Low-carbon concrete precast prestressed box girder and preparation method thereof

Abstract

The application discloses a low-carbon prefabricated prestressed box girder and a preparation method thereof, and comprises the following steps: S1, calculating the mass ratio of cementitious materials and other raw materials according to a preset concrete strength grade, i.e., a preliminary mixing proportion of the concrete; keeping the preliminary mixing proportion of the concrete unchanged, adjusting the mixing amount of cement, fly ash and mineral powder in the cementitious materials, and making the carbon emission of unit volume of the concrete lower than 330 kgCO2 / m 3 ; obtaining a low-carbon concrete mixing proportion; S2, dry mixing other raw materials with the cementitious materials according to the low-carbon concrete mixing proportion, and then adding an Al-doped C-S-H nanocrystalline core early strength agent to obtain the low-carbon concrete after stirring; and S3, compacting and curing the low-carbon concrete after pouring to obtain the prefabricated prestressed box girder.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering materials and prefabricated bridge components, and in particular to a low-carbon concrete precast prestressed box girder and its preparation method. Background Technology

[0002] Concrete, as the most widely used building material in engineering construction, primarily generates carbon emissions from the cement production process. Ordinary Portland cement produces significant amounts of carbon dioxide during its production, thus traditional high-strength concrete often has a high carbon emission level. In bridge engineering, precast prestressed box girder structures are widely used in highways and urban viaducts. These structures typically use C50 or higher strength concrete, which requires a large amount of cement, resulting in high carbon emissions during the concrete material stage.

[0003] To reduce carbon emissions from concrete, cement is often partially replaced by mineral admixtures such as fly ash and mineral powder in engineering projects. Increasing the mineral admixture replacement rate can effectively reduce cement usage and concrete carbon emissions. However, when the mineral admixture replacement rate exceeds 30%, the clinker content in the cementitious system decreases significantly, slowing down the hydration reaction rate and leading to insufficient early strength development in the concrete. The 5-7 day compressive strength often fails to reach 80% of the design strength, making it difficult to meet the early tensioning requirements during the construction of precast prestressed box girders. Furthermore, existing research on low-carbon concrete largely focuses on optimizing the mineral admixture replacement ratio, lacking a systematic mix design method under carbon emission limits, making it difficult to simultaneously achieve carbon emission control, high early strength, and good long-term structural performance.

[0004] Therefore, there is an urgent need to propose a method for preparing low-carbon concrete that can control carbon emissions from concrete while ensuring high early-stage strength of precast prestressed box girders. Summary of the Invention

[0005] In view of this, this application provides a low-carbon precast prestressed concrete box girder and its preparation method, which is used to solve the problem of how to ensure the early strength of concrete while meeting carbon emission limits.

[0006] To achieve the above technical objectives, this application adopts the following technical solution: This application provides a method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits, including the following steps: S1. Based on the preset concrete strength grade, design the mass ratio of cementitious materials to other raw materials, which is the preliminary concrete mix proportion; and determine the carbon emission limit per unit volume of concrete as 330 kg CO2 / m³. 3 And obtain the carbon emission factors of each material component; S2. Based on the carbon emission limits and mechanical performance requirements, a three-factor regulation system was constructed with cement content C, mineral admixture substitution rate R, and Al-doped CSH nanocrystalline nucleus early strength agent content N as variables, and the mix proportion parameters of low-carbon concrete were optimized. S3. The low-carbon concrete is used for casting and shaping precast prestressed box girders, and prestressed steel bars are tensioned and cured to achieve the design strength and durability requirements.

[0007] Preferably, other raw materials include water, fine aggregate, coarse aggregate, and water-reducing agent.

[0008] Preferably, the mass ratio of cementitious materials to other raw materials is calculated according to the preset concrete strength grade and the industry standard JGJ-2011 "Specification for Mix Proportion Design of Ordinary Concrete".

[0009] Preferably, in step S2, the basis for adjusting the content of cement, fly ash, and mineral powder in the cementitious material is as follows: According to the method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits, the basis for adjusting the content of cement, fly ash, and mineral powder in the cementitious material is as follows: The maximum allowable amount of cement is calculated based on the carbon emission limits, carbon emission factors, and low-carbon concrete mix proportions. And control the amount of cement C≤C max Increase the co-mixing substitution rate R of fly ash and mineral powder to reduce carbon emissions of the system; when the increased substitution rate R leads to insufficient early strength, add Al-doped CSH nanocrystal nuclei early strength agent and adjust its dosage N to promote the hydration reaction of the cementitious system.

[0010] Preferably, after the low-carbon concrete is poured, compacted, and cured, prestressing is performed when the compressive strength reaches more than 80% of the design strength, and then curing continues to be carried out to obtain the precast prestressed box girder.

[0011] Secondly, this application provides a low-carbon concrete precast prestressed box girder, comprising the following components in parts by weight: 290-310 parts cement, 85-95 parts fly ash, 85-95 parts mineral powder, 330-350 parts fine aggregate, 750-770 parts coarse aggregate, 144-150 parts water, 4-6 parts water-reducing agent, and 4-6 parts Al-doped CSH nanocrystalline nucleus early strength agent.

[0012] Preferably, the total mass of fly ash and mineral powder accounts for 35% to 40% of the total mass of cement, fly ash and mineral powder.

[0013] Preferably, the ratio of water to the total mass of cement, fly ash, and mineral powder is 0.3 to 0.32.

[0014] Preferably, the amount of Al-doped CSH nanocrystal nucleus early strength agent added is 1% to 1.5% of the total mass of cement, fly ash and mineral powder.

[0015] Preferably, the fineness modulus of the fine aggregate is 2.5% to 3.0%.

[0016] The beneficial effects of this application are as follows: This application constructs a cement-fly ash-mineral powder ternary cementitious system, which significantly reduces cement usage while maintaining concrete strength grade, thereby controlling carbon emissions per unit volume of concrete to 314~320 kgCO2 / m³. 3 It is significantly lower than the low-carbon limit of 330 kg CO2 / m³ for C50 concrete. 3 ; This application introduces an Al-doped CSH nanocrystal nucleus early-strength agent into a low-carbon cementitious system, effectively promoting the early hydration reaction of the cementitious system. This enables low-carbon concrete to achieve good early mechanical properties and durability while maintaining a low carbon emission level, specifically: 5-day compressive strength ≥ 40 MPa, 7-day compressive strength ≥ 45 MPa, 28-day carbonation depth ≤ 1 mm, and 28-day chloride ion diffusion coefficient ≤ 2.0 × 10⁻⁶. -12 m 2 ·s -1 28d electrical flux ≤743C.

[0017] This application reduces carbon emissions from concrete while addressing the problem of insufficient early strength development in C50 concrete under high mineral admixture conditions. This enables the concrete to meet the strength requirements of early tensioning of prestressed steel bars during the construction of precast prestressed box girders, thereby improving the adaptability of bridge structure construction and its engineering application value. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0019] Figure 1 This refers to the monitoring data of the material test beam in Example 1; Figure 2 This refers to the monitoring data of the material test beam in Example 2; Figure 3 This is a photograph of the material test beam used in this application. Detailed Implementation

[0020] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0021] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0022] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0024] Unless otherwise explicitly defined and specified herein, all technical and scientific terms used in this application shall have the generally accepted meanings understood by one of ordinary skill in the field of chemical and chemical materials technology (including but not limited to polymer chemistry, inorganic chemistry, organic synthesis, catalysis chemistry, materials processing, and chemical unit operations) based on their professional knowledge and conventional practice. The use of any terminology herein is intended to describe the specific embodiments of this application in the clearest and most accurate manner, so as to fully disclose the technical solution. Such use shall not in any way be construed as a limitation on the scope of the claims, nor does it imply the exclusion of equivalent technical solutions that could be reasonably known by one of skill in the art based on the concept of this application.

[0025] The terms "comprising," "including," "having," "containing," and any grammatical variations or similar expressions used in the specification, claims, and drawings of this application are all open-ended and non-exhaustive descriptive terms. Their purpose is to clearly describe the existence of technical features, components, steps, or parts, while explicitly allowing and covering the possibility that other features, components, steps, parts, or any combinations thereof not explicitly listed may exist or be added to the technical solution, as long as such additions do not destroy the integrity and inventiveness of the original technical solution.

[0026] When the terms "embodiments," "some embodiments," or "specific embodiments" are mentioned in the specification, they refer to examples that, in conjunction with the specific parameters, materials, steps, and results described in that section, constitute one or a group of examples for implementing the technical solutions of this application. These embodiments are used for full disclosure and illustrative purposes, not for exhaustive enumeration. Those skilled in the art should understand that, without departing from the overall inventive concept of this application, the various technical features disclosed in different embodiments can be combined, substituted, modified, or deleted to form other implementation methods that are not listed one by one in the specification but also fall within the protection scope of this application.

[0027] Unless otherwise expressly specified and limited, all terms related to chemical process operations, material preparation, processing and analytical testing involved in this application shall be interpreted in the broadest sense based on the conventional understanding of those skilled in the art.

[0028] Regarding performance testing and structural characterization, all testing and characterization methods involved in this application, unless otherwise specified, refer to conventional methods known in the art. Specific testing conditions may be selected and adjusted according to the sample properties and relevant national standards, international standards, or industry-standard methods. Test items may include mechanical properties (such as tensile, bending, and impact strength), thermal properties (such as DSC and TGA analysis), and chemical stability (such as solvent resistance and acid / alkali corrosion resistance). Structural characterization methods may include FT-IR, NMR, XRD, SEM, TEM, and BET. All test results should be understood to be within the allowable range of conventional experimental errors.

[0029] Regarding numerical values ​​and ranges, all parameter ranges expressed in this application in the form of "from a certain value to a certain value" should be understood as explicitly disclosing the endpoints of the range, each specific numerical point between the endpoints, and all sub-ranges formed by any two numerical points within the range. For example, "30℃ to 80℃" discloses 30, 31, ..., 80℃, as well as sub-ranges such as 30-50℃, 45-70℃, etc. When a numerical value is preceded by "about," "approximately," or similar words, it indicates that the numerical value is allowed to have reasonable errors recognized in the art under the measurement or control conditions, which can generally be understood as the deviation allowed by relevant standards or a normal fluctuation range of ±5% or ±10%.

[0030] A method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits includes the following steps: S1. Calculate the mass ratio of cementitious materials to other raw materials according to the preset concrete strength grade; this is the preliminary concrete mix proportion. Keep the amounts of other raw materials constant, adjust the amounts of cement, fly ash, and mineral powder in the cementitious materials, and ensure that the carbon emission per unit volume of concrete is below 330 kg CO2 / m³. 3 Thus, the mix proportion of low-carbon concrete was obtained; S2. According to the low-carbon concrete mix proportion, dry mix other raw materials and cementitious materials, then add Al-doped CSH nanocrystal nuclei early strength agent, and stir to obtain low-carbon concrete; S3. After pouring and compacting the low-carbon concrete, the precast prestressed box girder is obtained.

[0031] The preliminary concrete mix design is the mass ratio of cementitious materials, water, fine aggregate, coarse aggregate, and water-reducing agent; cementitious materials include cement, fly ash, and mineral powder. The low-carbon concrete mix design is the mass ratio of cement, fly ash, mineral powder, water, fine aggregate, coarse aggregate, and water-reducing agent. In the low-carbon concrete mix design, the sum of the amounts of cement, fly ash, and mineral powder is consistent with the amount of cementitious materials in the preliminary concrete mix design, and the amounts of other raw materials correspond one-to-one; other raw materials include water, fine aggregate, coarse aggregate, and water-reducing agent.

[0032] This application constructs a ternary low-carbon cementitious material system by adjusting the admixtures of cement, fly ash, and mineral powder, and controls the carbon emission of concrete per unit volume using carbon emission calculation methods. Based on this, an Al-doped CSH nanocrystalline nucleus early-strength agent is incorporated to prepare low-carbon concrete, which is then poured to form a precast prestressed box girder structure, ensuring that the carbon emission of concrete per unit volume does not exceed 330 kgCO2 / m³. 3 Under the premise of improving the early hydration reaction rate of concrete, the concrete can reach more than 80% of the design strength in 5-7 days, while having good workability and durability, thereby meeting the requirements for early tensioning construction of precast prestressed box girders.

[0033] In some embodiments, the mass ratio of cementitious materials to other raw materials is calculated according to the preset concrete strength grade and the industry standard JGJ-2011 "Specification for Mix Proportion Design of Ordinary Concrete".

[0034] In some embodiments, the method for adjusting the admixture amounts of cement, fly ash, and mineral powder is as follows: According to the method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits, the basis for adjusting the admixture amounts of cement, fly ash, and mineral powder in the cementitious materials is: based on the carbon emission limits and carbon emission... Factors and calculation of maximum allowable cement content for low-carbon concrete mix proportions and control Cement usage C≤C max To reduce carbon emissions, the replacement rate R of fly ash and mineral powder is increased. When the increased replacement rate R leads to insufficient early strength, Al-doped CSH nanocrystal nuclei are added, and the dosage N is adjusted to promote the hydration reaction of the cementitious system. Specifically, the maximum cement dosage is determined based on carbon emission limits, and the mineral admixture replacement rate is determined accordingly. The formula is used to determine the maximum cement dosage; if it is less than this value, the mineral admixture replacement rate is selected.

[0035] In some embodiments, after the low-carbon concrete is poured, compacted, and cured, prestressing is applied when the compressive strength reaches more than 80% of the design strength, and then curing continues to be carried out to obtain a precast prestressed box girder.

[0036] Specifically, the method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits includes the following steps: S1. Based on the required concrete strength grade for precast prestressed box girders, a preliminary mix design of the cementitious material system is conducted according to JGJ-2011 "Specification for Mix Proportion Design of Ordinary Concrete". The preliminary concrete mix proportion is calculated, which is the mass ratio of cementitious materials, water, fine aggregate, coarse aggregate, and water-reducing agent. Based on the preliminary concrete mix proportion, while keeping the mass ratio of cementitious materials, water, fine aggregate, coarse aggregate, and water-reducing agent constant, the admixtures of P.O42.5 cement, fly ash, and mineral powder in the cementitious materials are adjusted to construct a ternary low-carbon cementitious system. The carbon emission per unit volume of concrete is controlled through carbon emission calculation methods, and the carbon emission per unit volume of concrete is kept below 330 kgCO2 / m³. 3 The mix proportion at that time is used as the mix proportion for low-carbon concrete; S2. Based on the low-carbon concrete mix proportion, a combination of a vibrating table and an immersion vibrator is used to dry mix fine and coarse aggregates in a mixing device for 90-120 seconds to obtain a uniform aggregate mixture. Cement, fly ash, and mineral powder are then added to the aggregate mixture and dry-mixed for 180-240 seconds to obtain a uniform dry mixture. Water-reducing agent solution and mixing water are first added to the mixing device and stirred for 120 seconds. Then, Al-doped CSH nanocrystal nucleation early strength agent is added and stirred for another 60-90 seconds to form a large number of heterogeneous nucleation centers in the cementitious system, promoting CSH gel formation and thus improving the early strength of concrete, thereby preparing low-carbon concrete. S3. The low-carbon concrete mixture is poured into the precast box girder template coated with emulsified release agent and compacted by vibration. The poured concrete is then cured by watering at room temperature. When the curing period reaches 5-7 days and the strength meets the design tensioning requirements, prestressing is performed, followed by continued curing to form the precast prestressed box girder structure.

[0037] In some embodiments, the required concrete strength grade for precast prestressed box girders is C50.

[0038] This application provides a low-carbon concrete precast prestressed box girder, comprising the following components in parts by weight: 290-310 parts cement, 85-95 parts fly ash, 85-95 parts mineral powder, 330-350 parts fine aggregate, 750-770 parts coarse aggregate, 144-150 parts water, 4-6 parts water-reducing agent, and 4-6 parts Al-doped CSH nanocrystalline nucleus early strength agent.

[0039] In some embodiments, the ratio of water to the total mass of cement, fly ash, and mineral powder in the low-carbon concrete mix proportion is 0.3 to 0.32.

[0040] In some embodiments, the total mass of fly ash and mineral powder accounts for 35% to 40% of the total mass of cement, fly ash and mineral powder.

[0041] In some embodiments, the amount of Al-doped CSH nanocrystal nucleus early strength agent added is 1% to 1.5% of the total mass of cement, fly ash and mineral powder.

[0042] In some embodiments, the fine aggregate is manufactured sand, and the fineness modulus of the fine aggregate is 2.5% to 3.0%.

[0043] In some embodiments, the 5-day compressive strength of low-carbon concrete is ≥40MPa, and the 28-day compressive strength is ≥60MPa.

[0044] The following specific embodiments further illustrate this solution.

[0045] Example 1 A method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits includes the following steps: S1. Based on the required concrete strength grade C50 for the precast prestressed box girder, the preliminary mix design of the cementitious material system is carried out in accordance with JGJ-2011 "Specification for Mix Proportion Design of Ordinary Concrete". The preliminary mix proportion of concrete is calculated, namely the mass ratio of cementitious materials, water, fine aggregate, coarse aggregate, and water-reducing agent, as shown in Table 1. Table 1. Preliminary concrete mix proportions in Example 1

[0046] Keeping the initial concrete mix proportions unchanged, based on the aforementioned carbon emission limits, carbon emission factors, and low-carbon concrete... Calculation of maximum allowable cement content for initial concrete mix proportion ,in For concrete Material weight (kg) To calculate the CO2 emissions per unit of raw material produced (kgCO2 / kg), the cement dosage C was found to be ≤335.53 kg. Subsequently, the proportions of P.O42.5 cement, fly ash, and mineral powder in the cementitious materials were adjusted. The total mass of fly ash and mineral powder accounted for 35% of the total mass of cement, fly ash, and mineral powder. The calculated carbon emissions were then 315 kgCO2 / m³. 3 It is lower than the carbon emission limit for C50 concrete (330 kgCO2 / m³). 3 The carbon emission factors of each material are shown in Table 2, and the mix proportions of low-carbon concrete are shown in Table 3. Table 2 Carbon Emission Factors of Materials

[0047] Table 3. Mix proportions of low-carbon early-strength concrete in Example 1

[0048] S2. Based on the low-carbon concrete mix proportions obtained in Table 3, fine aggregates and coarse aggregates were added to a mixing device and dry-mixed for 100s to obtain a uniform aggregate mixture. Cement, fly ash, and mineral powder were then added to the aggregate mixture and dry-mixed for 200s to obtain a uniform dry mixture. The water-reducing agent solution and mixing water were first added to the mixing device and stirred for 120s. Then, Al-doped CSH nanocrystalline nucleus early strength agent (Jiangsu Bote New Material Co., Ltd. SBT®-511 ultra-early strength concrete water-reducing agent, dosage of 1wt% of cementitious material) was added and stirred for another 90s to form a large number of heterogeneous nucleation centers in the cementitious system, promote CSH gel formation, thereby improving the early strength of concrete and preparing low-carbon concrete. S3. The low-carbon concrete mixture is poured into the precast box girder template coated with emulsified release agent and compacted by vibration. The poured concrete is then cured by watering at room temperature. When the curing period reaches 6 days and the strength meets the design tensioning requirements, prestressing is performed, followed by continued curing to form the precast prestressed box girder structure.

[0049] Example 2 A method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits includes the following steps: S1. Based on the required concrete strength grade C50 for the precast prestressed box girder, the preliminary mix design of the cementitious material system is carried out in accordance with JGJ-2011 "Specification for Mix Proportion Design of Ordinary Concrete". The preliminary mix proportion of concrete is calculated, namely the mass ratio of cementitious materials, water, fine aggregate, coarse aggregate, and water-reducing agent, as shown in Table 1. Table 1. Preliminary concrete mix proportions in Example 1

[0050] Keeping the initial concrete mix proportions unchanged, based on the aforementioned carbon emission limits, carbon emission factors, and low-carbon concrete... Calculation of maximum allowable cement content for initial concrete mix proportion ,in For concrete Material weight (kg) To calculate the CO2 emissions per unit of raw material produced (kgCO2 / kg), the cement dosage C ≤ 335.53 kg was determined. Subsequently, the proportions of P.O42.5 cement, fly ash, and mineral powder in the cementitious materials were adjusted. The total mass of fly ash and mineral powder accounted for 40% of the total mass of cement, fly ash, and mineral powder. The calculated carbon emissions were then 296 kgCO2 / m³. 3 It is lower than the carbon emission limit for C50 concrete (330 kg CO2 / m³). 3 The carbon emission factors of each material are shown in Table 2, and the mix proportions of low-carbon concrete are shown in Table 3. Table 2 Carbon Emission Factors of Materials

[0051] Table 3. Mix proportions of low-carbon early-strength concrete in Example 1

[0052] S2. Based on the low-carbon concrete mix proportions obtained in Table 3, fine aggregates and coarse aggregates were added to a mixing device and dry-mixed for 120s to obtain a uniform aggregate mixture. Cement, fly ash, and mineral powder were then added to the aggregate mixture and dry-mixed for 240s to obtain a uniform dry mixture. The water-reducing agent solution and mixing water were first added to the mixing device and stirred for 120s. Then, Al-doped CSH nanocrystalline nucleus early strength agent (SBT®-511 ultra-early strength concrete water-reducing agent from Jiangsu Bote New Material Co., Ltd., with a dosage of 1wt% of the cementitious material) was added and stirred for another 90s to form a large number of heterogeneous nucleation centers in the cementitious system, promote CSH gel formation, thereby improving the early strength of concrete and preparing low-carbon concrete. S3. The low-carbon concrete mixture is poured into the precast box girder template coated with emulsified release agent and compacted by vibration. The poured concrete is then cured by watering at room temperature. When the curing period reaches 6 days and the strength meets the design tensioning requirements, prestressing is performed, followed by continued curing to form the precast prestressed box girder structure.

[0053] In this embodiment, the workability of the concrete was tested during the pouring of the prestressed concrete beam, and mechanical property and durability tests were conducted on specimens simultaneously. The results are shown in Tables 6-7. At the same time, the strain at half-span section and the deflection at the supports, quarter-span position, and mid-span section of the beam were monitored and analyzed. The results are as follows: Figure 2 As shown.

[0054] Comparative Example 1 A method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits is the same as in Example 1, except that it does not include the addition of Al-doped CSH nanocrystal nuclei early strength agent.

[0055] Testing and Evaluation Figure 3 This is a photograph of the material test beam used in this application. The workability of the concrete in Examples 1-2 and Comparative Example 1 was tested during the pouring of the prestressed concrete beam, and specimens were simultaneously placed for mechanical property and durability tests. The results are shown in Tables 6-7. Simultaneously, the strain at half-span section and the deflection changes at supports, quarter-span positions, and mid-span sections of the beam were monitored and analyzed. The results for Example 1 are as follows: Figure 1 As shown, the results of Example 2 are as follows Figure 2 As shown. (a) is a schematic diagram of the measuring points arranged from top to bottom at the mid-span section of the test beams YB1 to YB5; (b) is a displacement gauge (WY1) arranged at the mid-span position. In both embodiments, the beam surface is smooth and clean, with no obvious pores, honeycomb or surface defects, and the overall forming quality is good.

[0056] Table 6 Technical Indicators of Physical and Mechanical Properties of Concrete

[0057] The above results demonstrate that the low-carbon concrete of the precast prestressed box girder prepared according to the present invention exhibits excellent mechanical properties, with early strength and elastic modulus meeting the requirements for prestressing tension, and thus possesses superior workability.

[0058] Table 7 Technical indicators of durability performance of concrete in Examples 1-2

[0059] The above results demonstrate that the low-carbon concrete of the precast prestressed box girder prepared according to the present invention exhibits excellent resistance to chloride ion penetration and carbonation, and possesses good durability.

[0060] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits, characterized in that, Includes the following steps: S1. Based on the preset concrete strength grade, design the mass ratio of cementitious materials to other raw materials, which is the preliminary concrete mix proportion; and determine the carbon emission limit per unit volume of concrete as 330 kg CO2 / m³. 3 And obtain the carbon emission factors of each material component; S2. Based on the carbon emission limits and mechanical performance requirements, a three-factor regulation system was constructed with cement content C, mineral admixture substitution rate R, and Al-doped CSH nanocrystalline nucleus early strength agent content N as variables, and the mix proportion parameters of low-carbon concrete were optimized. S3. The low-carbon concrete is used for casting and shaping precast prestressed box girders, and prestressed steel bars are tensioned and cured to achieve the design strength and durability requirements.

2. The method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits according to claim 1, characterized in that, The other raw materials include water, fine aggregate, coarse aggregate, and water-reducing agent.

3. The method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits according to claim 1, characterized in that, Based on the preset concrete strength grade, the mass ratio of cementitious materials to other raw materials is calculated according to the industry standard JGJ-2011 "Specification for Mix Proportion Design of Ordinary Concrete".

4. The method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits according to claim 1, characterized in that, In step S2, the maximum allowable amount of cement is calculated based on the carbon emission limit, carbon emission factor, and low-carbon concrete mix proportion. And control the amount of cement C≤C max Increase the co-mixing substitution rate R of fly ash and mineral powder to reduce carbon emissions of the system; when the increased substitution rate R leads to insufficient early strength, add Al-doped CSH nanocrystal nuclei early strength agent and adjust its dosage N to promote the hydration reaction of the cementitious system.

5. The method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits according to claim 1, characterized in that, After the low-carbon concrete is poured, compacted, and cured, prestressing is performed when the compressive strength reaches more than 80% of the design strength, and then curing continues to obtain the precast prestressed box girder.

6. A low-carbon concrete precast prestressed box girder obtained by the preparation method according to any one of claims 1-5, characterized in that, The components include the following parts by weight: 290-310 parts cement, 85-95 parts fly ash, 85-95 parts mineral powder, 330-350 parts fine aggregate, 750-770 parts coarse aggregate, 144-150 parts water, 4-6 parts water-reducing agent, and 4-6 parts Al-doped CSH nanocrystal nucleation early strength agent.

7. The low-carbon precast prestressed concrete box girder based on carbon emission limits according to claim 6, characterized in that, The total mass of fly ash and mineral powder accounts for 35% to 40% of the total mass of cement, fly ash and mineral powder.

8. The low-carbon precast prestressed concrete box girder based on carbon emission limits according to claim 6, characterized in that, The ratio of water to the total mass of cement, fly ash, and mineral powder is 0.3 to 0.

32.

9. The low-carbon precast prestressed concrete box girder based on carbon emission limits according to claim 6, characterized in that, The amount of Al-doped CSH nanocrystal nucleus early strength agent added is 1% to 1.5% of the total mass of cement, fly ash and mineral powder.

10. The method for preparing low-carbon concrete precast prestressed box girders based on carbon emission limits according to claim 6, characterized in that, The fineness modulus of the fine aggregate is 2.5% to 3.0%.

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