Fabricated diaphragm wall vertical joint electric heating pre-tightening type connecting structure and construction method

Abstract

This invention discloses an electrically heated pre-tightening connection structure for vertical joints of prefabricated diaphragm walls and its construction method. The structure includes a vertical support unit, a positioning unit, a pre-tightening force generating unit, and a power supply unit. The vertical support unit is composed of threaded steel bars placed within a mating groove. The positioning unit is a positioning device welded to the threaded steel bars, which is a ring-shaped clamp made of iron-based shape memory alloy. The pre-tightening force generating unit is composed of electrothermal microfibers, with the rubber removed from both ends and wound around the threaded steel bars. The power supply unit consists of an iron plate, a power cord, and a generator installed on the upper part of the threaded steel bars. After heating to a specified temperature, the power is cut off. The electrothermal microfibers shrink during cooling, thus achieving active pre-tightening of the joint. This invention, through an adjustable electrothermal pre-tightening process, enables active control of the connection performance, significantly improving the joint stiffness and horizontal bearing capacity of prefabricated diaphragm walls, effectively ensuring the overall stability and long-term safety of deep foundation pit projects.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering and underground structure engineering, specifically to a prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure and its construction method. Background Technology

[0002] With the deepening of industrialized construction and green construction concepts, prefabricated diaphragm walls have been increasingly widely used in deep foundation pit engineering due to their advantages such as fast construction speed, minimal environmental impact, and easy quality control. However, their core technology and performance bottleneck lies in the connection nodes of the vertical joints between prefabricated wall panels. The performance of these nodes directly determines the overall rigidity, deformation coordination, waterproofing reliability, and even the overall safety of the foundation pit of the entire diaphragm wall.

[0003] Currently, common connection methods in engineering mainly include the following categories: First, grouting connection after rebar lap splicing. Although this method is relatively traditional, its performance heavily depends on the density of the grouting on site and the performance of the post-cast materials. It belongs to a passive force-bearing system, which has extremely high requirements for construction quality control and is difficult to avoid stiffness loss caused by shrinkage, micro-cracks, and other factors. Second, bolted mechanical connection. This method has extremely stringent requirements for the processing accuracy and installation alignment accuracy of precast components. In actual engineering, it has poor tolerance, and the stiffness of the joint area is often difficult to match with the wall itself, which can easily form weak links. Third, pre-tensioned prestressed connection with reserved ducts. Although this method can actively apply prestress and improve the stress, it has problems such as complex process, need to reserve complex ducts and anchoring systems in the wall panel, large space requirements for tensioning operations, and difficulty in accurately assessing long-term prestress loss, which restricts its widespread application.

[0004] In summary, existing technologies generally suffer from limitations such as limited improvement in connection stiffness, poor construction tolerance, complex processes, or inability to actively improve the stress state after the entire structure is formed. In deep, large, and complex foundation pit projects, the retaining structure needs to withstand enormous water and soil pressure and uneven loads, placing higher demands on the strength and stiffness of the connection nodes. Therefore, developing a new connection technology that can achieve efficient and reliable assembly, actively apply pre-tensioning force to significantly improve the performance of nodes and the whole structure, and simultaneously consider construction convenience and economy has become a key issue in promoting the development of prefabricated underground structures into deeper and broader fields. Summary of the Invention

[0005] Technical Problem: The purpose of this invention is to provide an electric heating pre-tightening connection structure and construction method for the vertical joint of prefabricated diaphragm walls, in order to solve the problems of low stiffness, poor force transmission performance, low construction efficiency and easy water seepage in the prefabricated diaphragm wall connection nodes mentioned in the background art, thereby achieving active pre-tightening of the connection nodes, controllable performance and high construction efficiency.

[0006] Technical solution: The present invention provides a prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure, which includes a prefabricated concave wall, a prefabricated convex wall and a pre-tightening connection device, wherein the prefabricated concave wall and the prefabricated convex wall are horizontally connected by the pre-tightening connection device; The pre-tightening connection device includes a vertical support unit, a positioning unit, a pre-tightening force generating unit, and a power supply unit; The vertical support unit includes two parallel threaded steel bars, which are placed in the connecting groove of the prefabricated concave wall and the prefabricated convex wall; The positioning unit includes four positioning devices, which are welded to the outer surface of two threaded steel bars respectively. They are arranged equidistantly from the bottom of the threaded steel bars upwards with a spacing of 1200mm, and the position of the vertical support unit is fixed in the connecting groove of the prefabricated concave wall and the prefabricated convex wall. The preload generating unit includes multiple electrothermal microfibers, each consisting of a carbon fiber core and an outer silicone rubber insulating layer. The electrothermal microfibers are arranged equidistantly from the bottom of the threaded steel bar upwards at a spacing of 1500mm, with their ends connected to the outer surfaces of two threaded steel bars. The preload is generated by the thermal expansion and contraction of the electrothermal microfibers. The power supply unit includes an iron plate fixed to the two threaded steel bars, a power cord connected thereto, and a power supply device for supplying power to the positioning device and the electrothermal microfiber. When the power is turned on, the positioning device and the electrothermal microfiber heat up simultaneously. The positioning device undergoes radial contraction to counteract the thermal expansion of the electrothermal microfiber during the heating stage, thus maintaining the stability of the space inside the trough. After the grout is poured, the power is turned off, causing the electrothermal microfiber to generate pre-tightening force during the cooling and contraction process.

[0007] The distance between the threaded steel bar and the edge of the wall shall not be less than 100mm; the length of the threaded steel bar shall be longer than the height of the prefabricated recessed wall to facilitate the installation of the power supply unit.

[0008] The positioning device is made of iron-based shape memory alloy to form a ring clamp. The initial inner diameter matches the diameter of the threaded steel. The length L2 of the two outer arms of the positioning device is less than the width of the connecting groove of the prefabricated concave wall. The outer arms point to the opening direction of the connecting groove, which makes it easy for the positioning device to be inserted into the mating groove of the prefabricated concave wall and the prefabricated convex wall.

[0009] The iron-based shape memory alloy is a smart material that uses iron as a matrix and incorporates manganese and silicon components. It achieves shape recovery through thermoelastic martensitic phase transformation. Its phase transformation temperature can be adjusted within the range of 100-200°C, and its tensile strength is typically about 500 MPa. At room temperature, a tensile force needs to be applied to the iron-based shape memory alloy to produce permanent plastic elongation.

[0010] The electrothermal microfiber typically has a tensile strength of 50-300 MPa and a carbon fiber volume ratio of 20%-60%. The electrothermal microfiber has a diameter of approximately 10-15 mm and a length L1 of 40d1, where d1 is the diameter of the threaded steel bar. 150 mm of silicone rubber layer is removed from both ends to expose the carbon fiber core, which is then tightly wound onto two threaded steel bars to ensure good electrical contact.

[0011] The connection area of ​​the electrothermal microfiber is coated with high-temperature resistant epoxy resin, which covers the winding part and extends to both ends. After it dries, it is then tightly wrapped with high-temperature resistant electrical tape to achieve insulation and protection.

[0012] The iron plate is a galvanized iron plate, with two plates on each threaded steel bar; the iron plate is fixed to the top of the threaded steel bar by high-strength bolts.

[0013] The power supply is an adjustable low-voltage high-current DC power supply with an output voltage range of 0-50V and an output current capability of not less than 350A. The power supply has constant current or constant voltage output modes and can perform precise control of current and time according to a preset program to achieve stable regulation of the positioning device and the electrothermal microfiber heating process.

[0014] The construction method of the prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure of the present invention includes the following steps: Step S1: Prefabricate the pre-tightening connection device. The inner diameter of the positioning device matches the diameter of the threaded steel. Four positioning devices are installed on the outer surface of each threaded steel, arranged equidistantly from the bottom of the threaded steel upwards at a spacing of 1200mm. At room temperature, apply tension to the outer extension arm of the iron-based shape memory alloy clamp of the positioning device to induce permanent plastic elongation. Next, install the pre-tightening force generation unit. Select multiple electrothermal microfibers composed of carbon fiber cores and silicone rubber insulation layers. Arrange each electrothermal microfiber equidistantly from the bottom of the threaded steel upwards at a spacing of 1500mm and temporarily fix them with non-metallic cable ties. Install the power supply unit. Fix four galvanized iron plates to the top of two threaded steels respectively with high-strength bolts. Wiring holes are reserved on the iron plates. Step S2: Fabrication of the prefabricated recessed wall. Three rows of vertical connecting grooves are reserved at the vertical joints of the wall panels. The two connecting grooves near the edge have rounded rectangular cross-sections and their depth extends through the height of the wall panel. The middle connecting groove has an isosceles trapezoidal cross-section and its depth extends through the height of the wall panel. Step S3: Fabrication of the precast protruding wall. Two rows of vertical connecting grooves and one row of connecting protrusions are reserved at the vertical joints of the wall panels. The two connecting grooves near the edge have rounded rectangular cross-sections and their depth extends through the height of the wall panel. The connecting protrusion in the middle has an isosceles trapezoidal cross-section and its depth extends through the height of the wall panel. Step S4: On-site installation and positioning. The precast concave wall and precast convex wall are hoisted to the design position in sequence. The mating groove and the mating convex groove are matched. The two connecting grooves of the precast concave wall and the precast convex wall are aligned to form a continuous cavity. Step S5: Pre-tightening force activation. Connect the power supply to the iron plate on top of the two threaded steel bars, turn on the power and slowly increase the output current to make the pre-embedded positioning device and the electrothermal microfiber heat up evenly and maintain this constant temperature state. During the heating process, the iron-based shape memory alloy undergoes radial contraction, thereby offsetting the thermal expansion generated during the heating process of the electrothermal microfiber. Step S6: After the positioning device and the electrothermal microfiber reach the specified temperature, grouting and curing are carried out, using high-strength grouting material that is resistant to high temperature and has micro-expansion. Step S7: Cooling and shrinkage to form pre-tightening force. After grouting is completed, the power is cut off, and the entire connection structure is allowed to cool slowly to ambient temperature under natural conditions. During the cooling process, the positioning device does not directly generate new pre-tightening force, while the electrothermal microfiber will shrink due to the thermal expansion and contraction effect. Since the two ends of the electrothermal microfiber are anchored to two threaded steel bars respectively, its shrinkage force is transmitted through the threaded steel bars and converted into horizontal tensile stress. Then, through the bonding friction between the grout and the wall of the connection groove, a continuous normal compressive stress is formed between the precast wall panels on both sides, realizing the active pre-tightening of the joint.

[0015] In step S5, the predetermined temperature is 80-120℃, which is lower than the tolerance temperature of the grouting material and the electrothermal microfiber insulation layer; during the power-on heating process in step S5, the constant temperature is maintained for 30 to 60 minutes.

[0016] Beneficial Effects: This invention utilizes the thermal expansion and contraction mechanism of electrothermal microfiber and precise control of an adjustable power supply, combined with the temperature response characteristics of an iron-based shape memory alloy positioning device, to achieve active and adjustable pre-tightening of the vertical joint connection performance of prefabricated diaphragm walls, significantly improving the overall stiffness and horizontal bearing capacity of the joints. The synergistic thermal response of the iron-based shape memory alloy and electrothermal microfiber during energization effectively offsets the thermal expansion effect during the heating stage, ensuring the initial stability of the connection structure before grouting. The vertical support unit and grouting material work together to form a composite force system, further enhancing the deformation coordination and shear resistance between wall panels. The modular construction method of factory prefabrication and on-site assembly greatly improves construction efficiency and installation convenience, ensuring smooth process connections and easy quality control. The system is equipped with multiple electrical protections, ensuring safe and reliable construction. It can flexibly adapt to different wall thicknesses and engineering conditions by adjusting component parameters, possessing both good technical versatility and comprehensive economic efficiency, aligning with the development direction of green construction and industrialized construction. Attached Figure Description

[0017] Figure 1This is a three-dimensional schematic diagram of the wall panel splicing of the prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure of the present invention; Figure 2 This is a three-dimensional schematic diagram of the pre-tightening connection device of the present invention; Figure 3 This is a top sectional view of the pre-tightening connection device of the present invention after it has been installed in place; Figure 4 This is a partially enlarged schematic diagram of the threaded steel positioning device of the present invention; Figure 5 This is a partially enlarged schematic diagram of the electrothermal microfiber of the present invention; Figure 6 This is a partially enlarged schematic diagram of the top iron plate of the rebar of the present invention; Among them are: 1. Threaded steel bar; 2. Positioning device; 3. Electrothermal microfiber; 4. Iron plate; 5. Precast concave wall; 51. Butt groove; 6. Precast convex wall; 61. Butt groove; 7. Connecting groove. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0019] Please see Figure 1-6 The present invention provides a prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure, which includes a prefabricated concave wall 5, a prefabricated convex wall 6 and a pre-tightening connection device. The prefabricated concave wall 5 and the prefabricated convex wall 6 are horizontally connected by the pre-tightening connection device. The pre-tightening connection device includes a vertical support unit, a positioning unit, a pre-tightening force generating unit, and a power supply unit; The vertical support unit includes two parallel threaded steel bars 1, which are placed in the connecting groove 7 of the prefabricated concave wall 5 and the prefabricated convex wall 6. The positioning unit includes four positioning devices 2, which are respectively welded to the outer surface of two threaded steel bars 1 and are arranged at equal intervals from the bottom of the threaded steel bars 1 upwards with a spacing of 1200mm. The position of the vertical support unit is fixed in the connecting groove 7 of the prefabricated concave wall 5 and the prefabricated convex wall 6. The preload generating unit includes multiple electrothermal microfibers 3, each consisting of a carbon fiber core and a silicone rubber insulating layer. The electrothermal microfibers 3 are arranged equidistantly from the bottom of the threaded steel bar 1 upwards at a spacing of 1500mm. Their ends are connected to the outer surfaces of the two threaded steel bars 1. The preload is generated by the thermal expansion and contraction of the electrothermal microfibers 3. The power supply unit includes an iron plate 4 fixed on the two threaded steel bars 1, a power cord connected thereto, and a power supply device, which is used to supply power to the positioning device 2 and the electrothermal microfiber 3. When the power is turned on, the positioning device 2 and the electrothermal microfiber 3 heat up simultaneously. The positioning device 2 undergoes radial contraction to counteract the thermal expansion of the electrothermal microfiber 3 during the heating stage, thus maintaining the stability of the space inside the cavity. After the grouting material is poured, the power is turned off, causing the electrothermal microfiber 3 to generate pre-tightening force during the cooling and contraction process.

[0020] The pre-tightening connection device is a core component of this invention. Its structural design fully considers the operability of on-site installation, the reliability of force transmission, and the controllability of pre-tightening force. This device is prefabricated in the factory and includes a vertical support unit, a positioning unit, a pre-tightening force generation unit, and a power supply unit.

[0021] The vertical support unit uses two HRB400 grade threaded steel bars 1, with the diameter selected according to the wall thickness. The length of the threaded steel bar 1 should be 150mm longer than the designed height of the precast wall panel to facilitate the installation of the power supply unit and subsequent end cutting. The two threaded steel bars 1 are arranged in parallel with a spacing of 40d1.

[0022] The positioning unit consists of four positioning devices 2. Each positioning device 2 is a ring-shaped clamp made of an iron-based shape memory alloy with a thickness of 6mm, a width of 20mm, and a length not exceeding 220mm. This iron-based shape memory alloy is a smart material with iron as the matrix, doped with manganese, silicon, etc., achieving shape recovery through thermoelastic martensitic phase transformation. Its phase transformation temperature can be adjusted within the range of 100-200°C, and its tensile strength is typically about 500MPa. The inner diameter of the positioning device 2 matches the diameter of the fixed threaded steel bar 1. The length of the two outer arms of the positioning device 2 is slightly smaller than the width of the prefabricated recessed wall 5, and the direction of the outer arms points outwards from the groove, facilitating the insertion of the positioning device 2 into the outer surface of the threaded steel bar 1. Each positioning device 2 is welded to the outer surface of two threaded steel bars 1, arranged equidistantly from the bottom of the threaded steel bar 1 upwards, with a spacing of 1200mm. The two outer arms of the positioning device 2, with a length slightly smaller than the width of the prefabricated recessed wall 5, primarily function to limit the horizontal displacement of the connecting device within the groove during installation, ensuring its centered positioning and providing a uniform thickness space for the grouting layer. At room temperature, a tensile force needs to be applied to the outer arm of the iron-based shape memory alloy clamp to produce permanent plastic elongation, resulting in a strain of 4%-6%.

[0023] The pre-tightening force generating unit is the core innovation of this invention, implemented by three electrothermal microfibers 3. The electrothermal microfibers 3 are composite wires, with an internal conductive heating core composed of high-strength carbon fiber bundles, and an external heat-resistant silicone rubber insulation layer, with an overall diameter of approximately 15mm. During installation, they are arranged equidistantly from the bottom of the threaded steel bar 1 upwards at a spacing of 1500mm, and temporarily secured with non-metallic cable ties. Approximately 150mm of insulation is removed from each end of the electrothermal microfiber 3, exposing the carbon fiber core. The exposed carbon fiber core is then tightly wound around the corresponding two threaded steel bars 1 to ensure reliable electrical connection and stable resistance. After winding, a layer of high-temperature resistant (temperature resistance > 150℃) epoxy resin adhesive is applied to the connection area, completely covering the wound area and extending 20mm to each end. After the epoxy resin has initially cured, two layers of high-temperature resistant electrical tape are tightly wrapped around it to achieve insulation protection and mechanical fixation. The three electrothermal microfibers 3 are connected in parallel to form an independent heating circuit.

[0024] The power supply unit provides a controllable power source for the electrothermal microfiber 3. At the top of each threaded steel bar 1, a 100mm×100mm×6mm galvanized steel plate is fixed as a terminal block using two M12 high-strength bolts. A 70mm² high-temperature resistant copper core flexible cable is used as the power cord, with copper terminals crimped to its ends and securely connected to the terminal block using bolts. The other end of the power cord is connected to an adjustable low-voltage, high-current DC power supply. This power supply has an output voltage of 0-50V and an output current capability of no less than 350A. It can operate in constant current mode and integrates overcurrent, short-circuit, and overheat protection, as well as a digital temperature feedback interface, making it suitable for fluctuating operating conditions at the construction site.

[0025] During factory prefabrication, the precast recessed wall 5 requires the pre-embedded core mold forming a horizontally continuous groove within the connecting side template. For the precast recessed wall 5, the cross-section of the two side grooves is a rounded rectangle, with a recommended width of 50mm, a recommended length greater than 20d1, a recommended corner radius of 25mm, and a depth the same as the wall panel height. For example, when d1 is 28mm, the length should be greater than 560mm; in this embodiment, it is 575mm. The cross-section of the middle groove is an isosceles trapezoid, with a recommended upper base of 100mm, a recommended lower base of 300mm, a recommended height of 100mm, and a depth the same as the wall panel height. The inner wall of the groove should be designed with a rough surface and pre-embedded with multiple rows of 8mm diameter HRB300 tie bars, extending 20mm beyond the inner wall to greatly enhance the bonding force between the grout and the precast concrete. The concrete strength grade of the wall panel should not be lower than C35, and during pouring, it should be ensured that the groove area is vibrated to ensure compaction and a regular shape.

[0026] During factory prefabrication, the precast protruding wall 6 requires the pre-embedding of a core mold forming a horizontally continuous groove within the connecting side formwork. For the precast protruding wall 6, the cross-section of the two side grooves is a rounded rectangle, with a recommended width of 50mm, a length greater than 20d1, a recommended corner radius of 25mm, and a depth equal to the wall panel height. For example, when d1 is 28mm, the length should be greater than 560mm; in this embodiment, it is 575mm. The cross-section of the middle protrusion is an isosceles trapezoid, with a recommended upper base of 100mm, a lower base of 300mm, a recommended height of 100mm, and a depth equal to the wall panel height. The inner wall of the groove should be designed with a rough surface and pre-embedded with multiple rows of 8mm diameter HRB300 tie bars, extending 20mm beyond the inner wall to greatly enhance the bonding force between the grout and the precast concrete. The concrete strength grade of the wall panel should not be lower than C35, and during pouring, it should be ensured that the groove area is vibrated to ensure compaction and a regular shape.

[0027] The implementation methods of this invention will be further described below in conjunction with the construction steps: Step S1: Factory prefabrication of pre-tightened connection devices. Based on the designed wall thickness, select HRB400 grade threaded steel 1 with a nominal diameter of 20-32mm as the vertical support unit. Its length is determined according to design requirements, typically 150mm longer than the height of the prefabricated wall panel. Install positioning device 2, using a ring clamp made of iron-based shape memory alloy with a thickness of 6mm, a width of 20mm, and a length not exceeding 220mm. The two outer arms of positioning device 2 are slightly shorter than the width of the groove in the prefabricated recessed wall 5, and the outer arms point outwards from the groove. At room temperature, a tensile force needs to be applied to the outer arms of the iron-based shape memory alloy clamp to induce permanent plastic elongation, stretching 4%-6%. % strain; four positioning devices 2 are installed on each threaded steel bar 1, respectively welded to the outer surface of two threaded steel bars 1, and arranged equidistantly from the bottom of the threaded steel bar 1 upwards at a spacing of 1200mm, to ensure that the threaded steel bar 1 is centered in the groove; a pre-tightening force generating unit is set up by selecting three electrothermal microfibers 3 composed of carbon fiber core and silicone rubber insulation layer, equidistantly arranged from the bottom of the threaded steel bar 1 upwards at a spacing of 1500mm, after removing 150mm of insulation layer from both ends, and then wrapping them around two threaded steel bars 1, coating them with high-temperature resistant epoxy resin and wrapping them with high-temperature resistant electrical tape; a power supply unit is installed by fixing four galvanized iron plates 4 to the top of the threaded steel bar 1 with M12 high-strength bolts; Step S2: Fabrication of precast recessed wall 5. Taking a 6000mm high precast single wall panel as an example, tie the main steel reinforcement cage of the wall panel. Reserve three rows of horizontal butt grooves on the connection side of the wall panel. The two grooves near the edge have rounded rectangular cross-sections, with a width of 50mm, a length of 575mm, a rounded corner radius of 25mm, and a depth the same as the height of the wall panel. The middle groove has an isosceles trapezoidal cross-section, with an upper base of 50-100mm, a lower base of 150-300mm, a height of 50-100mm, and a depth that extends through the height of the wall panel. Multiple rows of tie bars are pre-set in the grooves, extending 20mm out of the inner wall. Concrete with a strength grade of not less than C35 is poured, vibrated to compact, and cured until the concrete strength reaches 80% of the design value before demolding. Step S3: Fabrication of the precast protruding wall 6. Taking a 6000mm high precast single wall panel as an example, tie the main steel reinforcement cage of the wall panel. Reserve two rows of horizontal butt grooves and one row of horizontal butt protrusions on the connection side of the wall panel. The two grooves near the edge have a rectangular cross-section, with a width of 50mm, a length of 575mm, a corner radius of 25mm, and a depth the same as the height of the wall panel. The middle protrusion has an isosceles trapezoidal cross-section, with an upper base of 50-100mm, a lower base of 150-300mm, a height of 50-100mm, and a depth that extends through the height of the wall panel. Multiple rows of tie bars are pre-set in the grooves, extending 20mm out of the inner wall. Concrete with a strength grade of not less than C35 is poured, vibrated to compact, and cured until the concrete strength reaches 80% of the design value before demolding. Step S4: On-site installation and positioning. After the foundation pit is leveled, precise measurements and layout are performed. Using a crawler crane and specialized lifting equipment, prefabricated recessed walls 5 and prefabricated protruding walls 6 are installed sequentially according to the mating grooves and teeth. The verticality and planar position of the wall panels are finely adjusted using jacks and diagonal bracing systems to ensure that the horizontal grooves of the two wall panels are aligned, forming a continuous and elongated combined groove. Subsequently, the entire pre-tightened connection device is lifted and smoothly and vertically inserted into the combined groove, relying on the positioning device 2 to center it naturally. Step S5: Activate the preload by powering on, connect the power cord, set the DC power supply to constant current mode, set the initial current to 100A, and then slowly increase it to the target current value, such as 350A, at a rate of 50A / min; monitor the temperature by using a thermocouple or infrared thermometer pre-attached to the positioning device 2 and the electrothermal microfiber 3 to ensure it remains stable at 100℃±10℃. Maintain this constant temperature for 45 minutes to allow heat to be fully and evenly conducted to the entire frame and surrounding grouting body; during the heating process, the iron-based shape memory alloy undergoes radial shrinkage, thereby offsetting the thermal expansion generated during the heating of the electrothermal microfiber 3, further ensuring the stability of the structure; Step S6: After the positioning device 2 and the electrothermal microfiber 3 reach the specified temperature, inject high-strength grout. Use aluminate cement-based high-temperature resistant grout with a design strength grade of C50 and micro-expansion characteristics. Inject the grout from the grouting hole at the bottom of one side of the cavity until a uniform grout overflows from the top of the other side. During grouting, a small immersion vibrator can be used to gently vibrate the outside of the wall panel to ensure that the grout is dense and free of voids. Step S7: Cooling to form permanent pre-tightening force. After grouting is completed, disconnect the power supply and allow the system to cool slowly in the natural environment. The cooling time should be no less than 24 hours. During this period, the positioning device 2 does not directly generate new pre-tightening force, while the electrothermal microfiber 3 gradually cools and shrinks. Since the two ends of the electrothermal microfiber 3 are anchored on two threaded steel bars 1, the shrinkage force is converted into horizontal tensile stress through the threaded steel bars. This tensile stress is eventually converted into continuous normal compressive stress on the interface of the precast wall panels on both sides through the bonding friction between the grout and the concrete trench wall, thereby achieving active pre-tightening of the joint. Step S8: Finishing and Acceptance. After the pre-tightening process is completed, remove the power cord and exposed wiring plate 4, cut off the excess threaded steel 1 extending from the top of the wall panel, and smooth it with mortar; check for hollow areas in the connection area by tapping with a small hammer; select representative joints for pressure water testing to verify their sealing and waterproofing performance; vibrating wire surface strain gauges can also be installed at key wall panel connections to monitor the maintenance of pre-tightening force over a long period. After acceptance, proceed with subsequent earthwork excavation and structural construction.

[0028] This invention innovatively introduces electrothermal microfiber and iron-based shape memory alloy into the conventional reinforced concrete connection system through the aforementioned structure and construction method, achieving adjustable and controllable active pre-tightening reinforcement in the vertical joints of prefabricated diaphragm walls. This technology precisely controls the heating process using an adjustable low-voltage, high-current power supply, allowing the pre-tightening force to be flexibly adjusted according to project requirements. Simultaneously, the iron-based shape memory alloy positioning device undergoes radial contraction upon heating, effectively offsetting the thermal expansion effect of the electrothermal microfiber and ensuring the initial stability of the connection structure before grouting. This significantly improves the overall stiffness and horizontal bearing capacity of the joint, enhancing the diaphragm wall's resistance to lateral displacement and shear. During construction, the system possesses multiple electrical protection functions to ensure safe and reliable operation. Combined with modular technology of factory prefabrication and on-site assembly, construction efficiency is greatly improved, achieving convenient and efficient installation and activation. Through multi-material synergy and process innovation, this technology not only optimizes the stress performance of nodes but also promotes the development of prefabricated underground structures towards a more intelligent and reliable direction, possessing significant engineering application and promotion value.

[0029] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure, characterized in that: It includes a prefabricated concave wall (5), a prefabricated convex wall (6) and a pre-tightening connection device, wherein the prefabricated concave wall (5) and the prefabricated convex wall (6) are horizontally connected by the pre-tightening connection device; The pre-tightening connection device includes a vertical support unit, a positioning unit, a pre-tightening force generating unit, and a power supply unit; The vertical support unit includes two parallel threaded steel bars (1), which are placed in the connecting groove (7) of the prefabricated concave wall (5) and the prefabricated convex wall (6); The positioning unit includes four positioning devices (2), which are welded to the outer surface of two threaded steel bars (1) respectively. They are arranged equidistantly from the bottom of the threaded steel bars (1) with a spacing of 1200mm. The position of the vertical support unit is fixed in the connecting groove (7) of the prefabricated concave wall (5) and the prefabricated convex wall (6). The pre-tightening force generating unit includes multiple electrothermal microfibers (3). The electrothermal microfibers (3) are composed of a carbon fiber core and a silicone rubber insulating layer wrapped around it. The electrothermal microfibers (3) are arranged equidistantly from the bottom of the threaded steel (1) with a spacing of 1500mm. Their ends are connected to the outer surfaces of the two threaded steels (1). The pre-tightening force is generated under the physical characteristics of thermal expansion and contraction of the electrothermal microfibers (3). The power supply unit includes an iron plate (4) fixed on the two threaded steel bars (1), a power cord connected thereto, and a power supply device for supplying power to the positioning device (2) and the electrothermal microfiber (3). When the power is turned on, the positioning device (2) and the electrothermal microfiber (3) heat up at the same time; the positioning device (2) undergoes radial contraction to counteract the thermal expansion generated by the electrothermal microfiber (3) during the heating stage, and maintain the stability of the space inside the cavity. After the grouting material is poured, the power is turned off, so that the electrothermal microfiber (3) generates pre-tightening force during the cooling and contraction process.

2. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 1, characterized in that: The distance between the threaded steel (1) and the edge of the wall shall not be less than 100mm; the length of the threaded steel (1) shall be longer than the height of the prefabricated recessed wall (5) in order to facilitate the installation of the power supply unit.

3. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 1, characterized in that: The positioning device (2) is made of iron-based shape memory alloy to form a ring clamp. The initial inner diameter matches the diameter of the threaded steel (1). The length L2 of the two extension arms of the positioning device (2) is less than the width of the connecting groove (7) of the prefabricated concave wall (5). The extension arms point to the opening direction of the connecting groove (7), which makes it easy for the positioning device (2) to be inserted into the mating groove (7) of the prefabricated concave wall (5) and the prefabricated convex wall (6).

4. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 3, characterized in that: The iron-based shape memory alloy is a smart material that uses iron as a matrix and incorporates manganese and silicon components. It achieves shape recovery through thermoelastic martensitic phase transformation. Its phase transformation temperature can be adjusted within the range of 100-200°C, and its tensile strength is typically about 500 MPa. At room temperature, a tensile force needs to be applied to the iron-based shape memory alloy to produce permanent plastic elongation.

5. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 1, characterized in that: The tensile strength of the electrothermal microfiber (3) is usually 50-300MPa, and the carbon fiber volume ratio is 20%-60%. The diameter of the electrothermal microfiber (3) is about 10-15mm, and the length L1 is 40d1, where d1 is the diameter of the threaded steel (1). 150mm long silicone rubber layers are peeled off from both ends to expose the carbon fiber core, and the core is tightly wound on the two threaded steels (1) to ensure good electrical contact.

6. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 5, characterized in that: The connection area of ​​the electrothermal microfiber (3) is coated with high-temperature resistant epoxy resin, which covers the winding part and extends to both ends. After it dries, it is then tightly wrapped with high-temperature resistant electrical tape to achieve insulation and protection.

7. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 1, characterized in that: The iron plate (4) is a galvanized iron plate, and two plates are arranged on each threaded steel bar (1); the iron plate (4) is fixed to the top of the threaded steel bar (1) by high-strength bolts.

8. The prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to claim 1, characterized in that: The power supply is an adjustable low-voltage high-current DC power supply with an output voltage range of 0-50V and an output current capability of not less than 350A. The power supply has a constant current or constant voltage output mode and can perform precise control of current and time according to a preset program to achieve stable regulation of the heating process of the positioning device (2) and the electrothermal microfiber (3).

9. A construction method based on the prefabricated diaphragm wall vertical joint electric heating pre-tightening connection structure according to any one of claims 1-8, characterized in that: Includes the following steps: Step S1: Prefabricate a pre-tightening connection device. The inner diameter of the positioning device (2) matches the diameter of the threaded steel (1). Four positioning devices (2) are installed on the outer surface of each threaded steel (1) and are arranged equidistantly from the bottom of the threaded steel (1) with a spacing of 1200mm. At room temperature, apply tension to the iron-based shape memory alloy clamp extension arm of the positioning device (2) to generate permanent plastic elongation. Then, install the pre-tightening force generation unit. Select multiple electrothermal superfibers (3) composed of carbon fiber core and silicone rubber insulation layer. Arrange each electrothermal superfiber (3) equidistantly from the bottom of the threaded steel (1) with a spacing of 1500mm and temporarily fix it with non-metallic cable ties. Install the power supply unit. Fix four galvanized iron plates (4) to the top of two threaded steels (1) respectively with high-strength bolts. The iron plates (4) have reserved wiring holes. Step S2: The construction of the prefabricated recessed wall (5) involves reserving three rows of vertical connecting grooves at the vertical joint of the wall panel. The two connecting grooves (7) near the edge have a rounded rectangular cross-section and a depth that extends through the height of the wall panel. The connecting groove (51) in the middle has an isosceles trapezoidal cross-section and a depth that extends through the height of the wall panel. Step S3: Fabrication of the prefabricated protruding wall (6): Two rows of vertical connecting grooves and one row of connecting protrusions are reserved at the vertical joint of the wall panel. The two connecting grooves (7) near the edge have a rounded rectangular cross section and a depth that extends through the height of the wall panel. The connecting protrusion (61) in the middle has an isosceles trapezoidal cross section and a depth that extends through the height of the wall panel. Step S4: On-site installation and positioning. The precast concave wall (5) and precast convex wall (6) are hoisted to the design position in sequence. The mating groove (51) matches the mating convex groove (61). The two connecting grooves (7) of the precast concave wall (5) and precast convex wall (6) are aligned to form a continuous cavity. Step S5: Pre-tightening force activation, connect the power supply device to the iron plate (4) on the top of the two threaded steel bars (1), turn on the power and slowly increase the output current, so that the pre-embedded positioning device (2) and the electrothermal microfiber (3) are heated evenly and maintain this constant temperature state; during the heating process, the iron-based shape memory alloy undergoes radial contraction, thereby offsetting the thermal expansion generated during the heating process of the electrothermal microfiber (3); Step S6: After the positioning device (2) and the electrothermal microfiber (3) reach the specified temperature, grouting and curing are carried out, using high-strength grouting material that is resistant to high temperature and has micro-expansion. Step S7: Cooling and shrinkage to form pre-tightening force. After grouting is completed, the power is cut off and the entire connection structure is allowed to cool slowly to the ambient temperature in the natural environment. During the cooling process, the positioning device (2) does not directly generate new pre-tightening force, while the electrothermal microfiber (3) will shrink due to thermal expansion and contraction. Since the two ends of the electrothermal microfiber (3) are respectively anchored on two threaded steel bars (1), its shrinkage force is transmitted through the threaded steel bars (1) and converted into horizontal tensile stress. Then, through the bonding friction between the grout and the wall of the connecting groove (7), a continuous normal compressive stress is formed between the two precast wall panels, realizing the active pre-tightening of the joint.

10. The construction method according to claim 11, characterized in that: The predetermined temperature in step S5 is 80-120℃, which is lower than the tolerance temperature of the grouting material and the electrothermal microfiber (3) insulation layer; during the power-on heating process in step S5, the constant temperature state is maintained for 30 to 60 minutes.

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