Construction method of anti-cracking diaphragm in midspan of cable-stayed bridge based on multi-factor cause analysis

Abstract

The present application relates to the technical field of bridge construction engineering, in particular to a construction method of a cable-stayed bridge midspan anti-cracking diaphragm based on multi-factor cause analysis, which is as follows: first, a concrete damage plasticity model is used to establish a refined finite element model of the cable-stayed bridge midspan diaphragm and beam segment, the cracking law under the coupling action of multiple factors such as uneven settlement, temperature effect and prestress loss is quantified, and the main control cracking factor and safety control threshold are accurately identified; second, the combined diaphragm structure of a steel shell fully wrapping reinforced concrete core is optimized and designed, and shear studs and PBL shear keys are used for collaborative connection; finally, the safety control threshold is applied throughout the whole construction process of the hanging bracket erection, steel shell installation, concrete pouring, temperature maintenance, prestress tensioning and the like, forming a closed-loop management and control system. The present application can prevent and control cracking risks from the root cause, and improve the anti-cracking performance and durability of the diaphragm.

Description

Technical Field

[0001] This invention relates to the field of bridge construction engineering technology, and in particular to a method for constructing a mid-span anti-crack diaphragm for cable-stayed bridges based on multi-factor causal analysis. Background Technology

[0002] Prestressed concrete cable-stayed bridges are widely used in bridge engineering due to their advantages such as large span capacity, good structural stiffness, and aesthetically pleasing design. For box girder sections, the transverse diaphragms in the mid-span closure section are core load-bearing components. Their function is to enhance the longitudinal bending stiffness and transverse torsional stiffness of the main girder, prevent cross-sectional distortion, and ensure the overall structural performance. However, in engineering practice, cracking is a common problem in the transverse diaphragms of the mid-span closure section of prestressed concrete cable-stayed bridges. Cracks often extend outwards from the corner of the passageway, sometimes forming X-shaped through cracks. This not only weakens the load-bearing capacity of the diaphragms but also leads to steel corrosion and reduced concrete durability, seriously affecting the bridge's operational safety and service life.

[0003] Existing research and technologies have identified uneven settlement, temperature effects, insufficient early concrete strength, lack of transverse prestress, and insufficient diaphragm thickness as the main causes of diaphragm cracks. However, existing technologies have the following shortcomings: First, they do not systematically combine the analysis of multiple coupled causes with crack control throughout the entire construction process. Most adopt a "construction first, treatment later" approach, resulting in insufficient targeted crack control measures and an inability to prevent cracking risks at their root. Second, existing crack-resistant structures often employ thickened diaphragms or additional transverse prestress. Thickening the diaphragm increases the structure's self-weight and temperature shrinkage stress, making it difficult to fully control complex cracks using conventional prestressing construction, and prestress loss is difficult to predict. Third, existing steel-shell concrete diaphragm technology does not conduct preliminary quantitative analysis of the multi-factor cracking mechanism, making it impossible to optimize design and construction parameters based on the main cracking factors for different projects. Furthermore, it does not cover key aspects such as uneven settlement, temperature control, and concrete strength control throughout the entire construction process, thus failing to achieve closed-loop management throughout the entire process.

[0004] Therefore, this invention proposes a construction method for anti-crack transverse diaphragms at mid-span of cable-stayed bridges based on multi-factor causal analysis to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a construction method for mid-span crack-resistant diaphragms in cable-stayed bridges based on multi-factor causal analysis. This invention can prevent cracking risks at the source and improve the crack resistance and durability of diaphragms.

[0006] The technical solution of this invention to solve the technical problem is a construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis, comprising the following steps: S1. A refined finite element model of the mid-span transverse diaphragm and adjacent beam segments of the cable-stayed bridge was established using a concrete damage plasticity model. Multi-factor coupled loads were implemented in stages to quantify the influence of each factor on the cracking of the transverse diaphragm, determine the main controlling factors and the corresponding safety control thresholds, and obtain the causal analysis results. S2. Based on the causal analysis results, optimize the structural parameters of the diaphragm, determine the design requirements of the diaphragm, and transform the safety control thresholds of each factor into construction control indicators for each process in the entire construction process. S3. Based on the causal analysis results and the design requirements of the transverse diaphragm, the anti-crack transverse diaphragm at the mid-span of the cable-stayed bridge is constructed in sequence, including the erection and construction preparation of the hanger system, the installation and closure locking of the steel shell, the construction of the steel reinforcement and prestressing system, the concrete pouring, temperature control and curing, and the prestressing tensioning and structural demolition, so as to realize the construction of the transverse diaphragm. S4. Conduct visual and physical quality inspections on the diaphragms, and complete the final acceptance inspection based on the monitoring data throughout the entire process.

[0007] S1 is as follows: The multi-factor coupled construction load includes any three or more combinations of uneven settlement of the hanger, temperature gradient difference between the inside and outside of the concrete, overall temperature difference between new and old concrete, structural self-weight, transverse prestress, thickness of transverse diaphragm, and strength of concrete at different ages. The ABAQUS platform was used for modeling, C3D8R eight-node hexahedral elements were used to simulate concrete, and T3D2 truss elements were used to simulate steel reinforcement. The mesh was refined in the area of ​​the diaphragm passageway. The tensile and compressive damage evolution parameters of concrete were defined. When the tensile damage value was ≥90%, it was determined that there was a risk of cracking, and the safety control threshold was determined accordingly.

[0008] S2. The specific structure of the diaphragm is as follows: The diaphragm consists of a steel shell, a reinforced concrete core, steel-concrete shear connectors, and prestressed steel strands. The steel shell completely encloses the reinforced concrete core. The steel shell extends to both sides of the closure opening at the top plate, bottom plate, and web of the prestressed concrete main girder box girder where the closure section is located in the mid-span of the cable-stayed bridge and overlaps with the already poured beam segment. Steel-concrete shear connectors are welded to the inside of the steel shell, and the prestressed steel strands are laterally embedded inside the reinforced concrete core. The steel-concrete shear connector includes shear studs and PBL shear keys. The PBL shear key has an opening diameter of 50~60mm and a perforated steel bar diameter of 16~20mm. The interior of the reinforced concrete core is a steel skeleton made of multiple steel bars tied together. The concrete of the reinforced concrete core is self-compacting high-performance concrete.

[0009] S3 is as follows: Hanger system erection and construction preparation: Based on the cause analysis results and the design requirements of the diaphragm, carry out the preparation and inspection of construction materials, design, erection and pre-stressing of the hanger system, and deployment of the monitoring system, and output the accepted hanger system, the construction materials that have passed the on-site inspection, and the construction monitoring system that has been debugged. Steel shell installation and closure locking: Based on the design requirements of the diaphragm and the accepted hanger system, the steel shell is processed in the factory, hoisted and welded on site, and locked in place. The output is a steel shell with completed positioning welding and acceptance, and a closure structure with completed closure locking. Reinforcing steel and prestressed system installation: Based on the design requirements of the diaphragm and the accepted steel shell, carry out reinforcing steel binding, installation of the transverse prestressed system and acceptance of concealed works, and output the accepted reinforcing steel and prestressed system; Concrete pouring construction: Based on the cause analysis results, the design requirements of the diaphragm, and the accepted steel reinforcement and prestressing system, concrete mixing and symmetrical continuous pouring are carried out to output the closure section and diaphragm concrete structure that have been poured and are about to set. Temperature control and curing: Based on the causal analysis results and the poured concrete structure, real-time temperature monitoring, hydration heat and temperature difference control, moisture curing and strength monitoring of the concrete are carried out, and the output is a concrete structure that has reached the preset strength and is properly cured. Prestressing tensioning and structural demolition: Based on the causal analysis results, the design requirements of the diaphragm, and the properly cured concrete structure, transverse prestressing symmetrical tensioning, duct grouting, and graded demolding of formwork and hangers are carried out to output a crack-resistant diaphragm structure formed after tensioning and demolding.

[0010] The specific procedures for erecting and preparing the scaffolding system are as follows: According to the design requirements of the diaphragm, steel shell, reinforcing bars, self-compacting concrete raw materials, shear studs, PBL shear keys and prestressed steel strands are prepared, and all materials are tested according to specifications after they arrive on site. Based on the causal analysis, the steel truss hanger system for the closure section was designed. Eight precision-rolled threaded steel hangers were symmetrically arranged along the centerline of the main beam section. The bearing capacity of a single hanger was not less than 500kN. Then, a graded pre-stressing test of 1.2 times the total weight of the closure section was carried out to monitor the hanger deformation. After pre-stressing, the uneven settlement of the hanger was not greater than 1mm. The hangers were reinforced with double nuts and anti-loosening washers. Settlement measuring points, temperature measuring points, stress measuring points and displacement measuring points are set up at the locations of the hangers, steel shells and diaphragms. The monitoring system is debugged to ensure that the monitoring frequency and accuracy meet the construction control requirements. Output the accepted and qualified hanging system, the inspected and qualified construction materials, and the debugged construction monitoring system.

[0011] The specific procedures for steel shell installation and closure locking are as follows: The steel shell of the diaphragm is composed of multiple segmented steel shells. The segmented steel shells are processed in the factory according to the construction drawings and subjected to anti-corrosion treatment. After passing the inspection, the segmented steel shells are transported to the construction site and hoisted to the design position of the closure section. They are temporarily fixed by the hanging system, and the plane position, elevation and verticality of the steel shells are adjusted. Then the segmented steel shells are welded on site to form a fully enclosed steel shell. After the welding is completed, the weld flaw detection inspection is carried out. Within the closure temperature window required by the design, temporary I-beams are welded inside the steel shell. The two ends of the I-beams are welded to the inner wall of the steel shell to form a closure locking device, which constrains the relative displacement of the closure opening. Output the steel shell after positioning welding is completed and accepted, and the closure joint structure after closure and locking is completed.

[0012] The specific installation procedures for the reinforcing steel and prestressed system are as follows: Reinforcing bars are tied inside the steel shell. The specifications, spacing, and protective layer thickness of the reinforcing bars are set according to the design requirements of the diaphragm. The reinforcing bars are tied to the perforated reinforcing bars of the PBL shear key to form an integral reinforcing steel skeleton. According to the design requirements of the diaphragm, the prestressed system is installed horizontally. The prestressed steel strands are equipped with corrugated pipes. First, the corrugated pipes are installed, then the prestressed steel strands are inserted into the corrugated pipes, and anchors and matching pads are installed at both ends of the prestressed steel strands to form prestressed pipes. The position and sealing of the prestressed pipes are checked. After inspection, the steel reinforcement cage and prestressed system that have passed the acceptance test are output.

[0013] The specific concrete pouring operation is as follows: Mix self-compacting high-performance concrete according to the design requirements of the diaphragm, and control the slump, spread and temperature before placement. Before pouring, the steel shell, reinforcing bars, and prestressed ducts are moistened, debris at the closure joint is cleaned, the anchorage of the hangers is checked, the relative displacement at the closure joint is monitored, and after confirming that it meets the design requirements, within the design closure temperature window, a symmetrical and continuous pouring method is adopted. First, the bottom plate and web of the box girder of the closure section are poured, and then the diaphragm concrete is poured. During the pouring process, an immersion vibrator was used to assist in compaction. Throughout the pouring process, the settlement of the hanger, the deformation of the steel shell, and the displacement of the closure joint were monitored. If the deformation exceeded the control threshold, the pouring was stopped immediately and adjustments were made. After the concrete is poured, the concrete surface should be smoothed and finished in a timely manner. Output the closure section and transverse diaphragm concrete structure that has been poured and is about to set.

[0014] The specific temperature control and maintenance procedures are as follows: Temperature measuring points are set up inside and on the surface of the concrete in the diaphragm to monitor the internal temperature, surface temperature and ambient temperature of the concrete in real time. After the concrete has initially set, immediately cover the concrete surface with thermal insulation and moisture retention geotextile, and attach thermal insulation cotton to the outside of the steel shell. Control the temperature gradient difference between the inside and outside of the concrete to be no greater than ±20℃, and the overall temperature difference between the newly poured closure section concrete and the adjacent poured beam section to be no greater than ±25℃. Spray curing is used to keep the concrete surface continuously moist; During concrete pouring, reserve cubic and prism test blocks that are cured under the same conditions, and test the compressive strength of the test blocks daily. Output a self-compacting high-performance concrete structure that has reached the preset strength and has been properly cured.

[0015] The specific procedures for prestressed tensioning and structural demolition are as follows: When the compressive strength of the test blocks cured under the same conditions reaches 100% of the design strength and the elastic modulus reaches more than 90% of the design value, transverse prestressing is carried out. The transverse prestressed steel strands were tensioned using a symmetrical, staged tensioning method, employing a dual-control method with tension force control as the primary method and elongation verification as a secondary method. After tensioning, vacuum-assisted grouting was used for duct grouting, followed by anchor head sealing. Once the grout strength of the duct reaches more than 80% of the design strength, first remove the temporary I-beam closure locking device, and then dismantle the hanger system and outer formwork in stages and symmetrically. Output the crack-resistant transverse diaphragm structure formed after tensioning and demolding.

[0016] The effects described in the invention are merely those of the embodiments, and not all the effects of the invention. The above technical solutions have the following advantages or beneficial effects: This invention discloses a construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis. This invention establishes a closed-loop control system of analysis, design, and construction. Through multi-factor refined finite element analysis based on a concrete damage-plasticity model, it quantifies the cracking patterns under the coupled effects of multiple factors such as uneven settlement, gradient temperature difference, overall temperature difference, concrete strength, transverse diaphragm thickness, and transverse prestress. It accurately identifies the main controlling crack-causing factors and safety control thresholds, avoiding the blindness and lack of specificity of traditional crack-resistant measures, and identifying the root cause. To mitigate the risk of cracking, this invention optimizes the design of the diaphragm structure based on multi-factor causal analysis. Through full external steel plate constraint and the coordinated connection of shear studs and PBL shear keys, the tensile and crack resistance of the diaphragm are improved, structurally inhibiting crack initiation and propagation. Simultaneously, the steel shell can also serve as an internal formwork, eliminating the need for additional internal formwork, simplifying construction procedures, and reducing construction difficulty. Combined with an optimized transverse prestressing system, sufficient compressive stress reserves are provided for the diaphragm, effectively offsetting the tensile stress generated during construction, achieving dual crack resistance in both structure and process.

[0017] Furthermore, this invention incorporates the safety control threshold of multi-factor causal analysis throughout the entire construction process, setting clear control indicators for each procedure to comprehensively cover the prevention and control of all major crack-causing factors, effectively reducing the risk of cracking during construction. In addition, this invention strictly controls the timing of formwork removal and prestressing tensioning by real-time monitoring of the strength of test blocks cured under the same conditions, and strictly controls the temperature gradient difference between the inside and outside of the concrete and the overall temperature difference between the new and old concrete through full-cycle temperature monitoring and control, which can avoid the generation of early temperature cracks and improve the construction quality and structural durability of the diaphragm. Attached Figure Description

[0018] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0019] Figure 1 This is a schematic diagram of the method flow of the present invention.

[0020] Figure 2 This is a schematic diagram of a finite element model.

[0021] Figure 3 This is a schematic diagram of the steel reinforcement skeleton of the diaphragm.

[0022] Figure 4 The graph shows the variation of the suspender force and crack length with the uneven settlement of the suspender.

[0023] Figure 5 This is a graph showing the relationship between the maximum principal tensile stress of the diaphragm and the thickness of the diaphragm.

[0024] Figure 6 This is a graph showing the variation of concrete strength over age. Detailed Implementation

[0025] To clearly illustrate the technical features of this solution, the invention will be described in detail below through specific implementation methods and in conjunction with the accompanying drawings.

[0026] Example 1 Taking a prestressed concrete multi-tower, low-tower cable-stayed bridge as an example, this bridge adopts a semi-floating system with tower-pier consolidation and tower-beam separation. The main beam is a single-box, double-cell prestressed concrete box girder with a mid-span main beam section height of 7m, a bridge deck width of 13.8m, and a mid-span closure section length of 2m. The original design included 80cm thick ordinary reinforced concrete diaphragms, which were prone to defects such as chamfer cracks in the passageway and X-shaped through cracks after construction. This invention employs a mid-span crack-resistant diaphragm construction method based on multi-factor causal analysis for cable-stayed bridges. The method steps are as follows: Figure 1 As shown; S1. A refined finite element model of the mid-span transverse diaphragm and adjacent beam segments of the cable-stayed bridge is established using a concrete damage plasticity model, such as... Figure 2 As shown, multi-factor coupled loads were applied in stages to quantify the influence of each factor on the cracking of the diaphragm, determine the main cracking factors and the corresponding safety control thresholds of each factor, and obtain the causal analysis results. S2. Based on the causal analysis results, optimize the structural parameters of the diaphragm, determine the design requirements of the diaphragm, and transform the safety control thresholds of each factor into construction control indicators for each process in the entire construction process. S3. Based on the causal analysis results and the design requirements of the transverse diaphragm, the anti-crack transverse diaphragm at the mid-span of the cable-stayed bridge is constructed in sequence, including the erection and construction preparation of the hanger system, the installation and closure locking of the steel shell, the construction of the steel reinforcement and prestressing system, the concrete pouring, temperature control and curing, and the prestressing tensioning and structural demolition, so as to realize the construction of the transverse diaphragm. S4. Conduct visual and physical quality inspections on the diaphragms, and complete the final acceptance inspection based on the monitoring data throughout the entire process.

[0027] In a specific implementation, S1 is as follows: A solid element model including the mid-span diaphragm and adjacent 40m beam segments was built using the ABAQUS platform. C3D8R eight-node hexahedral elements were used to simulate C50 concrete, and T3D2 truss elements were used to simulate HRB400 steel reinforcement. Embedded constraints were used to simulate the bond coupling between the steel reinforcement and concrete. Stress concentration areas such as passageways and chamfers in the diaphragm were meshed with a 10mm mesh, while other areas were meshed with a 200mm mesh, resulting in a total of 126,000 elements.

[0028] The concrete constitutive relation was defined using a concrete damage plasticity model with the following input parameters: elastic modulus 3.45 × 10^4 MPa, Poisson's ratio 0.2, expansion angle 30°, flow potential offset 0.1, ratio of biaxial ultimate compressive strength to uniaxial ultimate compressive strength 1.16, ratio of tensile to compressive second stress invariant 0.667, and viscosity parameter 0.0005. Simultaneously, tensile and compressive damage evolution parameters were defined for the concrete, and C50 self-compacting high-performance concrete was selected.

[0029] Table 1. Compressive Damage Parameters of C50 Concrete Table 2. Tensile Damage Parameters of C50 Concrete Construction stage loads were applied in stages, including uneven settlement of the scaffolding (0-10mm), internal and external temperature gradient of concrete (5-40℃), overall temperature difference between new and old concrete (5-40℃), structural self-weight, transverse prestress of 0.7-0.9 times the design value, diaphragm thickness of 80-100cm, and concrete strength parameters at 1-8 days. Nonlinear calculations quantified the contribution of each factor to the principal tensile stress of the diaphragm: internal and external temperature gradient of concrete contributed 38%, uneven settlement of the scaffolding contributed 25%, transverse prestress loss contributed 18%, insufficient early strength of concrete contributed 12%, and other factors contributed 7%. The main controlling factors for cracking were determined to be internal and external temperature gradient of concrete and uneven settlement of the scaffolding. Specific safety control thresholds were defined: uneven settlement of the scaffolding ≤1mm, internal and external temperature gradient of concrete ≤20℃, transverse prestress loss ≤10%, and concrete tensile strength ≥100% of design strength. The optimal crack-resistant design parameters are as follows: the thickness of the transverse diaphragm is 90cm, the transverse prestressed steel strands are 4 strands of 15-7φ^s15.2 steel strands, and the tension control stress is 1395MPa.

[0030] In the specific implementation of prevention and control measures, S2 is as follows: Based on the causal analysis results, the transverse diaphragm structure was optimized: the steel shell is made of 12mm thick Q355B steel plate, fully encased in a reinforced concrete core, extending 60cm on each side of the closure joint at the top, bottom, and web of the box girder to overlap with the already cast beam segments; the steel-concrete shear connectors use shear studs and PBL shear keys in a coordinated arrangement, with shear studs having a diameter of 22mm and a quincunx arrangement of 200mm×200mm spacing; the PBL shear key opening diameter is 55mm, and the perforated reinforcement uses HRB400 grade φ18 steel bars, arranged at 300mm spacing along the height of the steel shell; the reinforced concrete core uses C50 self-compacting high-performance concrete with a spread of 700±50mm and a slump of 250±10mm; the transverse prestressed steel strands use 4 bundles of 15-7φ^s15.2 low-relaxation steel strands with a standard strength of 1860MPa and a tension control stress of 1395MPa.

[0031] Simultaneously, the construction control indicators for each process were determined: the preload of the hanger is 1.2 times the total weight of the closure section, and the uneven settlement after preloading is ≤1mm; the deviation of the installation plane of the steel shell is ≤±5mm, and the elevation deviation is ≤±5mm; the concrete temperature upon placement is 15~25℃, and the slump upon placement is 240~260mm; the temperature difference between the inside and outside of the concrete is ≤20℃, and the overall temperature difference between the new and old concrete is ≤25℃; the compressive strength of the concrete reaches 100% of the design strength during prestressing tension, and the elastic modulus is ≥90% of the design value.

[0032] In a specific implementation, S3 is as follows: (1) Preparation of steel shell, HRB400 steel bars, C50 self-compacting concrete raw materials, shear nails, PBL shear keys and 15.2mm prestressed steel strands were prepared for the erection of the hanging system. All materials were tested in accordance with GB50204-2015 "Code for Acceptance of Construction Quality of Concrete Structures" after they arrived on site, and all of them were qualified.

[0033] The closure section's steel truss hanger system was designed, using I40b I-beams to fabricate the truss. Eight φ32 precision-rolled threaded steel hangers were symmetrically arranged along the centerline of the main beam section, with a single hanger bearing capacity of 600kN. The hangers were reinforced with double nuts and anti-loosening washers for anchorage. A graded preloading test was conducted with 1.2 times the total weight of the closure section (120t). The graded loading proportions were 20% (24t), 40% (48t), 60% (72t), 80% (96t), 100% (120t), and 120% (144t). After each loading stage, the hanger deformation was monitored for 30 minutes. After preloading, the maximum settlement of the hanger was 8mm, and the uneven settlement was 0.6mm, meeting the control requirements.

[0034] Four settlement monitoring points were installed on the hanger, six displacement monitoring points were installed on the steel shell, twelve temperature monitoring points were installed inside and on the surface of the diaphragm concrete and in the environment, and four stress monitoring points were installed at the corner of the passageway. The monitoring system was debugged and tested at a frequency of once every 30 minutes, and the accuracy met the requirements.

[0035] (2) Steel Shell Installation and Closure Locking: The steel shell is processed in the factory in four pieces: top plate, bottom plate, left web plate, and right web plate. It undergoes sandblasting (Sa2.5 grade) for rust removal and epoxy zinc-rich anti-corrosion treatment. After processing, it is pre-assembled, with dimensional deviations ≤ ±3mm. After transportation to the construction site, a 25t truck crane is used to lift it to the designed position of the closure section, and it is temporarily fixed using a scaffolding system. A total station is used to adjust the plane position, elevation, and verticality of the steel shell, with deviations controlled within ±4mm. Carbon dioxide gas shielded welding is used for on-site welding. After welding, 100% ultrasonic testing is performed, and the weld quality meets the Class II standard.

[0036] Within the closure temperature window (16~18℃), two temporary I32b I-beams are welded inside the steel shell. The two ends of the I-beams are connected to the inner wall of the steel shell by bevel welding to form a closure locking device, which constrains the relative displacement of the closure opening.

[0037] (3) The reinforcing bars and prestressed system are installed inside the steel shell and HRB400 reinforcing bars are tied. The specifications, spacing and protective layer thickness deviations of the reinforcing bars are all ≤±8mm. The reinforcing bars are firmly tied to the perforated reinforcing bars of the PBL shear key to form an integral reinforcing bar skeleton, such as Figure 3 As shown.

[0038] The prestressed system is installed laterally using plastic corrugated pipes, with the joints sealed with three layers of sealing tape. Four bundles of 15-7φ^s15.2 prestressed steel strands are threaded through the pipes, and OVM15-7 anchors and matching pads are installed at both ends. The prestressed pipe position deviation is inspected to be ≤±8mm, the sealing performance is good, and the concealed works pass the acceptance inspection.

[0039] (4) The C50 self-compacting high-performance concrete was mixed according to the optimized mix proportion and the mixing time was 150s. The concrete slump was 250mm, the spread was 720mm, and the concrete temperature was 18℃, which met the requirements.

[0040] Before pouring, the steel shell, reinforcing bars, and prestressed ducts were thoroughly moistened, debris at the closure joint was cleaned, the anchorage of the hangers was checked, and the relative displacement at the closure joint was monitored to be ≤0.5mm. Within the closure temperature window of 16~18℃, a symmetrical and continuous pouring method was adopted, first pouring the bottom slab and web of the box girder of the closure section, and then pouring the transverse diaphragm concrete. During the pouring process, a φ50 immersion vibrator was used for auxiliary vibration, with a vibration time of 20~30s. Throughout the pouring process, the settlement of the hangers, the deformation of the steel shell, and the displacement at the closure joint were monitored, and the maximum deformation was within the control threshold.

[0041] After the concrete is poured, a second finishing and smoothing process should be carried out promptly.

[0042] (5) Temperature control and curing: Real-time monitoring of the internal temperature, surface temperature, and ambient temperature of the concrete, with a monitoring frequency of once per hour. After the initial setting of the concrete, two layers of thermal insulation and moisture-retaining geotextile were immediately applied to the surface, and 5cm thick thermal insulation cotton was pasted on the outside of the steel shell. During the curing period, the highest internal temperature of the concrete was 62℃, the surface temperature was 45℃, and the maximum temperature difference between the inside and outside was 17℃; the maximum overall temperature difference between the newly poured concrete and the adjacent poured beam segment was 22℃, all of which met the control requirements.

[0043] An automatic spray curing method was used to keep the concrete surface continuously moist for 14 days. During concrete pouring, three sets of 150mm×150mm×150mm cube test blocks and two sets of 150mm×150mm×300mm prism test blocks were reserved and cured under the same conditions. On day 7, the compressive strength of the test blocks was tested to be 52 MPa (104% of the design strength), and the elastic modulus was 3.52×10^4 MPa (92% of the design value), meeting the requirements for prestressing tension.

[0044] (6) For prestressing tensioning and structural demolition, transverse prestressed steel strands were tensioned using a symmetrical, graded tensioning method at both ends. The graded loading ratios were 10%, 20%, 50%, and 100%, and the load was held for 5 minutes before anchoring. A dual-control method was adopted, with tension force control as the primary method and elongation verification as a secondary method. The tension force was controlled at 1395 MPa, and the elongation deviation was between -3% and +4%, which met the requirements.

[0045] Within 12 hours after tensioning, vacuum-assisted grouting is used to grout the ducts. M50 cement grout is used, and the grouting pressure is 0.5~0.7MPa. After grouting, the anchor head is sealed promptly with C50 concrete.

[0046] After the grout strength of the duct reaches 42MPa (84% of the design strength), the temporary I-beam closure locking device is first removed. Then, the hanging system and outer formwork are dismantled in stages and symmetrically according to the principle of first erecting and then dismantling, and the last erected and then dismantled. The structural deformation is monitored during the dismantling process, and no abnormalities are found.

[0047] In a specific implementation, S4 is as follows: The appearance quality of the diaphragm was inspected, and no defects such as cracks, honeycombing, or pitting were found on the surface. The internal density of the concrete was tested using ultrasonic testing, and no defects such as voids or looseness were found. The concrete strength was tested using the rebound method, and the average strength was 54 MPa, which meets the design requirements. The effective prestress was tested using the tensioning method, and the average effective stress was 1268 MPa, with a prestress loss of 9.1%, which meets the control requirements.

[0048] Based on the settlement, temperature, stress, and displacement monitoring data throughout the construction process, the construction quality of the diaphragm fully meets the design and specification requirements, and the project has passed the final acceptance inspection.

[0049] Example 2 To address the cracking phenomenon in the mid-span transverse diaphragm of a single-box double-cell prestressed concrete beam during construction, a local solid element finite element model was established based on ABAQUS software to analyze the cause of the cracking. (1) Uneven settlement The mid-span closure section is constructed using a hanging scaffold. The bottom formwork platform bears the weight of the concrete, and the force is then transferred to the hanging scaffold via the hangers. During actual construction, there may be instances of loosening of the anchorages of the hanging scaffold and hangers, leading to uneven bottom support of the concrete in the mid-span closure section and uneven settlement of the beam. To calculate the impact of this factor on the stress of the transverse diaphragm in the mid-span closure section, the uneven support at the bottom of the concrete is simulated by applying displacement to the four hangers on the right side of the web plate, gradually increasing the displacement from 0mm to 10mm. The tensile damage diagram of the transverse diaphragm concrete is observed. When the tensile damage exceeds 90%, the concrete in that section is considered to be cracked. The variation patterns of boom force and crack length with uneven boom settlement are as follows: Figure 4As shown, before the uneven settlement of the diaphragm is 3mm, the diaphragm force and the uneven settlement are basically linearly related, and the diaphragm concrete has not yet cracked. In the range of 3mm to 7mm uneven settlement, due to tensile damage (cracking) of some diaphragm concrete, it enters a plastic state. Therefore, the rate of change of the diaphragm force with uneven settlement gradually decreases, while the rate of change of the crack length with uneven settlement gradually increases. After the uneven settlement of the diaphragm is 7mm, the portion of the diaphragm concrete that has entered the plastic state increases, the rate of change of the diaphragm force with uneven settlement further slows down, the crack penetrates the diaphragm, and the crack length gradually stabilizes.

[0050] (2) Temperature This study analyzed the effects of gradient temperature difference and overall temperature difference on cracking damage of the mid-span transverse diaphragm of concrete. Under gradient temperature difference conditions, there was no damage at 5℃, and the damage gradually increased within 15℃; at 20℃, the damage at the chamfer of the near-mid web of the passageway exceeded 90%, and cracks began to appear; at 25~35℃, cracks extended towards the top and bottom slabs and the outer web; at 40℃, the cracks continued to extend along the chamfer. When the overall temperature difference was applied to the closure section, there was no damage at 5℃, and the damage increased within 20℃; at 25℃, the damage exceeded 90%, and microcracks appeared; at 30℃, visible fine cracks appeared at the chamfer of the inner web side; at 35℃, cracks appeared and extended at both the inner and outer web side chamfers; at 40℃, cracks extended outwards along the four chamfers, forming an overall "X" shape distribution. Both temperature differences showed that cracks continued to develop with increasing temperature, with significant cracking of the transverse diaphragm under high temperature difference conditions, and the chamfer being a critical weak point.

[0051] (3) Transverse prestressing Using the baseline working condition of 5mm uneven settlement of the hanger rod, a gradient temperature difference of 25℃, and an overall temperature difference of 30℃, a pressure of 6MPa in the direction of the middle web was applied to both sides of the outer web of the diaphragm to simulate the effect of transverse prestressing. After adding transverse prestressing, compared with the case without transverse prestressing steel bars, the maximum principal tensile stress value of the diaphragm was reduced, which was much less than its tensile ultimate strength. This indicates that transverse prestressing steel bars can effectively improve the stress distribution of the diaphragm and reduce the risk of cracking.

[0052] (4) Thickness of the diaphragm Assuming a 6mm uneven settlement of the hanger, and considering diaphragm thicknesses of 80cm, 85cm, 90cm, 95cm, and 100cm, the maximum principal tensile stress of the diaphragms with different thicknesses is calculated as follows: Figure 5 As shown, when the thickness of the diaphragm is increased from 80cm to 100cm, the maximum principal tensile stress decreases from 4.26MPa to 3.53MPa. The increase in the thickness of the diaphragm increases the cross-sectional area and moment of inertia of the diaphragm in the transverse direction, which has a certain effect on reducing its principal tensile stress.

[0053] (5) Self-weight Under the self-weight of the mid-span closure section, the maximum principal tensile stress of the main beam and the maximum principal tensile stress of the transverse diaphragm did not reach the ultimate tensile strength of the concrete, thus ruling out self-weight as a factor causing the cracking of the mid-span transverse diaphragm.

[0054] (6) Concrete age and strength During concrete pouring, concrete cube specimens (150mm×150mm×150mm) and prism specimens (150mm×150mm×300mm) were prepared using the same batch of concrete. For each test, eight groups of cube and prism specimens were prepared, with three specimens in each group. The age of each group of specimens ranged from 1 to 8 days. The compressive strength of the concrete cube and the axial compressive strength of the concrete were tested at these ages. The average value of the three specimens in each group was taken. The variation patterns of the C55 concrete cube compressive strength and axial compressive strength at different ages are shown below. Figure 6 As shown, the concrete cube compressive strength and axial compressive strength are both positively correlated with the concrete age. The concrete can only reach the strength grade required by the concrete specification after 7 to 8 days of age. Demolding the concrete too early may cause the concrete strength to fail to meet the design requirements, resulting in tensile cracking.

[0055] The above systematic analysis of the factors influencing diaphragm cracking clarifies the impact of key parameters such as temperature stress, construction deviations, prestressing layout, and early concrete performance on crack initiation and development. This provides important theoretical basis and data support for the diaphragm structure design and construction method of this invention. Based on the above factor analysis results, the diaphragm structure, reinforcement layout, and prestressing system are optimized in a targeted manner, and matching construction control measures are formulated. This can synergistically reduce stress concentration and inhibit early crack initiation and propagation from both structural design and construction technology perspectives, thereby significantly improving the crack resistance and long-term durability of the diaphragm.

[0056] Although the specific embodiments of the invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the invention. Based on the technical solutions of the invention, various modifications or variations that can be made by those skilled in the art without creative effort are still within the scope of protection of the invention.

Claims

1. A construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis, characterized in that, Includes the following steps: S1. A refined finite element model of the mid-span transverse diaphragm and adjacent beam segments of the cable-stayed bridge was established using a concrete damage plasticity model. Multi-factor coupled loads were implemented in stages to quantify the influence of each factor on the cracking of the transverse diaphragm, determine the main cracking factors and the corresponding safety control thresholds of each factor, and obtain the causal analysis results. S2. Based on the causal analysis results, optimize the structural parameters of the diaphragm, determine the design requirements of the diaphragm, and transform the safety control thresholds of each factor into construction control indicators for each process in the entire construction process. S3. Based on the causal analysis results and the design requirements of the transverse diaphragm, the anti-crack transverse diaphragm at the mid-span of the cable-stayed bridge is constructed in sequence, including the erection and construction preparation of the hanger system, the installation and closure locking of the steel shell, the construction of the steel reinforcement and prestressing system, the concrete pouring, temperature control and curing, and the prestressing tensioning and structural demolition, so as to realize the construction of the transverse diaphragm. S4. Conduct visual and physical quality inspections on the diaphragms, and complete the final acceptance inspection based on the monitoring data throughout the entire process.

2. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 1, characterized in that, The multi-factor coupled construction load includes any three or more combinations of uneven settlement of the hanger, temperature gradient difference between the inside and outside of the concrete, overall temperature difference between new and old concrete, structural self-weight, transverse prestress, thickness of transverse diaphragm, and strength of concrete at different ages. The ABAQUS platform was used for modeling, C3D8R eight-node hexahedral elements were used to simulate concrete, and T3D2 truss elements were used to simulate steel reinforcement. The mesh was refined in the area of ​​the diaphragm passageway. The tensile and compressive damage evolution parameters of concrete were defined. When the tensile damage value was ≥90%, it was determined that there was a risk of cracking, and the safety control threshold was determined accordingly.

3. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 1, characterized in that, The specific structure of the diaphragm is as follows: The diaphragm consists of a steel shell, a reinforced concrete core, steel-concrete shear connectors, and prestressed steel strands. The steel shell completely encloses the reinforced concrete core. The steel shell extends to both sides of the closure opening at the top plate, bottom plate, and web of the prestressed concrete main girder box girder where the closure section is located in the mid-span of the cable-stayed bridge and overlaps with the already poured beam segment. Steel-concrete shear connectors are welded to the inside of the steel shell, and the prestressed steel strands are laterally embedded inside the reinforced concrete core. The steel-concrete shear connector includes shear studs and PBL shear keys. The PBL shear key has an opening diameter of 50~60mm and a perforated steel bar diameter of 16~20mm. The interior of the reinforced concrete core is a steel skeleton made of multiple steel bars tied together. The concrete of the reinforced concrete core is self-compacting high-performance concrete.

4. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 3, characterized in that, S3 is as follows: Hanger system erection and construction preparation: Based on the cause analysis results and the design requirements of the diaphragm, carry out the preparation and inspection of construction materials, design, erection and pre-stressing of the hanger system, and deployment of the monitoring system, and output the accepted hanger system, the construction materials that have passed the on-site inspection, and the construction monitoring system that has been debugged. Steel shell installation and closure locking: Based on the design requirements of the diaphragm and the accepted hanger system, the steel shell is processed in the factory, hoisted and welded on site, and locked in place. The output is a steel shell with completed positioning welding and acceptance, and a closure structure with completed closure locking. Reinforcing steel and prestressed system installation: Based on the design requirements of the diaphragm and the accepted steel shell, carry out reinforcing steel binding, installation of the transverse prestressed system and acceptance of concealed works, and output the accepted reinforcing steel and prestressed system; Concrete pouring construction: Based on the cause analysis results, the design requirements of the diaphragm, and the accepted steel reinforcement and prestressing system, concrete mixing and symmetrical continuous pouring are carried out to output the closure section and diaphragm concrete structure that have been poured and are about to set. Temperature control and curing: Based on the causal analysis results and the poured concrete structure, real-time temperature monitoring, hydration heat and temperature difference control, moisture curing and strength monitoring of the concrete are carried out, and the output is a concrete structure that has reached the preset strength and is properly cured. Prestressing tensioning and structural demolition: Based on the causal analysis results, the design requirements of the diaphragm, and the properly cured concrete structure, transverse prestressing symmetrical tensioning, duct grouting, and graded demolding of formwork and hangers are carried out to output a crack-resistant diaphragm structure formed after tensioning and demolding.

5. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 4, characterized in that, The specific procedures for erecting and preparing the scaffolding system are as follows: According to the design requirements of the diaphragm, steel shell, reinforcing bars, self-compacting concrete raw materials, shear studs, PBL shear keys and prestressed steel strands are prepared, and all materials are tested according to specifications after they arrive on site. Based on the causal analysis, the steel truss hanger system for the closure section was designed. Eight precision-rolled threaded steel hangers were symmetrically arranged along the centerline of the main beam section. The bearing capacity of a single hanger was not less than 500kN. Then, a graded pre-stressing test of 1.2 times the total weight of the closure section was carried out to monitor the hanger deformation. After pre-stressing, the uneven settlement of the hanger was not greater than 1mm. The hangers were reinforced with double nuts and anti-loosening washers. Settlement measuring points, temperature measuring points, stress measuring points and displacement measuring points are set up at the locations of the hangers, steel shells and diaphragms. The monitoring system is debugged to ensure that the monitoring frequency and accuracy meet the construction control requirements. Output the accepted and qualified hanging system, the inspected and qualified construction materials, and the debugged construction monitoring system.

6. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 5, is characterized in that, The specific procedures for steel shell installation and closure locking are as follows: The steel shell of the diaphragm is composed of multiple segmented steel shells. The segmented steel shells are processed in the factory according to the construction drawings and subjected to anti-corrosion treatment. After passing the inspection, the segmented steel shells are transported to the construction site and hoisted to the design position of the closure section. They are temporarily fixed by the hanging system, and the plane position, elevation and verticality of the steel shells are adjusted. Then the segmented steel shells are welded on site to form a fully enclosed steel shell. After the welding is completed, the weld flaw detection inspection is carried out. Within the closure temperature window required by the design, temporary I-beams are welded inside the steel shell. The two ends of the I-beams are welded to the inner wall of the steel shell to form a closure locking device, which constrains the relative displacement of the closure opening. Output the steel shell after positioning welding is completed and accepted, and the closure joint structure after closure and locking is completed.

7. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 6, characterized in that, The specific installation procedures for the reinforcing steel and prestressed system are as follows: Reinforcing bars are tied inside the steel shell. The specifications, spacing, and protective layer thickness of the reinforcing bars are set according to the design requirements of the diaphragm. The reinforcing bars are tied to the perforated reinforcing bars of the PBL shear key to form an integral reinforcing steel skeleton. According to the design requirements of the diaphragm, the prestressed system is installed horizontally. The prestressed steel strands are equipped with corrugated pipes. First, the corrugated pipes are installed, then the prestressed steel strands are inserted into the corrugated pipes, and anchors and matching pads are installed at both ends of the prestressed steel strands to form prestressed pipes. The position and sealing of the prestressed pipes are checked. After inspection, the steel reinforcement cage and prestressed system that have passed the acceptance test are output.

8. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 7, characterized in that the concrete... The specific pouring and casting operations are as follows: Mix self-compacting high-performance concrete according to the design requirements of the diaphragm, and control the slump, spread and temperature before placement. Before pouring, the steel shell, reinforcing bars, and prestressed ducts are moistened, debris at the closure joint is cleaned, the anchorage of the hangers is checked, the relative displacement at the closure joint is monitored, and after confirming that it meets the design requirements, within the design closure temperature window, a symmetrical and continuous pouring method is adopted. First, the bottom plate and web of the box girder of the closure section are poured, and then the diaphragm concrete is poured. During the pouring process, an immersion vibrator was used to assist in compaction. Throughout the pouring process, the settlement of the hanger, the deformation of the steel shell, and the displacement of the closure joint were monitored. If the deformation exceeded the control threshold, the pouring was stopped immediately and adjustments were made. After the concrete is poured, the concrete surface should be smoothed and finished in a timely manner. Output the closure section and transverse diaphragm concrete structure that has been poured and is about to set.

9. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 8, characterized in that, The specific temperature control and maintenance procedures are as follows: Temperature measuring points are set up inside and on the surface of the concrete in the diaphragm to monitor the internal temperature, surface temperature and ambient temperature of the concrete in real time. After the concrete has initially set, immediately cover the concrete surface with thermal insulation and moisture retention geotextile, and attach thermal insulation cotton to the outside of the steel shell. Control the temperature gradient difference between the inside and outside of the concrete to be no greater than ±20℃, and the overall temperature difference between the newly poured closure section concrete and the adjacent poured beam section to be no greater than ±25℃. Spray curing is used to keep the concrete surface continuously moist; During concrete pouring, reserve cubic and prism test blocks that are cured under the same conditions, and test the compressive strength of the test blocks daily. Output a self-compacting high-performance concrete structure that has reached the preset strength and has been properly cured.

10. The construction method for mid-span crack-resistant transverse diaphragms of cable-stayed bridges based on multi-factor causal analysis according to claim 9, characterized in that, The specific procedures for prestressed tensioning and structural demolition are as follows: When the compressive strength of the test blocks cured under the same conditions reaches 100% of the design strength and the elastic modulus reaches more than 90% of the design value, transverse prestressing is carried out. The transverse prestressed steel strands were tensioned using a symmetrical, staged tensioning method, employing a dual-control method with tension force control as the primary method and elongation verification as a secondary method. After tensioning, vacuum-assisted grouting was used for duct grouting, followed by anchor head sealing. Once the grout strength of the duct reaches more than 80% of the design strength, first remove the temporary I-beam closure locking device, and then dismantle the hanger system and outer formwork in stages and symmetrically. Output the crack-resistant transverse diaphragm structure formed after tensioning and demolding.

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