Cement concrete surface layer function gradient material and preparation method thereof

Abstract

The application discloses a cement concrete surface layer functional gradient material and a preparation method thereof, and has a composite surface layer structure, comprising a first concrete layer and a second concrete layer; the first concrete layer comprises cement, mineral admixture, synthetic fiber, coarse aggregate, fine aggregate, water and a first water reducing agent; the second concrete layer comprises cement, coarse aggregate, fine aggregate, water and a second water reducing agent; the coarse aggregates of the first concrete layer and the second concrete layer are of the same lithology; the linear expansion coefficient of the first concrete layer is 10*10 ‑6 / °C±0.5*10 ‑6 / °C, the linear expansion coefficient of the second concrete layer is 8*10 ‑6 / °C±0.5*10 ‑6 / °C, and the flexural elastic modulus ratio is 1.05-1.1. Through interlayer material collaborative design, the application remarkably improves the anti-freezing, anti-permeation, wear resistance and interlayer deformation coordination, prolongs the service life of the pavement, and is especially suitable for high requirement engineering such as airport pavement.

Description

Technical Field

[0001] This invention relates to the field of civil engineering materials technology, specifically to a functionally graded cement concrete surface material and its preparation method, which is particularly suitable for large hub airports and airports with high requirements for pavement frost resistance and wear resistance. Background Technology

[0002] Airport cement concrete pavements have advantages such as high load-bearing capacity, low construction cost, and low maintenance cost. Over 95% of newly built civil airport pavements in my country use dry-hardened cement concrete. However, numerous domestic and international studies have shown that dry-hardened cement concrete pavements experience accelerated surface functional failure under the high tire pressure, strong impact, and salt corrosion-freeze-thaw coupling effects of aircraft, resulting in an average pavement life of less than 70% of the designed 30-year lifespan. High-performance cement concrete (HPC) has been applied in the field of civil engineering materials, particularly in key structural nodes of water conservancy and building engineering, exhibiting good durability and impact toughness. However, the construction cost of full-thickness airport pavement high-performance cement concrete is high, requiring a large amount of high-quality aggregate.

[0003] Therefore, to improve the durability and service life of airport concrete pavements while reducing construction costs and the amount of natural sand and gravel used, some studies have proposed layered functional gradient materials. For example, patent document CN115717347A discloses an ultra-high performance concrete gradient composite structure and its preparation method, which includes a first ultra-high performance concrete layer, a connecting layer, and a second ultra-high performance concrete layer. The first ultra-high performance concrete layer is an ultra-high performance concrete curing layer doped with steel fibers; the second ultra-high performance concrete layer is a second ultra-high performance concrete curing layer doped with coarse aggregates and polymer fibers; the connecting layer includes steel fiber mesh, diatomaceous earth, calcined gypsum, and silica sol, etc. However, both the upper and lower layers in this publication use ultra-high performance concrete, and the bonding relies on the connecting layer. The interfaces of different materials are prone to deformation incoordination under extreme environments (such as high temperature, low temperature, and freeze-thaw cycles), leading to interlayer cracking and a significant decrease in freeze-thaw resistance and impermeability.

[0004] Patent document CN206591385U discloses a double-layer cement concrete pavement structure that can withstand vehicle loads well and saves engineering costs by using manufactured sand. However, it can only meet the requirements of ordinary pavement load-bearing capacity and durability, and it is still significantly insufficient for the comprehensive durability performance such as high impact resistance, high freeze-thaw resistance, and high wear resistance required for airport pavements.

[0005] Therefore, developing a graded functional material that combines excellent mechanical properties, high durability, good interlayer synergistic deformation capability, and controllable cost has become an urgent technical challenge to be solved in this field. Summary of the Invention

[0006] This invention addresses the problems of poor interlayer deformation synergy, insufficient frost resistance and impermeability, and inability to meet the stringent service requirements of airport pavements in existing functionally graded materials. It provides a functionally graded material for airport cement concrete pavements and its preparation method. Compared to existing materials, this invention, through the synergistic design of the interlayer material system, improves the frost resistance, impermeability, and wear resistance of airport cement concrete pavements, extends pavement service life, reduces engineering costs, and is suitable for large-scale mechanized construction of airport pavements.

[0007] The present invention adopts the following technical solution: The first objective of this invention is to provide a functionally graded material for airport cement concrete surface layers, the functionally graded material having a composite surface layer structure, the composite surface layer structure comprising a first concrete layer (a top high-performance concrete layer) and a second concrete layer (a bottom green concrete support layer), such as... Figure 1 As shown in the figure. Each cubic meter of the first concrete layer includes 330-380 kg of cement, 100-200 kg of mineral admixtures, 1-3 kg of synthetic fibers, 1100-1400 kg of coarse aggregates, 650-700 kg of fine aggregates, 130-180 kg of water, and also includes a first water-reducing agent; The second concrete layer per cubic meter includes 320-350 kg of cement, 1335-1400 kg of coarse aggregate, 600-715 kg of fine aggregate, 130-145 kg of water, and also includes a second water-reducing agent; The difference in crushing value between the coarse aggregate in the first concrete layer and the coarse aggregate in the second concrete layer is less than or equal to 3%, and the difference in apparent density is less than or equal to 100 kg / m³. 3 ; The difference in the coefficients of linear expansion between the first concrete layer and the second concrete layer is less than or equal to 2 × 10⁻⁶. -6 / ℃, and the ratio of the flexural elastic modulus of the first concrete layer to that of the second concrete layer is 0.9~0.95.

[0008] The present invention has the following reinforcement design for the first concrete layer (the upper high-performance concrete layer): 1. Increase the amount of cementitious materials to 330-380 kg of cement and 100-200 kg of mineral admixtures per cubic meter of concrete. This is nearly 100 kg more than traditional airport pavement concrete (310-330 kg of cement per cubic meter), thereby reducing the water-cement ratio to 0.35-0.38, which is significantly lower than 12.5% ​​compared to traditional airport pavement concrete (above 0.4). This inhibits moisture migration and the formation of weak water pockets on the concrete surface, improves surface density, and thus enhances the freeze-thaw resistance and impermeability.

[0009] 2. Medium-heat silicate cement is used, and ultrafine active mineral admixtures such as silica fume, ground granulated blast furnace slag, and fly ash are added. Currently, airport pavements mostly use road silicate cement or ordinary silicate cement, with a high proportion of C3A (mass ratio ≤7.0%) and C3S (unrestricted), which are high-heat hydration cementitious materials (the proportions of the two components in medium- and low-heat cement are: C3A≤5.0%, 2.0% and C3S≤43.0%, 27.0%, respectively). This results in a certain degree of initial damage to the internal structure of the cementitious material and the interface structure with coarse aggregates during the curing process, making it prone to cracking under strong impact loads. This invention selects medium-heat silicate cement to reduce cracking caused by early temperature rise. At the same time, the addition of ultrafine active mineral admixtures such as silica fume, ground granulated blast furnace slag, and fly ash significantly improves the compactness of the concrete, enhances its impermeability, impact toughness, and wear resistance.

[0010] 3. Adding synthetic fibers. Traditional dry-hard concrete for airport pavements generally does not contain fibers. In this invention, 1-3 kg of synthetic fibers are added per cubic meter of high-performance cement concrete to the top layer, which acts as a bridge for micro-cracks and improves impact resistance and toughness. 4. High-efficiency water-reducing agent is used. Because this invention has made significant improvements to the original airport concrete mix proportions, such as increasing the amount of cement and cementitious materials and incorporating fibers, the viscosity of the cement mixture has increased significantly, posing challenges to the workability and constructability of the cement mixture. Therefore, in this invention, a first water-reducing agent is added to the first concrete layer, requiring a water reduction rate of more than 30% to reduce water consumption and ensure workability.

[0011] This invention features the following green and low-carbon design for the second concrete layer (the underlying green concrete layer): 1. Coarse aggregate is recycled aggregate, and the difference between its crushing value and that of the coarse aggregate in the first concrete layer is less than or equal to 3%, and the difference in apparent density is less than or equal to 100 kg / m³. 3 The fine aggregate in the second concrete layer is selected from manufactured sand. Both recycled aggregate and manufactured sand are green and environmentally friendly materials, reducing the amount of natural aggregate used and lowering the overall cost of cement concrete pavement.

[0012] 2. The water-cement ratio of the second concrete layer is 0.40~0.42, with each cubic meter of concrete containing 320~350kg of cement and 130~145kg of water. By optimizing the material design, the linear expansion coefficient and elastic modulus of the upper and lower layers are ensured.

[0013] This invention offers the following synergistic design for the interlayer interface of double-layer concrete: 1. By combining the above materials, the linear expansion coefficient of the first concrete layer is controlled within 1×10⁻⁶. -6 / ℃±0.5×10 -6 / ℃, the second concrete layer is controlled at 8×10-6 / ℃±0.5×10 -6 / ℃, the ratio of the flexural elastic modulus of the first and second concrete layers is controlled between 1.05 and 1.10.

[0014] 2. Through a specialized construction process, the second layer of concrete is first poured and vibrated to a level, followed by the pouring of the first layer of concrete using a wet-jointing technique. This process involves roughening, grooving, curing, and joint cutting to create a functional gradient material on the cement concrete surface. This ensures the overall volumetric stability and deformation coordination of the gradient material under temperature changes and aircraft loads, significantly outperforming existing gradient structures that rely on connecting layers.

[0015] As a preferred design, the mass of the first water-reducing agent accounts for 1% to 3% of the sum of the mass of cement and mineral admixtures in the first concrete layer.

[0016] As a preferred design, the mass of the second water-reducing agent accounts for 0 to 2% of the mass of cement in the second concrete layer.

[0017] In this invention, by adding a first reducing agent and a second reducing agent, the porosity of concrete can be reduced and the linear expansion coefficient can be lowered.

[0018] As a preferred design, the thickness of the first concrete layer accounts for 1 / 5 to 1 / 3 of the total thickness of the composite surface layer structure, and the thickness of the second concrete layer accounts for 2 / 3 to 4 / 5 of the total thickness of the composite surface layer structure.

[0019] As a preferred design, the cement for the first concrete layer is 42.5 grade medium-heat silicate cement. The cement for the second concrete layer is selected from either 42.5 grade or 32.5 grade road silicate cement.

[0020] As a preferred design, the ultrafine active mineral admixture is any one of fly ash, silica fume, and slag. The use of ultrafine active mineral admixtures such as fly ash, silica fume, and slag in this invention can reduce early cracking.

[0021] As a preferred design, the coarse aggregate in the first concrete layer is selected from any one of limestone, basalt, and granite, and the maximum particle size is less than or equal to 19 mm; The coarse aggregate in the second concrete layer is selected from recycled aggregate; And / or, the fine aggregate in the second concrete layer is selected from manufactured sand.

[0022] The maximum particle size of coarse aggregate in the first concrete layer is limited to ≤19mm, which further improves the bulk density and uniformity, thereby enhancing impermeability and wear resistance.

[0023] As a preferred design, the synthetic fiber is any one of modified polyester, polyacrylonitrile, polyamide, and polyvinyl alcohol; And / or, the water reduction rate of both the first water-reducing agent and the second water-reducing agent is greater than 30%.

[0024] A second objective of this invention is to provide a method for preparing a functionally graded cement concrete surface layer material as described in any of the above claims, comprising the following steps: (1) Preparation of the second concrete: After dry mixing coarse aggregate, cement and fine aggregate for 5~10s, add water and water-reducing agent and mix for 50~55s, and control the slump to 10~15mm; (2) Preparation of the first concrete: Dry mix coarse aggregate, cement, mineral admixture, fine aggregate and fiber for 5~10s, add water and water-reducing agent and mix for 75~80s, control the slump to 10~20mm and the Vebe consistency to 15~20s; (3) Layered paving: First, pour the second concrete at the bottom and vibrate it level. Then, pour the first concrete on top through wet bonding process to complete the roughening, grooving, curing and cutting process to form a functional gradient material for cement concrete surface layer.

[0025] As a preferred design, a double-layer slipform mechanical paving and / or manual paving method is adopted, and the time interval between the preparation of the double-layer slipform mechanical paving on the same cross section does not exceed 30 minutes.

[0026] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. Excellent durability: The high-performance concrete in the top layer significantly improves the resistance to freezing, impermeability, wear and impact through the synergistic effect of low water-cement ratio, mineral admixtures and synthetic fibers.

[0027] 2. Significant interlayer synergistic deformation capacity: The upper and lower layers of cement concrete in this invention have coordinated coefficients of linear expansion and flexural modulus through reasonable material design, with the coefficient of linear expansion of the upper layer cement concrete controlled at 10×10⁻⁶. -6 / °C±0.5×10 -6 / °C, with the lower and upper layers controlled at 8×10. -6 / °C±0.5×10 -6 The temperature is / °C, and the ratio of the flexural elastic modulus of the upper and lower layers is 0.9 to 0.95, which fully utilizes the material advantages of each gradient layer while ensuring the overall volume stability and deformation coordination of the gradient material.

[0028] 3. Economic and environmental benefits: The lower layer uses recycled aggregate and manufactured sand to reduce the amount of natural aggregate used and the project cost, while ensuring the structural support function.

[0029] 4. Applicable to high-frequency load scenarios on airport pavements: The comprehensive performance of the pavement structure layer far exceeds that of ordinary double-layer pavement, and can meet the harsh service conditions of high tire pressure, strong impact, and salt corrosion-freeze-thaw coupling of aircraft. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of a functionally graded cement concrete surface layer material provided in an embodiment of the present invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. The illustrative embodiments and descriptions of this invention are only used to explain this invention and are not intended to limit this invention.

[0033] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. The range defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range.

[0034] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0035] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0036] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other substances not listed may also be included, or that only the listed substances may be included.

[0037] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc. Example 1:

[0038] A gradient cement concrete surface material for airport pavement has a composite surface structure, comprising a first concrete layer (upper high-performance cement concrete layer) and a second concrete layer (lower green cement concrete layer), wherein the thickness ratio of the upper high-performance cement concrete layer to the lower green cement concrete layer is 1:4. The total thickness is 38 cm.

[0039] (1) Raw materials: Cement: The top layer is 42.5 grade medium-heat Portland cement; the bottom layer is 42.5 grade road Portland cement. Mineral admixtures: silica fume, fly ash (Grade I); Coarse aggregate: The top layer consists of limestone crushed stone with a continuous gradation of 5-19mm; recycled aggregate with a continuous gradation of 5-30mm. Fine aggregate: The top layer is natural river sand, continuously graded 0-5mm; the bottom layer is manufactured sand, continuously graded 0-5mm. Synthetic fiber: 12mm modified polyester fiber; Water-reducing agent: Polycarboxylate water-reducing agent, water reduction rate >30%; (2) The preparation method includes the following steps: 1. Preparation of the lower layer of green cement concrete By weight, 1389 parts of recycled aggregate, 330 parts of road silicate cement and 694 parts of manufactured sand are first mixed and dry-mixed for 5 seconds, then 139 parts of water are added and mixed for 55 seconds. After mixing evenly, the lower layer of green cement concrete is obtained with a slump of 10~15mm.

[0040] 2. Preparation of the top layer of high-performance cement concrete By weight, first mix 1204 parts coarse aggregate, 340 parts medium-heat silicate cement, 70 parts fly ash, 70 parts silica fume, 1.5 parts modified polyester fiber, and 648 parts natural river sand and dry mix for 5 seconds. Then add 84 parts water and 12 parts water-reducing agent and mix for 80 seconds. Control the slump to 10-20 mm and the Vebe consistency to 15-20 seconds. Stir evenly to obtain the surface high-performance cement concrete.

[0041] 3. Forming of functionally graded cement concrete materials Layered paving: First, pour the lower layer of manufactured sand cement concrete. After the lower layer of concrete mixture is vibrated and leveled and no obvious air bubbles escape from the surface, immediately pour the upper layer of high-performance cement concrete on top using a wet process. After the upper layer of high-performance cement concrete is laid, roughen, groove, cure, and cut the surface to obtain a gradient cement concrete material for airport pavement.

[0042] During the paving process, a double-layer slipform mechanical paving and / or manual paving method is adopted, and the time interval between the preparation of the double-layer slipform mechanical paving on the same cross section shall not exceed 30 minutes. Example 2:

[0043] The difference between this embodiment and Embodiment 1 is that: The thickness ratio of the upper high-performance cement concrete layer to the lower high-performance cement concrete layer is 1:2. Everything else is the same as in Example 1.

[0044] Comparative Example 1: This comparative example provides a C40 cement concrete commonly used in airport pavements, with a thickness of 38 cm, the full thickness of an airport concrete surface layer. The raw materials include road silicate cement, natural sand, crushed stone, water-reducing agent, and water. By weight, the composition is: 330 parts road silicate cement, 1400 parts crushed stone, 572 parts natural sand, 132 parts water, and 6.6 parts water-reducing agent.

[0045] Furthermore, the concrete in this comparative example does not require layering; it consists of only one layer, without any layers.

[0046] Comparative Example 2: Comparative Example 2 provides a high-performance cement concrete for airport pavement, with a thickness of 38 cm, representing the full thickness of the airport concrete surface layer. The raw materials include road silicate cement, natural sand, crushed stone, water-reducing agent, water, fly ash, and silica fume. By weight, it comprises 1204 parts coarse aggregate, 340 parts road silicate cement, 70 parts fly ash, 70 parts silica fume, 1.5 parts modified polyester fiber, and 648 parts natural sand, along with 84 parts water and 12 parts water-reducing agent.

[0047] Furthermore, the concrete in this comparative example does not require layering; it consists of only one layer, without any layers.

[0048] Comparative Example 3: A double-layer concrete pavement structure, with the upper layer being high-performance concrete and the lower layer being green concrete, and the only difference from Example 1 is that the water-cement ratio of the upper layer is changed to 0.42.

[0049] Comparative Example 4: A double-layer concrete pavement structure, with a high-performance concrete upper layer and a green concrete lower layer, differing from Example 1 only in the selection of coarse aggregates for the upper and lower layers, resulting in different lithologies. The upper layer uses granite as coarse aggregate, while the lower layer uses limestone as recycled aggregate. The difference in lithology results in a 10% difference in crushing value and a 300 kg / m³ difference in density. 2 .

[0050] Comparative Example 5: A double-layer concrete pavement structure with a high-performance concrete upper layer and a green concrete lower layer, and the only difference from Example 1 is that no mineral admixtures are added to the upper layer.

[0051] Comparative Example 6: A double-layer concrete pavement structure with a high-performance concrete upper layer and a green concrete lower layer, and the only difference from Example 1 is that the upper layer does not contain synthetic fibers.

[0052] Performance testing: The following performance tests were performed on the concrete materials prepared in the examples and comparative examples: (1) Volume stability and mechanical properties: The flexural strength of concrete was tested according to GB / T 50081-2002 "Standard for Test Methods of Mechanical Properties of Ordinary Concrete"; (2) Durability: The wear resistance, resistance to single-sided salt freezing and impermeability of concrete were tested according to JTG3420-2020 "Test Procedures for Cement and Cement Concrete in Highway Engineering". The test results are shown in Table 1 and Table 2.

[0053] Table 1. Concrete performance test results of the examples.

[0054] Table 2. Concrete performance test results of different embodiments and comparative examples.

[0055] As can be seen from Examples 1 and 2 and Comparative Examples 1 and 2, the gradient material of the present invention, while maintaining a low cost, has significantly better antifreeze, antipermeability and wear resistance than existing airport concrete materials, and has good interlayer synergy.

[0056] As can be seen from Comparative Example 3, a water-cement ratio deviating from the range of the present invention will lead to an imbalance between the coefficient of linear expansion and the modulus ratio, a significant increase in interlayer shear stress, and a decrease in antifreeze performance.

[0057] As shown in Comparative Example 4, when the lithology of the upper and lower coarse aggregates is different, the difference in linear expansion coefficients increases, and the interlayer bonding performance is significantly reduced.

[0058] As can be seen from Comparative Examples 5 and 6, the lack of mineral admixtures or synthetic fibers will lead to a significant decrease in durability, proving that the synergistic effect between the components of the present invention is irreplaceable.

[0059] In summary, compared with existing airport pavement materials, the gradient material for the airport pavement cement concrete surface layer provided in this application exhibits several advantages. Firstly, in terms of material optimization design, it utilizes high-strength polymer fibers and mineral admixtures to prepare a high-strength, high-toughness, and high-performance concrete surface layer, improving the mechanical and durability properties of the pavement concrete. Secondly, in terms of economical concrete material design, the lower layer rationally incorporates manufactured sand, recycled concrete, and other recycled aggregates to prepare low-cost, green cement concrete that provides structural support. Thirdly, in terms of optimized airport pavement structure design, the gradient design of the high-performance concrete material for airport pavement, by controlling the coordination of the linear expansion coefficient and flexural modulus of the two layers, ensures that the airport pavement concrete possesses excellent overall performance, fully utilizing the performance of different functional layers and significantly extending the service life of the airport pavement.

[0060] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A functionally graded material for cement concrete surface layers, characterized in that, The functionally graded material has a composite surface layer structure, which includes a first concrete layer and a second concrete layer. Each cubic meter of the first concrete layer includes 330-380 kg of cement, 100-200 kg of ultrafine active mineral admixture, 1-3 kg of synthetic fiber, 1100-1400 kg of coarse aggregate, 650-700 kg of fine aggregate, and 130-180 kg of water, and also includes a first water-reducing agent. The second concrete layer per cubic meter includes 320-350 kg of cement, 1335-1400 kg of coarse aggregate, 600-715 kg of fine aggregate, 130-145 kg of water, and also includes a second water-reducing agent; The difference in crushing value between the coarse aggregate in the first concrete layer and the coarse aggregate in the second concrete layer is less than or equal to 3%, and the difference in apparent density is less than or equal to 100 kg / m³. 3 ; The difference in the coefficients of linear expansion between the first concrete layer and the second concrete layer is less than or equal to 2 × 10⁻⁶. -6 / ℃, and the ratio of the flexural elastic modulus of the first concrete layer to that of the second concrete layer is 1.05~1.

10.

2. The functionally graded cement concrete surface layer material according to claim 1, characterized in that, The mass of the first water-reducing agent accounts for 1% to 3% of the total mass of cement and mineral admixtures in the first concrete layer.

3. The functionally graded cement concrete surface layer material according to claim 1, characterized in that, The mass of the second water-reducing agent accounts for 0 to 2% of the mass of cement in the second concrete layer.

4. The functionally graded cement concrete surface layer material according to claim 1, characterized in that, The thickness of the first concrete layer accounts for 1 / 5 to 1 / 3 of the total thickness of the composite surface layer structure, and the thickness of the second concrete layer accounts for 2 / 3 to 4 / 5 of the total thickness of the composite surface layer structure.

5. The functionally graded cement concrete surface layer material according to claim 1, characterized in that, The cement used for the first concrete layer is grade 42.5 medium-heat Portland cement; The cement used for the second concrete layer is selected from either grade 42.5 or grade 32.5 road silicate cement.

6. The functionally graded cement concrete surface layer material according to claim 1, characterized in that, The ultrafine active mineral admixture is any one of fly ash, silica fume, and slag.

7. The functionally graded cement concrete surface layer material according to claim 1, characterized in that, The coarse aggregate in the first concrete layer is selected from any one of limestone, basalt, and granite, and the maximum particle size is less than or equal to 19 mm. The coarse aggregate in the second concrete layer is selected from recycled aggregate; And / or, the fine aggregate in the second concrete layer is selected from manufactured sand.

8. A functionally graded cement concrete surface layer material according to claim 1, characterized in that, The synthetic fiber is any one of modified polyester, polyacrylonitrile, polyamide, and polyvinyl alcohol; And / or, the water reduction rate of both the first water-reducing agent and the second water-reducing agent is greater than 30%.

9. A method for preparing a functionally graded cement concrete surface layer material according to any one of claims 1 to 8, characterized in that, Includes the following steps: (1) Preparation of the second concrete: After dry mixing coarse aggregate, cement and fine aggregate for 5~10s, add water and water-reducing agent and mix for 50~55s, and control the slump to 10~15mm; (2) Preparation of the first concrete: Dry mix coarse aggregate, cement, mineral admixture, fine aggregate and fiber for 5~10s, add water and water-reducing agent and mix for 75~80s, control the slump to 10~20mm and the Vebe consistency to 15~20s; (3) Layered paving: First, pour the second concrete at the bottom and vibrate it level. Then, pour the first concrete on top through wet bonding process to complete the roughening, grooving, curing and cutting process to form a functional gradient material for cement concrete surface layer.

10. The method for preparing a functionally graded material for cement concrete surface layer according to claim 9, characterized in that, The time interval between mechanical paving and / or manual paving of double-layer slipform on the same cross section shall not exceed 30 minutes.

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