Fabricated diaphragm wall Fe-SMA pre-tightening type connecting structure and construction method thereof

Abstract

The invention discloses a fabricated diaphragm wall Fe-SMA pre-tightening type connecting structure and a construction method thereof. The fabricated diaphragm wall Fe-SMA pre-tightening type connecting structure comprises a vertical supporting unit, a horizontal pre-tightening force generating unit, a positioning unit and a power supply unit. The vertical supporting units are composed of deformed steel bars and placed in the butt joint grooves. The horizontal pre-tightening force generating unit is a Fe-SMA rib which is welded on the deformed steel bar; the positioning unit is a positioning device welded on the deformed steel bar, and the positioning device is an annular hoop made of Fe-SMA; the power supply unit comprises an iron plate, a power line and a power generation device arranged at the upper part of the deformed steel bar; after being powered on, the pre-tightening type connecting device contracts in the radial direction, and therefore active pre-tightening of the connector is achieved. Through an electric heating pre-tightening process with adjustable parameters, active regulation and control of the connection performance are realized, and the pre-tightening force can be flexibly set, so that the node connection rigidity and the horizontal bearing capacity of the fabricated underground diaphragm wall are remarkably improved, and the overall stability and long-term safety of a deep foundation pit project are effectively guaranteed.

Description

Technical Field

[0001] This invention relates to the fields of geotechnical engineering and underground structure engineering, specifically to a prefabricated diaphragm wall Fe-SMA (iron-based shape memory alloy) pre-tightening connection structure and its construction method. Background Technology

[0002] As modern urban underground space development expands to deeper and wider areas, prefabricated diaphragm wall technology has become the mainstream choice for deep foundation pit support engineering due to its significant advantages in construction efficiency, quality consistency, and environmental friendliness. However, efficient and reliable connections between prefabricated components remain a core challenge restricting the technology's development towards high performance and long service life, with the mechanical and sealing performance of vertical joints being particularly critical.

[0003] In current engineering practice, the mainstream connection technologies for vertical joints all have corresponding limitations. Traditional grouting methods for rebar lap splices are highly dependent on the quality of on-site work, resulting in significant performance variability. Furthermore, being essentially a passive load-bearing system, it is difficult to actively adjust or compensate for performance during the structure's service life. Mechanical connections, such as high-strength bolts, are convenient to install, but they require extremely strict prefabrication accuracy and installation alignment. Sudden stiffness changes are prone to occur in the joint area, often becoming weak points in the structure. While pre-embedded prestressed connections can actively introduce compressive stress, they face practical difficulties such as complex processes, high precision in duct pre-reservation, difficulty in quantifying long-term stress loss, and limited space for on-site tensioning operations.

[0004] In summary, existing connection technologies generally face a common challenge: it is difficult to actively and controllably enhance the mechanical properties of connection nodes after the overall structure has been formed and bears loads. Given the stringent requirements of complex geological conditions and deep foundation pits, developing a new intelligent connection technology that can achieve "ready-to-use assembly with on-demand reinforcement" has become an urgent need to promote the advancement of prefabricated underground structure technology and engineering innovation. Summary of the Invention

[0005] Technical Problem: The purpose of this invention is to provide a prefabricated diaphragm wall Fe-SMA pre-tightening connection structure and its construction method, so as to achieve active and adjustable pre-tightening reinforcement of vertical joint nodes after the wall panel is assembled and the grout is fully cured, thereby significantly improving the overall stiffness, horizontal bearing capacity and long-term working performance of the nodes.

[0006] Technical solution: To achieve the above objectives, the present invention provides a prefabricated diaphragm wall electric heating pre-tightening connection structure using Fe-SMA, comprising 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 horizontal pre-tightening force generating unit, a positioning unit, and a power supply unit; The vertical support unit includes two pairs of threaded steel bars arranged in parallel vertically, which are used to be placed in the strip grooves of the prefabricated concave wall and the prefabricated convex wall; The horizontal preload generating unit includes horizontal Fe-SMA ribs, the two ends of which are welded to two threaded steel bars located in the same strip groove. They are arranged equidistantly from the bottom of the threaded steel bars with a spacing of 1500mm, forming a grid frame with the two threaded steel bars. The positioning unit includes positioning devices, which are welded to the outer surface of the threaded steel bar and arranged at equal intervals from the bottom of the threaded steel bar upwards, with a spacing of 1200mm. The power supply unit includes an iron plate fixed to the upper end of the threaded steel bar and a power supply device for supplying power to the Fe-SMA rib and the positioning device. After the grouting material is poured and cured, the power is turned on, and the Fe-SMA reinforcement and positioning device heat up simultaneously. Due to the physical properties of Fe-SMA's thermal expansion and contraction, a continuous normal compressive stress is formed between the precast wall panels on both sides, thereby achieving active pre-tightening of the joint.

[0007] The diameter of the threaded steel bar is 20-32mm, and HRB400 grade threaded steel bar is used. It is suitable for walls with a thickness of 800-1000mm. The distance between the threaded steel bar and the edge of the wall should not be less than 100mm. The length of the threaded steel bar should be 150mm longer than the height of the prefabricated recessed wall to facilitate the installation of the power supply unit.

[0008] The diameter of the horizontal Fe-SMA reinforcement is 16-22 mm; the length L1 of the horizontal Fe-SMA reinforcement is 50d2, where d2 is the diameter of the horizontal Fe-SMA reinforcement.

[0009] The Fe-SMA is a smart material with iron as the matrix and manganese and silicon doped into it. It achieves shape recovery through thermoelastic martensitic phase transformation. Its phase transformation temperature can be adjusted in the range of 100-200°C. Its tensile strength is 500MPa. At room temperature, tensile force needs to be applied to the Fe-SMA ribs to make them produce permanent plastic elongation.

[0010] The positioning device is made of Fe-SMA with a thickness of 6mm, a width of 20mm, and a length not exceeding 220mm to form a ring clamp. The initial inner diameter matches the diameter of the threaded steel. The length L2 of the outer arm of the positioning device is less than the width of the strip groove of the precast concave wall. The outer arm points towards the center of the strip groove, which facilitates the insertion of the positioning device into the strip groove of the precast concave wall and the precast convex wall. At room temperature, a tensile force needs to be applied to the outer arm of the Fe-SMA clamp to produce permanent plastic elongation.

[0011] The iron plate is a galvanized iron plate with dimensions of 100mm×100mm×6mm, and two plates are arranged at the top of each threaded steel bar; the two iron plates are fixed to the top of the threaded steel bar with bolts.

[0012] The power cord is a high-temperature resistant flexible cable with a nominal cross-sectional area of ​​50mm² to 95mm² for the core. The core is covered with a high-temperature resistant insulation layer and a flame-retardant sheath. The end of the power cord is connected to an iron plate via copper terminals and bolts, and is equipped with insulation protection. The power supply unit 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 unit 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 heating process of Fe-SMA ribs and positioning devices.

[0013] The construction method of the prefabricated diaphragm wall Fe-SMA pre-tightened connection structure of the present invention includes the following steps: Step S1: Prefabricated pre-tightening connection device: Select threaded steel according to the designed wall thickness. Its length is usually longer than the height of the prefabricated wall panel. The distance between two threaded steels is 40d1, where d1 is the diameter of the threaded steel. Select horizontal Fe-SMA ribs. At room temperature, tensile force needs to be applied to the Fe-SMA ribs to make them produce permanent plastic elongation. Fix the two threaded steels in parallel with a distance of 50d2, which is the length of the horizontal Fe-SMA ribs. Weld three horizontal Fe-SMA ribs between the two threaded steels respectively. Arrange them at equal intervals from the bottom of the threaded steels upwards to form a grid frame with the vertical support unit. Then, arrange the positioning device. Finally, install the power supply unit. Fix four iron plates to the top of the two threaded steels respectively with bolts. The iron plates have reserved wiring holes. Step S2: Construction of prefabricated recessed walls: Reserve three vertical butt grooves at the vertical joints of the wall panels. The two grooves near the edge have rounded rectangular cross-sections and extend to the height of the wall panel. The middle butt groove has an isosceles trapezoidal cross-section and extends to the height of the wall panel. Step S3: Construction of prefabricated protruding wall: Reserve two vertical strip grooves and one intermediate connecting protrusion at the vertical joint of the wall panel. The two grooves near the edge have rounded rectangular cross-sections and their depth extends through the height of the wall panel; the intermediate connecting protrusion has an isosceles trapezoidal cross-section and its depth extends through the height of the wall panel. Step S4: On-site installation and positioning: Hoist the precast recessed wall and precast protruding wall sequentially to the designed position. The central connecting groove of the precast recessed wall should align with the central connecting boss of the precast protruding wall. The strip groove of the precast protruding wall should be aligned with the strip groove of the precast recessed wall to form a continuous cavity. Smoothly insert the pre-tightening connection device into the strip groove. Step S5: Grouting and Curing: High-temperature resistant, micro-expansion high-strength grouting material is continuously injected into the strip groove until uniform grout overflows from the top of the groove; Step S6: Pre-tightening activation: Connect the power supply to the pre-reserved wiring hole on the iron plate, and slowly increase the output current to make the pre-embedded Fe-SMA ribs and positioning devices heat up evenly to 100℃-120℃, and maintain the constant temperature for 30 to 60 minutes; due to the physical properties of Fe-SMA thermal expansion and contraction, the Fe-SMA ribs and positioning devices will generate radial contraction, which will generate horizontal tensile stress in the entire grid frame, and then through the bonding friction between the grout and the groove wall, a continuous normal compressive stress will be formed between the precast wall panels on both sides, realizing the active pre-tightening of the joint; Step S7: Cooling to form permanent prestress: After heating is complete, cut off the power and allow the entire connection structure to cool slowly to ambient temperature under natural conditions to form long-term effective prestress.

[0014] in, The high-strength grouting material with high temperature resistance and micro-expansion mentioned in step S5 is aluminate cement-based high-temperature resistant mortar with a 28-day compressive strength of not less than 50 MPa, a long-term temperature resistance of not less than 150℃, and micro-expansion characteristics.

[0015] Beneficial Effects: This invention, through an innovative "grouting and curing followed by electrothermal activation" process, achieves active and adjustable pre-tightening of vertical joints in prefabricated diaphragm walls, significantly improving the overall stiffness and horizontal bearing capacity of the joints. This technology utilizes the unique thermo-induced shape recovery effect of Fe-SMA material. After the grout reaches its design strength, it is electrically activated, allowing the pre-tightening force to act directly on the formed structure. The force transmission path is clear and reliable, effectively avoiding the risks of thermal disturbance or uncertain stress on the grout during the plastic stage in traditional methods. Precise control of the heating process via an adjustable power supply allows for flexible setting of the pre-tightening force according to project requirements, endowing the joint with the ability of "post-construction intelligent reinforcement." The Fe-SMA reinforcement and positioning device work synergistically during heating, simultaneously achieving structural fastening and prestress application. This results in high material utilization efficiency, and the generated recovery stress is permanently locked after cooling, forming a long-term stable prestressed system. The construction process clearly separates dry and wet operations, ensuring smooth process transitions and simplified on-site operation. This not only improves work efficiency but also facilitates phased quality control, guaranteeing the integrity and final strength of the grout. The entire system boasts a high degree of factory prefabrication, strong adaptability, and reliable construction safety. While improving structural performance, it reduces reliance on complex, large-scale tensioning equipment, aligning with the development trends of industrialized, intelligent, and green construction in prefabricated buildings. It enables active and adjustable pre-tightening reinforcement of vertical joints after wall panel assembly and grouting have fully cured, significantly enhancing the overall stiffness, horizontal bearing capacity, and long-term performance of the joints. Attached Figure Description

[0016] Figure 1 This is a three-dimensional schematic diagram of the wall panel splicing of the prefabricated diaphragm wall Fe-SMA 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 horizontal Fe-SMA reinforcement 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. Fe-SMA rib, 3. Positioning device, 4. Iron plate, 5. Precast concave wall, 51. Intermediate connecting groove, 6. Precast convex wall, 61. Intermediate connecting boss, 7. Strip groove. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings. This description will focus on the unique features of this invention; common prefabricated construction processes such as precast wall panel fabrication and on-site hoisting and positioning will not be elaborated upon, as those skilled in the art can implement them using conventional techniques.

[0018] See Figure 1-6 The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure 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 horizontal pre-tightening force generating unit, a positioning unit, and a power supply unit. The vertical support unit includes two pairs of vertically parallel threaded steel bars 1, which are placed in the strip grooves 7 of the prefabricated concave wall 5 and the prefabricated convex wall 6. The horizontal pre-tightening force generating unit includes horizontal Fe-SMA ribs 2, the two ends of which are welded to two threaded steel bars 1 located in the same strip groove 7, starting from the bottom of the threaded steel bars 1. The Fe-SMA reinforcing bars 2 and the positioning devices 3 are arranged equidistantly upwards at a spacing of 1500mm, forming a grid frame with the two threaded steel bars 1. The positioning unit includes a positioning device 3, which is welded to the outer surface of the threaded steel bars 1 and arranged equidistantly upwards from the bottom of the threaded steel bars 1 at a spacing of 1200mm. The power supply unit includes an iron plate 4 fixed to the upper end of the threaded steel bars 1 and a power supply device for supplying power to the Fe-SMA reinforcing bars 2 and the positioning devices 3. After the grouting material is poured and cured, the power is turned on, and the Fe-SMA reinforcing bars 2 and the positioning devices 3 heat up simultaneously. Due to the physical properties of Fe-SMA thermal expansion and contraction, a continuous normal compressive stress is formed between the precast wall panels on both sides, thereby achieving active pre-tightening of the joint.

[0019] 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. The device is prefabricated in the factory and includes a vertical support unit, a horizontal pre-tightening force generation unit, a positioning unit, and a power supply unit.

[0020] 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.

[0021] The horizontal preload generation unit is the core innovation of this invention, implemented by three horizontal Fe-SMA ribs 2. The Fe-SMA ribs 2 have a diameter ranging from 16-22 mm and a recommended length of 1000 mm. Fe-SMA is a smart material with iron as the matrix, incorporating manganese, silicon, and other components, 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 around 500 MPa. At room temperature, tensile force needs to be applied to the Fe-SMA ribs 2 to induce permanent plastic elongation, resulting in a strain of 4%-6%. On a dedicated welding table, the three horizontal Fe-SMA ribs 2 are welded between two threaded steel bars 1, equidistantly spaced from the bottom of the threaded steel bars upwards at a spacing of 1500 mm, forming a grid frame with the two threaded steel bars 1. All welds must be full and continuous. After welding, this frame serves as a key load-bearing skeleton for transmitting horizontal preload and resisting shear deformation.

[0022] The positioning unit consists of four positioning devices 3. Each positioning device 3 is a ring-shaped clamp made of Fe-SMA with a thickness of 6mm, a width of 20mm, and a length not exceeding 220mm. Its inner diameter matches the diameter of the threaded steel bar 1 being fixed. The two outer arms of the positioning device 3 are slightly shorter than the width of the precast recessed wall 5, and their direction points outwards from the recess, facilitating insertion of the positioning device 3 into the outer surface of the threaded steel bar 1. At room temperature, a tensile force is applied to the outer arms of the Fe-SMA clamp to induce permanent plastic elongation, resulting in a strain of 4%-6%. Each positioning device 3 is welded to the outer surface of two threaded steel bars 1, arranged equidistantly from the bottom of the threaded steel bars 1 at a spacing of 1200mm. The two outer arms of the positioning device 3, slightly shorter than the width of the precast recessed wall 5, primarily function to limit the horizontal displacement of the connecting device within the cavity during installation, ensuring its centered positioning and providing a uniform thickness space for the grouting layer.

[0023] The power supply unit provides controllable power to the Fe-SMA reinforcing bars 2 and the positioning device 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 line, with copper terminals crimped to its ends and securely connected to the terminal block using bolts. The other end of the power line 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 working conditions at the construction site.

[0024] 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.

[0025] 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.

[0026] The implementation methods of this invention will be further described below in conjunction with the construction steps: Step S1: The factory prefabricates the pre-tightened connection device. Based on the designed wall thickness, select HRB400 grade threaded steel bar 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. Select horizontal Fe-SMA reinforcing bars 2 with a diameter of 20mm. At room temperature, apply tension to the Fe-SMA reinforcing bars 2 to induce permanent plastic elongation, resulting in a strain of 4%-6%. Fix two threaded steel bars parallel to each other on a dedicated welding platform, with a spacing of 1000mm between the horizontal Fe-SMA reinforcing bars 2. Weld three horizontal Fe-SMA reinforcing bars 2 between the two threaded steel bars 1, equidistantly arranged from the bottom of the threaded steel bars 1 upwards, with a spacing of 1500mm. A grid frame is formed; a positioning device 3 is installed, which is a ring clamp made of Fe-SMA with a thickness of 6mm, a width of 20mm, and a length not exceeding 220mm. The length of the two outer arms of the positioning device 3 is slightly smaller than the width of the prefabricated concave wall 5, and the direction of the outer arms points out of the groove. At room temperature, a tensile force needs to be applied to the outer arms of the Fe-SMA clamp to produce permanent plastic elongation and tensile strain of 4%-6%; four positioning devices 3 are installed on each threaded steel bar 1, which are welded to the outer surface of two threaded steel bars 1 respectively, and are arranged equidistantly from the bottom of the threaded steel bar 1 upwards with a spacing of 1200mm to ensure that the threaded steel bar 1 is centered in the groove; a power supply unit is installed, and four galvanized iron plates 4 are fixed to the top of the threaded steel bar 1 with M12 high-strength bolts.

[0027] 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 special lifting tools, 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 3 to center it naturally. Step S5: Inject high-strength grout using aluminate cement-based high-temperature resistant grout with a design strength grade of C50 and micro-expansion characteristics. Before grouting, thoroughly clean the trench cavity and moisten the inner wall; inject the grout from the grouting hole at the bottom of one side of the trench cavity until uniform grout overflows from the vent hole at 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 the grout is dense and free of voids; after grouting, keep the grouted area moist for at least 3 days. Step S6: Activate the pre-tightening force by powering on, connecting the power cord, setting the DC power supply to constant current mode, setting the initial current to 100A, and then slowly increasing 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 middle of the Fe-SMA rib 2 and the positioning device 3, ensuring it remains stable at 100℃±10℃; maintain this constant temperature for 45 minutes to allow the heat to be fully and evenly conducted to the entire frame and the surrounding grouting body. During the heating process, due to the thermal expansion and contraction properties of Fe-SMA, the Fe-SMA rib 2 and the positioning device 3 undergo radial contraction, causing horizontal tensile stress to be generated in the entire grid frame. This, in turn, through the adhesive friction between the grouting material and the groove wall, forms a continuous normal compressive stress between the precast wall panels on both sides, achieving active pre-tightening of the joint. Step S7: Cooling to form permanent preload. After completing the constant temperature heating, disconnect the power and allow the entire connection system to cool naturally for no less than 24 hours. During the cooling process, Fe-SMA does not directly generate new preload, as its preload has been basically fully established during the heating activation stage. The cooling process firmly maintains the generated recovery stress and preload, forming a long-term effective preload. 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 test 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 of time; after acceptance, proceed with subsequent earthwork excavation and structural construction.

[0028] This invention integrates Fe-SMA material into the vertical joint connection system of prefabricated diaphragm walls through innovative structure and process, using reinforcement and clamps. This achieves adjustable and controllable active pre-tightening reinforcement of the joint after molding. The technology employs a "grouting and curing first, followed by electrothermal activation" sequence, precisely controlling the heating process of the Fe-SMA components through an adjustable low-voltage, high-current power supply. This allows for flexible setting and application of pre-tightening force according to actual engineering needs. During energization, the horizontal Fe-SMA reinforcement and the annular positioning device synergistically generate shape-restoring force through heating. This force is effectively transferred to the wall panels on both sides through the cured grout, significantly improving the overall stiffness of the joint, horizontal bearing capacity, and the structure's resistance to lateral displacement and shear. The entire system has multiple electrical protection functions, ensuring safe and reliable construction. Combined with modular operation methods of factory prefabrication and on-site assembly, it greatly improves construction efficiency, achieving a convenient and efficient process from installation to activation. This technology, through collaborative innovation in materials, structure, and processes, not only improves the stress performance of nodes but also provides key technical support for the development of prefabricated underground structures towards intelligence and high reliability, and has 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 Fe-SMA 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 horizontal pre-tightening force generating unit, a positioning unit, and a power supply unit; The vertical support unit includes two pairs of threaded steel bars (1) arranged in parallel vertically, which are used to be placed in the strip grooves (7) of the prefabricated concave wall (5) and the prefabricated convex wall (6); The horizontal preload generating unit includes a horizontal Fe-SMA rib (2), the two ends of which are welded to two threaded steel bars (1) located in the same strip groove (7), and are arranged equidistantly from the bottom of the threaded steel bars (1) with a spacing of 1500mm, forming a grid frame with the two threaded steel bars (1). The positioning unit includes a positioning device (3), which is welded to the outer surface of the threaded steel (1) and arranged at equal intervals from the bottom of the threaded steel (1) upwards, with a spacing of 1200mm. The power supply unit includes an iron plate (4) fixed to the upper end of the threaded steel (1) and a power supply device for supplying power to the Fe-SMA rib (2) and the positioning device (3); After the grouting material is poured and cured, the power is turned on, and the Fe-SMA reinforcement (2) and the positioning device (3) heat up at the same time. Due to the physical properties of Fe-SMA thermal expansion and contraction, a continuous normal compressive stress is formed between the precast wall panels on both sides, thereby realizing the active pre-tightening of the joint.

2. The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claim 1, characterized in that: The diameter of the threaded steel (1) is 20-32mm. HRB400 grade threaded steel (1) is used, which is suitable for walls with a thickness of 800-1000mm. The distance between the threaded steel (1) and the edge of the wall is not less than 100mm. The length of the threaded steel (1) should be 150mm 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 Fe-SMA pre-tightening connection structure according to claim 1, characterized in that: The diameter of the horizontal Fe-SMA reinforcement (2) is 16-22 mm; the length L1 of the horizontal Fe-SMA reinforcement is 50d2, where d2 is the diameter of the horizontal Fe-SMA reinforcement.

4. The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claim 3, characterized in that: The Fe-SMA is a smart material with iron as the matrix and manganese and silicon as the dopant. It achieves shape recovery through thermoelastic martensitic phase transformation. Its phase transformation temperature can be adjusted in the range of 100-200°C. Its tensile strength is 500MPa. At room temperature, a tensile force needs to be applied to the Fe-SMA rib (2) to make it produce permanent plastic elongation.

5. The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claim 1, characterized in that: The positioning device (3) is made of Fe-SMA with a thickness of 6mm, a width of 20mm, and a length of no more than 220mm to form a ring clamp. The initial inner diameter matches the diameter of the threaded steel (1). The length L2 of the outer arm of the positioning device (3) is less than the width of the strip groove (7) of the prefabricated concave wall (5). The outer arm points to the center of the strip groove (7) to facilitate the insertion of the positioning device (3) into the strip groove (7) of the prefabricated concave wall (5) and the prefabricated convex wall (6). At room temperature, a tensile force needs to be applied to the outer arm of the Fe-SMA clamp to produce permanent plastic elongation.

6. The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claim 1, characterized in that: The iron plate (4) is a galvanized iron plate (4) with dimensions of 100mm×100mm×6mm. Two plates are arranged on the top of each threaded steel bar (1). The two iron plates (4) are fixed to the top of the threaded steel bar (1) by bolts.

7. The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claim 1, characterized in that: The power cord is a high-temperature resistant flexible cable with a nominal cross-sectional area of ​​50mm² to 95mm². The core is covered with a high-temperature resistant insulation layer and a flame-retardant sheath. The end of the power cord is connected to the iron plate (4) through copper terminals and bolts, and is provided with insulation protection.

8. The prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claim 1, characterized in that: The power supply unit 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 unit 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 Fe-SMA ribs (2) and positioning device (3).

9. A construction method based on the prefabricated diaphragm wall Fe-SMA pre-tightening connection structure according to claims 1-8, characterized in that: Includes the following steps: Step S1: Prefabricated pre-tightening connection device: Select threaded steel (1) according to the design wall thickness. Its length is usually longer than the height of the prefabricated wall panel. The distance between the two threaded steel (1) is 40d1, where d1 is the diameter of the threaded steel (1). Select horizontal Fe-SMA ribs (2). At room temperature, the Fe-SMA ribs (2) need to be tensile to produce permanent plastic elongation. Fix the two threaded steel (1) in parallel with a distance of 50d2, which is the length of the horizontal Fe-SMA ribs (2). Weld the three horizontal Fe-SMA ribs (2) between the two threaded steel (1) respectively. Arrange them at equal intervals from the bottom of the threaded steel (1) upwards to form a grid frame with the vertical support unit. Then arrange the positioning device (3). Finally, install the power supply unit. Fix the four iron plates (4) to the top of the two threaded steel (1) respectively with bolts. The iron plates (4) have reserved wiring holes. Step S2: Fabrication of prefabricated recessed wall (5): Three vertical butt grooves are reserved at the vertical joint of the wall panel. The two strip grooves (7) near the edge have rounded rectangular cross-sections and their depth extends through the height of the wall panel; the middle butt groove (51) has an isosceles trapezoidal cross-section and its depth extends through the height of the wall panel. Step S3: Fabrication of prefabricated protruding wall (6): Two vertical strip grooves (7) and one intermediate connecting protrusion (61) are reserved at the vertical joint of the wall panel. The two strip grooves (7) near the edge have rounded rectangular cross sections and their depth extends through the height of the wall panel. The intermediate connecting protrusion (61) has an isosceles trapezoidal cross section and its depth extends through the height of the wall panel. Step S4: On-site installation and positioning: Hoist the precast concave wall (5) and precast convex wall (6) to the design position in sequence. The middle mating groove (51) of the precast concave wall (5) matches the middle mating boss (61) of the precast convex wall (6). The strip groove (7) of the precast convex wall (6) is aligned with the strip groove (7) of the precast concave wall (5) to form a continuous cavity. Insert the pre-tightening connection device smoothly into the strip groove (7). Step S5: Grouting and Curing: High-strength grout with high temperature resistance and micro-expansion is continuously injected into the strip groove (7) until uniform grout overflows from the top of the groove; Step S6: Pre-tightening activation: Connect the power supply to the pre-reserved wiring hole on the iron plate (4), and slowly increase the output current so that the pre-embedded Fe-SMA ribs (2) and positioning device (3) are heated evenly to 100℃-120℃, and the constant temperature is maintained for 30 to 60 minutes; due to the physical properties of Fe-SMA thermal expansion and contraction, Fe-SMA ribs (2) and positioning device (3) generate radial contraction, which causes the entire grid frame to generate horizontal tensile stress, and then through the bonding friction between the grout and the groove wall, a continuous normal compressive stress is formed between the precast wall panels on both sides, so as to realize the active pre-tightening of the joint; Step S7: Cooling to form permanent prestress: After heating is complete, cut off the power and allow the entire connection structure to cool slowly to ambient temperature under natural conditions to form long-term effective prestress.

10. The construction method according to claim 9, characterized in that: The high-strength grouting material with high temperature resistance and micro-expansion mentioned in step S5 is aluminate cement-based high-temperature resistant mortar with a 28-day compressive strength of not less than 50 MPa, a long-term temperature resistance of not less than 150℃, and micro-expansion characteristics.

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