A method for controlling cracks in the end anchorage zones of prestressed concrete I-shaped and T-shaped beams

Abstract

The present invention discloses a method for controlling cracks in the end anchorage zone of prestressed concrete I-shaped and T-shaped beams, which includes arranging a vertical prestressed reinforcement area in a certain area near the tensioning and anchoring position of the prestressed steel bundle at the beam end while keeping the reinforcement layout in the original beam-end stirrup dense area unchanged; extending the length of the bearing plate, and using the manufactured new bearing plate to provide anchorage for the vertical prestressed reinforcement. By arranging a vertical prestressed reinforcement area in a certain area near the tensioning and anchoring position of the prestressed steel bundle at the beam end while keeping the reinforcement layout in the original beam-end stirrup dense area unchanged, the present invention simultaneously adopts two technologies of active crack prevention and passive crack control, effectively reducing the appearance and expansion of horizontal cracks; the beam-end bearing plate extends to the beam end, achieving the purpose of preventing the appearance of vertical cracks at the beam end and reducing the later maintenance and repair costs.

Description

Technical Field

[0001] The present invention relates to a method for controlling cracks in the end anchorage area of prestressed concrete I-shaped and T-shaped beams, belonging to the technical field of crack control in the end anchorage of prestressed concrete beams of bridges. Background Technique

[0002] In prestressed concrete I-shaped beams, the prestressed anchorage area is a typical D area (the area where the cross-section strain does not conform to the plane section assumption and the geometric dimensions are irregular) of concrete bridges. The anchoring force not only generates a strong local pressure on the concrete below the anchor, but also generates splitting forces and cleavage forces at the edge of the beam end. He Zhiqi et al. determined the magnitude of the splitting force generated by the anchoring force in the anchorage area through the methods of elastic stress and principal stress trajectories, and established a strut-and-tie model according to the principal stress trajectory method to determine the magnitude of the splitting force in the anchorage area. In 2009, Liu Zhao et al. aimed at the working condition of a rectangular cross-section beam end bearing a horizontal or inclined anchoring force, and constructed a strut-and-tie model for the post-tensioned anchorage area according to the principal stress trajectory. On this basis, using the equilibrium condition of the nodal forces in the strut-and-tie model and the geometric relationship in the model, the calculation formula for the magnitude of the splitting force in the post-tensioned anchorage area was derived. At the same time, using the finite element numerical analysis results, the calculation formula for the centroid position of the splitting stress resultant force was fitted. Zhao established analyzed and studied common prestressed concrete anchorage areas on the basis of introducing the basic theory of the strut-and-tie model, and gave a strut-and-tie model for the design of the anchorage area. The research content mainly includes: finite element analysis of the end and internal central anchorage areas, and a correction method for the existing strut-and-tie model was proposed. American scholars Stone and Breen carried out extensive analysis and experimental research on the problem of cracking along the prestressing tendon direction in the post-tensioned prestressed anchorage area of thin-web concrete box girders, and learned that the splitting tensile stress is located on the load axis and perpendicular to the loading axis, and the spalling tensile stress is located on or near the loading surface and parallel to the loading surface. Marshall and Mattock proposed an equation for designing vertical stirrups to limit the size of the horizontal cracks in the beam end web. Usually, these forces will generate three types of cracks in the anchorage area, namely splitting cracks, spalling cracks and vertical tearing cracks, as Figure 9 shown;

[0003] For splitting cracks and vertical cracks, the current "Bridge Code" gives corresponding calculation formulas and control methods for the safety check of local bearing in the anchorage area and the cross-sectional dimensions of concrete components. For horizontal spalling cracks, the "Bridge Code" gives the calculation formula (1) for the peripheral spalling force caused by local depression of the anchor plate:

[0004] T s,d =0.02max{P di} (1)

[0005] In the formula: P di is the design value of the i-th anchoring force on the same end face.

[0006] When the center distance between two anchoring forces is greater than 1 / 2 of the height of the anchoring end section, the magnitude of the splitting force generated on the end face between the large-spacing anchor heads is calculated according to formula (2).

[0007]

[0008] In the formula, Pd is the average value of the design value of the anchoring force, S is the center distance between two anchoring forces, and h is the height of the anchoring end section.

[0009] Regarding the splitting cracks caused by the anchoring force, Article 3 of Section 9.4.18 of the current "Bridge Code" states that "crack-resistant steel bars to resist surface splitting forces should be arranged at the beam end section", but no specific arrangement form is given. Therefore, it is particularly important to reasonably arrange steel bars at the prestressed anchoring end to control splitting cracks.

[0010] Regarding the splitting cracks caused by prestress, the "Bridge Code" gives the design value T of the splitting force b,d and the horizontal distance d from the position where the splitting force acts to the anchoring surface b which is calculated according to the following formula:

[0011]

[0012] d b = 0.5(h - 2e) + 5e·sin a

[0013] In the formula: P d ——The design value of the prestressed anchoring force, taking twice the tension control force;

[0014] a——The width of the anchor backing plate;

[0015] h——The height of the anchoring surface section;

[0016] e——The eccentricity of the anchoring force, taking the distance from the action point of the anchoring force to the centroid of the net section;

[0017] y——The eccentricity of the anchoring force on the section, y = 2e / h;

[0018] a——The inclination angle of the prestressed steel bar, generally taking -5° to 20°. When the action line of the anchoring force points from the starting point to the centroid of the section, it takes a positive value, and when it gradually moves away from the centroid of the section, it takes a negative value.

[0019] The "Highway Bridge Code" stipulates that the steel bar spacing of the closed stirrups configured under the anchor to resist the splitting force under the anchor should not be greater than 100 mm, and crack-resistant steel bars to resist surface splitting forces should be arranged at the beam end section. It can be seen that the "Highway Bridge Code" only makes qualitative requirements on how to arrange, and the specific form still needs to be further analyzed and given. Summary of the Invention

[0020] The object of the present invention is to overcome the deficiencies in the prior art and provide a method for controlling cracks in the end anchorage zones of prestressed concrete I- and T-shaped beams, which greatly improves the structural safety and durability and reduces the later maintenance costs of the beams, makes full use of the performance of the materials, and effectively reduces the degree of crack development.

[0021] To achieve the above object, the present invention is implemented by the following technical solutions:

[0022] In the first aspect, the present invention provides a method for controlling cracks in the end anchorage zones of prestressed concrete I- and T-shaped beams, including:

[0023] While keeping the reinforcement layout in the original stirrup densification zone at the beam end unchanged, a vertical prestressed reinforcement zone is configured in a certain area near the tensioning and anchoring position of the prestressed steel bundle at the beam end;

[0024] The length of the bearing plate is extended, and the obtained new bearing plate is used to provide anchorage for the vertical prestressed reinforcement.

[0025] Further, the stirrup densification zone at the beam end starts from the beam end along the beam length direction, and in the set anchorage zone stirrup densification zone with a length of 1.5d b The diameter of the stirrups is φ12mm - φ16mm, and the spacing is 80mm - 100mm, where d b is the horizontal distance from the position where the splitting force acts to the anchorage surface.

[0026] Further, the bearing plate in the anchorage zone is further extended to the beam end. The width of the bearing plate is the width of the I- and T-shaped beam, the length is taken as 450mm - 600mm, and the thickness is taken as 20mm.

[0027] Further, when the bearing plate is extended, the length of the bearing plate along the mid-span side of the bearing center line of the bearing plate is retained, but the length of the bearing plate along the beam end side of the bearing center line is changed and extended to the beam end.

[0028] Further, when vertical prestressed reinforcement is configured in the beam end anchorage zone, the prestressed reinforcement is arranged in 2 - 4 rows along the beam length, 2 roots in each row, and symmetrically arranged.

[0029] Further, the prestressed reinforcement is made of fine rolled threaded steel, with a diameter of 20mm - 25mm and a spacing of 150mm.

[0030] Further, the prestressed reinforcement is made of 40Si2MnMoV material, with a surface hardness of HB280 - 350, a yield strength of not less than 835MPa, and an ultimate strength of not less than 980Mpa.

[0031] Furthermore, the lower end of the vertical prestressed reinforcement is welded to the new bearing plate to provide anchorage. The upper end is successively arranged with a backing plate, a washer, a nut, and a check nut. First, the prestress is tensioned to 10% of the design load, and then it is loaded in stages to 50%, 70%, and 100% of the design load to complete the configuration.

[0032] Furthermore, a protective sleeve is arranged outside the vertical prestressed reinforcement. The structure of the protective sleeve from the inside out is epoxy primer, the first layer of polysulfide sealant, high-strength fiberglass cloth, the second layer of polysulfide sealant, and polyurethane topcoat.

[0033] Furthermore, the thickness of each layer of the protective sleeve adopts the following parameters: 80μm for the epoxy primer, 2000 - 3000μm for the first layer of polysulfide sealant, 600μm for the high-strength fiberglass cloth, 2000 - 3000μm for the second layer of polysulfide sealant, and 120μm for the polyurethane topcoat.

[0034] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0035] 1. The present invention provides a method for controlling cracks in the anchorage area at the beam ends of prestressed concrete I and T-shaped beams. By keeping the reinforcement layout in the original beam end stirrup densification area unchanged, a vertical prestressed reinforcement area is configured in a certain area near the prestressed steel bundle tensioning and anchoring position at the beam end. Both active crack prevention and passive crack control technologies are adopted simultaneously, effectively reducing the appearance and expansion of horizontal cracks; the beam end bearing plate extends to the beam end, achieving the purpose of preventing the appearance of vertical cracks at the beam end and reducing the later maintenance and repair costs.

[0036] 2. On the basis of retaining the original local bearing reinforcement configuration, the present invention sets up a split reinforcement densification area to prevent excessive splitting force caused by prestress from leading to the appearance of horizontal cracks, and quantitatively defines the layout area, position, diameter, and spacing of the split reinforcement, giving the layout of the densified reinforcement in the anchorage area theoretical and physical significance.

[0037] 3. The present invention sets vertical prestressed reinforcement at the beam ends of prestressed concrete I & T-shaped beams, establishing a vertical pre-compressive stress reserve in the beam end anchorage area after the prestressed reinforcement is tensioned, preventing the horizontal cracks at the beam end from changing from passive crack prevention to active crack prevention, greatly improving the ability of the beam end to resist horizontal cracking. A protective sleeve is arranged around the vertical prestressed reinforcement to make it non-bonded with the surrounding concrete, ensuring the uniformity of the prestressed reinforcement force, reliably transferring the prestress evenly to the beam end concrete, and also avoiding local bond failure of the beam end concrete.

[0038] 4. The present invention extends the length of the beam end bearing plate to the beam end face, while ensuring the local bearing of the bearing, also avoiding the problem of vertical cracks caused by local stress during prestress tensioning due to beam end slip and tensioning camber.

[0039] 5. Through the above technologies, the peeling cracks, splitting cracks and vertical cracks at the ends of prestressed concrete I&T beams are controlled at the same time, improving the safety and durability of the beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a schematic diagram of the steel bar arrangement in the anchorage area of the original beam end of this patent;

[0041] Figure 2 This is the schematic diagram of the cross section of the original steel bar arrangement;

[0042] Figure 3 This is a schematic diagram of the steel bar arrangement in the anchorage area at the end of the beam of this patent;

[0043] Figure 4 This is a schematic cross-sectional view of the anchorage area at the end of the beam of this patent;

[0044] Figure 5 This is a schematic diagram of the prestressed tendons in the anchorage area of the beam end of this patent;

[0045] Figure 6 This is a schematic diagram of the prestressed tendon protective layer in the anchorage area of the beam end of this patent;

[0046] Figure 7 This is a schematic diagram of the split steel bar sample in the anchorage area of the beam end of this patent;

[0047] Figure 8 This is a schematic diagram of the dimensions of the support pad in the anchorage area of the beam end of this patent;

[0048] Figure 9 This is a schematic diagram of three typical cracks in the background technology of this patent. DETAILED DESCRIPTION

[0049] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0050] Example 1

[0051] like Figures 3 to 8 As shown, this embodiment introduces a method for controlling cracks in the anchorage zone at the ends of prestressed concrete I-shaped and T-shaped beams, comprising:

[0052] While keeping the original reinforcement arrangement of the beam end stirrup densification area unchanged, a vertical prestressed steel bar area is arranged in a certain area near the prestressed steel strand tensioning and anchoring position at the beam end, extending the length of the support pad to avoid horizontal and vertical cracks in the concrete in this area.

[0053] The beam end stirrup densification zone refers to the area along the beam length starting from the beam end at a set 1.5d in order to prevent the prestressed tendons from splitting cracks in the anchorage area of the beam end.b In the stirrup dense area of the anchorage zone of the length, the diameter of the stirrup is φ12mm - φ16mm, and the spacing is 80mm - 100mm. Among them, d b refers to the horizontal distance from the position where the splitting force acts to the anchorage surface.

[0054] In this method, in order to prevent the vertical cracks caused by the slip and local stress of the concrete at the support due to the camber of the beam body after the prestressed tendon is tensioned, the support backing plate in the anchorage zone is further extended to the beam end. The width of the support backing plate generally adopts the width of the I-shaped and T-shaped beams, the length generally takes 450mm - 600mm, and the thickness takes 20mm. This patent retains the length of the backing plate along the mid-span side of the support center line of the backing plate, but changes the length of the backing plate along the beam end side of the support center line and extends it to the beam end. Such a design change, on the one hand, avoids that during the tensioning process, the beam body undergoes elastic compression under the action of prestress, the beam end will slip, and the friction force causes the concrete at the beam end to spall and vertical cracks to occur; on the other hand, it overcomes that after the prestress of the beam body is tensioned, the beam body undergoes camber, resulting in local stress at the beam end and easy spalling of the concrete.

[0055] Vertical prestressed tendons are arranged in the beam end anchorage zone. The prestressed tendons are arranged in 2 - 4 rows along the beam length, 2 in each row, and symmetrically arranged. The prestressed tendons adopt precision rolled thread steel, with a diameter of 20mm - 25mm and a spacing of 150mm.

[0056] The prestressed tendons adopt 40Si2MnMoV material, with a surface hardness of HB280 - 350, a yield strength of not less than 835MPa, and an ultimate strength of not less than 980MPa;

[0057] The lower end of the vertical prestressed tendon is welded to the new support backing plate to provide anchorage, and the upper end is successively arranged with a backing plate, a washer, a nut (35CrMo), and a locking nut. First, the prestress is tensioned to 10% of the design load, and then it is loaded in stages to 50%, 70% to 100% of the design load.

[0058] In order to prevent the prestressed tendon from bonding with the surrounding concrete and causing local bonding failure during tensioning, a protective sleeve is arranged outside the vertical prestressed tendon. The structure of the protective sleeve from the inside out is epoxy primer, the first layer of polysulfide sealant, high-strength fiberglass cloth, the second layer of polysulfide sealant, and polyurethane topcoat;

[0059] The thickness of each layer of the above protective sleeve adopts the following parameters: epoxy primer 80μm, the first layer of polysulfide sealant 2000 - 3000μm, high-strength fiberglass cloth 600μm, the second layer of polysulfide sealant 2000 - 3000μm, and polyurethane topcoat 120μm.

[0060] The following further details the use and process of the method for controlling cracks in the beam end anchorage zone of the prestressed concrete I-shaped and T-shaped beams of the present invention with reference to the attached drawings.

[0061] The reinforcement in the anchorage area at the beam ends of common existing prestressed concrete I- and T-shaped beams is as Figure 1 shown. It can be seen that vertical prestressed reinforcement is not arranged at the beam ends, the bearing plates at the supports do not extend to the beam ends, and a crack-splitting reinforcement densification zone is arranged at the beam ends, but the length of the densification zone and the central position of the reinforcement in the densification zone are not clear. Figure 2 is the sectional reinforcement in the anchorage area at the beam ends of existing prestressed concrete I- and T-shaped beams. It can be seen that the crack-splitting reinforcement is a closed stirrup.

[0062] According to this patent, the arrangement area of the crack-splitting reinforcement at the beam ends of prestressed concrete I- and T-shaped beams is adjusted, and the distance of 1.5d b from the beam end is specified. Where d b is determined by the following formula:

[0063]

[0064] |sin a|d b = 0.5(h - 2e) + 5e·sin a

[0065] In the formula: P d —— Design value of the prestressed anchoring force, taking twice the tension control force;

[0066] a—— Width of the bearing plate;

[0067] h—— Section height of the anchorage surface;

[0068] e—— Eccentricity of the anchoring force, taking the distance from the action point of the anchoring force to the centroid of the net section;

[0069] y—— Eccentricity of the anchoring force on the section, y = 2e / h;

[0070] a—— Inclination angle of the prestressed reinforcement, generally taking -5° to 20°. It takes a positive value when the action line of the anchoring force points from the starting point to the centroid of the section, and a negative value when it gradually moves away from the centroid of the section.

[0071] The diameter of the stirrup is φ12mm, and the spacing is 100mm. Such an arrangement makes the diameter of the reinforcement in the crack-splitting reinforcement densification zone smaller, but the reinforcement spacing is densified, which can better prevent the cracking and expansion of cracks.

[0072] Vertical prestressed reinforcement is arranged at the beam ends of prestressed concrete I- and T-shaped beams. The outermost first row of prestressed reinforcement is arranged 100mm away from the beam end, and then 3 rows are arranged along the beam length direction, with two prestressed reinforcement in each row. One end of the prestressed reinforcement is welded to the bearing plate, and the other end is successively arranged with bearing plates, washers, nuts and anti-loosening nuts. A protective sleeve is arranged outside the prestressed reinforcement to prevent it from bonding with the surrounding concrete.

[0073] The width of the bearing pad is kept unchanged, and the length is extended to the end of the beam. The pad is close to the end of the beam and welded to the corresponding prestressed tendons. Anchor bars are set at the center of the bearing pad along the length of the beam with a length of not less than 15d, where d is the diameter of the anchor bar.

[0074] After completing the steel bar binding, welding and prestressed tendon arrangement, and accurately positioning various embedded parts, concrete pouring is carried out, and curing is carried out after vibrating and compacting. After the curing is completed, before the prestressed steel strands are tensioned, the vertical prestressed tendons are tensioned first, so that the prestressed tendons are evenly transferred from the pad to the concrete, so that the concrete at the beam end builds up reserve compressive stress and improves crack resistance, and then the tensioning construction of the prestressed tendons at the beam end is carried out.

[0075] By changing the diameter and spacing of the split reinforcement densification zone at the beam end of the prestressed concrete I&T beam, adding vertical prestressed tendons, extending the pad length of the support, and improving the stress on the beam end, the number and length of cracks near the anchor end after tensioning are effectively controlled, thereby improving the safety and durability of the prestressed components, extending their service life, and reducing the cost of subsequent repair and maintenance.

[0076] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for controlling cracks in the end anchorage zones of prestressed concrete I and T-shaped beams, characterized in that, Including: Under the condition of keeping the reinforcement layout in the stirrup - densified area at the beam end unchanged, a vertical prestressed steel bar area is configured within a certain area near the tensioning and anchoring position of the prestressed steel tendon at the beam end; the stirrup - densified area at the beam end starts from the beam end along the beam length direction, and is the stirrup - densified area in the anchorage area with a length of 1.5d_b. The diameter of the stirrups is φ12mm~φ16mm, and the spacing is 80mm - 100mm, where d_b is the horizontal distance from the position of the splitting force to the anchoring surface. The length of the bearing plate is extended, and the manufactured new bearing plate is used to provide anchorage for the vertical prestressed steel bars. The bearing plate in the anchorage area is further extended to the beam end. The width of the bearing plate is the width of the I - shaped and T - shaped beam, the length is taken as 450mm - 600mm, and the thickness is taken as 20mm. When vertical prestressed steel bars are configured in the beam - end anchorage area, the prestressed steel bars are arranged in 2 - 4 rows along the beam length, 2 bars in each row, and symmetrically arranged. The prestressed steel bars adopt 40Si2MnMoV material, with a surface hardness of HB280 - 350, a yield strength of not less than 835MPa, and an ultimate strength of not less than 980MPa. The lower end of the vertical prestressed steel bar is welded to the new bearing plate to provide anchorage. The upper end is successively arranged with a bearing plate, a washer, a nut, and a locking nut. First, the prestress is tensioned to 10% of the design load, and then it is loaded in stages to 50%, 70%, and 100% of the design load to complete the configuration. A protective sleeve is arranged outside the vertical prestressed steel bar. The structure of the protective sleeve from the inside out is epoxy primer, the first layer of polysulfide sealant, high - strength fiberglass cloth, the second layer of polysulfide sealant, and polyurethane topcoat. The thickness of each layer of the protective sleeve adopts the following parameters: epoxy primer 80μm, the first layer of polysulfide sealant 2000 - 3000μm, high - strength fiberglass cloth 600μm, the second layer of polysulfide sealant 2000 - 3000μm, and polyurethane topcoat 120μm.

2. The method for controlling cracks in the end anchorage zone of prestressed concrete I and T-shaped beams according to claim 1, characterized in that When the bearing plate is extended, the length of the bearing plate along the mid - span side of the beam of the bearing plate center line is reserved, but the length of the bearing plate along the beam - end side of the bearing plate center line is changed and extended to the beam end.

3. The method for controlling cracks in the end anchorage zones of prestressed concrete I and T-shaped beams according to claim 2, characterized in that, The prestressed steel bars adopt precision - rolled threaded steel, with a diameter of 20mm~25mm and a spacing of 150mm.

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