UHPC steel bar box culvert structure and prefabricating method thereof

Abstract

The invention relates to the technical field of box culvert structures, in particular to a UHPC steel bar box culvert structure and a prefabrication method thereof. The box culvert main body is mainly formed by pouring a cementing material, aggregate, filler, an additive, water and ultra-high performance concrete added with mixed fibers. The structural strength is improved by adjusting the water-binder ratio of concrete raw materials, the structural density is reduced by using pre-wetted shale ceramsite, hollow glass beads and other light materials in the concrete raw materials, and the tensile property of the structure is improved by optimizing the adding proportion of steel fibers and polypropylene fibers. In the prefabrication process, an overturning inner mold is adopted as a reinforcing steel bar binding platform, and high-temperature steam is used for concrete curing. Through the optimization, the high-strength characteristic of the concrete box culvert is ensured, meanwhile, the overall density and the wall thickness of the box culvert are effectively reduced, and the tensile capacity of the box culvert structure is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of box culvert technology, and in particular to a UHPC reinforced box culvert structure and its prefabrication method. Background Technology

[0002] Box culverts, as an important underground structure, are widely used in highway or railway tunnels, culverts, and urban integrated pipe corridors. They primarily serve to support roads and distribute loads, thus requiring high strength. Box culvert structures can be specifically divided into cast-in-place reinforced concrete box culverts and precast box culverts. To ensure high structural strength, traditional precast concrete box culverts often have thicker walls, typically exceeding 25cm. This results in individual box culvert segments weighing tens or even hundreds of tons, making transportation and hoisting extremely difficult. Furthermore, the low tensile strength of ordinary concrete means that under the pressure of backfill soil and repeated vehicle loads, cracks can easily develop in the tension areas, affecting the long-term service life of the structure.

[0003] Chinese patent document CN119373035A discloses "An RPC Reinforced Box Culvert Structure," which uses reactive powder concrete as the main material for the production of precast box culverts and sets up an internal support frame on the steel reinforcement structure of the box culvert. Through improvements in concrete raw materials and steel reinforcement structure, a preliminary reduction in the wall thickness of the precast box culvert has been achieved. Reactive powder concrete is a form of ultra-high performance concrete. Although the precast box culverts made according to this patent can guarantee the structural strength of the box culvert, the high density of ultra-high performance concrete still presents the problem that the self-weight of the box culvert structure cannot be significantly reduced. Furthermore, under the raw material ratio of this patent, there is still room for improvement in the tensile properties of the concrete.

[0004] Therefore, it is necessary to propose a new type of reinforced box culvert structure that can significantly reduce weight while maintaining high strength, and at the same time improve the tensile performance of the box culvert. Based on this lightweight reinforced box culvert structure, a prefabrication method should be proposed to ensure that the structural strength of the box culvert is further improved during the prefabrication process. Summary of the Invention

[0005] This invention primarily addresses the issues of large wall thickness and heavy weight in traditional box culverts while maintaining strength, and also further improves the tensile performance of precast box culvert structures made of high-performance concrete.

[0006] To address the aforementioned technical problems, this invention provides a lightweight UHPC reinforced box culvert structure. The box culvert includes a main body, a reinforcing steel frame inside the main body, and socket structures integrally formed at both ends of the main body for splicing multi-segment box culverts. The main body is constructed from ultra-high performance concrete, and the main material mass ratio of the ultra-high performance concrete is as follows:

[0007] Cementitious materials: 800-850 parts of ordinary Portland cement with a strength grade of 52.5, 180-200 parts of silica fume, 100-120 parts of ultrafine mineral powder, and 60-70 parts of limestone powder.

[0008] Aggregates: 250-280 parts of quartz sand with a particle size of 0.5-1mm, and 350-400 parts of pre-wetted shale ceramsite with a particle size of 5-10mm;

[0009] Filler: 50-120 parts of hollow glass microspheres;

[0010] Water and admixtures: 140-150 parts water, 15-20 parts polycarboxylate superplasticizer, and 12-15 parts expansion agent;

[0011] Meanwhile, steel fibers with a volume fraction of 1.5%-2.0% and polypropylene fibers with a volume fraction of 0.1%-0.3% are added to the raw material according to the volume of the box culvert body.

[0012] Furthermore, the pre-wetted shale ceramsite has a pre-wetted water absorption rate of 4%-10%. The steel fibers are hook-shaped, with a length of 10mm-15mm and an equivalent diameter of 0.18mm-0.25mm.

[0013] Furthermore, to improve the later strength of the box culvert and further reduce its density, the present invention also uses fly ash microspheres to replace part of the cementitious material. The cementitious material includes 700-750 parts of ordinary Portland cement with a strength grade of 52.5, 100-120 parts of silica fume, and 200-250 parts of fly ash microspheres. At the same time, the content of hollow glass microspheres in the filler is adjusted to 100-120 parts.

[0014] Furthermore, the present invention also provides a method for prefabricating the above-mentioned UHPC reinforced box culvert structure, comprising the following steps:

[0015] Step S1, Pre-fabrication preparation;

[0016] According to the prefabrication construction drawings, non-magnetic stainless steel box culvert molds are customized and produced. The molds include an inner mold, an outer mold, a closed inner mold, and sealing molds at the ends of the outer mold. The top of the outer mold is provided with several concrete pouring ports. The mold surface is ground and a release agent is applied. An attached vibrator base is welded onto the mold. After the mold is transported to the prefabrication area, the height difference is adjusted and the mold is inspected for gaps. The attached vibrator is installed on the vibrator base, connected to power and tested. A steam generator is also installed in the prefabrication area in advance and connected to power and water for testing.

[0017] Step S2: Rebar tying and formwork sealing;

[0018] Concrete protective layer pads are fixed on the mold walls of the inner and outer molds that are in contact with the concrete. The inner mold is tilted and flipped over, and the inner mold is used as a platform for tying reinforcing bars. The reinforcing bar skeleton is tied on the inner mold wall. Then the inner mold and the reinforcing bar skeleton are hoisted together in the outer mold and the outer mold is assembled. Finally, the edge sealing mold is installed at the ends of the inner and outer molds to complete the overall assembly of the mold.

[0019] Step S3, UHPC concrete preparation;

[0020] Prepare raw materials according to the material ratio and mix them. First, put the cementitious materials, aggregates and fillers into the mixer and dry mix for about 10 minutes. Then add water and admixtures and wet mix for about 30 minutes. During the wet mixing process, steel fibers and polypropylene fibers are evenly added until the fibers are evenly mixed.

[0021] Step S4, concrete pouring;

[0022] The concrete mixture produced in step S3 is poured into the mold layer by layer from top to bottom through the concrete pouring port at the top of the mold. During the pouring process, the attached vibrator is turned on throughout and maintains a vibration frequency greater than 50Hz. When the concrete mixture is about 20cm away from the top of the formwork, the concrete pouring speed is slowed down until the pouring is completed. Afterwards, the concrete at the pouring port is smoothed by hand, water is sprinkled, and then the curing geotextile is covered.

[0023] Step S5, concrete curing;

[0024] After the concrete is poured and covered with geotextile for curing, it is left to stand until the concrete initially sets before the formwork is removed. Then, the steam generator is connected to the steel pipe and moved to the inner wall of the finished box culvert. The finished box culvert is covered with high-temperature resistant fabric, the steam generator is started, and the steam curing temperature is set to 70℃. The finished box culvert is continuously steam cured for 48-72 hours. During the steam curing process, a dedicated person monitors it regularly to prevent air leakage, power outages, water shortages, etc., from affecting the steam curing effect. After the steam curing is completed, the box culvert is transferred to a standard curing room and cured for 28 days while maintaining a humidity of not less than 95%.

[0025] Furthermore, the concrete curing process includes the following four stages:

[0026] Static curing stage: After the concrete is poured, the box culvert is allowed to stand still for 24 hours in a normal temperature environment with a relative humidity of over 70% to allow the concrete to initially set.

[0027] Heating stage: Start the steam generator to heat up the temperature. The heating rate is controlled to be no more than 12°C per hour until the steam curing space covered by the high-temperature resistant sewing cloth reaches 70°C. During this stage, the ambient humidity is maintained above 95%.

[0028] Constant temperature stage: The steam curing process maintains a constant temperature of 70℃, and the ambient humidity is kept above 95% during this stage; the total time for the heating and constant temperature stages is 48-72 hours.

[0029] Cooling stage: After shutting down the steam generator, the box culvert is moved to the curing room for cooling and subsequent curing. The cooling rate is controlled to be no more than 15°C per hour until the difference between the surface temperature of the box culvert and the ambient temperature does not exceed 20°C.

[0030] Furthermore, in the above prefabrication method, an external magnetic device can be used to guide the distribution of steel fibers during the concrete pouring process, specifically including the following steps:

[0031] In step S1, the pre-prefabrication preparation stage, a box culvert model is constructed according to the various design parameters, material parameters, and construction scenario parameters of the box culvert. The stress analysis is then performed using finite element software to obtain the specific distribution of the box culvert's tensile-prone parts.

[0032] In the UHPC concrete preparation stage of step S3, when mixing raw materials, the mixture is divided into two batches according to the different steel fiber addition amounts. The steel fiber addition amount of the first batch of mixture is lower than that of the second batch of mixture.

[0033] In step S4, during the concrete pouring stage, when pouring in layers, the second batch of concrete mixture is poured into the tension-prone parts of the box culvert; during the pouring process, a magnetic device is placed in the tension-prone parts to guide the steel fibers to concentrate in the tension-prone parts of the box culvert.

[0034] Furthermore, the tensile-prone parts include the outer wall of the upper part of the box culvert sidewall and the bottom surface of the top slab in the middle span.

[0035] Compared with the prior art, the beneficial effects of this invention are as follows:

[0036] 1. By adjusting the mix proportions of UHPC concrete, using a lower water-cement ratio, and incorporating lightweight materials such as pre-wetted shale ceramsite and hollow glass microspheres into the concrete raw materials, the high strength characteristics of the concrete are ensured while effectively reducing the overall density of the box culvert, allowing the box culvert structure to be reduced to a wall thickness of 10 cm. Furthermore, this invention further enhances the toughness and crack resistance of the concrete by optimizing the addition ratio of steel fibers and polypropylene fibers, significantly improving the tensile strength of the box culvert structure.

[0037] 2. In the prefabrication method of UHPC reinforced concrete box culvert, the inner mold is flipped as a platform for tying the reinforcing bars, which realizes the molding of the reinforcing bar skeleton of the lightweight box culvert with a small wall thickness. In the concrete curing stage, high temperature steam curing measures are used to ensure the full hydration and strength development of the concrete during the curing process, which further improves the strength and durability of the lightweight box culvert. Attached Figure Description

[0038] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention.

[0039] Figure 1 This is a schematic diagram of the overall structure of a UHPC reinforced box culvert according to the present invention;

[0040] Figure 2 This is a schematic diagram of the process for a prefabrication method of UHPC reinforced box culvert structure according to the present invention;

[0041] Figure 3 This is a schematic diagram of step S2 in the prefabrication method of UHPC reinforced box culvert structure according to the present invention;

[0042] Figure 4 This is a finite element stress analysis diagram of a UHPC reinforced box culvert structure according to the present invention.

[0043] In the attached diagram: 1. Box culvert main body; 11. Top slab; 12. Bottom slab; 13. Side wall; 2. Inner formwork; 3. Protective layer pad; 4. Reinforcing steel frame; 5. Socket structure. Detailed Implementation

[0044] The technical solution of the present invention will now be described with reference to the accompanying drawings. However, the described embodiments are only some embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0045] It should be noted that in the description of this invention, the terms "upper," "lower," "inner," "outer," "front," and "rear," etc., indicating directional or positional relationships, are based on the directional or positional relationships shown in the accompanying drawings, for example, the drawings... Figure 1 , 3 This describes the placement of a UHPC reinforced box culvert structure under normal conditions according to the present invention; this is merely for ease of description and is not intended to indicate or imply that the device or component must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0046] The accompanying drawings show schematic diagrams of structures according to embodiments disclosed in this invention. These drawings are not to scale, and some details may be enlarged or omitted for illustrative purposes.

[0047] It should be noted that the punctuation mark "-" used in this invention means "to", such as "60-70 parts of limestone powder" means "60 to 70 parts of limestone powder", which includes 60 and 70 as well as any value within the range of 60 to 70. Other expressions are similar.

[0048] like Figure 1 The image shows a UHPC reinforced box culvert of the present invention, used for flood control and drainage channels under a highway. It is designed to be integrally cast from UHPC ultra-high performance concrete. The precast box culvert has a cross-sectional dimension of 2m×2m and a wall thickness of 10cm. The box culvert structure includes a box culvert body 1, a steel reinforcement skeleton 4 constituting the box culvert body 1, and a socket structure 5 integrally formed at both ends of the box culvert body 1 for splicing multi-segment box culverts.

[0049] like Figure 2 As shown, the specific manufacturing process of the box culvert structure is as follows:

[0050] Step S1, Pre-fabrication preparation;

[0051] Customize non-magnetic stainless steel molds for box culverts according to prefabrication construction drawings. Grind the mold surface and apply release agent. Weld attached vibrator bases to the molds. Based on the shape of the box culvert structure, the molds are divided into inner mold 2, outer mold, and sealing molds at the ends of the inner and outer molds. The distance between the inner mold 2 and the outer mold is the wall thickness. The top of the outer mold has a concrete pouring port. After the molds are transported to the prefabrication area, they are leveled using a laser level, channel steel, and steel plates to ensure that the overall height difference of the molds is no more than 1mm. The molds are also inspected to ensure there are no gaps and no grout leakage. The attached vibrator is installed on the vibrator base, connected to power and tested. A steam generator is also installed in the prefabrication area in advance, connected to power and water, and tested.

[0052] Step S2: Rebar tying and formwork sealing;

[0053] This embodiment adopts a double-layer frame-shear wall reinforcement structure. The box culvert is designed to be thin, with a protective layer thickness of only 2.5cm. After the steel reinforcement cage 4 is tied on the outside, it is difficult to hoist it into the mold. This is because the box culvert wall thickness is small, resulting in a small spacing between the inner and outer layers of steel reinforcement in the frame-shear wall reinforcement. The traditional steel reinforcement tying method requires the steel reinforcement cage 4 to be placed horizontally between the inner and outer molds, which is extremely difficult and easily damages the steel reinforcement cage 4.

[0054] like Figure 3 As shown, the present invention adopts the process of tilting and flipping the inner mold 2, using the inner mold 2 as a platform for tying the reinforcing bars, and tying the reinforcing bar skeleton 4 on the inner mold 2. Then, the inner mold 2 is hoisted and placed in the outer mold, so that the reinforcing bar skeleton 4 is also hoisted into the mold at the same time. Finally, the outer mold is assembled, and the edge sealing mold is installed on the inner mold 2 and the end of the outer mold to complete the assembly of the overall mold.

[0055] In addition, considering the small thickness of the box culvert, there is not enough operating space to place the concrete protective layer spacers 3 after the reinforcement is tied. Therefore, before tying the reinforcement, concrete protective layer spacers 3 need to be fixed in a quincunx pattern on the inner formwork 2 and the formwork wall surface of the outer formwork that are in contact with the concrete to ensure that the thickness requirements of the concrete protective layer are met after the reinforcement is tied.

[0056] Step S3, UHPC concrete preparation;

[0057] In the first embodiment of the present invention, raw materials are prepared according to the following mass ratio of the main materials for ultra-high performance concrete:

[0058] Cementitious materials: 800-850 parts of ordinary Portland cement with a strength grade of 52.5, 180-200 parts of silica fume, 100-120 parts of ultrafine mineral powder, and 60-70 parts of limestone powder.

[0059] Aggregate: 250-280 parts of quartz sand with a particle size of 0.5-1mm, 350-400 parts of pre-wetted shale ceramsite with a particle size of 5-10mm, the pre-wetted water absorption rate of the shale ceramsite is 4%-10%;

[0060] Filler: 50-120 parts of hollow glass microspheres;

[0061] Water and admixtures: 140-150 parts water, 15-20 parts polycarboxylate superplasticizer, and 12-15 parts expansion agent;

[0062] In accordance with the volume ratio of the main body 1 of the box culvert, steel fibers with a volume fraction of 1.5%-2.0% and polypropylene fibers with a volume fraction of 0.1%-0.3% are added. The steel fibers are hook-shaped with a length of 10mm-15mm and an equivalent diameter of 0.18mm-0.25mm.

[0063] Under this mix proportion, the high-strength cementitious material system composed of ordinary silicate cement, silica fume, and ultrafine mineral powder ensures the strength of the material matrix. Secondly, regarding lightweighting, replacing most traditional aggregates with shale ceramsite achieves the main weight reduction effect; while adding an appropriate amount of hollow glass microspheres to the slurry utilizes their extremely low density to further reduce the overall weight and improve slurry fluidity. The addition of fiber groups can form a mesh within the concrete, reducing the possibility of cracks during tensile processes and assisting in the toughening and crack resistance of the box culvert structure. Finally, through precise control of high-efficiency water-reducing agents and expanding agents, on the one hand, the box culvert structure is ensured to be more dense and stronger at a low water-cement ratio; on the other hand, good fluidity is guaranteed after the raw materials are mixed, avoiding mixing difficulties. Furthermore, the pre-wetting treatment of shale ceramsite prevents it from absorbing large amounts of water during mixing and allows it to slowly release internal moisture during concrete hardening, playing a role in internal curing, further assisting in reducing the water-cement ratio and enhancing the strength of the box culvert structure.

[0064] After the materials are prepared, a twin-shaft forced mixer is used to mix the raw materials. First, the cementitious materials, aggregates, and fillers are added to the mixer for about 10 minutes of dry mixing until uniform. Then, a wet mixing process is carried out for about 30 minutes, during which water and admixtures are added to the mixer for wet mixing. During the mixing process, mixed fibers are evenly added and the mixing continues until the fibers are evenly mixed in. The uniformity of the mixture directly affects the strength and durability of the final concrete. Therefore, the mixing process needs to be strictly controlled and monitored to ensure that all components are fully mixed to form a uniform, lump-free mixture.

[0065] Step S4, concrete pouring;

[0066] The concrete mixture produced in step S3 is poured into the mold layer by layer from top to bottom through the concrete pouring port at the top of the mold. During the pouring process, the attached vibrator on the mold is turned on throughout and the vibration frequency of the vibrator is kept greater than 50Hz. When the concrete is poured to a height of about 20cm from the top of the mold, the concrete pouring speed is slowed down until the pouring is completed. Afterwards, the concrete at the pouring port is smoothed by hand, water is sprinkled, and then the curing geotextile is covered.

[0067] Because the box culvert walls are relatively thin, immersion vibrators cannot effectively compact the concrete during the pouring process. Therefore, this embodiment uses an attached vibrator, which is fixed to the mold to expel air bubbles from the concrete and ensure its density. Furthermore, the frequency and amplitude of the attached vibrator can be adjusted according to the pouring conditions to meet the vibration requirements of different areas.

[0068] Step S5, concrete curing;

[0069] After pouring the concrete and covering it with geotextile for 24 hours, the formwork was removed. Then, a steam generator was connected to a steel pipe and moved to the inner wall of the finished culvert. High-temperature resistant fabric was used to cover the finished culvert, creating a relatively closed steam curing environment. The steam generator was started and the curing temperature was set to 70℃, and the finished culvert was continuously steam-cured for 48-72 hours. During the steam curing process, a designated person monitored the process regularly to prevent leaks, power outages, or water shortages from affecting the curing effect. After 48-72 hours of continuous curing observation, the overall curing effect of the culvert was good. After steam curing, the culvert was transferred to a standard curing room and cured for 28 days while maintaining a humidity level of no less than 95%.

[0070] High-temperature curing can more fully activate the pozzolanic activity of cementitious materials, especially silica fume and mineral powder, promoting the formation of hydration products and crystal transformation, thereby forming a denser and more stable microstructure. It is particularly effective for UHPC concrete systems containing hollow glass microspheres, promoting the pozzolanic reaction on the surface of the hollow glass microspheres, improving interfacial bonding, and compensating for the strength loss caused by the introduction of lightweight components, thus further enhancing the long-term strength and durability of UHPC concrete. In this embodiment, to ensure the effectiveness of high-temperature curing, the curing process is further divided into the following four stages:

[0071] Static curing stage: After the concrete is poured, the box culvert is allowed to stand still for 24 hours in a normal temperature environment with a relative humidity of over 70% to allow it to initially solidify and prevent cracking during the heating process.

[0072] Heating stage: Start the steam generator to heat up the temperature. The heating rate is controlled to be no more than 12°C per hour until the steam curing space covered by the high-temperature resistant fabric reaches 70°C to avoid temperature stress cracking caused by excessive temperature difference between the inside and outside.

[0073] Constant temperature stage: The steam curing process maintains a constant temperature of 70℃, and the total time for the heating and constant temperature stages is 48-72 hours.

[0074] Cooling stage: After turning off the steam generator, the box culvert is moved to the curing room for cooling and subsequent curing. The cooling rate is controlled to be no more than 15°C per hour to prevent surface cracking caused by rapid cooling, until the difference between the surface temperature of the box culvert and the ambient temperature does not exceed 20°C.

[0075] Throughout the steam curing process, from the heating stage to the cooling stage, the ambient humidity is maintained above 95% to prevent moisture evaporation from the concrete surface.

[0076] In the second embodiment of the present invention, to further assist in reducing the weight of the box culvert, lowering the material density, and improving its later-stage strength, fly ash microspheres are selected to replace part of the cement and other cementitious materials, and the use of quartz sand is eliminated. This is mainly due to the pozzolanic activity of fly ash microspheres, which can further improve the later-stage strength of the concrete. Eliminating quartz sand simultaneously increases the amount of hollow glass microspheres added, which can further reduce the concrete density. The ratio of fly ash microspheres to hollow glass microspheres can be flexibly adjusted to suit different density and strength requirements. Specifically, the mass ratio of UHPC concrete in this embodiment is:

[0077] Cementitious materials: 700-750 parts of ordinary Portland cement with a strength grade of 52.5, 100-120 parts of silica fume, and 200-250 parts of fly ash microspheres;

[0078] Aggregate: 250-280 parts of quartz sand with a particle size of 0.5-1mm, 350-400 parts of pre-wetted shale ceramsite with a particle size of 5-10mm, the pre-wetted water absorption rate of the shale ceramsite is 4%-10%;

[0079] Microsphere filler: Hollow glass microspheres 80-120 kg / m³ 3 ;

[0080] Water and admixtures: 140-150 parts water, 15-20 parts polycarboxylate superplasticizer, and 12-15 parts expansion agent;

[0081] In addition, steel fibers with a volume fraction of 1.5%-2.0% and polypropylene fibers with a volume fraction of 0.1%-0.3% are added according to the volume ratio of the main body of the box culvert. The steel fibers are hook-shaped with a length of 10mm-15mm and an equivalent diameter of 0.18mm-0.25mm.

[0082] In the two embodiments described above, by utilizing the tensile properties and physical properties of steel fibers, the tensile performance of the box culvert can be improved by further optimizing its distribution within the concrete, thereby preventing the box culvert surface from cracking due to tension. Specifically:

[0083] In the prefabrication preparation stage, a box culvert model is constructed according to the various design parameters, material parameters, and construction scenario parameters. Then, stress analysis is performed using ABAQUS finite element software to obtain the specific distribution of tension components within the box culvert. In this embodiment, as shown... Figure 4 The figure shown is a finite element stress analysis diagram of the box culvert in this invention. The analysis results show that the stress deformation area of ​​the box culvert is concentrated in the top plate and side wall of the box culvert, and the tensile-prone parts are the upper outer wall of the side wall 13 and the bottom surface of the top plate 11 at the mid-span. Therefore, the local tensile strength of the box culvert can be increased by concentrating the fiber material in the tensile-prone parts.

[0084] When mixing raw materials, the mixture is divided into two batches according to the different amounts of steel fiber added. The amount of steel fiber added in the first batch of the mixture is lower than that in the second batch of the mixture.

[0085] During the layered pouring, the first batch of concrete mixture is poured to the lower middle part of the box culvert bottom slab 12 and side wall 13, and the second batch of concrete mixture is poured to the upper part of the box culvert side wall 13 and top slab 11. During the pouring process, strong magnets are placed on the upper outer wall of the box culvert side wall 13 and the bottom surface of the top slab 11 at the mid-span. Since the mold is not magnetic, the steel fibers in the mixture will be attracted by the strong magnets outside the mold and tend to concentrate towards the significantly tensile parts such as the upper outer wall of the box culvert side wall 13 and the bottom surface of the top slab 11, thereby increasing the tensile strength of these parts.

[0086] Before formal implementation, experiments are required to determine the appropriate placement time for different magnetic devices. Selected magnetic devices include electromagnetic coils and strong magnets. By observing the concentration trend of steel fibers under different magnetic devices at different times in the experimental scenario, the optimal placement time for each magnetic device can be determined. This avoids situations where the placement time is too short to achieve the expected effect, or too long, causing all the steel fibers to be magnetically adsorbed onto the concrete surface, affecting the overall performance of the box culvert body 1.

[0087] After completing the prefabrication of the UHPC concrete box culvert as described in the above embodiments, the present invention also conducts load tests on the box culvert structure, the main deflection test being as follows:

[0088] An LVDT displacement sensor was installed inside the finished box culvert body 1 after curing to monitor the deflection and displacement of the box culvert during loading. Stress calculations were performed simulating a backfill of 8m, a dead load safety factor of 1.5, and a live load safety factor of 1.3, yielding a required uniformly distributed load of 210kPa for the top slab 11. The box culvert was hoisted onto the reaction frame base, and a uniformly distributed load support was installed on top of the box culvert. A manual jack was placed in the center of the support, and the load was increased incrementally at 30kN / min increments for a total of 7 increments. After reaching the uniformly distributed load value for the top slab 11, an additional 10kN was applied, for a total load of 220kN. The deflection and strain of the finished box culvert were then observed.

[0089] The test results show that the maximum deformation during the loading process occurred at the midpoint of the bottom surface of the top plate 11, with a displacement of 6.88 mm, which is less than the allowable deflection limit of L / 300≈7.33 mm for the span in the "Design Specification for Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts". Where L is the span of the box culvert, which is 2 m. Furthermore, no obvious cracks occurred during the loading process, proving that the strength of the box culvert with this wall thickness can meet the requirements for commissioning and has excellent tensile strength, which meets the expected results.

[0090] It should be noted that the above-disclosed embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Those skilled in the art can understand that implementing all or part of the above-described embodiments and making equivalent changes in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A UHPC reinforced box culvert structure, characterized in that, The culvert includes the main body of the box culvert, the steel reinforcement cage inside the main body, and the socket structures integrally formed at both ends of the main body. The main body of the box culvert is made of ultra-high performance concrete, and the main material mass ratio of the ultra-high performance concrete is as follows: Cementitious materials: 800-850 parts of ordinary Portland cement with a strength grade of 52.5, 180-200 parts of silica fume, 100-120 parts of ultrafine mineral powder, and 60-70 parts of limestone powder. Aggregates: 250-280 parts of quartz sand with a particle size of 0.5-1mm, and 350-400 parts of pre-wetted shale ceramsite with a particle size of 5-10mm; Filler: 50-120 parts of hollow glass microspheres; Water and admixtures: 140-150 parts water, 15-20 parts polycarboxylate superplasticizer, and 12-15 parts expansion agent; Add steel fibers with a volume fraction of 1.5%-2.0% and polypropylene fibers with a volume fraction of 0.1%-0.3% according to the volume of the main body of the box culvert.

2. The UHPC reinforced box culvert structure according to claim 1, characterized in that, The cementitious material is replaced by fly ash microspheres, which includes 700-750 parts of ordinary Portland cement with a strength grade of 52.5, 100-120 parts of silica fume, and 200-250 parts of fly ash microspheres. The content of hollow glass microspheres in the filler is adjusted to 100-120 parts.

3. A UHPC reinforced box culvert structure according to claim 1 or 2, characterized in that, The pre-wetted shale ceramsite has a pre-wetted water absorption rate of 4%-10%.

4. A UHPC reinforced box culvert structure according to claim 3, characterized in that, The steel fiber is hook-shaped, with a length of 10mm–15mm and an equivalent diameter of 0.18mm–0.25mm.

5. A prefabrication method for a UHPC reinforced box culvert structure according to claim 4, characterized in that, Includes the following steps: Step S1, Pre-fabrication preparation; According to the prefabrication construction drawings, non-magnetic stainless steel box culvert molds are customized and produced. The molds include an inner mold, an outer mold, a closed inner mold, and sealing molds at the ends of the outer mold. The top of the outer mold is provided with several concrete pouring ports. The mold surface is ground and a release agent is applied. An attached vibrator base is welded onto the mold. After the mold is transported to the prefabrication area, the height difference is adjusted and the mold is inspected for gaps. The attached vibrator is installed on the vibrator base, connected to power and tested. A steam generator is also installed in the prefabrication area in advance and connected to power and water for testing. Step S2: Rebar tying and formwork sealing; Concrete protective layer pads are fixed on the mold walls of the inner and outer molds that are in contact with the concrete. The inner mold is tilted and flipped over, and the inner mold is used as a platform for tying reinforcing bars. The reinforcing bar skeleton is tied on the inner mold wall. Then the inner mold and the reinforcing bar skeleton are hoisted together in the outer mold and the outer mold is assembled. Finally, the edge sealing mold is installed at the ends of the inner and outer molds to complete the overall assembly of the mold. Step S3, UHPC concrete preparation; Prepare raw materials according to the material ratio and mix them. First, put the cementitious materials, aggregates and fillers into the mixer and dry mix for about 10 minutes. Then add water and admixtures and wet mix for about 30 minutes. During the wet mixing process, steel fibers and polypropylene fibers are evenly added until the fibers are evenly mixed. Step S4, concrete pouring; The concrete mixture produced in step S3 is poured into the mold layer by layer from top to bottom through the concrete pouring port at the top of the mold. During the pouring process, the attached vibrator is turned on throughout and maintains a vibration frequency greater than 50Hz. When the concrete mixture is about 20cm away from the top of the formwork, the concrete pouring speed is slowed down until the pouring is completed. Afterwards, the concrete at the pouring port is smoothed by hand, water is sprinkled, and then the curing geotextile is covered. Step S5, concrete curing; After the concrete is poured and covered with curing geotextile, it is left to stand until the concrete initially sets before demolding. Then, the steam generator is connected to the steel pipe and moved to the inner wall of the finished box culvert. The finished box culvert is covered with high-temperature resistant fabric, the steam generator is started, and the steam curing temperature is set to 70℃. The finished box culvert is continuously steam cured for 48-72 hours. During the steam curing process, it is monitored regularly by a designated person. After the steam curing is completed, the box culvert is transferred to a standard curing room and cured for 28 days while maintaining the humidity of the curing room at no less than 95%.

6. A prefabrication method for a UHPC reinforced box culvert structure according to claim 5, characterized in that, The concrete curing process includes the following four stages: Static curing stage: After the concrete is poured, the box culvert is allowed to stand still for 24 hours in a normal temperature environment with a relative humidity of over 70% to allow the concrete to initially set. Heating stage: Start the steam generator to heat up the temperature. The heating rate is controlled to be no more than 12°C per hour until the steam curing space covered by the high-temperature resistant sewing cloth reaches 70°C. During this stage, the ambient humidity is maintained above 95%. Constant temperature stage: The steam curing process maintains a constant temperature of 70℃, and the ambient humidity is kept above 95% during this stage; the total time for the heating and constant temperature stages is 48-72 hours. Cooling stage: After shutting down the steam generator, the box culvert is moved to the curing room for cooling and subsequent curing. The cooling rate is controlled to be no more than 15°C per hour until the difference between the surface temperature of the box culvert and the ambient temperature does not exceed 20°C.

7. A prefabrication method for a UHPC reinforced box culvert structure according to claim 5, characterized in that, The distribution of steel fibers during concrete pouring is guided using a magnetic device, specifically including the following steps: In step S1, the pre-prefabrication preparation stage, a box culvert model is constructed according to the various design parameters, material parameters, and construction scenario parameters of the box culvert. The stress analysis is then performed using finite element software to obtain the specific distribution of the box culvert's tensile-prone parts. In the UHPC concrete preparation stage of step S3, when mixing raw materials, the mixture is divided into two batches according to the different steel fiber addition amounts. The steel fiber addition amount of the first batch of mixture is lower than that of the second batch of mixture. In step S4, during the concrete pouring stage, when pouring in layers, the second batch of concrete mixture is poured into the tension-prone parts of the box culvert; during the pouring process, a magnetic device is placed in the tension-prone parts to guide the steel fibers to concentrate in the tension-prone parts of the box culvert.

8. A prefabrication method for a UHPC reinforced box culvert structure according to claim 7, characterized in that, The tensile-prone parts include the outer wall of the upper part of the box culvert sidewall and the bottom surface of the top slab in the middle span.

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