Method for adhesive strengthening of CFRP plate for curved steel structure

Abstract

The application discloses a CFRP plate adhesive reinforcement method for curved steel structure and relates to the field of structure reinforcement. The method comprises the following steps: applying a release agent on a reinforced surface; placing a CFRP base material on a reinforced surface to be reinforced, wherein the CFRP base material comprises CFRP cloth and release cloth which are laid from bottom to top; fixing a vacuum bag on the reinforced surface to form a sealing device; and injecting impregnated glue resin into the sealing device to form a CFRP plate. The material for manufacturing the CFRP plate is placed on the surface of the reinforced surface, and a sealing space is formed, and then the impregnated glue resin is injected into the sealing space, so that compared with the existing glue injection method for reinforcement, the method is simple, convenient to operate, and the performance of the formed CFRP plate is more excellent.

Description

Technical Field

[0001] This invention relates to the field of structural reinforcement, and in particular to a method for adhesive reinforcement of curved steel structures using CFRP (Carbon Fibre-reinforced Polymer) plates. Background Technology

[0002] Bridge structures, subjected to natural environmental influences and external loads during their service life, inevitably suffer damage and defects. Therefore, employing appropriate reinforcement techniques to restore or enhance their load-bearing capacity is essential to ensure the bridge's service life and structural safety. Traditional reinforcement methods for concrete bridge structures include cross-section enlargement, steel plate bonding, and prestressed reinforcement. For steel structures, traditional methods include steel plate bonding, bolted connections, steel plate welding, and riveting. While these methods offer some reinforcement, they each have their drawbacks, such as inconvenience in operation, high cost, and damage to the original structure. CFRP (Cemented Polymer Reinforced Plastic), with its lightweight, high strength, and high modulus of elasticity, exhibits excellent fatigue resistance and durability, and is gradually replacing traditional reinforcement methods, finding widespread application in bridge structure reinforcement.

[0003] Currently, curved structural members are a crucial component of bridge structures and also sensitive areas prone to structural defects. There are two existing methods for their reinforcement: one is direct bonding reinforcement with CFRP fabric, and the other is reinforcement using CFRP sheets formed through pultrusion. The problems with these two methods for reinforcing curved structures are as follows:

[0004] 1. Compared to using CFRP sheets for reinforcing curved structures, CFRP fabric has a poorer reinforcement effect because its performance and structure are inferior to CFRP sheets.

[0005] 2. When CFRP sheets formed by pultrusion are used to fabricate curved structures, the bending curvature of the CFRP sheet must be tailored to the overall curvature of the structure. Therefore, this method results in high cost, difficult manufacturing processes, and a high susceptibility to manufacturing errors, leading to a decrease in the quality of the CFRP sheet itself. Furthermore, when bonding the curved CFRP sheet to the surface being reinforced, the difficulty in fabricating the curved CFRP sheet and the resulting poor fit can lead to gaps between the CFRP sheet and the reinforced surface, resulting in poor reinforcement effectiveness. Therefore, fabricating CFRP sheets suitable for reinforcing curved structures is quite challenging. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the technical problem solved by this invention is: how to manufacture CFRP boards suitable for reinforcing curved structures while ensuring the quality of curved structure reinforcement.

[0007] To achieve the above objectives, the present invention provides a method for adhesive reinforcement of curved steel structures using CFRP sheets, the method comprising the following steps:

[0008] Apply a release agent to the surface to be reinforced; place the CFRP substrate on the area to be reinforced, the CFRP substrate including CFRP cloth and release cloth laid from bottom to top; fix the vacuum bag to the surface to be reinforced to form a sealing device; inject the sealing device with impregnating resin to form a CFRP board.

[0009] Based on the above technical solution, after forming the sealing device and before injecting the adhesive, the following steps are also included: constructing an adhesive injection channel, which includes an impregnating resin container, a sealing device, a resin collector and a vacuum pump connected in sequence, and performing an airtightness check on the device.

[0010] Based on the above technical solution, the process of forming the sealing device specifically includes the following steps: The CFRP substrate also includes a flow guide net, a spiral tube, an inlet glue injection seat, and an outlet glue injection seat laid from bottom to top. First, the inlet glue injection seat and the outlet glue injection seat are placed on the flow guide net parallel to the fiber direction. Then, the spiral tube is arranged below the inlet glue injection seat and the outlet glue injection seat respectively along the direction perpendicular to the fiber direction. The inlet glue injection seat and the outlet glue injection seat are placed on the left and right sides of the flow guide net along the direction of resin flow with the fiber. The arranged flow guide net, inlet glue injection seat, outlet glue injection seat, and spiral tube are fixed. Several sealing components are arranged around the flow guide net. Each sealing component is provided with a vacuum bag overlap area to avoid the vacuum bag being suspended. When sealing the vacuum bag, the sealing starts from both ends, and the remaining vacuum bag in the middle section is sealed by the sealing components in the overlap area.

[0011] Based on the above technical solution, the glue injection process specifically includes the following steps: turn on the vacuum pump and vacuum clamp, and drive the flow of impregnating resin under vacuum negative pressure. The impregnating resin flows from the glue injection seat into the sealing device, and then through the spiral tube connected to the glue injection seat, it is evenly distributed inside the sealing device. When the impregnating resin flows out from the glue outlet, turn off the vacuum pump and vacuum clamp.

[0012] Based on the above technical solution, before the sealing device is glued with impregnating resin to form a CFRP plate, the following steps are also included: mixing 0.2wt% functionalized carbon nanotubes, 2wt% silane coupling agent and impregnating resin, wherein the functionalized carbon nanotubes are selected from any one or more of carboxylated carbon nanotubes, hydroxylated carbon nanotubes and aminated carbon nanotubes.

[0013] Based on the above technical solution, the process of forming the CFRP board specifically includes the following steps: the CFRP cloth after injection is first cured in a closed environment of 10℃~30℃, and after curing, it is demolded to obtain a nano-modified CFRP board. Then, the obtained CFRP board is placed in an environment of 50℃~60℃ for 8~12 hours to cure.

[0014] Based on the above technical solution, after forming the CFRP board, the following steps are also included: cleaning the CFRP board and the reinforced surface, bonding the CFRP board to the reinforced surface with adhesive, and squeezing out excess adhesive by applying pressure to increase the interfacial adhesion.

[0015] Based on the above technical solution, after cleaning the CFRP board and the reinforced surface and before bonding the CFRP board to the reinforced surface with adhesive, the following steps are also included: sprinkling several steel balls of any type with a diameter of 1mm to 1.5mm into the reinforced surface to control the thickness of the adhesive.

[0016] Based on the above technical solution, after squeezing out the excess adhesive by applying pressure, the following steps are also included: waiting for the adhesive to cure at an environment of 10℃~30℃, and after curing, applying at least two layers of acrylic waterproof coating continuously along the surface of the CFRP board, the outer peripheral surface of the adhesive and the reinforced surface.

[0017] Based on the above technical solution, before bonding the CFRP board to the reinforced surface with adhesive, the following steps are also included: the adhesive is Sika30 epoxy resin adhesive, and 2wt% of liquid nitrile rubber polymer with active groups is dispersed in the adhesive. The liquid rubber polymer with active groups is any one or more of terminal carboxyl nitrile rubber, terminal hydroxyl nitrile rubber, and terminal amino nitrile rubber.

[0018] Compared with the prior art, the advantages of the present invention are as follows:

[0019] (1) This invention places the material for making the CFRP board on the surface of the surface to be reinforced, forming a sealed space, and then injects impregnating resin into the sealed space. Compared with existing reinforcement injection methods, this method is simpler, easier to operate, and produces a CFRP board with superior performance. The CFRP board of this invention is manufactured using the surface to be reinforced as a mold, thus allowing for better fit between the CFRP board and the structure of the reinforced surface. The gap between the CFRP board and the reinforced surface is very small, thereby improving the reinforcement effect. Therefore, while ensuring the reinforcement quality of the structure, this CFRP board is suitable for reinforcing curved steel structures.

[0020] (2) This invention achieves selectable adhesives and controllable adhesive layer thickness by first fabricating CFRP boards and then bonding them to the damaged structure. Compared to traditional reinforcement methods that use impregnating adhesives, resulting in thin and uneven adhesive layers and weak adhesion, this invention allows for the selection of high-strength, high-adhesion structural adhesives. Furthermore, by sprinkling fine steel balls, the adhesive layer thickness is controlled within an optimal range, ensuring the performance of the adhesive layer. In addition, addressing the brittleness of traditional epoxy resins, this invention modifies the adhesive with rubber particles containing active groups, improving the adhesive's toughness. The active groups on the rubber particles can react with the epoxy groups within the adhesive, promoting the formation of curing products and further enhancing the performance of the adhesive layer.

[0021] (3) The present invention first selects a special adhesive for the structure, which has excellent bonding performance; by applying pressure, the adhesion between the adhesive and the reinforced structure and the CFRP board is increased, and at the same time, the air bubbles at the interface are squeezed out, ensuring the smoothness of the adhesive layer at the interface; secondly, the rubber particles attached to the surface of the adhesive layer and the carbon nanotubes on the surface of the CFRP board increase the roughness of the adhesive layer and the surface of the CFRP board at the interface, thereby improving the mechanical interlocking force between the CFRP board and the adhesive at the interface; the active groups of the carbon nanotubes on the surface of the CFRP board, the active groups on the rubber particles on the surface of the adhesive, and the epoxy groups in the epoxy resin react with each other at the interface, further improving the adhesion at the interface.

[0022] (4) The present invention is based on the low porosity of the CFRP board formed under vacuum negative pressure, the strong bonding force between the CFRP cloths in the board, and the performance of the reinforcement material is significantly better than that of the reinforcement material using the manual pasting method of existing processes; by nano-modifying the impregnating adhesive to introduce functional carbon nanotubes into the CFRP board, the operation is simple, which not only improves the performance of the impregnating adhesive as the matrix of the CFRP board, but also the unique tubular slender structure of the carbon nanotubes interspersed between the carbon fibers, making the carbon fibers tightly connected, further improving the performance of the CFRP board and improving the problems of low interlayer shear strength and low transverse strength of the CFRP board; the CFRP board is first cured at room temperature to form a curved structure corresponding to the reinforced structure, and then placed at high temperature for secondary curing to improve its performance. Compared with room temperature, the epoxy groups in the CFRP board react more fully at high temperature, and the cross-linking degree of the cured product is higher, thereby obtaining better performance. At the same time, the functional carbon nanotubes can improve the thermal stability of the CFRP board to a certain extent, making the CFRP board less prone to deformation in high temperature environment and maintaining the curvature line of the original curved structure.

[0023] (5) Based on the reinforced structure, the present invention makes the adhesive layer at the interface flat and the interface adhesion strong, ensuring the tight fit between the adhesive layer and the reinforced structure and the reinforcing material, thereby reducing the probability of harmful molecules penetrating, ensuring the tight connection between the adhesive and the CFRP material, and the outer periphery of the adhesive layer is coated with a waterproof coating, further reducing the penetration of water molecules and the like into the interface area.

[0024] Furthermore, the adhesive used in this invention possesses excellent heat resistance. Simultaneously, the modification of the adhesive with rubber particles not only improves its toughness and alleviates the phenomenon of brittle fracture of the adhesive under load in the later stages of the structure, thus rapidly reducing the load-bearing capacity of the reinforcement material, but also increases the glass transition temperature of the adhesive, further improving its heat resistance. Similarly, the CFRP board used in this invention, after modification with functionalized carbon nanotubes and high-temperature secondary curing, also exhibits improved glass transition temperature and heat resistance, thereby enhancing the heat resistance and stability of the entire reinforcement system. Attached Figure Description

[0025] Figure 1 This is a flowchart of the CFRP board adhesive reinforcement method for curved steel structures in an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram illustrating the fabrication of the CFRP board in an embodiment of the present invention;

[0027] Figure 3 for Figure 2 Schematic diagram of the reinforced structure of the fabricated CFRP plate;

[0028] Figure 4 This is a schematic diagram of the experimental curved surface structure in an embodiment of the present invention.

[0029] In the diagram: 1-Impregnating resin container, 2-Impregnating resin, 3-Vacuum guide tube, 4-Vacuum tube clamp, 5-Reinforced surface, 6-CFRP cloth, 7-Release cloth, 8-Guide net, 9-Vacuum bag, 10-Inlet injection seat, 11-Spiral tube, 12-Outlet injection seat, 13-Sealing tape, 14-Vacuum gauge, 15-Resin collector, 16-Vacuum pump, 17-Structural adhesive, 18-CFRP board. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] See Figure 1 As shown, the method for adhesively reinforcing curved steel structures with CFRP sheets in this embodiment of the invention includes the following steps:

[0032] Apply a release agent to the reinforced surface 5; place the CFRP substrate on the reinforced surface 5 to be reinforced, and tightly attach the outer edge of the vacuum bag 9 to the reinforced surface 5 with sealing tape 13. Lay the CFRP substrate from bottom to top, including CFRP cloth 6 and release cloth 7, to form a sealing device with a sealed internal space; inject the sealing device with impregnating resin 2 to form a CFRP board 18.

[0033] Therefore, this invention places the material for making the CFRP board 18 on the surface of the reinforced surface 5 to form a sealed space, and then injects the impregnating resin 2 into the sealed space. Compared with existing reinforcement injection methods, this method is simpler, more convenient to operate, and produces a CFRP board 18 with superior performance. The CFRP board 18 of this invention is manufactured using the reinforced surface 5 as a mold, thus allowing for better fit between the CFRP board 18 and the reinforced surface 5. The gap between the CFRP board 18 and the reinforced surface 5 is very small, thereby improving the reinforcement effect. Therefore, while ensuring the reinforcement quality of the structure, this CFRP board 18 is suitable for reinforcing curved steel structures.

[0034] Preferably, the above steps also include the following step: See Figure 2 As shown, the impregnating resin container 1, sealing device, resin collector 15 and vacuum pump 16 are connected in sequence through vacuum guide pipe 3 to form a glue injection channel, and the airtightness of the device is checked.

[0035] The advantage of this design is that during the process of injecting the impregnating resin 2 into the sealing device, it is necessary to keep the inside of the sealing device sealed. Therefore, this injection channel is designed so that the impregnating resin 2 can be injected into the sealing device through vacuum pressure, while ensuring that the inside of the sealing device remains sealed.

[0036] Preferably, before applying the release agent to the reinforced surface 5, the following steps are specifically included: grinding the reinforced surface 5 with a grinder, filling and smoothing its holes with glue, and then cleaning off the impurities and dust on its surface with alcohol.

[0037] The advantage of this design is that it avoids the adverse effects of holes or impurities on the reinforced surface 5 on the reinforcement effect.

[0038] Preferably, the process of applying the release agent to the reinforced surface 5 includes the following steps: applying the release agent to the reinforced surface 5 at least three times, with an interval of 3-5 minutes between each application, and waiting 5-10 minutes after applying the last release agent to ensure that the release agent has fully evaporated and the mold surface is dry.

[0039] The benefits of this design are: by selecting the best release agent and optimizing the release method, it effectively prevents the impregnating adhesive from sticking to the reinforced surface 5 during the release process. At the same time, it reduces the micro-bubbles adhering to the underlying CFRP material, ensuring the surface properties of the CFRP after curing.

[0040] Preferably, the process of forming the sealing device specifically includes the following steps: The CFRP substrate is further laid from bottom to top with a flow guide net 8, a spiral tube 11, an inlet glue injection seat 10, and an outlet glue injection seat 12. First, the inlet glue injection seat 10 and the outlet glue injection seat 12 are placed on the flow guide net 8 parallel to the fiber direction. Then, the spiral tube 11 is arranged laterally under the inlet glue injection seat 10 and the outlet glue injection seat 12, respectively, along the direction perpendicular to the fiber direction. The inlet glue injection seat 10 and the outlet glue injection seat 12 are placed on the left and right sides of the flow guide net 8 along the direction of resin flow along the fiber direction. The arranged flow guide net 8, inlet glue injection seat 10, outlet glue injection seat 12, and spiral tube 11 are fixed with pressure-sensitive adhesive tape. Four sealing tapes 13 are arranged around the flow guide net 8. Each sealing tape 13 has an overlap area for a vacuum bag 9 in the middle section to prevent the vacuum bag 9 from being suspended. When sealing the vacuum bag 9, the tape is pasted from both ends. Finally, the remaining vacuum bag 9 in the middle section is completely sealed by the sealing tape 13 in the overlap area.

[0041] The advantage of this design is that the impregnation process relies solely on the negative pressure provided by the vacuum pump 16 to drive the flow of the impregnating adhesive. This requires the impregnating adhesive to have a low viscosity and a certain working time. As the impregnation time increases, the curing degree of the epoxy resin and curing agent in the impregnating adhesive gradually increases, causing the viscosity of the impregnating adhesive to gradually increase. On the one hand, this hinders the flow of the impregnating adhesive during the impregnation process; on the other hand, the impregnating adhesive with excessive viscosity is difficult to penetrate into the underlying CFRP cloth 6, resulting in insufficient wetting of the CFRP cloth 6. Therefore, when the reinforcement area is large and the working time of the impregnating adhesive is not long enough, the impregnation time can be reduced by optimizing the injection seat and the spiral tube 11.

[0042] Furthermore, the horizontal arrangement of the spiral tube 11 under the injection seat can change the flow direction of the impregnating adhesive and accelerate the flow speed. When the impregnating adhesive enters the injection seat, it quickly spreads along the spiral tube 11 to the entire injection end of the CFRP material. The optimized position of the injection seat and the spiral tube 11 ensures that the impregnating adhesive flows in the same direction as the carbon fiber in the CFRP, which reduces the resistance encountered by the impregnating adhesive during the flow process, makes the impregnating adhesive flow smoothly, and improves the wettability of the impregnating adhesive on the CFRP material.

[0043] Preferably, the process of forming the glue injection channel specifically includes the following steps: the lower end of the guide tube connecting the glue injection seat 10 and the glue dispensing seat 12 is cut at a 45° bevel, and the hole of the guide tube and the glue injection seat is first sealed with three layers of sealing tape 13, and then the outer layer of sealing tape 13 is pasted with pressure-sensitive tape.

[0044] The advantage of this design is that by modifying the lower end of the guide tube, the glue injection channel is prevented from being blocked due to the guide tube tightly fitting against the guide mesh 8 during glue injection. During the glue injection process, the low-viscosity impregnating adhesive provides a certain lubrication effect and easily penetrates the sealing tape 13 at the perforation. The perforation of the guide tube and the glue injection seat is first sealed with three layers of sealing tape 13, and then completely sealed with pressure-sensitive tape, thereby reducing the risk of the sealing tape 13 at the perforation loosening and leaking air during glue injection.

[0045] Preferably, the process of checking the airtightness of the device includes the following steps: turn on the vacuum pump 16 and vacuum clamp 4 to evacuate the air, turn off the vacuum pump 16 and the corresponding vacuum clamp 4 after the evacuation is completed, wait for 20 minutes, and observe whether the reading of the vacuum gauge 14 on the resin collector 15 drops. If the reading is stable, it means that the seal is complete.

[0046] The benefits of this design are as follows: During the CFRP board 18 injection and curing process, the sealing performance of the device is a key factor affecting the quality of the CFRP board 18. When the device is not completely sealed, on the one hand, the vacuum negative pressure that drives the impregnation adhesive flow will be too low, resulting in insufficient wetting of the underlying CFRP cloth 6 by the impregnation adhesive, leading to insufficient adhesive and missing adhesive. On the other hand, the introduction of a large amount of air will result in high porosity and low quality of the molded CFRP board 18. At the same time, during the curing process, insufficient negative pressure will result in insufficient bonding between the CFRP cloths 6, resulting in poor interlayer shear performance of the cured CFRP board 18.

[0047] Furthermore, the bonding interface between the vacuum bag 9 and the sealing tape 13, as well as the perforation point between the guide tube and the injection seat, are the most prone to causing air leakage in the device. During the bonding process, the excess in the vacuum bag 9 can easily lead to gaps between the vacuum bag 9 and the sealing tape 13. The low-viscosity impregnating adhesive has a certain lubricating effect and low flow resistance, making it easy to penetrate into the perforation points of the guide tube and the injection seat, thus causing the sealing tape to loosen. Therefore, this invention effectively avoids the gap phenomenon of the vacuum bag 9 by setting an overlap area, ensuring complete bonding between the vacuum bag 9 and the sealing tape 13; optimizes the tape sealing method at the perforation points to reduce the loosening of the sealing tape 13 caused by the impregnating adhesive at the perforation points; and adds an airtightness check to the device before injection to ensure the sealing performance of the molding device.

[0048] Preferably, before applying the impregnating resin 2 to the sealing device to form the CFRP plate 18, the following steps are specifically included: (Nano-modification of the impregnating resin 2) 0.2 wt% functionalized carbon nanotubes, 2 wt% silane coupling agent and impregnating resin 2 are uniformly mixed, wherein the functionalized carbon nanotubes are selected from any one or more of carboxylated carbon nanotubes, hydroxylated carbon nanotubes and aminated carbon nanotubes.

[0049] The advantages of this design are as follows: epoxy resin possesses excellent mechanical properties such as strength and modulus, stable thermal and chemical properties, and good processability, making it commonly used as the impregnating agent for CFRP board 18. CFRP board 18 uses the impregnating agent as the matrix and carbon fiber as the reinforcement. The performance of carbon fiber is significantly superior to that of the impregnating agent; therefore, the performance of the impregnating agent largely determines the performance of CFRP board 18. However, epoxy resin, due to its high degree of curing and cross-linking, suffers from insufficient toughness and high brittleness. Furthermore, while unidirectional CFRP board 18 exhibits excellent stress performance along the fiber direction, its performance perpendicular to the fiber direction depends solely on the adhesive strength and properties of the impregnating agent. This results in low interlaminar shear strength and transverse tensile strength of the CFRP board 18. When used for structural reinforcement, insufficient interlaminar shear strength often leads to CFRP material failure. Moreover, the stress at crack locations in actual engineering structures is complex, and the principal stresses do not completely coincide with the fiber direction of the CFRP material. This necessitates that CFRP board 18 possess a certain degree of multidirectional stress resistance.

[0050] Furthermore, this invention achieves nano-modification of the CFRP plate 18 through nano-modification of the impregnating adhesive, thereby improving the interlaminar shear properties and transverse tensile properties of the CFRP plate 18 and enhancing its multi-directional stress resistance. The principle is as follows: on one hand, functionalized carbon nanotubes are uniformly dispersed in the impregnating adhesive. Due to the microcrack inhibition effect of the carbon nanotubes, the toughness of the epoxy resin is significantly improved. Moreover, the active groups on the carbon nanotubes can react with epoxy groups, increasing the crosslinking degree of the epoxy resin and the adhesion between the carbon nanotubes and the resin. Finally, the silane coupling agent reduces the carbon nanotube... The aggregation property increases the homogeneity of carbon nanotubes within the impregnating resin, thereby improving the performance of the impregnating resin and consequently enhancing the performance of the CFRP plate 18. On the other hand, the functionalized carbon nanotubes, evenly distributed within the CFRP along with the impregnating resin, fill the micropores within the CFRP, improving the connection between the carbon nanotubes and the impregnating resin. The unique slender tubular structure of the carbon nanotubes acts as a "binding" mechanism for the carbon fibers, resulting in a tighter connection between the carbon fibers and increasing the interlayer strength and transverse strength of the CFRP plate 18. At the same time, the functionalized carbon nanotubes also improve the heat resistance of the CFRP plate 18 to some extent.

[0051] Furthermore, the carbon nanotubes deposited on the surface of the CFRP plate 18 increase the surface roughness of the CFRP plate 18, improve the surface properties of the CFRP plate 18, and thus improve the interfacial properties between the CFRP plate 18 and the adhesive.

[0052] Preferably, the glue injection process includes the following steps: turning on the vacuum pump 16 and vacuum clamp 4, driving the impregnation resin 2 to flow under vacuum negative pressure, the impregnation resin 2 flows into the sealing device from the glue injection seat 10, and then is evenly distributed inside the sealing device through the spiral tube 11 connected to the glue injection seat 10. When the impregnation resin 2 flows out from the glue outlet seat 12, the vacuum pump 16 and vacuum clamp 4 are turned off.

[0053] The advantages of this design are: under vacuum negative pressure, the impregnating resin 2 can flow at a uniform speed, and the opening and closing of the flow is relatively convenient; the spiral tube 11 set inside the sealing device changes the point inflow to a linear inflow, which can make the impregnating resin 2 evenly distributed inside the sealing device, thereby ensuring sufficient glue injection inside the sealing device.

[0054] Preferably, the process of forming the CFRP board 18 includes the following steps: the CFRP cloth 6 after injection is first cured in a closed, room temperature (10℃~30℃) environment for 3 days, and after curing, it is demolded to obtain the nano-modified CFRP board 18. Then, the obtained CFRP board 18 is placed in a high temperature (50℃~60℃) environment for 8~12 hours to cure.

[0055] The benefits of this design are as follows: high-temperature curing can enhance the activity of the internal groups of epoxy resin, accelerate the reaction rate, improve the crosslinking degree and density of the cured product, improve the heat resistance of the resin, and increase the adhesion between the impregnating adhesive and carbon fiber, thereby further improving the performance of CFRP board 18 while shortening the curing time. Compared with existing reinforcement methods, this invention can achieve high-temperature curing of CFRP materials. First, it cures for 3 days, allowing the CFRP board 18 to form a curved structure. Then, a second curing at high temperature improves its strength and other mechanical properties, as well as its heat resistance. Furthermore, the functionalized carbon nanotubes improve the thermal stability of the CFRP board 18 to some extent, making it less prone to deformation under the high-temperature environment during curing, thus maintaining the curvature of the original curved structure.

[0056] Preferably, after forming the CFRP board 18, the specific steps include: cleaning the CFRP board 18 and the reinforced surface 5, see [reference]. Figure 3 As shown, the CFRP board 18 is bonded to the reinforced surface 5 using structural adhesive 17, and excess structural adhesive 17 is squeezed out by applying pressure to increase the interfacial adhesion.

[0057] The advantage of this design is that the finished CFRP board 18 is bonded to the reinforced surface 5 to complete the reinforcement of the reinforced surface 5.

[0058] Preferably, before bonding the CFRP board 18 to the reinforced surface 5 using structural adhesive 17, the following steps are specifically included: the structural adhesive 17 is Sika30 epoxy resin adhesive, and 2wt% of liquid nitrile rubber polymer with active groups is uniformly dispersed in the adhesive. The liquid rubber polymer with active groups is any one or more of terminal carboxyl nitrile rubber, terminal hydroxyl nitrile rubber, and terminal amino nitrile rubber.

[0059] The benefits of this design are as follows: Sika30 adhesive has strong bonding strength, good heat resistance, excellent performance, and moderate viscosity, making it particularly suitable for reinforcing various structures. However, Sika30 adhesive is an epoxy resin adhesive, which is characterized by high brittleness. Compared to nanomaterials, rubber particle modification of epoxy resin significantly improves the toughness of epoxy resin while also enhancing its heat resistance. Furthermore, rubber particles have a significant inhibitory effect on microcracks, and their active groups can react with epoxy groups, promoting the cross-linking of the cured product, thereby improving the adhesive's performance.

[0060] Preferably, after cleaning the CFRP board 18 and the reinforced surface 5, and before using structural adhesive 17 to attach the CFRP board 18 to the reinforced surface 5, the following steps are specifically included: sprinkling several steel balls of any type with a diameter of 1mm to 1.5mm into the reinforced surface 5 to control the thickness of the structural adhesive 17.

[0061] The benefits of this design are as follows: the thickness of the structural adhesive 17 is a major factor affecting the performance of the adhesive layer and the interface. Too thin an adhesive layer leads to low load-bearing capacity and weak interfacial adhesion, while too thick an adhesive layer results in a longer stress transmission path, causing the adhesive layer to transform from a load-bearing structure into a primary load-bearing structure, making it difficult to efficiently transfer stress to the CFRP board 18, which has superior load-bearing capacity. Therefore, controlling the adhesive layer to an optimal thickness allows the reinforced structure to achieve maximum load-bearing capacity. By sprinkling fine steel balls of appropriate diameter onto the adhesive layer, the thickness can be quantitatively controlled. This method is simple and convenient, does not damage the structure, and during the pressure application process after CFRP bonding, the pressure is borne by the small steel balls, facilitating the pressure application operation and avoiding the influence of pressure on the adhesive layer thickness.

[0062] Preferably, the excess structural adhesive 17 is squeezed out by applying pressure, which specifically includes the following steps: pressure needs to be applied to the surface of the pasted CFRP board 18, and the pressure application method and time are selected according to different reinforced structures. For example, steel structures and concrete structures can be pressured by magnets and straps respectively, and the pressure application time is 3 to 5 hours.

[0063] The benefits of this design are: by applying pressure to squeeze out excess structural adhesive 17, the smoothness of the adhesive layer at the interface is ensured, and at the same time, air bubbles at the interface are squeezed out, which increases the interfacial adhesion between the structural adhesive 17 and the CFRP board 18 and the reinforced surface 5, thereby improving the interfacial performance.

[0064] Preferably, after squeezing out excess structural adhesive 17 by applying pressure, the following steps are specifically included: waiting for the structural adhesive 17 to cure at room temperature (the curing time depends on the adhesive used), and after curing, applying at least two layers of acrylic waterproof coating continuously and evenly along the surface of the CFRP board 18, the outer peripheral surface of the structural adhesive 17 and the reinforced surface 5.

[0065] The advantage of this design is that by applying a waterproof coating, the penetration of water molecules into the interface between the structural adhesive 17, the CFRP board 18, and the reinforced surface 5 is reduced, thereby improving the long-term durability of the interface.

[0066] To compare the superior reinforcement effect of this invention with existing direct external CFRP fabric reinforcement methods and the reinforcement method of patent publication number CN107053702A, and to verify the effectiveness of the present invention, a curved surface component was fabricated for experimental research. The curved surface component used C40 concrete, and the reinforcement material was four layers of CFRP fabric or a CFRP board made of four layers of CFRP fabric. An initial crack with a depth of 50mm and a width of 2mm was pre-set in the mid-span of the specimen. The form and dimensions of the component are described in [reference needed]. Figure 4 As shown.

[0067] 1) Test Plan

[0068] Examples 1-9 are curved surface components reinforced using the present invention. The only difference between Examples 1-5 is the thickness of the adhesive, which ranges from 0.5 to 2.5 mm. The difference between Examples 6-9 and Examples 1-5 is as follows: In Example 6, the CFRP board was cured at room temperature (10℃-30℃) for 7 days, while in Examples 1-5, the CFRP board was cured at room temperature for 3 days + 60℃ / 12 hours. In Example 7, the adhesive was not modified, but the CFRP board underwent nano-modification. In Example 8, the adhesive underwent functional modification, but the CFRP board was not modified. In Example 9, the adhesive was not modified, and the CFRP board was not modified. The adhesive thickness in Examples 6-9 is the same as in Example 2.

[0069] Comparative Example 1 shows a curved surface component fabricated using a direct external CFRP fabric reinforcement method.

[0070] Comparative Example 2 shows a curved surface component fabricated based on the reinforcement method disclosed in CN107053702A.

[0071] The experimental designs for Examples 1-9 and Comparative Examples 1-2 are detailed in Table 1, as follows:

[0072] Table 1. Experimental Design of Examples 1-9 and Comparative Examples 1-2

[0073]

[0074] 2) Preparation of test specimens

[0075] C40 concrete was used to make curved components. A crack was left in the middle of the span of the specimen, and different methods were used to reinforce the crack.

[0076] The reinforcement method for the experimental examples is the same as that for the examples. The differences between the specimens in each experimental example are as follows: In step S3, the CFRP material used in Experiments 1 to 5 is cured at room temperature for 3 days to obtain CFRP boards, and then the CFRP boards are cured at 60°C for 12 hours. Both the adhesive and the CFRP boards are modified. The CFRP material used in Experiment 6 is cured at room temperature for 3 days and then cured at room temperature for another 4 days. In Example 7, the adhesive is not modified, but the CFRP board is nano-modified. In Example 8, the adhesive is functionalized, but the CFRP board is not modified. In Example 9, the adhesive is not modified, and the CFRP board is not modified. In step S4, the adhesive thickness of Experiments 1 to 5 is 0.5–2.5 mm, and the adhesive thickness of Experiments 6 to 9 is 1 mm. No coating protection is required in step S5. The remaining steps are the same.

[0077] The reinforcement steps for Comparative Example 1 are as follows: 1. Clean, fill, and level the surface of the curved component; 2. Apply GE-7118A / 7114B impregnating adhesive to both the component and the CFRP cloth, then adhere the CFRP cloth to the surface of the component and roll it back and forth with a roller to squeeze out excess resin; after the first layer of CFRP cloth is adhered, apply impregnating adhesive to the second layer of CFRP cloth, adhere it to the first layer of CFRP cloth, and roll it back and forth with a roller; repeat the above steps until all four layers of CFRP cloth are adhered; 3. Curing at room temperature for 7 days is sufficient.

[0078] The reinforcement steps for Comparative Example 2 are described in the patent with publication number CN107053702A, but the test materials used are consistent with those in the experimental example. The CFRP cloth is 4 layers, the impregnating adhesive is GE-7118A / 7114B vacuum-infused epoxy resin, and the curing regime is room temperature curing for 7 days.

[0079] Tables 2 and 3 show the parameters for GE-7118A / 7114B impregnating adhesive and Sika30 adhesive, Table 4 shows the parameters for CFRP materials, and Table 5 shows the parameters for steel plates.

[0080] Table 2 Impregnating adhesive parameters

[0081]

[0082] 3) Loading of test specimens and analysis of results

[0083] The curved component was loaded using a 300kN quasi-static tensile testing machine at a loading rate of 0.3mm / min. The parameter settings and test results of the specimens are shown in Table 6.

[0084] Table 6. Parameter settings and test results for specimens in Experimental Examples 1-6 and Comparative Examples 1-2.

[0085]

[0086] Compared with Comparative Example 1, the ultimate bearing capacity of the specimen in Comparative Example 2 increased by 27.3%.

[0087] Compared with Comparative Example 1, the ultimate bearing capacity of specimens in Experimental Examples 1 to 8 increased by 51.2% to 138.8%.

[0088] Compared with Comparative Example 2, the ultimate bearing capacity of specimens in Experimental Examples 1 to 8 increased by 18.8% to 87.9%.

[0089] Compared with Test Example 2, the ultimate bearing capacity of the specimen in Test Example 6 decreased by 16.6%.

[0090] Compared with Test Example 2, the ultimate bearing capacity of the specimen in Test Example 7 decreased by 10.8%.

[0091] Compared with Test Example 2, the ultimate bearing capacity of the specimen in Test Example 8 decreased by 15.5%.

[0092] Compared with Test Example 2, the ultimate bearing capacity of the specimen in Test Example 9 decreased by 19.5%.

[0093] The only difference between Experiments 1-5 is the adhesive thickness. It can be seen that when the adhesive thickness is less than 1.5 mm, the ultimate load-bearing capacity of the specimen increases with increasing thickness. The increased adhesive layer thickness enhances the interfacial and adhesive layer properties, thus improving the load-bearing capacity. However, when the adhesive layer thickness exceeds 1.5 mm, the ultimate load-bearing capacity decreases with further increases in thickness. When the adhesive thickness is too large, the stress transmission path becomes longer, and the adhesive layer changes from a load-bearing structure to a primary load-bearing structure, leading to a decrease in the specimen's load-bearing capacity. Based on the ultimate load-bearing capacity, a preferred adhesive layer thickness of 1-1.5 mm is recommended for Sika30 adhesive.

[0094] The only difference between Experiment 6 and Experiment 2 lies in the curing regime of the CFRP board. Experiment 6 uses room temperature curing, while Experiment 2 uses a combination of room temperature and high-temperature curing. Compared to Experiment 2, the ultimate load-bearing capacity of the specimen in Experiment 6 decreased by 16.6%. Compared to room temperature curing, the high-temperature environment can increase the activity of the internal groups of the epoxy resin, accelerate the reaction rate, increase the crosslinking degree and density of the cured product, improve its thermal properties, and increase the adhesion between the impregnating adhesive and the carbon fiber, thereby improving the performance of the CFRP board and shortening the curing time. With the improvement of the CFRP board performance, the ultimate load-bearing capacity of the specimen is enhanced. The CFRP board in Experiment 6 is only cured at room temperature, and its performance is not as good as the CFRP board cured at high temperature.

[0095] The difference between Experiment 7 and Experiment 2 is that the adhesive used in Experiment 7 was unmodified. Compared to Experiment 2, the ultimate load-bearing capacity of the specimen in Experiment 7 decreased by 10.8%. Sika30 adhesive, an epoxy resin adhesive, is characterized by insufficient toughness and high brittleness. Modification with rubber particles containing active groups significantly improved the adhesive's toughness. The reaction between the active groups and epoxy groups enhanced the strength of the cured product. With the improvement in adhesive layer performance, the ultimate load-bearing capacity of the specimen was increased. The unmodified adhesive in Experiment 7 performed worse than the rubber particle-modified adhesive.

[0096] The difference between Experiment 8 and Experiment 2 is that the CFRP plate used in Experiment 8 was unmodified. Compared with Experiment 2, the ultimate load-bearing capacity of the specimen in Experiment 8 decreased by 15.5%. The CFRP plate uses an impregnating resin as the matrix and carbon fiber as the reinforcement. The performance of carbon fiber is significantly better than that of the impregnating resin; therefore, the performance of the impregnating resin largely determines the performance of the CFRP plate. However, epoxy resin impregnating resins suffer from insufficient toughness and high brittleness due to their high degree of curing and cross-linking. Modifying the impregnating resin with functionalized carbon nanotubes thus modifies the CFRP plate. On the one hand, the performance of the impregnating resin is enhanced, thereby improving the performance of the CFRP plate. On the other hand, the carbon nanotubes fill the micropores within the CFRP plate, improving the connection between the carbon nanotubes and the impregnating resin. The unique slender tubular structure of the carbon nanotubes acts as a "binding" mechanism for the carbon fibers, resulting in a tighter connection between the carbon fibers and further improving the performance of the CFRP plate, especially increasing its interlaminar shear strength. With the improvement of the CFRP plate performance, the ultimate load-bearing capacity of the specimen is enhanced. The CFRP plate in Experimental Example 8 was unmodified, and its performance was inferior to that of the CFRP plate modified with functionalized carbon nanotubes.

[0097] The difference between Experimental Example 9 and Experimental Example 2 is that neither the adhesive nor the CFRP board used in Experimental Example 9 was modified. Compared to Experimental Example 2, the ultimate load-bearing capacity of the specimen in Experimental Example 9 decreased by 19.5%. For reinforced structures where both the adhesive and the CFRP board were modified, not only were the properties of the adhesive layer and the reinforcing material improved, but the modified particles adhering to the surfaces of the adhesive layer and the CFRP board increased the mechanical interlocking force at the interface. Simultaneously, the active groups and epoxy groups aggregated at the interface cross-linked, improving the adhesion between the interfaces, thereby enhancing the interfacial properties between the adhesive and the CFRP board, and ultimately increasing the load-bearing capacity of the specimen. In Experimental Example 9, where neither the adhesive nor the CFRP board was modified, the adhesive layer properties, CFRP board properties, and interfacial properties of the specimen were inferior to those of the specimen with modified adhesive and CFRP board.

[0098] The above data shows that in the reinforcement method of this patent, the adhesive is optional, the adhesive layer thickness is controllable, and the modified CFRP board after secondary high-temperature curing has excellent performance. Based on the reinforcement structure of this patent, especially since both the adhesive and the CFRP board have been modified and the adhesive layer thickness is within the preferred adhesive layer thickness range, its interface performance, adhesive layer performance and CFRP material performance are significantly improved, the ultimate bearing capacity is significantly improved, and it has excellent heat resistance and long-term durability. The reinforcement effect is significantly better than the existing reinforcement methods.

[0099] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A method for adhesively reinforcing curved steel structures with CFRP sheets, characterized in that, The method includes the following steps: Apply a release agent to the reinforced surface (5); place the CFRP substrate on the reinforced surface (5) to be reinforced. The CFRP substrate includes CFRP cloth (6) and release cloth (7) laid from bottom to top. Fix the vacuum bag (9) on the reinforced surface (5) to form a sealing device; inject the sealing device with impregnating resin (2) to form a CFRP board (18); clean the CFRP board (18) and the reinforced surface (5), and adhere the CFRP board (18) to the reinforced surface (5) with adhesive. Excess adhesive is squeezed out by applying pressure to increase the interfacial adhesion. The process of forming the CFRP board (18) includes the following steps: the CFRP cloth (6) after injection is first cured in a closed environment of 10℃~30℃, and after curing, the nano-modified CFRP board (18) is obtained by demolding. The obtained CFRP board (18) is then placed in an environment of 50℃~60℃ for 8~12 hours to cure. Before the sealing device is glued with impregnating resin (2) to form CFRP plate (18), the following steps are also included: mixing 0.2wt% functionalized carbon nanotubes, 2wt% silane coupling agent and impregnating resin (2), wherein the functionalized carbon nanotubes are selected from any one or more of carboxylated carbon nanotubes, hydroxylated carbon nanotubes and aminolated carbon nanotubes. After the excess adhesive is squeezed out by applying pressure, the following steps are also included: waiting for the adhesive to cure at an environment of 10℃~30℃, and after curing, applying at least two layers of acrylic waterproof coating continuously along the surface of the CFRP board (18), the outer peripheral surface of the adhesive and the reinforced surface (5); Before bonding the CFRP board (18) to the reinforced surface (5) with adhesive, the following steps are also included: the adhesive is Sika30 epoxy resin adhesive, and 2wt% of liquid nitrile rubber polymer with active groups is dispersed in the adhesive. The liquid rubber polymer with active groups is any one or more of terminal carboxyl nitrile rubber, terminal hydroxyl nitrile rubber, and terminal amino nitrile rubber.

2. The method for adhesive reinforcement of curved steel structures using CFRP sheets as described in claim 1, characterized in that, After the sealing device is formed and before the glue is injected, the following steps are also included: building a glue injection channel, which includes an impregnating resin container (1), a sealing device, a resin collector (15) and a vacuum pump (16) connected in sequence, and performing an airtightness check on the device.

3. The method for adhesive reinforcement of curved steel structures using CFRP sheets as described in claim 2, characterized in that, The process of forming the sealing device specifically includes the following steps: The CFRP substrate also includes a guide net (8), a spiral tube (11), an inlet glue injection seat (10), and an outlet glue injection seat (12) laid from bottom to top. First, the inlet glue injection seat (10) and the outlet glue injection seat (12) are placed on the guide net (8) parallel to the fiber direction. Then, the spiral tube (11) is arranged under the inlet glue injection seat (10) and the outlet glue injection seat (12) respectively along the direction perpendicular to the fiber. The inlet glue injection seat (10) and the outlet glue injection seat (12) are then placed on the guide net (8). (12) Place the resin along the direction of fiber flow on both sides of the guide net (8) and fix the arranged guide net (8), glue inlet seat (10), glue outlet seat (12) and spiral tube (11); arrange several sealing components around the guide net (8), and set up a vacuum bag (9) overlap area on each sealing component to avoid the vacuum bag (9) from being suspended. When sealing the vacuum bag (9), start pasting from both ends, and seal the remaining vacuum bag (9) in the middle section through the sealing component of the overlap area.

4. The method for adhesive reinforcement of curved steel structures using CFRP sheets as described in claim 3, characterized in that, The process of injecting adhesive includes the following steps: turn on the vacuum pump (16) and vacuum clamp (4), and drive the impregnation resin (2) to flow under vacuum negative pressure. The impregnation resin (2) flows from the glue injection seat (10) into the sealing device, and then through the spiral tube (11) connected to the glue injection seat (10), it is evenly distributed inside the sealing device. When the impregnation resin (2) flows out from the glue outlet injection seat (12), turn off the vacuum pump (16) and vacuum clamp (4).

5. The method for adhesive reinforcement of curved steel structures using CFRP sheets as described in claim 1, characterized in that, After cleaning the CFRP board (18) and the reinforced surface (5), and before bonding the CFRP board (18) to the reinforced surface (5) with adhesive, the following steps are also included: sprinkle several steel balls of any type with a diameter of 1mm to 1.5mm into the reinforced surface (5) to control the thickness of the adhesive.

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