superior performance to the main pipe material during repair. After repair, perform non-destructive testing on the repair area to ensure repair quality; if necessary, perform localized heat treatment on the repair area to eliminate residual stress. After smoothing, restore the surface of the main pipe to its original integrity.

[0050] S8. Graded stress release and temporary device removal: Gradually release the preload of the temporary stress diversion device in stages. After each stage of unloading, monitor the node strain and displacement. Continue unloading only after confirming that the node is in an elastic state. Remove all temporary diversion components after the load is completely reset. In the specific implementation, the retractable temporary member 10 undergoes graded stress release. Stress release is performed in three stages: the first stage releases 30% of the preload, the second stage releases another 30%, and the third stage releases the remaining 40%. After each stage of stress release, the stress is held for 10 minutes, and strain gauges and displacement meters are used to monitor node deformation and stress changes in real time to confirm that the node is in an elastic working state. After stress release is completed, the retractable temporary member 10, the upper chord outer sleeve 7, and the lower chord outer sleeve 8 are removed sequentially.

[0051] S9. Surface anti-corrosion and fireproof protection treatment: Grind and smooth all areas of additive repair and pipe wall welding, and spray anti-corrosion coating and fireproof coating in sequence to complete node protection.

[0052] In practice, the additive manufacturing repair area 11 and the additive manufacturing repair opening area 29 are finally polished to remove weld slag and spatter, ensuring a smooth surface transition. Epoxy zinc-rich primer and intumescent fire-retardant coating are then sprayed to restore the original protective layer of the joint.

[0053] At this point, the overall repair process for the damaged K-type steel pipe node is complete.

[0054] This invention achieves an integrated repair process of local stress control, geometric repair, and internal reinforcement at the node through the coordinated operation of the above steps. The temporary stress diversion device reduces the stress concentration in the damaged area by changing the local force transmission path, providing low-stress construction conditions for subsequent additive manufacturing repair; additive manufacturing technology restores the node's geometric shape; internally expanded steel fiber reinforced concrete and double-sided sand-filled grouting bags form an internal reinforcement system, which, combined with electromagnetic heating to accelerate curing, achieves efficient in-situ repair of the damaged steel pipe node.

[0055] Example 2 This embodiment also provides a double-sided sand-attached grouting bag, which is applicable to the above-mentioned repair method. It is used for local injection of expandable steel fiber reinforced concrete to reinforce damaged steel pipe joints. The bag includes a sealed bag body, a grouting port, and an air vent. The bag body is made of double-sided sand-attached cloth, which is composed of a middle polyester woven matrix, two sides of flexible epoxy resin layers, and an outer fine sand layer. The two ends of the bag body are provided with integrated metal pull rings, and the grouting port is embedded with a metal ring. The metal pull rings are used for the bag body to pass through the pipe, tension, and position.

[0056] In the specific implementation, the main body of the bag in this embodiment is sewn with double-sided sandpaper 20. The double-sided sandpaper 20 has a three-layer composite structure: the middle polyester woven fabric 25 is a tensile base, the double-sided continuously coated flexible epoxy resin layer 26 is a layer of fine sand 27 is bonded on the outside; the bag body is reserved with grouting port 22 and venting hole 21; the two ends have integrated metal pull rings 23, and the grouting port is embedded with a metal ring 24.

[0057] The double-sided sand-attached grouting bag of this embodiment can be folded and inserted into the opening 17 of the narrow pipe wall. After being inflated, it completely fits the inner wall of the main pipe. It integrates the three functions of water isolation, water retention and bidirectional interlocking, which existing single-layer plastic grouting bags cannot achieve at the same time.

[0058] Example 3 To achieve the above objectives, this embodiment also provides an in-situ repair device for damaged steel pipe nodes based on stress field reconstruction, used to implement the above repair method, including: The detachable outer sleeve is installed in pairs on both sides of the intact main pipe section of the damaged node. It is spliced ​​by two semi-arc steel sleeves and locked with anchor bolts. A rubber pad layer is set on the inner side of the sleeve. A retractable temporary rod connects to detachable outer sleeves on both sides to form a temporary stress diversion device; Double-sided sand-lined grouting bags are installed inside the repair section of the main pipe; A spiral coil, wound around the outer wall of the main tube and connected to a high-frequency induction heating power supply, is used for electromagnetic co-curing of expansion-type steel fiber reinforced concrete.

[0059] In the specific implementation, the detachable outer sleeves are: upper chord outer sleeve 7 (upper chord arc-shaped steel sleeve 12 + rubber pad 13), lower chord outer sleeve 8 (lower chord arc-shaped steel sleeve 14 + rubber pad 13), and are locked with matching anchor bolts 9. Telescopic temporary member 10: threaded rod 15 + positive and negative threaded sleeve 16, connecting two sets of outer sleeves to form a stress diversion system; Double-sided sand-lined grouting bag 18, used as a special consumable for internal pipe reinforcement; Helical coil 28, connected to an external high-frequency induction heating power supply, is used for electromagnetic curing of expansion steel fiber concrete 19. Overall effect of the device: all components work together and match, modular assembly is carried out on site, no large hoisting equipment is required, and it can be operated at high altitudes, in small factories, and on offshore platforms; the whole device realizes a complete closed loop of "stress pre-diversion - internal grouting - rapid curing", and no single sleeve, coil, or grouting bag can independently complete the node reinforcement and repair.

[0060] Effect comparison and verification: To verify the technical effectiveness of the temporary stress diversion control and internal local reinforcement methods in the in-situ repair method for damaged steel pipe nodes based on stress field reconstruction of the present invention, a finite element analysis model of the steel pipe truss was established and verified by numerical calculation method.

[0061] See Figures 13-17 A finite element model of a steel pipe truss with a total length of 5000mm was established. The truss contains 3 K-type nodes. The overall structure is made of Q355 steel. The main pipe has a cross-sectional dimension of Φ200×8mm and the branch pipe has a cross-sectional dimension of Φ120×6mm.

[0062] In the model, the left end of the truss is fixed and the right end is free. A vertical displacement load of 15mm is applied at a position 1700mm from the right end.

[0063] Subsequently, the temporary stress diversion device described in this invention is installed near the middle K-shaped node, with the center of the device 510mm away from the center of the node.

[0064] During construction, a 2.8mm contraction displacement is applied to the telescopic temporary members on the tension side, and a 2.8mm elongation displacement is applied to the telescopic temporary members on the compression side, in order to form a local stress regulation effect at the nodes.

[0065] The structural response was calculated using the finite element analysis software ABAQUS, and... Figure 14 Stress data for the critical area of ​​node AK is extracted at the location shown.

[0066] The analysis results show that: After adopting the technical solution described in this invention, the stress level in the dangerous area of ​​the node is significantly reduced, while the stress level of the main pipe near the installation area of ​​the temporary stress diversion device is correspondingly increased, indicating that the local force transmission path of the node has changed and stress redistribution has occurred.

[0067] Further analysis shows that: The peak stress in the critical area of ​​the node is reduced by approximately 31%.

[0068] The above results demonstrate that the temporary stress diversion device of the present invention can effectively reduce the stress concentration in the damaged area and form a low-stress working state during the repair stage, providing favorable construction conditions for subsequent additive manufacturing repair and internal reinforcement.

[0069] In another embodiment, to verify the technical effect of internal local concrete reinforcement, a K-type node model with main pipe size Φ400×8mm and branch pipe size Φ190×6mm was established.

[0070] In the model, the two ends of the branch pipe are hinged, and the end of the main pipe is subjected to displacement load along the axis of the main pipe.

[0071] Comparative analysis was conducted using different local filling lengths, where the filling lengths were taken as 1, 1.25, 1.5, 1.75, and 2 times the diameter of the main pipe, respectively.

[0072] Numerical analysis was performed using the finite element analysis software ABAQUS.

[0073] The results show that: When the local filling length reaches 1 times the diameter of the main pipe, the load-bearing capacity of the node increases by about 55%; when the filling length is further increased, the increase in load-bearing capacity gradually decreases, because the load-bearing capacity is dominated by the failure of the branch pipe.

[0074] Furthermore, to determine the change in the failure control mode of the node after reinforcement, a branch pipe yield criterion was established based on the stress state of the branch pipe. Since the two branch pipes are symmetrically arranged, and each branch pipe forms a 45° angle with the main pipe, the peak axial load of the main pipe is... P u The axial force of a single branch pipe can be approximated by the axial components of the two branch pipes, therefore the equivalent axial force of a single branch pipe can be expressed as: N b = P u / (2cos45°). The cross-sectional area of ​​the branch pipe is A b =π[ d b ²-( d b -2 t b )²] / 4, where, db and t b These refer to the pipe diameter and wall thickness, respectively. For a Φ190×6mm branch pipe, A b =3468.32mm². Axial yield bearing capacity of the branch pipe. N b,y Cross-sectional area A b With yield strength f y The product of. Taking Q355 steel as an example, N b,y = A b f y =1231.25kN, corresponding to the main pipe load when both branch pipes reach axial yield. P b,y =2 N b,y cos45° = 1741.25 kN.

[0075] Finite element analysis results show that the peak load of the unreinforced node is 1105.00 kN, which translates to a nominal axial stress of 225.28 MPa for a single branch pipe, only 63.5% of the yield strength of Q355 steel. This indicates that local failure of the main pipe node area occurs in the unreinforced node before the branch pipe yields. When the reinforcement length is one time the diameter of the main pipe, the peak load increases to 1712.84 kN, and the nominal axial stress of a single branch pipe is 349.21 MPa, reaching 98.4% of the yield strength of Q355 steel, indicating that the branch pipe has essentially entered the yield critical state. When the reinforcement length is 1.5 times the diameter of the main pipe, the peak load is 1744.16 kN, and the nominal axial stress of a single branch pipe is 355.59 MPa, reaching the yield strength of Q355 steel. Furthermore, the finite element contour plots also show the failure mode of branch pipe yielding.

[0076] This indicates that after local filling with expansive steel fiber reinforced concrete, the local failure of the main pipe at the node is suppressed, and the branch pipe participates more in the stress and enters the yield state. The failure control mode of the node changes from local failure of the main pipe to yield control of the branch pipe.

[0077] The above results indicate that: Using locally filled expansive steel fiber reinforced concrete can effectively improve the load-bearing capacity of joints, while avoiding the material waste caused by full-length filling, thus balancing the requirements of reinforcement effect and economy.

[0078] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for in-situ repair of damaged steel pipe joints based on stress field reconstruction, characterized in that, Includes the following steps: S1. Three-dimensional digital modeling of damage: Three-dimensional scanning is performed on the concave and cracked damage areas of steel pipe nodes to collect geometric data, construct a three-dimensional damage model, and plan the subsequent additive manufacturing repair path. S2. Temporary stress diversion and control: Install detachable outer sleeves on the intact main pipe sections on both sides of the damaged node, and set up telescopic temporary rods between the two detachable outer sleeves to form a temporary stress diversion device; by adjusting the telescopic temporary rods, change the force on the branch pipe, transfer part of the load of the branch pipe to the intact section of the main pipe, so that the damaged area maintains a low stress repair condition. S3. Pretreatment of damaged substrate: Clean the anti-corrosion coating and oxide rust layer on the surface of the recessed and cracked damaged areas of the nodes, and perform crack arrest and beveling treatment on the cracks. S4. Additive manufacturing restoration of metal defects: Based on the 3D model, additive manufacturing process is used to fill the recessed area of ​​the main pipe in layers, repair the cracks of the branch pipe, and restore the original shape of the node; pipe wall openings are made in the repaired section of the main pipe wall as a channel to connect the inside and outside of the pipe. S5. Built-in double-sided sand-attached grouting installation: The foldable double-sided sand-attached grouting bag is sent into the inner cavity of the main pipe through the opening in the pipe wall. The bag body is axially unfolded, tensioned and positioned by the metal pull rings at both ends of the bag body. The bag body is inflated so that the outer wall of the bag body fits the inner wall of the main pipe. S6. Local composite grouting inside the pipe: Inject expandable steel fiber concrete into the bag, and after the vent hole overflows with continuous grout, seal the grouting port to form an internal concrete reinforcement section inside the main pipe. S7. Electromagnetic induction co-curing: A spiral coil is installed in the area of ​​the internal concrete reinforcement section corresponding to the outer wall of the main pipe. An alternating current is passed through it. Eddy current heat is generated by the steel pipe wall and the steel fibers inside the concrete, which accelerates the hydration and expansion reaction of the concrete. After the concrete reaches the set strength, the exposed bag accessories are cut off and the pipe wall opening is sealed by additive manufacturing process. S8. Graded stress release and temporary device removal: Gradually release the preload of the temporary stress diversion device in stages. After each stage of unloading, monitor the node strain and displacement. Continue unloading only after confirming that the node is in an elastic state. Remove all temporary diversion components after the load is completely reset. S9. Surface anti-corrosion and fireproof protection treatment: Grind and smooth all areas of additive repair and pipe wall welding, and spray anti-corrosion coating and fireproof coating in sequence to complete node protection.

2. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 1, characterized in that, The retractable temporary rod in S2 is a forward and reverse threaded adjustment rod, which includes a left-hand screw section, a right-hand screw section, and a matching forward and reverse threaded sleeve. Rotating the forward and reverse threaded sleeve can simultaneously extend and retract the two screw sections, applying tension to the compressed branch pipe and pressure to the tensioned branch pipe. Furthermore, the extension and retraction of the rod is precisely controlled by finite element pre-calibration and torque wrench quantitative control, thereby achieving quantitative stress redistribution.

3. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 1, characterized in that, The base material of the double-sided sand-attached grouting bag in S5 is double-sided sand-attached cloth; the double-sided sand-attached cloth uses polyester woven fabric as the middle matrix, and both sides of the matrix are coated with a flexible epoxy resin layer and a bonding fine sand layer in sequence; the flexible epoxy resin layer forms a closed water barrier to isolate the pipe wall corrosion and lock in the concrete moisture to achieve self-curing, and the fine sand layer forms a mechanical interlocking force transmission structure with the inner wall of the steel pipe and the concrete respectively.

4. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 3, characterized in that, The S5 double-sided sand-attached grouting bag has integrated metal pull rings at both ends, and a metal ring is embedded in the grouting port. The metal pull ring and the embedded metal ring are integrally formed and used for bag traction, tensioning and positioning.

5. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 1, characterized in that, The raw materials of S6 expansive steel fiber reinforced concrete include cement, fine aggregate, steel fiber, expansive agent, water-reducing agent and water; the steel fiber is uniformly dispersed in the concrete, with a steel fiber volume content of 1% to 2% and an expansive agent content of 8% to 12% of the cement mass. The steel fiber simultaneously achieves the dual functions of matrix toughening and electromagnetic heating medium; during the hardening process, the expansive agent generates radial expansion extrusion force, which improves the bonding strength between the concrete and the main pipe and the double-sided sand-attached grouting bag.

6. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 1, characterized in that, The spiral coil in S7 is a copper spiral coil with a coil turn spacing of 5-10mm, which is wound around the outer wall of the main pipe corresponding to the internal concrete reinforcement section; the heating temperature is constantly controlled at 50℃-60℃, and the heating is carried out for 3-12 hours. After reaching the preset time, it is naturally cooled; the alternating magnetic field synchronously excites the main pipe wall and internal steel fibers to generate eddy current heat, so as to achieve uniform heating of the concrete inside and out.

7. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 1, characterized in that, The S8 stress release is divided into three levels, releasing 30%, 30%, and 40% of the total preload in sequence. After each level of stress release, the system is left to stand for 10 minutes, and the deformation and stress data of the nodes are collected in real time using strain gauges and displacement meters. Only after no abnormalities are found can the next level of unloading be carried out.

8. The in-situ repair method for damaged steel pipe nodes based on stress field reconstruction according to claim 1, characterized in that, In S1, 3D scanning is performed using a handheld laser scanner, and the data is imported into reverse engineering software to generate a defect model. The S3 pre-processing process includes grinding and rust removal with an angle grinder and cleaning with acetone. The S4 additive manufacturing process uses an electric arc wire melting process, and after every 3 layers are printed, interlayer hammering is performed to eliminate residual stress, and the interlayer temperature is controlled to ≤150℃. The pipe wall holes opened in S4 are narrow pipe wall openings extending along the pipe axis, and the edges of the openings are smoothly ground to avoid scratching the double-sided sand-coated grouting bags.

9. An in-situ repair device for damaged steel pipe joints based on stress field reconstruction, used to implement the repair method according to any one of claims 1 to 8, characterized in that, include: The detachable outer sleeve is installed in pairs on both sides of the intact main pipe section of the damaged node. It is spliced ​​by two semi-arc steel sleeves and locked with anchor bolts. A rubber pad layer is set on the inner side of the sleeve. A retractable temporary rod connects to detachable outer sleeves on both sides to form a temporary stress diversion device; Double-sided sand-lined grouting bags are installed inside the main pipe repair section for localized injection of expandable steel fiber reinforced concrete to reinforce damaged steel pipe joints. The bags include a sealed bag body, grouting port, and vent. The bag body is made of double-sided sand-lined fabric, which is a composite of a polyester woven matrix in the middle, flexible epoxy resin layers on both sides, and a fine sand layer on the outer side. Integrated metal pull rings are installed at both ends of the bag body, and a metal ring is embedded in the grouting port. The metal pull rings are used for inserting the bag into the pipe and for positioning during tensioning and unfolding. A spiral coil, wound around the outer wall of the main tube and connected to a high-frequency induction heating power supply, is used for electromagnetic co-curing of expansion-type steel fiber reinforced concrete.

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