Frp profile-concrete composite beam with built-in damage self-monitoring and construction method thereof

Abstract

This invention discloses a FRP-concrete composite beam with built-in damage self-monitoring and its construction method, belonging to the field of civil engineering structural technology. The composite beam includes a concrete slab, an FRP profile, a shear reinforcement layer, intelligent connectors, and a protective shell. The intelligent connectors penetrate the shear reinforcement layer and are anchored to the FRP profile. Strain sensing elements are embedded internally, and driving elements are externally located. When the preload of the connector decreases, the driving elements actively extend to restore the preload. The shear reinforcement layer is a perforated plate filled with a tough material to enhance the shear resistance of the interface. The protective shell is a U-shaped precast concrete shell that wraps around the FRP web, providing lateral restraint and protection. This invention achieves real-time monitoring of FRP-concrete interface damage and automatic preload compensation, significantly improving the shear performance, integrity, and durability of the structure. It has advantages such as self-sensing, self-repair, and easy construction, and is suitable for long-span bridges, marine engineering, and intelligent building structures.

Description

Technical Field

[0001] This invention relates to the field of civil engineering structures, and in particular to an FRP profile-concrete composite beam with built-in damage self-monitoring and its construction method. Background Technology

[0002] Fiber-reinforced polymer / plastics (FRP) possess excellent properties such as lightweight, high strength, corrosion resistance, and fatigue resistance, leading to their increasingly widespread application in civil engineering. With the growing demand for lightweight, corrosion-resistant, and high-strength materials in bridge engineering and high-rise building structures, FRP is increasingly being used in composite structures due to its superior mechanical properties and durability. FRP profile-concrete composite beams combine the advantages of FRP's lightweight and high strength with the good compressive strength of concrete, becoming a research hotspot in structural engineering.

[0003] However, existing FRP profile-concrete composite beams mostly use mechanical connectors (such as bolts and shear studs) or adhesive layers to achieve interface connections, which still have the following key technical problems in practical applications: (1) Interface slippage and damage concealment: The shear connector between the FRP profile and the concrete slab is the key to ensuring their coordinated operation. Under long-term loads, fatigue loads, or extreme loads, the connector is prone to loosening, plastic elongation, or even fracture. These damages occur inside the structure and cannot be directly detected by external observation, posing a great safety hazard.

[0004] (2) Irreversible relaxation of preload: During the use of bolted connectors, the preload will gradually decrease due to the creep, shrinkage, vibration of concrete and the creep characteristics of FRP material, which will weaken the combined effect, increase the interface slip, and ultimately affect the structural bearing capacity and stiffness.

[0005] (3) FRP is prone to local buckling: The web of thin-walled FRP profiles is prone to local buckling when under pressure, which limits its application in high stress areas.

[0006] (4) Lack of proactive repair capability: Existing composite beams cannot recover on their own after the performance of the connectors deteriorates, resulting in high maintenance and reinforcement costs and often requiring the interruption of structural use.

[0007] To address the aforementioned issues, existing research has attempted to monitor bolts by attaching strain gauges or embedding fiber optic gratings, or by using shape memory alloy washers to compensate for preload. However, these technical solutions are often limited in function and lack system integration design tailored to the interface characteristics of FRP-concrete composite beams.

[0008] Therefore, developing an FRP-concrete composite beam that integrates real-time monitoring, automatic compensation, and shear reinforcement has significant engineering value and innovative implications. Summary of the Invention

[0009] The purpose of this invention is to provide an FRP profile-concrete composite beam with built-in damage self-monitoring and its construction method, aiming to solve the problems of invisible FRP-concrete interface damage, irreversible relaxation of preload, and easy buckling of FRP web in the prior art. It realizes real-time perception, active early warning and automatic compensation of preload for interface damage of composite beam, while significantly improving the integrity, shear performance and durability of the structure.

[0010] To achieve the above objectives, the present invention provides an FRP profile-concrete composite beam with built-in damage self-monitoring, comprising concrete and FRP profile, and further comprising a shear reinforcement layer, smart connectors and a protective shell; A shear reinforcement layer is placed between the concrete slab and the FPR profile; The smart connector penetrates the shear reinforcement layer and is anchored to the FPR profile. The smart connector includes: The connector body is used to transfer interfacial stress; The strain sensing element is embedded inside the connector body to sense the strain of the connector body in real time. A drive element, disposed outside the connector body, is configured to actively extend when the preload of the connector decreases, in order to restore or compensate for the preload. The protective shell should at least cover the outer side of the web of the FPR profile.

[0011] Preferably, the driving element is a shape memory alloy spring, the connecting body is a bolt, the shape memory alloy spring is sleeved on the shank of the bolt, one end of which abuts against the nut or the head of the connecting part, and the other end abuts against the surface of the shear reinforcement layer or the FRP profile.

[0012] Preferably, the strain sensing element is a fiber Bragg grating sensor, which is embedded in the center of the connector body along the axial direction.

[0013] Preferably, the shear reinforcement layer includes an open-cell FPR plate and a tough filler material filling the pores of the open-cell FPR plate; the tough filler material is a high-toughness cement-based composite material.

[0014] Preferably, the protective shell is a precast high-performance concrete shell with a U-shaped cross section, and the precast concrete shell is fixedly connected to the web of the FRP profile by shear studs.

[0015] Preferably, an isolation sleeve is fitted outside the section of the connector body located inside the shear reinforcement layer to prevent the section from adhering to the surrounding material; the isolation sleeve is a rubber sleeve.

[0016] Preferably, the anchoring depth of the smart connector in the concrete slab is 1 / 2 to 2 / 3 of the thickness of the concrete slab, so as to ensure sufficient anchoring force while avoiding penetration through the top of the slab.

[0017] Preferably, the cross-sectional shape of the FRP profile is I-shaped.

[0018] This invention also provides a construction method for the aforementioned FRP profile-concrete composite beam with built-in damage self-monitoring, comprising the following steps: Step 1: Prefabricate FRP profiles, protective shells, shear reinforcement layers, and smart connectors with strain sensing and drive elements; Step 2: Install the protective casing onto the outside of the FRP profile; Step 3: Apply the shear reinforcement layer to the upper surface of the FRP profile; Step 4: Install the smart connector, ensuring it passes through the shear reinforcement layer and connects to the FRP profile, and apply preload. Step 5: Pour concrete slabs to anchor the upper part of the smart connectors within the concrete slabs.

[0019] Therefore, the FRP profile-concrete composite beam with built-in damage self-monitoring and its construction method of the present invention have the following beneficial effects: (1) By using fiber optic strain sensors embedded in the core stress area of ​​the smart connector, the strain changes of the connector can be monitored accurately and in real time, directly reflecting the interface slippage and preload loss state, realizing early warning of damage, and greatly improving the structural safety level. Compared with traditional manual inspection or post-inspection, this invention realizes uninterrupted monitoring throughout the entire life cycle.

[0020] (2) A shape memory alloy spring is introduced as the driving element. The spring is configured in stress-triggered mode: when the bolt loosens due to vibration or material creep and the preload decreases, the spring senses the decrease in stress and actively elongates, pushing the nut towards the concrete slab, thereby automatically restoring most of the lost preload. This is a fundamental innovation of traditional passive connectors, enabling the structure to have the intelligent characteristic of "self-repair" without the need for external energy or human intervention.

[0021] (3) The shear reinforcement layer filled with tough material through the perforated plate, combined with the intelligent connector, improves the interface connection stiffness; the protective shell and the web of the FRP profile are connected by studs, which further contributes to the shear resistance. The three work together to achieve a shear bearing capacity far exceeding that of traditional designs, and the tough filling material can adapt to deformation and delay crack propagation.

[0022] (4) The precast concrete protective shell not only provides lateral support for the FRP web and prevents local buckling, but also serves as an excellent fireproof and corrosion-resistant protective layer, solving the problems of poor fire resistance and susceptibility to ultraviolet / chemical corrosion in the application of FRP structures. The shell is reliably connected to the FRP web through studs, forming a true composite section.

[0023] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the cross-sectional structure of the FRP profile-concrete composite beam with built-in damage self-monitoring according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of the smart connector according to an embodiment of the present invention; Figure 3 This is a cross-sectional view of the smart connector according to an embodiment of the present invention; Figure label: 1. Concrete slab; 2. Shear reinforcement layer; 3. Fiber optic strain sensor; 4. Shear stud; 5. FRP profile; 6. Smart connector; 7. Protective housing; 8. Memory alloy spring; 9. Rubber sleeve. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0027] Example Please see Figures 1-3 This embodiment provides an FRP profile 5-concrete composite beam with built-in damage self-monitoring, including a concrete slab 1, an FRP profile, a shear reinforcement layer 2, a smart connector 6, and a protective shell 7.

[0028] Concrete slab 1 is cast-in-place using ordinary concrete of strength grade C40, with a thickness of 150mm. Concrete slab 1 contains conventional reinforcing bars and distribution bars (not shown in the figure), and the reinforcing cage is tied according to current specifications.

[0029] FRP profile 5 is made of glass fiber reinforced polymer (GFRP) through a pultrusion molding process. The profile has an I-shaped cross-section with a flange width of 150mm, a web height of 300mm, and a wall thickness of 10mm at all parts. The upper flange of FRP profile 5 has pre-drilled bolt holes at the designed spacing (300mm in this embodiment), with the hole diameter slightly larger than the diameter of the screw of the smart connector 6.

[0030] The shear reinforcement layer 2 is a perforated FRP plate, 10 mm thick, with shear reinforcement holes of 50 mm diameter and 100 mm spacing arranged in a rectangular array. Each hole is pre-filled with high-toughness cement-based composite material (ECC) to form ECC pins. The ultimate tensile strain of ECC can reach over 3%, exhibiting multi-crack characteristics and capable of withstanding large deformations without brittle failure.

[0031] The shear reinforcement layer 2 also includes a connector through-hole, slightly larger than an M20 bolt, located directly above the bolt holes on the upper flange of the FRP profile, offset from the shear reinforcement holes. This hole is not filled with any material, allowing the smart connector 6 to pass through freely. The perforated FRP plate is placed on the top surface of the upper flange of the FRP profile 5, ensuring precise alignment between the connector through-hole and the bolt holes on the upper flange of the FRP profile.

[0032] The intelligent connector 6 includes a connector body, a strain sensing element, and a driving element. The connector body uses M20 high-strength bolts (grade 8.8), and the total length is determined based on the thickness of the concrete slab (1mm) + the shear reinforcement layer (2mm) + the upper flange of the FRP + the height of the nut and spring. For example, the length is 240mm, the diameter is 20mm, the pitch is 2.5mm, and the threaded section is 100mm.

[0033] The strain sensing element is a fiber Bragg grating strain sensor 3 (range ±5000με). During fabrication, a 2mm diameter through hole is drilled along the central axis of the bolt shank. The fiber Bragg grating sensor is inserted into the hole, ensuring the grating region is located in the middle of the bolt shank (i.e., the expected maximum strain zone). It is then filled and fixed with epoxy resin. The optical fiber extends from the bolt head, is protected by a stainless steel flexible conduit, and is ultimately connected to an external fiber Bragg grating demodulator.

[0034] The driving element is a compression spring made of Ni-Ti shape memory alloy. It has a wire diameter of 2.5 mm, a free length of 25 mm, a compressed length of 17 mm, a total of 6 coils, and an effective number of coils of 4. The spring undergoes a "training" process: it is heated to 120°C, compressed to 17 mm, held for 2 hours, and then cooled to room temperature, so that it is in a compressed state when subjected to the design preload (90 kN). When the preload drops to the set threshold (75 kN), the spring will extend to approximately 22 mm, thereby pushing the nut to move.

[0035] A rubber sleeve 9 is fitted around the high-strength bolt rod at the height of the shear reinforcement layer 2. The sleeve has an inner diameter of 28 mm, an outer diameter of 34 mm, and a length equal to the thickness of the shear reinforcement layer 2. The function of the rubber sleeve 9 is to isolate the bolt rod from the ECC and concrete, ensuring that the bolt can deform freely axially in this section, while providing the necessary space for the drive of the shape memory alloy spring 8 and preventing it from being "jammed" by the surrounding materials.

[0036] The protective outer shell 7 is a precast high-performance concrete shell with a U-shaped cross-section and a thickness of 40mm. The concrete strength grade is C60, and it contains steel fiber (1% by volume). The cross-sectional shape of the shell matches the outer surface of the web and flange of the FRP profile 5. The distance between the inner walls of the two webs is slightly greater than the thickness of the FRP web, and the width of the bottom support plate is the same as the width of the lower flange of the FRP. Multiple shear studs 4 are pre-welded or pre-embedded on the inner web and bottom support plate of the shell. They are 13mm in diameter, 60mm in length, with an adjacent horizontal spacing of 120mm, a vertical spacing of 120mm, and a side distance of 50mm.

[0037] The construction method of the above-mentioned device is as follows: 1. Factory prefabrication stage: 1) FRP I-shaped profiles are prefabricated using the pultrusion process, and bolt holes are drilled at 300mm intervals on the upper flange.

[0038] 2) Precast U-shaped high-performance concrete shell: formwork erection, binding of steel mesh (including pre-embedded studs), pouring of C60 steel fiber concrete, and curing to design strength.

[0039] 3) Fabrication of perforated FRP boards: Cut the FRP board, drill holes (50mm diameter, 100mm spacing), place the board into the mold, inject ECC into the holes, and cure for 28 days to form shear reinforcement layer 2.

[0040] 4) Fabrication of intelligent connector 6: Drill a hole in the center of the M20 high-strength bolt rod, insert a fiber optic grating sensor and fix it with epoxy resin; fit the rubber sleeve 9 into the corresponding section of the bolt rod; fit the shape memory alloy spring 8 into the bolt rod.

[0041] On-site assembly stage: 1) Insert the U-shaped protective shell 7 into the web of the FRP profile 5 from the end, adjust the position so that the shear studs 4 on the inside of the shell are in close contact with the web of the FRP profile 5, and then fix the shear studs 4 to the FRP web by epoxy structural adhesive.

[0042] 2) Lay the shear reinforcement layer 2 on the top surface of the upper flange of the FRP profile 5, so that the connection holes on the perforated FRP plate are aligned with the bolt holes of the upper flange of the FRP.

[0043] 3) Tie the steel cage of the concrete slab 1 above the shear reinforcement layer 2 and set up spacers to ensure the thickness of the protective layer.

[0044] 4) Install the smart connector 6: Pass the bolt shank through the connecting hole of the shear reinforcement layer 2 and the bolt hole of the upper flange of the FRP profile 5 from top to bottom, and screw the nut on below the upper flange of the FRP profile 5. At this time, the shape memory alloy spring abuts against the bolt head and the lower surface of the shear reinforcement layer respectively. Apply the designed preload using a torque wrench, at which point the shape memory alloy spring 8 is compressed to the designed length. Record the initial reading of the fiber optic grating sensor.

[0045] 5) Pour concrete slab 1 and compact it using an immersion vibrator, taking care to avoid direct contact with the smart connector 6 and the fiber optic lead. Cur the concrete for 28 days, keeping it moist during this period.

[0046] 6) Connect the optical fibers led out from each smart bolt to the fiber optic demodulator via a coupler, and connect it to the structural health monitoring system, and set early warning thresholds (such as strain changes exceeding ±2000με or preload decrease exceeding 20%).

[0047] The working principle and process of this device are as follows: (1) Normal service phase: When the composite beam is under load, the interfacial shear force is transferred from the concrete slab 1 to the FRP profile 5 through the intelligent connector 6. The high-strength bolts generate tensile strain, and fiber optic grating sensors monitor strain changes in real time. The data is processed by a demodulator to display the current preload level and interfacial slip state. The ECC filler in the shear reinforcement layer 2, together with the perforated plate, resists part of the shear force, while the ductile deformation capacity of the ECC buffers stress concentration. The U-shaped protective shell 7 works in conjunction with the FRP web through studs to improve bending stiffness and lateral stability, while also providing fire protection for the FRP.

[0048] (2) Self-compensation process when preload decreases: When the preload of the bolts decreases due to long-term loading, concrete creep, FRP creep, or vibration in the composite beam, the axial pressure sensed by the shape memory alloy spring 8 decreases. Because the spring is pre-trained to undergo phase transformation recovery (from martensite to austenite) when stress decreases, the spring actively extends, pushing the nut towards the concrete slab 1, thereby tightening the bolt and restoring most of the lost preload. This process requires no external energy or human intervention, achieving "self-healing."

[0049] (3) Damage monitoring and early warning: If extreme loads (such as overload or impact) cause the bolts to plastically elongate or the nuts to loosen significantly, the strain reading of the fiber optic grating sensor will show a sudden change or a continuous drop exceeding the set threshold. The monitoring system will immediately issue an alarm signal, prompting management personnel to conduct inspection or maintenance. Simultaneously, the presence of the U-shaped protective housing 7 prevents sudden instability of the FRP web, providing a safety redundancy for the structure.

[0050] Therefore, this invention provides an FRP profile-concrete composite beam with built-in damage self-monitoring and its construction method. This achieves real-time monitoring of interface damage and automatic compensation of preload in the FRP-concrete composite beam. Employing a "factory prefabrication + on-site assembly" model, the FRP profile, protective shell, shear reinforcement layer, and intelligent connectors can all be prefabricated with high precision in the factory. The encapsulation of the strain sensor can be completed under controlled conditions, resulting in high survival rate and reliability. On-site work requires only simple assembly, bolt pre-tightening, and concrete pouring (or precast slab installation), leading to high construction efficiency and easily guaranteed quality.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A composite beam of FRP profile and concrete with built-in damage self-monitoring, comprising concrete and FRP profile, characterized in that: It also includes a shear reinforcement layer, smart connectors, and a protective housing; A shear reinforcement layer is placed between the concrete slab and the FPR profile; The smart connector penetrates the shear reinforcement layer and is anchored to the FPR profile. The smart connector includes: The connector body is used to transfer interfacial stress; The strain sensing element is embedded inside the connector body to sense the strain of the connector body in real time. A drive element, disposed outside the connector body, is configured to actively extend when the preload of the connector decreases, in order to restore or compensate for the preload. The protective shell should at least cover the outer side of the web of the FPR profile.

2. The FRP profile-concrete composite beam with built-in damage self-monitoring according to claim 1, characterized in that: The driving element is a shape memory alloy spring, the connecting body is a bolt, and the shape memory alloy spring is sleeved on the shank of the bolt. One end of the spring abuts against the nut or the head of the connecting part, and the other end abuts against the surface of the shear reinforcement layer or FRP profile.

3. The FRP profile-concrete composite beam with built-in damage self-monitoring as described in claim 2, characterized in that: The strain sensing element is a fiber Bragg grating sensor, which is embedded in the center of the connector body along the axial direction.

4. The FRP profile-concrete composite beam with built-in damage self-monitoring as described in claim 3, characterized in that: The shear reinforcement layer includes an open-cell FPR plate and a tough filler material filling the pores of the open-cell FPR plate; the tough filler material is a high-toughness cement-based composite material.

5. The FRP profile-concrete composite beam with built-in damage self-monitoring as described in claim 4, characterized in that: The protective shell is a precast high-performance concrete shell, which is fixedly connected to the web of the FRP profile by shear studs.

6. The FRP-concrete composite beam with built-in damage self-monitoring as described in claim 5, characterized in that: An isolation sleeve is fitted outside the section of the connector body located inside the shear reinforcement layer to prevent the section from bonding with the surrounding materials; the isolation sleeve is a rubber sleeve.

7. The FRP-concrete composite beam with built-in damage self-monitoring as described in claim 6, characterized in that: The anchorage depth of the smart connector in the concrete slab is 1 / 2 to 2 / 3 of the thickness of the concrete slab.

8. A construction method for an FRP profile-concrete composite beam with built-in damage self-monitoring as described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Prefabricate FRP profiles, protective shells, shear reinforcement layers, and smart connectors with strain sensing and drive elements; Step 2: Install the protective casing onto the outside of the FRP profile; Step 3: Apply the shear reinforcement layer to the upper surface of the FRP profile; Step 4: Install the smart connector, ensuring it passes through the shear reinforcement layer and connects to the FRP profile, and apply preload. Step 5: Pour concrete slabs to anchor the upper part of the smart connectors within the concrete slabs.

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