A composite repair and reinforcement technology for non-uniformly exposed steel tubular concrete columns after fire

Abstract

This invention discloses a composite post-fire repair and reinforcement technology for non-uniformly exposed concrete-filled steel tube columns. The technology includes a concrete-filled steel tube column, a wall, CFRP cloth, GFRP plate, SMA reinforcement, a fiber grid, and a UHPC surface layer. The concrete-filled steel tube column is inspected and assessed after a fire. CFRP cloth is applied to severely damaged areas, such as bulging, extending to the walls on both sides. Studs are welded to the steel tube surface, and L-shaped GFRP plates are installed at the junction of the column and wall and secured with tension bolts. SMA reinforcement is secured to the studs with a wire lock. Electrical current is applied to the SMA reinforcement to a preset temperature, applying a circumferential restraining force to the concrete-filled steel tube column. A fiber grid is suspended from the wall surface and secured with screws. A 30mm-thick UHPC containing mixed fibers is sprayed onto the wall and column surfaces. Because the CFRP cloth, SMA reinforcement, and UHPC provide three lines of defense for the structure, the technology effectively improves the mechanical properties of concrete-filled steel tube columns after a fire, while ensuring the durability requirements of the reinforced structure and enhancing its fire resistance to secondary fire.

Description

Technical Field

[0001] The present invention relates to the technical field of building structures, in particular to a composite repair and reinforcement technology for non-uniformly exposed steel tube concrete columns after fire. Background Art

[0002] Concrete-filled steel tube (CFST) columns have been widely used in high-rise and super-high-rise buildings and long-span structures due to their excellent bearing capacity, seismic performance, and ease of construction. Fire is a common and recurring disaster, and the steel tubes of CFST columns may be directly exposed to fire. Under the coupled effects of load and temperature fluctuations of fire, the material properties of the steel tubes and concrete are severely degraded. The steel tubes may buckle locally or even detach from the core concrete. Even if a CFST column does not fail completely in a fire, its mechanical properties, such as bearing capacity, stiffness, and ductility, may be significantly reduced after the fire, rendering the exposed structure unusable or posing serious safety hazards, necessitating damage assessment and repair and reinforcement. In particular, in actual building fires, the walls adjacent to the columns block the spread of fire, and the column components are subjected to non-uniform fire, forming real non-uniform fire boundaries on two or three sides. This leads to non-uniform stress and non-uniform material degradation caused by the non-uniform temperature field in the fire-exposed columns, which in turn causes non-uniform constraint effects, additional eccentricity, and non-uniform residual deformation of the components after the fire, making the damage assessment and repair and reinforcement of non-uniformly fired steel tube concrete columns relatively complex and difficult. Currently, relevant technologies are still blank.

[0003] For steel tube concrete columns exposed to fire, post-fire repair and reinforcement requires comprehensive consideration of multiple factors. In addition to the degree of fire, structural deformation, interface damage, etc., the mechanical properties of the reinforced structure after secondary fire exposure must also be considered. Currently, commonly used structural reinforcement technologies at room temperature can improve the mechanical properties of the structure, but they have certain limitations when used for steel tube concrete structures that have been subjected to non-uniform fire and may be exposed to secondary fire:

[0004] 1) FRP Sheet Reinforcement: FRP is lightweight, high-strength, corrosion-resistant, easy to construct and maintain, and has excellent fatigue resistance. It can be flexibly installed to adapt to structural configurations. However, the epoxy resin adhesive used to bond FRP loses its adhesion at a conversion temperature of approximately 100°C, significantly reducing the performance of FRP-reinforced structures after secondary fire exposure. Furthermore, due to the presence of partition walls, the FRP sheet has difficulty effectively bonding to the reinforced surface at the internal corners where the wall connects to the column, and is prone to bulging and falling off. Therefore, it is urgent to develop FRP sheet wrapping methods and technologies that can effectively reinforce columns subjected to uneven fire on three or two sides, while taking into account the influence of the wall.

[0005] 2) Steel plate reinforcement: This effectively improves the bearing capacity of components without significantly increasing their cross-section. However, unprotected steel has poor fire resistance and high thermal conductivity, leading to significant degradation of its mechanical properties at high temperatures. This affects the performance of the reinforced structure in secondary fires, making it difficult to meet the primary fire resistance requirements for columns. Therefore, an additional fireproofing layer is required, which is time-consuming, labor-intensive, and costly.

[0006] 3) Prestressed reinforcement: By applying circumferential prestressing, the structure's load-bearing capacity and seismic performance are improved. However, traditional prestressed reinforcement technology places high demands on the structure's original form and stress state. During secondary fire, the prestressed structure may be exposed to high temperatures, resulting in severe prestress loss and brittle failure. This makes it difficult to use in steel tube concrete-filled structures that lack a secondary fire protection line.

[0007] 4) Reinforcement by increasing cross-sections: This method increases the bearing capacity and stiffness of components by encasing them in concrete. However, on-site concrete pouring is a wet operation, requiring additional formwork support. The increased cross-section size also occupies more usable building space. Using ultra-high performance concrete (UHPC) instead of conventional concrete to reinforce existing structures effectively reduces the added cross-section area and has minimal impact on the stiffness of the original structure. However, UHPC has dense pores and is prone to cracking and shedding at high temperatures, affecting the fire resistance of the reinforced structure.

[0008] Therefore, it can be seen that the current single traditional reinforcement method cannot achieve the ideal repair effect for steel tube concrete columns damaged by non-uniform fire. There is an urgent need for a composite reinforcement technology that is convenient to operate, effective in repair, and can meet the structural durability and fire resistance requirements of secondary fire. In recent years, a variety of new high-performance materials have emerged in the civil engineering field, such as ultra-high performance concrete (UHPC), shape memory alloys (SMA), and fiber-reinforced plastics (FRP). Their application in the post-fire repair and reinforcement of steel tube concrete columns with non-uniform fire damage has broad prospects. They can significantly improve the mechanical properties of the fire-damaged structure and meet the first-level fire resistance requirements of the reinforced columns without the need for external fire protection materials. Summary of the Invention

[0009] In view of the deficiencies of the prior art, the present invention discloses a composite repair and reinforcement technology for non-uniformly exposed steel tubular concrete columns after fire, so as to solve the problems raised in the above background technology.

[0010] To achieve the above objectives, the present invention provides the following technical solution: a composite repair and reinforcement technology for non-uniformly exposed steel tube concrete columns after fire, comprising the following steps:

[0011] S1. First, after the fire, the damaged areas of the steel tube concrete columns were inspected and identified, and the walls that had surface peeling after the fire were cleaned and leveled with repair mortar;

[0012] S2. Apply epoxy resin glue to the damaged parts of the steel tube concrete column such as buckling, and then glue the CFRP sheet. The glued area extends to the walls on both sides.

[0013] S3. At the inner corner where the concrete-filled steel tube column and the wall meet, studs are locally welded in the non-reinforced area of ​​the steel tube surface. Holes are punched in the non-reinforced areas of the walls on both sides to reserve tension bolt holes for installing tension bolts. An L-shaped GFRP plate is installed at the inner corner where the concrete-filled steel tube column and the wall meet. The L-shaped GFRP plate has a hole on the side where it connects to the steel tube and the wall. Epoxy resin glue is applied to the back of the L-shaped GFRP plate in the area not in contact with the CFRP sheet as a flexible cushioning layer. The L-shaped GFRP plate is fixed with tension bolts on the wall side.

[0014] S4. Mechanically stretch the SMA reinforcement and slowly unload it after reaching a certain strain. Fix the SMA reinforcement to the studs at the inner corner where the steel tube and the wall meet using a wire lock. Electrically excite the SMA reinforcement at both ends to heat it to a preset temperature, so that the SMA reinforcement generates prestress and exerts a circumferential restraint force on the concrete-filled steel tube column.

[0015] S5. Drill holes in the wall surface at the junction of the horizontal and vertical bars of the fiber grid mesh. Screw the holes into the holes to secure the fiber grid mesh to the wall surface. Apply epoxy resin glue to the CFRP sheet in the overlapping area to adhere the fiber grid mesh to the CFRP sheet.

[0016] S6. Weld short steel bar heads on the surface of the steel pipe, spray UHPC containing mixed fibers on the surface of the steel tube concrete column and wall to form a reinforced surface layer, and finally complete the repair and reinforcement of the steel tube concrete column.

[0017] Preferably, step S1 specifically includes the following steps: according to the actual non-uniform fire boundary of the building, including the situation of two-side fire or three-side fire, a post-fire reinforcement scheme for steel tube concrete columns is selected, and a non-destructive testing method for steel tube concrete columns is used to determine the seriously damaged parts such as steel tube bulging and steel tube-concrete delamination, and then determine the CFRP cloth reinforcement position; clean the oxide layer produced by the fire on the surface of the steel tube at the reinforcement position, and for steel tube concrete structures with fire protection layers, if it is confirmed after the fire that the structure does not need to be reinforced, the original fire protection layer is still used to replace the small amount of fire protection material missing in the burned area. Repair and strengthen. If the fire protection layer is damaged and falls off on a large scale after the fire, replace the fire protection material in the burned area, remove the original fire protection material and re-construct. For the unburned area of ​​the steel tube concrete column, repair the material that is not water-resistant after being soaked in water. The fire protection structure and material should be the same as the original component. If it is necessary to change, the fire resistance limit of the repaired component should not be lower than that of the original component. Clean and polish the surface peeling area of ​​the wall connected to the steel tube concrete column caused by fire, remove impurities and dust, fill cracks and defects with repair mortar or repair glue, and ensure that the wall surface is flat.

[0018] Preferably, the non-destructive testing method for the steel tube concrete column includes traditional methods such as manual tapping method, induced vibration method, ultrasonic testing method and infrared thermal imaging method, as well as emerging technologies such as ultrasonic phased array.

[0019] Preferably, in step S2, several groups of CFRP sheets are pasted at intervals along the height direction of the steel tube concrete column, and at the same time, the CFRP sheets are pasted in the middle and end parts of the steel tube concrete column where the stress is greater. The pasting direction of the CFRP sheets is horizontal and extends to the walls on both sides.

[0020] Preferably, precise and targeted reinforcement is carried out according to the actual damage degree of the steel tube concrete column, that is, the number of CFRP cloth layers is determined according to the damage degree, and the CFRP cloth is extended to the areas where the walls on both sides are more severely damaged. When the damage to the wall after the fire is not serious, the extension length of the CFRP cloth is 1000mm, and the CFRP cloth is pasted with the formulated epoxy resin glue, which includes colorless and transparent epoxy glue and light yellow curing agent.

[0021] Preferably, in step S3, studs are locally welded on the surface of the non-CFRP cloth reinforced area of ​​the steel pipe inner corner. The welding studs can be welded by manual arc welding, and the welding equipment is simple and convenient and flexible to operate; holes are punched in the non-CFRP cloth reinforced area of ​​the wall to reserve tension bolt holes, and holes are opened in the L-shaped GFRP plate. The hole diameter position and size are respectively consistent with the ends of the welding studs on the steel pipe surface and the tension bolt holes in the wall. Epoxy resin glue is applied to the area on the back of the L-shaped GFRP plate that is not in contact with the CFRP cloth as a flexible cushion layer. The long side of the L-shaped GFRP plate is fixed to the wall and fixed by tension bolts. The short side is fixed to the surface of the steel pipe. The reinforced steel tube concrete column includes a two-sided non-uniformly fire-exposed column and a three-sided non-uniformly fire-exposed column.

[0022] 1) When repairing and reinforcing columns with uneven fire damage on both sides, the studs are respectively fixed on the non-CFRP reinforced areas on the adjacent sides of the concrete-filled steel tube column;

[0023] 2) When repairing and reinforcing a column that is unevenly exposed to fire on three sides, the studs are respectively fixed on the non-CFRP cloth reinforced areas on the opposite sides of the concrete-filled steel tube column.

[0024] Preferably, in step S4, the SMA bar is an iron-based SMA (Fe-SMA) bar. Fe-SMA is inexpensive, cost-effective, and has good fatigue resistance. Other types of shape memory alloys can also be used. At a temperature lower than the phase transition temperature of the SMA bar, a jack is used to mechanically stretch the SMA bar to 6%, and the stretching length does not exceed the recoverable range. The SMA bar is fixed to the peg through the wire locker proposed by the present invention. The wire locker has two holes. One end of the SMA bar passes through the upper hole and passes through the lower hole to form a ring sleeve. The SMA reinforcement is fixed by tightening the wire lock on the bolts on the surface of the steel pipe; the two ends of the SMA reinforcement are energized by an arc welding machine. When the SMA reinforcement reaches 200℃ and shows a shrinkage and tensioning trend and begins to stabilize, the heating is stopped, so that the SMA reinforcement generates a certain degree of prestress, thereby exerting a circumferential constraint force on the steel tube concrete column; when the reinforced steel tube concrete column is subjected to a secondary fire, the SMA reinforcement indirectly heats up due to the fire and reaches the complete phase change temperature (400℃), generating a secondary constraint force on the steel tube concrete column, thereby improving the fire resistance limit of the column.

[0025] Preferably, in step S5, the fiber grid mesh is arranged along the wall, and the width extends to the severely damaged area of ​​the wall or is not less than 1m. The fiber grid mesh is a bidirectional glass fiber grid, and installation slots are opened at the connection between the horizontal bars and the vertical bars of the fiber grid mesh. Holes are drilled at the corresponding installation positions of the wall, and residual debris in the holes are cleaned. Structural adhesive is injected into the holes, and screws are screwed into the holes in the wall to fix the fiber grid mesh.

[0026] Preferably, in step S6, the welded steel bar head is welded in the non-CFRP cloth reinforcement area on the surface of the steel pipe, using a small-diameter threaded steel bar head for welding to achieve a reliable connection between the UHPC surface layer and the steel tube concrete column interface. The sprayed UHPC adopts a rapid spraying equipment such as a hose peristaltic mortar sprayer. The thickness of the UHPC does not exceed 30 mm, and the mixed fibers therein include recycled tire polymer (RTP) fibers, polyvinyl alcohol (PVA) fibers, and polyethylene (PE) fibers. While ensuring the reinforcement effect of the UHPC, the incorporation of the mixed fibers can significantly reduce the density and thermal conductivity of the UHPC, and can melt to form channels when the reinforced structure is subjected to a secondary fire, thereby discharging internal high-temperature water vapor and preventing the UHPC from bursting.

[0027] Compared with the existing single structure room temperature reinforcement technology, the present invention has the following beneficial effects:

[0028] 1. The composite reinforcement technology proposed in the present invention for the repair of steel tube concrete columns after fire can achieve rapid and effective repair of composite columns under the actual non-uniform fire conditions of the wall, while eliminating the adverse effects of additional eccentricity and non-uniform constraint effects inside the burned column caused by non-uniform fire.

[0029] 2. The present invention adopts CFRP cloth, SMA bars, UHPC and other new high-performance materials to form three lines of defense for repair and reinforcement, providing sufficient redundancy and safety for improving the mechanical properties of the structure after fire and the fire resistance performance of secondary fire. Among them, the UHPC surface layer is the first line of defense for reinforcement and repair and secondary fire, which improves the bearing capacity and stiffness of the column components after fire, while reducing the newly added cross-sectional area. The UHPC is mixed with mixed fibers such as RTP, PVA and PE to effectively prevent the occurrence of bursts and reduce the thermal conductivity of UHPC. When the fire occurs again, it effectively reduces the heating rate of the internal structure and meets the thermal insulation performance requirements of the structure during normal use. As a second line of defense for reinforcement, repair, and secondary fire exposure, the SMA bars are electrically heated to generate partial prestress, applying a first-stage circumferential constraint to the CFST columns, improving their mechanical properties after fire. Furthermore, during secondary fire exposure, the SMA heats up at a controlled rate, remaining within the complete phase transition temperature, providing secondary circumferential constraint for the columns exposed to fire. CFRP sheets, a third line of defense for reinforcement, repair, and secondary fire exposure, directly improve the mechanical properties of the CFST columns after fire. Furthermore, during secondary fire exposure of the external structure, the insulation provided by the UHPC surface layer allows the CFRP sheets to maintain a relatively low temperature for extended periods, providing continuous and effective constraint on the CFST columns. These three lines of defense organically combine, work together, and reinforce each other, forming a new trinity composite reinforcement technology that balances improved mechanical properties of CFST columns after non-uniform fire exposure with fire resistance performance after secondary fire exposure.

[0030] 3. The present invention uses CFRP sheets to reinforce the fire-exposed surface of the non-uniformly exposed concrete-filled steel tube columns and extends this to the walls on both sides. The pasting area and number of CFRP sheets are determined comprehensively based on the stress characteristics and damage degree of the exposed columns. This can effectively improve the bearing capacity, stiffness, and ductility of the concrete-filled steel tube columns after fire, while also achieving the effects of saving materials, reducing costs, and facilitating construction.

[0031] 4. The present invention installs unequal-sided L-shaped GFRP plates at the inner corners of the concrete-filled steel tube column and the wall to fix the CFRP cloth, effectively solving the problems of difficult bonding and fixing of the CFRP cloth at the inner corners, poor reinforcement effect, and easy loosening and falling off. The L-shaped GFRP plates are effectively connected to the wall and the fire-affected column respectively by tension bolts and studs. The tension bolts anchor the FRP plates on the wall side. For irregular deformation of the fire-affected surface of the concrete-filled steel tube column, the L-shaped GFRP plates can be effectively connected to the structure to constrain and limit deformation. FRP material has the characteristics of light weight, high strength, corrosion resistance, etc., and its thermal conductivity is relatively low, which avoids the problems of using angle steel to fix it in the structure to form thermal bridges, and effectively takes into account the structural bearing capacity and thermal performance during normal use.

[0032] 5. Compared to conventional prestressed steel bars, the present invention employs cost-effective Fe-SMA prestressing tendons, achieving a two-stage prestressing effect on fire-exposed columns, thereby balancing the structural component's room-temperature mechanical properties and secondary fire resistance. Because SMA tendons lack a distinct yield point, they are first prestressed to a strain of 6%. After unloading, the SMA tendons are fixed at both ends. By applying electrical current to the SMA tendons, the tendons are heated above 200°C, generating maximum restoring force during the heating process. Furthermore, during the SMA tendon cooling phase, its restoring stress continues to increase, further imparting a first-stage circumferential constraint to the concrete-filled steel tube column, effectively improving the bearing capacity of the reinforced structural component at room temperature. When the structure is subjected to a secondary fire, after undergoing the complete heating and cooling process, the SMA tendons are heated again to the complete phase transition temperature. The resulting restoring force acts as a permanent prestress, enhancing structural performance. This in turn imparts a secondary circumferential constraint to the concrete-filled steel tube column, improving both the structural fire resistance during secondary fire and the mechanical properties after secondary fire. In addition, the wire locker has a simple structure and is easy to construct, which can achieve reliable fixation of SMA tendons and effective application of two-stage prestressing.

[0033] 6. The present invention sprays a layer of UHPC surface layer on the surface of steel tube concrete columns and walls. UHPC has the advantages of high strength, impact resistance, and good durability. Construction is fast and convenient. The thickness of the UHPC surface layer does not exceed 30 mm. Without affecting the usable space of the building, it can effectively improve the mechanical properties of the fire-exposed columns and have an aesthetically pleasing appearance. It can also serve as a fireproof protective layer, reducing the temperature of the internal steel pipes, CFRP cloth and SMA reinforcement and significantly improving their durability. At the same time, it can also increase the thermal resistance of the wall and reduce the overall energy consumption of the building.

[0034] 7. In the present invention, a hybrid fiber of RTP fiber, PVA fiber and PE fiber is added to the UHPC surface layer. While ensuring the strength and reinforcement effect of UHPC, the hybrid fiber can melt at high temperature to form channels, emitting internal high-temperature water vapor to prevent the UHPC layer from bursting. At the same time, it reduces the thermal conductivity of UHPC and the heating rate under fire, effectively lowering the temperature of the internal SMA reinforcement and CFRP cloth, ensuring that they can still function when the structure is subjected to secondary fire. It can also reduce the dead weight of the structure and reduce the reinforcement cost. At the same time, the recycled tire fiber can realize solid waste utilization, carbon reduction and energy saving, and green environmental protection, which meets the national dual carbon strategic needs.

[0035] 8. To ensure the effective function of the UHPC reinforced surface layer, this invention welds steel bar heads to the surface of the concrete-filled steel tube columns to enhance the interface between the steel tube and the UHPC. A fiber grid mesh is used to strengthen the bond between the UHPC surface layer and the reinforced wall. Furthermore, the studs and tension bolts used to secure the L-shaped GFRP panels also serve as connectors, further enhancing the interface between the UHPC surface layer, the wall, and the concrete-filled steel tube columns. The fiber grid mesh is made of bidirectional glass fiber and features high strength, lightweight, corrosion resistance, excellent thermal stability, and low thermal conductivity. Mounting slots are provided at the junctions between the horizontal and vertical bars of the fiber grid mesh, making installation more convenient and quicker while also helping to improve the accuracy and stability of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0037] In the attached figure:

[0038] Figure 1 It is a plan view of the composite reinforcement method of a three-sided fire-exposed steel tubular concrete column according to the present invention;

[0039] Figure 2 This is an overall schematic diagram of the composite reinforcement method for three-side-fired steel tube concrete columns of the present invention;

[0040] Figure 3 It is a plan view of the composite reinforcement method of the double-sided fire-exposed steel tube concrete column of the present invention;

[0041] Figure 4 This is an overall schematic diagram of the composite reinforcement method for double-sided fire-exposed steel tube concrete columns of the present invention;

[0042] Figure 5 This is a cross-sectional view of the composite reinforcement tension bolts of the steel tube concrete column of the present invention;

[0043] Figure 6 It is a schematic diagram of SMA tendon fixation;

[0044] Figure 7 This is a schematic diagram of the connection structure between the steel tube concrete column and the wall;

[0045] Figure 8 is a schematic diagram of a fiber grid mesh;

[0046] Figure 9 This is a diagram of the hydraulic jack equipment for tensioning SMA tendons.

[0047] Numbers in the figure: 1. Core concrete; 2. Steel pipe; 3. Wall; 4. Flexible joint; 401. PE rod; 402. Alkali-resistant glass fiber mesh; 403. Special sealant; 5. CFRP cloth; 6. Tension bolt; 601. Tension screw; 602. Nut; 603. Cross brace; 7. L-shaped GFRP plate; 8. Stud; 9. Wire locker; 10. SMA reinforcement; 11. Rebar head; 12. Fiber grid mesh; 13. Screw; 14. UHPC; 15. Arc welder; 16-a. Fixing plate; 16-b. Reaction plate; 16-c. Hydraulic jack; 16-d. Displacement meter. DETAILED DESCRIPTION

[0048] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0049] Example: Figures 1-9 As shown, a composite repair and reinforcement technology for non-uniformly exposed steel tube concrete columns after fire includes the following steps:

[0050] S1. First, inspect the damaged area of ​​a concrete-filled steel tube column after a fire. The concrete-filled steel tube column includes a core concrete 1 and a steel tube 2. The steel tube 2 is located between two walls 3 and fixed to the walls 3 via a flexible joint 4. The core concrete 1 is located inside the steel tube 2. The internal structure of the flexible joint 4 includes a PE rod 401 installed on the surface of the steel tube, an alkali-resistant glass fiber mesh cloth 402 laid on the surface, and a layer of special sealant 403.

[0051] Specifically, based on the actual non-uniform fire boundary of the building, including fire on three sides or two sides, the post-fire reinforcement plan of the steel tube concrete column is judged, and the non-destructive testing method of the steel tube concrete column is adopted. The non-destructive testing method of the steel tube concrete column includes traditional methods such as manual tapping method, induced vibration method, ultrasonic testing method, infrared thermal imaging method, or emerging technologies such as ultrasonic phased array, to detect the locations such as steel tube bulging and steel tube-concrete voids, and then select the key reinforcement locations of the CFRP cloth 5, clean the peeling oxide layer on the surface of the steel tube 2 at the reinforcement location caused by the fire, and for the wall 3 where the surface has peeled off after the fire, the severely damaged and cracked parts are chiseled out, the chiseled interface is cleaned, and the cracks and defects are filled with repair mortar or repair glue.

[0052] S2. CFRP sheets 5 are then pasted on areas of the concrete-filled steel tube column where the steel tube 2 is severely damaged, such as where buckling occurs. Several groups of CFRP sheets 5 are pasted at intervals along the height of the concrete-filled steel tube column. CFRP sheets 5 are also pasted on the middle and end portions of the steel tube 2, where stress is greater. The number of layers of CFRP sheets 5 is determined based on the extent of damage. The CFRP sheets 5 are pasted in a horizontal direction and extend to the severely damaged areas of the walls 3 on both sides. If the post-fire damage to the walls 3 is not severe, the CFRP sheets 5 are extended to a length of 1000 mm. The sheets are pasted using a prepared epoxy resin adhesive comprising a colorless, transparent epoxy adhesive (A) and a light yellow curing agent (B).

[0053] S3. Studs 8 are welded at the inner corner where the steel tube 2 of the steel tube concrete column connects with the wall 3. Holes are punched on the side of the wall 3 to reserve holes for tension bolts 6 for installing the tension bolts 6. Holes are opened at specific positions on the L-shaped GFRP plate 7 and set at the inner corner where the steel tube 2 and the wall 3 intersect. The L-shaped GFRP plate 7 is pressed on the surface of the CFRP cloth 5. The tension bolts 6 are installed. First, the tension screws 601 are inserted into the wall, and the two ends are inserted into the cross braces 603. The nuts 602 are screwed in to fix the tension screws 601. The L-shaped GFRP plate 7 is anchored by the tension bolts 6 and studs 8 to achieve compaction and fixation of the CFRP cloth 5 at the inner corner.

[0054] Among them, the treated steel tube concrete columns include two fire-exposed surfaces and three fire-exposed surfaces;

[0055] 1) When repairing and reinforcing a column exposed to fire on both sides, the studs 8 are respectively fixed on the adjacent sides of the steel tube 2 of the concrete-filled steel tube column in the non-CFRP cloth 5 reinforcement area;

[0056] 2) When repairing and reinforcing a column exposed to fire on three sides, the studs 8 are respectively fixed on the non-CFRP cloth 5 reinforced areas on the opposite sides of the steel tube 2 of the concrete-filled steel tube column.

[0057] S4. Use a jack to tension the SMA tendon 10 to a strain of 6%. The working principle of the jack is as follows: the SMA tendon 10 is inserted into the fixed plates 16-a at both ends, one end is fixed, and the other end is fixed to the reaction plate 16-b, the hydraulic jack 16-c is started, and the displacement meter 16-d is used to observe that the tensioning strain reaches 6% and then slowly unloads; the SMA tendon 10 is a Fe-SMA tendon, and its two ends are fixed to the bolt 8 by the provided wire locker 9, and the wire locker 9 has a double hole, and the hole diameter is selected. The specification is slightly larger than the diameter of the SMA tendon 10. One end of the SMA tendon 10 is inserted into the upper hole of the wire lock 9 and comes out from the lower hole to form a ring sleeve on the bolt 8 on the surface of the steel pipe. The wire lock 9 is tightened to fix the SMA tendon 10. The two ends of the SMA tendon 10 are energized by the arc welder 15. When the SMA tendon 10 reaches 200°C and produces a shrinkage and tightening trend and begins to stabilize, the heating is stopped, so that the SMA tendon 10 generates prestress and then applies the first stage of circumferential constraint force to the steel tube concrete column.

[0058] S5. The fiber grid mesh 12 is a bidirectional glass fiber grid. Mounting slots are provided at the junctions of the horizontal and vertical bars of the fiber grid mesh. Drill holes in the mounting position of the wall 3, clean the debris in the hole of the wall 3, inject structural adhesive into the hole, screw the screws 13 into the hole of the wall to fix the fiber grid mesh 12. In the overlapping area of ​​the fiber grid mesh 12 and the CFRP cloth 5, apply epoxy resin adhesive on the CFRP cloth 5 to ensure that the fiber grid mesh 12 is evenly adhered to the CFRP cloth 5.

[0059] S6. Weld the steel bar head 11 on the surface of the steel pipe 2 to enhance the interface performance between the UHPC14 and the steel pipe 2. Spray the UHPC14 on the surface of the steel pipe 2 and the wall 3 to form a reinforced surface layer. The spraying of the UHPC14 is carried out using a hose peristaltic mortar sprayer. The thickness of the UHPC14 reinforced surface layer does not exceed 30 mm. The UHPC14 includes mixed fibers such as RTP fiber, PVA fiber and PE fiber. Under high temperature, the mixed fibers in the UHPC14 reinforced surface layer melt to form channels, discharge the internal high-temperature water vapor, prevent the UHPC from bursting, and finally complete the repair and reinforcement of the steel tube concrete column.

[0060] The specific working principle is to improve the mechanical properties of steel tube concrete columns after non-uniform fire exposure and take into account the secondary fire resistance by setting up three lines of defense. By inspecting and identifying the damaged areas of the steel tube concrete columns after fire, the scope and method of reinforcement are determined, the damaged areas of the wall 3 are cleaned and leveled with repair mortar to restore the integrity and continuity of the structure; CFRP cloth 5 is pasted in the middle and end areas of the steel tube concrete columns where the stress is greater and the areas with serious damage. The pasting direction is horizontal and extends to the areas of the walls 3 on both sides to ensure reliable connection and coordinated force between the wall and the column; in the non-CFRP cloth 5 pasting area at the inner corner where the steel tube 2 and the wall 3 are connected, bolts 8 are welded on the surface of the steel tube 2, and holes are drilled in the wall 3 to reserve 6 holes for tension bolts, L-type GFR After the holes in the P plate 7 are opened, the CFRP cloth 5 is pressed and fixed, effectively solving the problems of difficult bonding and fixing of the CFRP cloth 5 at the internal corners, poor reinforcement effect, and easy bulging and debonding. The short side of the L-shaped GFRP plate 7 is inserted into the stud 8, and the long side is fixed with the tension bolt 6, so that the L-shaped GFRP plate 7 is effectively connected to the wall and the fire column; the SMA reinforcement 10 is inserted into the locking wire 9 to form a ring on the stud 8, and the arc welding machine 15 is connected to the two ends of the SMA reinforcement 10 for electric excitation, so that the SMA reinforcement 10 generates the first stage of circumferential constraint force on the steel tube concrete column, effectively improving the bearing capacity of the component after the fire, and During a fire, the SMA bars 10 heat up to the complete phase change temperature to generate recovery stress, thereby applying a secondary circumferential constraint force to the steel tube concrete column to prevent the component from being damaged by the fire for the second time, and applying permanent effective prestress to the steel tube concrete column after the structure cools down; the steel bar head 11 is welded to the non-CFRP cloth 5 reinforcement area on the surface of the steel tube 3, and a fiber grid mesh 12 is set on the surface of the wall 3, which is combined with studs and tension bolts to enhance the interface connection performance between the UHPC 14 and the steel tube 2 and the wall 3. The connection between the horizontal and vertical bars of the fiber grid mesh 12 is provided with installation slots to facilitate the installation of screws 13 to fix the fiber grid mesh The UHPC14 surface layer has high strength, impact resistance and good durability. It is mixed with mixed fibers such as RTP fiber, PVA fiber, and PE fiber. While ensuring the strength and reinforcement effect of UHPC14, the mixed fibers melt at high temperatures to form channels, discharge internal high-temperature water vapor, prevent the UHPC14 layer from bursting, reduce the thermal conductivity of UHPC and the heating rate under fire, and effectively reduce the temperature of the internal SMA reinforcement 10 and CFRP cloth 5, ensuring that they can still play a role when the structure is subjected to secondary fire, thereby improving the fire resistance of the steel tube concrete column subjected to secondary fire, and can also reduce the dead weight of the structure and reduce the reinforcement cost.

[0061] It should be noted that the above description is merely intended to illustrate preferred embodiments of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A composite repair and reinforcement method for non-uniformly exposed steel tube concrete columns after fire, characterized in that: The following steps are involved: S1. First, after the fire, the damaged areas of the steel tube concrete columns were inspected and identified, and the walls that had surface peeling after the fire were cleaned and leveled with repair mortar; S2. Apply epoxy resin glue to the damaged part of the steel tube concrete column where the steel tube is buckled, and then glue the CFRP sheet. The glued area extends to the walls on both sides. S3. At the inner corner where the concrete-filled steel tube column and the wall meet, studs are locally welded in the non-CFRP cloth-reinforced area on the steel tube surface. Holes are punched in the non-reinforced areas of the walls on both sides to reserve tension bolt holes for installing tension bolts. An L-shaped GFRP plate is installed at the inner corner where the concrete-filled steel tube column and the wall meet. The L-shaped GFRP plate has a hole on the side connecting to the steel tube and the wall. Epoxy resin glue is applied to the back of the L-shaped GFRP plate in the area not in contact with the CFRP cloth as a flexible cushioning layer. The L-shaped GFRP plate is fixed with tension bolts on the wall side. S4. Mechanically stretch the SMA reinforcement and slowly unload it after reaching a certain strain. Fix the SMA reinforcement to the studs at the inner corner where the steel tube and the wall meet using a wire lock. Electrically excite the SMA reinforcement at both ends to heat it to a preset temperature, so that the SMA reinforcement generates prestress and exerts a circumferential restraint force on the concrete-filled steel tube column. S5. Drill holes in the wall surface at the junction of the horizontal and vertical bars of the fiber grid mesh. Screw the holes into the holes to secure the fiber grid mesh to the L-shaped GFRP plate and the wall surface. Apply epoxy resin glue to the CFRP sheet in the overlapping area to adhere the fiber grid mesh to the CFRP sheet. S6. Weld short steel bar heads on the surface of the steel pipe, and spray UHPC containing mixed fibers on the surface of the steel tube concrete column and wall to form a reinforced surface layer, finally completing the repair and reinforcement of the fire-damaged steel tube concrete column.

2. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 1, characterized in that: Step S1 specifically includes the following steps: according to the actual non-uniform fire boundary of the building, including two-sided fire or three-sided fire, a post-fire reinforcement plan for the steel tube concrete column is selected, a non-destructive testing method for the steel tube concrete column is used to determine the seriously damaged parts such as steel tube bulging and steel tube-concrete delamination, and then the CFRP cloth reinforcement position is determined, the oxide layer produced by the fire on the surface of the steel tube at the reinforcement position is cleaned, the surface peeling area of ​​the wall connected to the steel tube concrete column caused by the fire is cleaned and polished, impurities and dust are removed, and cracks and defects are filled with repair mortar or repair glue to ensure that the wall surface is flat.

3. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 2, characterized in that: The non-destructive testing method for steel tube concrete columns includes traditional methods such as manual tapping method, induced vibration method, ultrasonic testing method and infrared thermal imaging method, as well as the emerging technology of ultrasonic phased array.

4. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 1, characterized in that: In step S2, CFRP sheets are pasted on the outer surface of the steel tube in the severely damaged area of ​​the steel tube concrete column. Several groups of CFRP sheets are pasted at intervals along the height direction of the steel tube concrete column. At the same time, CFRP sheets are pasted in the middle and ends of the steel tube concrete column. The CFRP sheets are pasted in the horizontal direction and extend to the walls on both sides.

5. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 4, characterized in that: Accurate and targeted reinforcement is carried out according to the actual damage level of the steel tube concrete column. That is, the number of CFRP sheet layers is determined according to the damage level. The CFRP sheet is extended to the areas on both sides where the damage is more serious. When the wall damage after the fire is not serious, the extension length of the CFRP sheet is 1000mm. The CFRP sheet is pasted using the formulated epoxy resin glue, which includes colorless and transparent epoxy glue and light yellow curing agent.

6. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 1, characterized in that: In step S3, an L-shaped GFRP plate is set to fix the CFRP cloth. The L-shaped GFRP plate has unequal sides, with the long side arranged along the wall and the short side arranged along the steel tube concrete column. The L-shaped GFRP plate is pre-drilled with an electric drill at the installation position of the tension bolts. The hole diameter of the L-shaped GFRP plate is consistent with the hole diameter of the wall. The reinforced steel tube concrete column includes a two-sided non-uniform fire column and a three-sided non-uniform fire column. 1) When repairing and reinforcing a column with uneven fire damage on both sides, the studs are respectively fixed on the wall surfaces on adjacent sides of the concrete-filled steel tube column; 2) When repairing and reinforcing a column that is unevenly exposed to fire on three sides, the studs are respectively fixed on the wall surfaces on opposite sides of the steel tube concrete column.

7. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 1, characterized in that: In step S4, the SMA bar is a Fe-SMA bar. A jack device is used to tension the SMA bar to a strain of 6% and slowly unload it. The SMA bar is fixed to the stud through a provided wire locker. The wire locker has two holes. One end of the SMA bar is inserted into the upper hole and out from the lower hole to form a ring-shaped sleeve on the stud on the surface of the steel pipe. The wire locker is tightened to fix the SMA bar. The two ends of the SMA bar are energized by an arc welder. When the temperature of the SMA bar reaches 200°C, a shrinkage and tightening trend is generated and the heating is stopped after it begins to stabilize, so that the SMA bar undergoes partial phase change to generate prestress and thus exerts a circumferential constraint force on the steel tube concrete column. When the steel tube concrete column is subjected to fire for the second time, the SMA bar is heated to the complete phase change temperature, and a secondary constraint force is generated on the steel tube concrete column.

8. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tubular columns after fire according to claim 1, characterized in that: In step S5, the fiber grid mesh is set along the wall, and the width extends to the severely damaged area of ​​the wall or is not less than 1m. The fiber grid mesh is a bidirectional glass fiber grid. Installation slots are opened at the connection between the horizontal bars and the vertical bars of the fiber grid mesh. Holes are drilled at the corresponding installation positions of the wall, and residual debris in the holes are cleaned. Structural adhesive is injected into the holes, and screws are screwed into the holes in the wall to fix the fiber grid mesh.

9. The composite repair and reinforcement method for non-uniformly exposed concrete-filled steel tube columns after fire according to claim 1, characterized in that: In step S6, the welded short steel bar heads are arranged at the center of the steel tube surface and above and below the SMA bars, avoiding the CFRP cloth pasting area, to enhance the interface connection performance between the UHPC surface layer and the steel tube concrete column. The UHPC is sprayed using a hose peristaltic mortar sprayer. The spraying thickness of the UHPC on the steel tube concrete column and wall surface does not exceed 30 mm, and the spraying width along the wall direction covers the fiber grid mesh. The mixed fibers in the UHPC include recycled tire polymer fibers, PVA fibers, and PE fibers. When the structure is subjected to secondary fire, the fibers in the UHPC surface layer melt to form channels, thereby discharging internal high-temperature water vapor and preventing the UHPC from bursting.

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