New and old steel bar connecting mode suitable for replacing bridge expansion device

Abstract

The invention discloses a new and old steel bar connecting mode suitable for replacing a bridge expansion device, and relates to the technical field of highway bridge maintenance. Comprising the following steps: S1, prefabricating a reinforcing steel bar lock catch in a factory; s2, chiseling away concrete in an anchoring area of the old telescopic device; s3, the old telescopic device is dismantled; s4, a new telescopic device is installed; s5, the old anchoring steel bars are connected with horizontal full-length steel bars preset in anchoring steel bar rings of the new telescopic device profile steel through steel bar lock catches; s6, concrete is poured into an anchoring area of the new telescopic device; and step S7, preserving. Rapid and reliable connection between a new telescopic device and an old anchoring steel bar is achieved through the steel bar lock catch, the defects of air pollution, original structure damage and the like of traditional steel bar planting and welding are overcome, the construction period is greatly shortened, the construction difficulty and cost are reduced, meanwhile, the connection firmness and construction safety are improved, and the replacement quality of the telescopic device is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of highway bridge maintenance technology, and in particular to a method for connecting old and new steel bars for replacing bridge expansion joints. Background Technology

[0002] Expansion joints are a key component of bridge structures, primarily used to accommodate deformations caused by temperature changes, vehicle loads, and foundation settlement, ensuring bridge structural safety and driving comfort. However, expansion joints are constantly exposed to the natural environment, enduring repeated impacts and friction from vehicles, as well as erosion from rain and snow. They are highly susceptible to aging, cracking, damage, and leakage, making them vulnerable components of bridges that require regular replacement and maintenance. During the replacement of expansion joints, the removal of the old expansion joints inevitably disturbs the old anchoring steel bars at the bottom of the groove, causing them to easily shift, deform, or even be damaged. Simultaneously, the old anchoring steel bars, embedded in the beam or back wall, form a closed steel bar structure with fixed spacing. Their position and spacing are difficult to perfectly match the steel profiles of the newly installed expansion joints, making it impossible to establish an effective connection directly. This problem has become a key bottleneck restricting the quality and efficiency of expansion joint replacement work. In existing technologies, the main methods for addressing the connection between new and old reinforcing bars are rebar anchoring or welding. However, both methods have significant drawbacks: (1) Rebar installation method: This method involves multiple steps such as drilling, cleaning, gluing, and inserting new rebars, making the construction process complex. The bonding performance of the adhesive is affected by various factors such as concrete strength, moisture content, and ambient temperature, resulting in poor stability. Furthermore, the rebar installation depth is limited by the thickness of the top slab of the beam. When the top slab of the beam is thin, it is difficult to guarantee the rebar installation depth, leading to unreliable connection. After long-term use, problems such as loosening and failure may occur, affecting the safety of the bridge structure. (2) Welding method: It is necessary to weld the new and old steel bars on site. The workload is large and the welding quality is greatly affected by factors such as the skill level of the operator and the degree of corrosion of the steel bars. It is easy to have defects such as false welding and missing welding. The high temperature generated during the welding process will damage the performance of the steel bars and accelerate the subsequent corrosion of the steel bars. At the same time, the welding construction period is long, the labor and material costs are high, and the sparks and fumes generated by welding will pollute the construction environment.

[0003] Therefore, there is an urgent need to develop a new steel bar connection method that is easy to construct, reliable in connection, economical and environmentally friendly, and can protect the original bridge structure, so as to solve the shortcomings of existing technologies and improve the quality and efficiency of bridge expansion joint replacement construction. Summary of the Invention

[0004] This invention addresses the problems of poor reliability, complex construction, low efficiency, high cost, damage to the original structure, and environmental pollution associated with existing methods for connecting old and new steel bars when replacing bridge expansion joints. It provides a new method for connecting old and new steel bars suitable for replacing bridge expansion joints. By using prefabricated steel bar locking clips, the new and old steel bars can be quickly and reliably connected without the need for rebar installation and welding. This improves construction efficiency and connection quality, reduces construction costs, and protects the integrity of the original bridge structure.

[0005] The objective of this invention is primarily achieved through the following approach: A method for connecting old and new steel bars for replacing bridge expansion joints includes the following steps: Step S1: Factory prefabrication of steel bar locks; Step S2: Remove the concrete in the anchorage area of ​​the old expansion joint; Step S3: Remove the old telescopic device; Step S4: Install the new telescopic device; Step S5: Use a rebar lock to connect the old anchoring rebar to the pre-set horizontal continuous rebar inside the anchoring rebar ring of the new expansion joint steel. Step S6: Pour concrete in the anchorage zone of the new expansion joint; Step S7: Health Preservation.

[0006] Preferably, in step S1, the rebar lock is made of HRB400 steel bar and includes three bent hooks. The first hook is used to hook the old anchoring rebar, and the second and third hooks are used to hook the horizontal continuous rebar preset in the expansion joint steel anchoring rebar ring. The hook structure can be anchored in the concrete.

[0007] Preferably, step S2 includes: before chiseling away the concrete in the anchorage zone, first mark the concrete to be chiseled away by laying out and marking the part to be chiseled away, and then use a cutting machine to cut along the line; the asphalt concrete between the cuts is chiseled away with a pneumatic pick, and during the chiseling process, try to avoid damaging the original embedded steel bars and ensure that the base concrete is solid. If there is a loose base, the weak base concrete must be removed.

[0008] Preferably, step S3 includes: removing the old expansion joint by gas cutting and hoisting it out of the construction site. During gas cutting, the old anchoring steel bars must be protected and preserved intact. After the removal is completed, a compressed air pump is used to remove debris and scum from the trench. If the concrete at the trench opening or beam end is damaged during construction, it should be repaired and reinforced.

[0009] Preferably, step S4 includes the following steps: Step S41: Insert the horizontal continuous reinforcing bar into the steel anchoring reinforcing bar ring of the new expansion joint; Step S42: Use a crane or manual labor to place the new expansion joint into the slot, ensuring that the new expansion joint is straight and at the same elevation as the existing expansion joint, and adjust the expansion joint width to match the original width and position accurately.

[0010] Preferably, in step S41, the length of the horizontal continuous steel bar matches the length of the new expansion joint steel, and HRB400 steel bars are used. The number of bars is determined according to the design requirements as one or two.

[0011] Preferably, in step S42, after the new telescopic device is hoisted into place, a steel plate is welded to the joint between it and the existing telescopic device to reinforce it; after welding, the top reinforcing steel plate is ground to ensure that there are no sharp corners or protrusions and the surface is smooth; the width of the top welded steel plate does not exceed the width of the telescopic device's steel.

[0012] Preferably, step S5 includes: firstly, hooking the first hook of the rebar lock to the old anchor rebar, then hooking the second and third hooks of the rebar lock to the horizontal continuous rebar, and fixing them with steel wire; the horizontal continuous rebar is placed in the lower corner of the steel anchor rebar ring of the expansion joint. When one horizontal continuous rebar is set in one steel anchor rebar ring, one rebar lock is used to connect each old anchor rebar; when two horizontal continuous rebars are set, two rebar locks are used to connect each old anchor rebar.

[0013] Preferably, in step S6, the concrete in the anchorage zone of the expansion joint is poured with cement-based quick-setting repair material; before pouring, anti-crack steel mesh is set in the groove, and the net protective layer thickness of the anti-crack steel mesh is not less than 3.5cm; the initial setting time of the cement-based quick-setting repair material is not earlier than 20min, and the compressive strength after 1 day is not less than 40MPa.

[0014] Preferably, in step S7, curing is carried out after the concrete is poured, and a covering film is used for curing; the time from the completion of concrete pouring to the opening of traffic is not less than 4 hours, and the concrete strength at the time of opening to traffic is not less than 30MPa.

[0015] Therefore, compared with the prior art, the present invention has the following advantages: (1) The present invention has reliable connection and high safety. The mechanical connection between the old and new steel bars is achieved by prefabricated steel bar locks. The three hooks of the steel bar locks are tightly hooked and tied to the old anchor steel bars and the horizontal continuous steel bars respectively. At the same time, the hook structure can be anchored in the concrete, which significantly enhances the locking ability and pull-out resistance of the connection. It does not rely on adhesives or welding quality, is not affected by environmental factors, and has higher connection stability and reliability, effectively ensuring the safety of the bridge structure. (2) The present invention is simple to construct and improves efficiency. The steel bar lock is prefabricated in the factory and only needs to be hooked and tied on site to complete the connection. The operation is simple and convenient, without the need for complex processes such as drilling, gluing, and welding. It greatly shortens the construction period. According to calculations, when replacing the expansion joint of a two-lane highway, the construction period can be shortened by more than 30%, which significantly improves the construction efficiency. (3) This invention protects the original structure, is environmentally friendly and economical, does not require drilling holes in the original beam, avoids damage to the original bridge structure, and maintains the integrity of the bridge structure; at the same time, it avoids the environmental impact of welding sparks, dust pollution and rebar adhesive, making it green and environmentally friendly; the rebar locks can be reused, and the construction process is simplified, reducing labor and material costs. When replacing the expansion joint of a two-lane highway, the cost can be reduced by more than 25%, which is economically significant. (4) The present invention is highly adaptable. The connection method of the steel bar lock can adapt to the displacement and deformation of the old anchor steel bar. It does not require strict alignment of the old and new steel bars, and can effectively solve the problem of mismatch between the old anchor steel bar and the new expansion device steel. It is applicable to various bridge expansion device replacement projects and has a wide range of applications. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the steel bar locking structure in this invention; Figure 2 This is a side view of the connection between the new telescopic device and the old anchoring steel bar in this invention; Figure 3 This is an elevation view of the connection between the new telescopic device and the old anchoring steel bars in this invention. Detailed Implementation

[0017] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0018] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art. Unless otherwise specified, the components or equipment in the following embodiments are general standard parts or components known to those skilled in the art, and their structures and principles can be learned by those skilled in the art through technical manuals or conventional experimental methods.

[0019] like Figure 1-3 As shown, a method for connecting old and new steel bars for replacing bridge expansion joints includes the following steps: Step S1: Factory prefabrication of steel bar locks; Step S2: Remove the concrete in the anchorage area of ​​the old expansion joint; Step S3: Remove the old telescopic device; Step S4: Install the new telescopic device; Step S5: Use a rebar lock to connect the old anchoring rebar to the pre-set horizontal continuous rebar inside the anchoring rebar ring of the new expansion joint steel. Step S6: Pour concrete in the anchorage zone of the new expansion joint; Step S7: Health Preservation.

[0020] Further optimization of the above scheme is carried out in step S1. The rebar lock is made of HRB400 steel bar and includes three bent hooks. The first hook is used to hook the old anchor rebar, and the second and third hooks are used to hook the horizontal continuous rebars preset in the expansion joint steel anchor rebar ring. At the same time, its hook structure can be anchored in the concrete to enhance the locking and pull-out resistance of the connection. The bending angle of the first hook is preferably 180°, the bending angle of the second and third hooks is preferably 20°, the straight section length of the second and third hooks after bending is preferably 15cm, and the straight section between the first hook and the second hook, and between the first hook and the third hook, is preferably 8cm.

[0021] Further optimization of the above scheme is carried out in step S2. Before chiseling away the concrete in the anchorage zone, the area to be chiseled away is marked with lines. A cutting machine is used to cut along the lines, ensuring that the edges of the cuts are neat and undamaged. The asphalt concrete between the cuts is removed with a pneumatic hammer. When removing the concrete, damage to the original embedded steel bars should be avoided as much as possible. The chiseling operation should proceed from the end of the groove towards the middle. A handheld breaker should be used first, and when the depth reaches 50mm from the bridge structure, a pneumatic hammer should be used to remove it to the bottom. The use of large machinery such as vehicle-mounted breakers is prohibited. The width and depth of the groove should be determined according to the site conditions and meet the requirements for expansion joint installation.

[0022] Further optimization of the above scheme is carried out in step S3. The old expansion joint is removed by gas cutting and lifted out of the construction site. When gas cutting, care should be taken to preserve the complete old anchoring steel bars. After removing the old expansion joint, a compressed air pump is used to remove debris and scum from the groove. If the groove opening and beam end concrete are found to be damaged during construction, they should be repaired and reinforced.

[0023] The above scheme will be further optimized in the next step, which specifically includes the following steps: Step S41: Insert the horizontal continuous reinforcing bar into the steel anchoring reinforcing bar ring of the new expansion joint; Step S42: Use a crane or manual labor to place the new expansion joint into the slot, ensuring that the new expansion joint is straight and at the same elevation as the existing expansion joint, and adjust the expansion joint width to be consistent with the original width and the position to be accurate.

[0024] Further optimization of the above scheme is carried out in step S41. The length of the horizontal continuous steel bar should match the length of the new expansion joint steel. HRB400 steel bars should be used, and the number of bars should be determined according to the design requirements, which can be one or two.

[0025] Further optimization of the above scheme is carried out in step S42. After the new telescopic device is hoisted into place, a steel plate is welded to the joint between the new telescopic device and the existing telescopic device to strengthen it and prevent the steel from breaking. After welding, the top reinforcing steel plate is ground to ensure that there are no sharp corners or protrusions and that the surface is smooth. The length of the top welded steel plate is 15cm and the thickness of the steel plate is 4mm. The width of the steel plate should not exceed the width of the telescopic device steel.

[0026] Further optimization of the above scheme is carried out in step S5. First, the first hook of the rebar lock is hooked and connected to the old anchor rebar. Then, the second and third hooks of the rebar lock are hooked and connected to the horizontal continuous rebar inside the new expansion joint steel anchor rebar ring, and fixed with steel wire. The horizontal continuous rebar is preferably placed in the lower corner of the expansion joint steel anchor rebar ring. When only one horizontal continuous rebar is set in a steel anchor rebar ring, one rebar lock is used to connect each old anchor rebar. When two horizontal continuous rebars are set in a steel anchor rebar ring, two rebar locks are used to connect each old anchor rebar.

[0027] Further optimization of the above scheme is carried out in step S6, where the concrete in the anchorage zone of the expansion joint is poured using cement-based rapid repair material. Before pouring the concrete, it is advisable to install anti-crack steel mesh in the slot of the expansion joint, and the net protective layer thickness of the anti-crack steel mesh should not be less than 3.5 cm; the initial setting time of the cement-based rapid repair material should not be earlier than 20 minutes, and its 1-day compressive strength should not be less than 40 MPa.

[0028] Further optimization of the above scheme is then carried out in step S7. Curing should be implemented after the concrete pouring in the anchorage zone of the expansion joint is completed, using a covering film for curing. The time between the completion of the concrete pouring in the anchorage zone and the opening of traffic should not be less than 4 hours, and the concrete strength at the time of opening to traffic should not be less than 30 MPa.

[0029] Example 1: Replacement project of expansion joint on a highway bridge Step S1: Factory prefabrication of rebar locks; HRB400φ16 rebar is selected to process and manufacture rebar locks. The first hook has a bending angle of 180° and is used to hook old anchor rebars; the second and third hooks have bending angles of 20° and a straight section length of 15cm after bending, and are used to hook horizontal continuous rebars; the straight section length between the first and second hooks and between the first and third hooks is 8cm to ensure connection stability.

[0030] Step S2: Remove the concrete in the old expansion joint anchorage area; First, mark the area to be removed according to the design requirements; Use a cutting machine to cut along the marked lines to the old concrete base layer, ensuring the edges of the cut are neat and undamaged; Then, use a pneumatic hammer to remove the asphalt concrete and old concrete between the cuts, advancing the removal work from the end of the trench towards the middle. First, use a handheld rock breaker to remove to 50mm from the bridge structure, then switch to a small pneumatic hammer to remove to the bottom of the trench; During the removal process, assign a dedicated person to observe and avoid damaging the old anchorage reinforcement; If two loose concrete areas are found at the bottom of the trench, promptly remove the weak concrete until the solid base is exposed.

[0031] Step S3: Remove the old expansion joint; use an oxy-fuel cutting torch to cut the anchor bolts and connections of the old expansion joint. During the cutting process, adjust the flame size to avoid high-temperature damage to the old anchor steel bars and ensure that the old anchor steel bars are preserved intact; use a crane to lift the removed old expansion joint out of the construction site and stack it in the designated location; then use an air compressor to thoroughly remove debris and slag from the groove. Inspection revealed that there was local damage to the concrete at the beam end on one side of the groove, which was repaired and reinforced with epoxy mortar.

[0032] Step S4: Install the new expansion joint; First, select HRB400φ18 steel bars as horizontal continuous steel bars. Cut two horizontal continuous steel bars according to the length of the new expansion joint steel profile and insert them into the anchoring steel bar ring of the new expansion joint steel profile, placing them at the lower corner of the anchoring steel bar ring. Then, use a crane to lift the new expansion joint into the slot and adjust the position of the expansion joint to make the lines of the new expansion joint straight with the existing expansion joint, the elevation level with the bridge deck, the joint width of the expansion joint and the original joint width both 20mm, and the position accurate. Weld a 15cm×10cm×4mm (length×width×thickness) steel plate at the joint between the new expansion joint and the existing expansion joint steel profile for reinforcement. After welding, use an angle grinder to grind the top reinforcing steel plate to remove sharp corners and protrusions, so that the surface transitions smoothly with the steel profile.

[0033] Step S5: Connect the old and new reinforcing bars; use two reinforcing bar hooks at each old anchoring reinforcing bar. First, hook the first hook of the reinforcing bar hook firmly to the old anchoring reinforcing bar. Then, hook the second and third hooks of the two reinforcing bar hooks to the two horizontal continuous reinforcing bars respectively, ensuring that the hooks are tight. Then, use φ1.2mm steel wire to double-strand tie and fix each hook connection, with no less than 3 turns, to ensure a reliable connection.

[0034] Step S6: Pour concrete in the anchorage zone; select cement-based rapid repair material with an initial setting time of 25 min and a 1-day compressive strength of 45 MPa; before pouring, lay φ6@100×100 anti-crack steel mesh in the groove, with a net protective layer thickness of 4 cm; use a layered pouring method to pour the cement-based rapid repair material into the groove, and use a vibrator to compact it to ensure that the concrete is tightly bonded to the steel bars and structural steel, and that the surface is flat.

[0035] Step S7: Curing; Immediately after the concrete is poured, cover it with plastic film for curing to prevent moisture evaporation; vehicles are prohibited from passing during the curing period. Five hours after pouring, the concrete strength is tested and found to be 32MPa, meeting the requirements for opening to traffic. Traffic is then allowed.

[0036] Example 2: Replacement project of expansion joints on a bridge in a certain city Step S1: Factory prefabrication of steel bar locks; HRB400φ14 steel bars are selected for processing, the first hook bend angle is 180°, the second and third hook bend angles are 20°, the straight section length of the second and third hooks is 15cm, and the straight section length between the first hook and the second and third hooks is 8cm.

[0037] Steps S2-S3: The concrete in the anchorage zone was removed and the old expansion joint was dismantled according to the method of Example 1. No loosening of the base was found during the removal process, and the concrete at the groove and beam end was undamaged.

[0038] Step S4: Install the new expansion joint; select one HRB400φ16 horizontal continuous steel bar and insert it into the steel anchoring steel ring of the new expansion joint. Manually place the new expansion joint into the slot and adjust it into place. Weld a reinforcing steel plate at the joint and grind it smooth.

[0039] Step S5: Connect the old and new reinforcing bars; use one reinforcing bar lock at each old anchoring reinforcing bar, hook the old and new reinforcing bars and then tie them with steel wire to fix them.

[0040] Step S6: Pour cement-based rapid repair material (initial setting time 22 min, 1-day compressive strength 42 MPa), set anti-crack steel mesh (net protective layer 3.5 cm) in the trench and vibrate to compact it.

[0041] Step S7: After curing under a film for 4.5 hours, the concrete strength was tested and found to be 31 MPa. Traffic was then opened. After use, all performance characteristics were stable and met the project requirements.

[0042] It should be understood that this embodiment is for illustrative purposes only and is not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for connecting old and new steel bars for replacing bridge expansion joints, characterized in that, Includes the following steps: Step S1: Factory prefabrication of steel bar locks; Step S2: Remove the concrete in the anchorage area of ​​the old expansion joint; Step S3: Remove the old telescopic device; Step S4: Install the new telescopic device; Step S5: Use a rebar lock to connect the old anchoring rebar to the pre-set horizontal continuous rebar inside the anchoring rebar ring of the new expansion joint steel. Step S6: Pour concrete in the anchorage zone of the new expansion joint; Step S7: Health Preservation.

2. The connection method for replacing old and new steel bars in bridge expansion joints according to claim 1, characterized in that, In step S1, the rebar lock is made of HRB400 steel bar and includes three bent hooks. The first hook is used to hook the old anchoring rebar, and the second and third hooks are used to hook the horizontal continuous rebar preset in the expansion joint steel anchoring rebar ring. The hook structure can be anchored in the concrete.

3. The connection method for replacing old and new steel bars in bridge expansion joints according to claim 1, characterized in that, Step S2 includes: before chiseling away the concrete in the anchorage zone, first mark the concrete to be chiseled away by laying out and marking the lines, and then use a cutting machine to cut along the lines; the asphalt concrete between the cuts is chiseled away with a pneumatic pick, and during the chiseling process, try to avoid damaging the original embedded steel bars and ensure that the base concrete is solid. If there is a loose base, the weak base concrete must be removed.

4. The connection method for replacing old and new steel bars in bridge expansion joints according to claim 1, characterized in that, Step S3 includes: removing the old expansion joint by gas cutting and hoisting it out of the construction site. During gas cutting, the old anchoring steel bars must be protected and preserved intact. After the removal is completed, a compressed air pump is used to remove debris and scum from the trench. If the concrete at the trench opening or beam end is damaged during construction, it should be repaired and reinforced.

5. The method for connecting old and new steel bars for replacing bridge expansion joints according to claim 1, characterized in that, Step S4 includes the following steps: Step S41: Insert the horizontal continuous reinforcing bar into the steel anchoring reinforcing bar ring of the new expansion joint; Step S42: Use a crane or manual labor to place the new expansion joint into the slot, ensuring that the new expansion joint is straight and at the same elevation as the existing expansion joint, and adjust the expansion joint width to match the original width and position accurately.

6. A method for connecting old and new reinforcing bars for replacing bridge expansion joints according to claim 5, characterized in that, In step S41, the length of the horizontal continuous steel bar is matched with the length of the new expansion joint steel, and HRB400 steel bars are used. The number of bars is determined according to the design requirements as one or two.

7. A method for connecting old and new steel bars for replacing bridge expansion joints according to claim 5, characterized in that, In step S42, after the new telescopic device is hoisted into place, a steel plate is welded to the joint between it and the existing telescopic device to reinforce it; after welding, the top reinforcing steel plate is ground to ensure that there are no sharp corners or protrusions and the surface is smooth; the width of the top welded steel plate does not exceed the width of the telescopic device's steel.

8. A method for connecting old and new steel bars for replacing bridge expansion joints according to claim 1, characterized in that, Step S5 includes: firstly, hooking the first hook of the rebar lock to the old anchor rebar, then hooking the second and third hooks of the rebar lock to the horizontal continuous rebar, and fixing them with steel wire; the horizontal continuous rebar is placed in the lower corner of the steel anchor rebar ring of the expansion joint. When one horizontal continuous rebar is set in one steel anchor rebar ring, one rebar lock is used to connect each old anchor rebar; when two horizontal continuous rebars are set, two rebar locks are used to connect each old anchor rebar.

9. A method for connecting old and new reinforcing bars for replacing bridge expansion joints according to claim 1, characterized in that, In step S6, the concrete in the anchorage zone of the expansion joint is poured with cement-based quick-setting repair material; before pouring, anti-crack steel mesh is set in the groove, and the net protective layer thickness of the anti-crack steel mesh is not less than 3.5cm; the initial setting time of the cement-based quick-setting repair material is not earlier than 20min, and the compressive strength after 1 day is not less than 40MPa.

10. A method for connecting old and new reinforcing bars for replacing bridge expansion joints according to claim 1, characterized in that, In step S7, curing is carried out after the concrete is poured, and a covering film is used for curing; the time from the completion of concrete pouring to the opening of traffic is not less than 4 hours, and the concrete strength at the time of opening to traffic is not less than 30MPa.

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