Preparation method of a tunnel pavement fire-retardant asphalt mixture

Abstract

The application discloses a preparation method of a flame-retardant asphalt mixture for a tunnel pavement, and relates to the technical field of road engineering materials. In the traditional asphalt mixture system, a modified flame-retardant additive is introduced, the mineral powder is reasonably replaced, the mixing amount of the flame-retardant additive is controlled, the mixing temperature is optimized, the compaction process and the curing process are controlled, and the efficient dispersion and function exertion of the flame-retardant additive are realized. The prepared mixture not only has the road performances such as the high-temperature rut resistance, the low-temperature crack resistance and the water stability of a conventional asphalt mixture, but also significantly improves the ignition temperature, delays the combustion spread and reduces the smoke release. The method has simple construction technology and is easy to be combined with the existing asphalt pavement construction process, and is particularly suitable for the pavement safety improvement of tunnels and high fire risk places.

Description

Technical Field

[0001] This invention relates to the field of road engineering materials technology, and in particular to a method for preparing flame-retardant asphalt mixture for tunnel pavement. Background Technology

[0002] In recent years, with the significant increase in the scale of urban transportation infrastructure construction, especially the increasing prevalence of underground tunnels, urban expressways, and underpasses, the safety requirements for pavement materials in these enclosed or semi-enclosed spaces have become increasingly stringent. Traditional asphalt mixtures are widely used in urban roads and tunnel pavements due to their advantages such as convenient construction, good smoothness, low noise, and easy maintenance. However, in the event of a fire (such as a vehicle fire or fuel spill), traditional asphalt pavement can also become a source of combustion, or exacerbate the spread of the fire, releasing large amounts of heat, smoke, and toxic gases, thus seriously endangering the evacuation of personnel and fire rescue operations within the tunnel. Literature indicates that "although asphalt mixtures contain a large amount of inert aggregate, they can still ignite and release large amounts of smoke in a tunnel fire, necessitating the development of fire-resistant / flame-retardant pavement materials."

[0003] Currently, numerous studies and engineering projects have attempted to add flame retardants, inorganic mineral fillers, mineral fibers, graphite, and metal hydroxides to asphalt or asphalt mixtures to enhance their flame retardant, smoke reduction, and high-temperature resistance properties. For example, a patent discloses a flame retardant combination for asphalt pavements in long tunnels (patent number: CN102173631B), which includes expandable graphite, aluminum hydroxide, red phosphorus, organic expandable flame retardants, and smoke inhibitors, specifying the proportions of each component. Another patent discloses a combination of "inorganic flame retardant + mineral fiber" for asphalt pavements in SMA tunnels (patent number: CN100432001C), with a formulation of: 0.3–0.5% mineral fiber and 1.2–1.5% inorganic flame retardant (by mass, based on the overall mixture). In addition, a US patent proposes a "fuel-resistant asphalt binder" modification system (patent number: US20160108241A1). Although it is not specifically designed for tunnel fire scenarios, it can be regarded as a reference for the direction of fire resistance / anti-combustion of road asphalt. Its modified asphalt contains SBS, EVA and fatty amine derivatives to improve fire resistance or oil resistance.

[0004] Although the aforementioned research and patents have revealed the potential of flame-retardant asphalt systems in road applications, the following major problems or technological gaps still exist in tunnel pavement applications: The trade-off between road performance and flame retardant properties. Adding flame retardants, mineral fibers, or other non-traditional fillers can alter the high-temperature rutting resistance, low-temperature crack resistance, water stability, and mixing / compaction properties of asphalt mixtures. Literature indicates that the incorporation of flame retardants can lead to decreased permeability, reduced ductility, increased softening point, and increased viscosity. Therefore, in high-safety-risk environments such as tunnels, how to improve flame retardant properties while ensuring conventional road performance is a pressing technical challenge.

[0005] Flame retardants exhibit poor dispersibility and compatibility. The compatibility, interfacial bonding, and durability of flame retardants, inorganic mineral fibers, graphite, etc., incorporated into asphalt mixtures with aggregates, mineral powder, and the asphalt binder system are not yet fully mature. Some research reports indicate that in tunnel asphalt fires, although flame retardants can delay ignition and reduce smoke, long-term vehicle loads, cyclic temperatures, and water-freeze alternation may weaken their effectiveness.

[0006] There is a lack of construction adaptability and process parameters. The current road construction system (mixing temperature, paving temperature, compaction temperature, compaction degree requirements, etc.) is designed for conventional asphalt mixtures and does not specifically optimize the process for flame-retardant asphalt mixtures. Summary of the Invention

[0007] Based on the problems raised in the background art mentioned above, the present invention proposes a method for preparing flame-retardant asphalt mixture for tunnel pavement.

[0008] The technical solution is as follows: A method for preparing a flame-retardant asphalt mixture for tunnel pavement includes the following steps, in parts by weight: (1) Mix the aggregate, mineral powder, and flame retardant according to the following proportions by weight: 45-50 parts coarse aggregate; 25-35 parts medium aggregate; 20-30 parts fine aggregate; 2.0-4.0 parts mineral powder; and 0.1-0.5 parts flame retardant. (2) Heat 5.0 to 6.0 parts of road asphalt to 150°C to 170°C, add dry mix and mix at 145°C to 165°C for 2 to 4 minutes; (3) Spread and compact the mixed material, turn on the vibrating plate to vibrate, and complete the compaction before the temperature cools down to below 70°C. (4) After the mixture is compacted, it is naturally cooled to room temperature and cured for at least 24 hours, which is the flame-retardant asphalt mixture for tunnel pavement.

[0009] Furthermore, the coarse aggregate particles are 10-20 mm; the medium aggregate particles are 5-10 mm; and the fine aggregate particles are 0-5 mm.

[0010] Furthermore, the mineral powder is limestone powder or granite powder with a particle size of less than 75 μm.

[0011] Furthermore, the road asphalt is conventional natural petroleum asphalt 70# or 90#.

[0012] Furthermore, the aggregates, mineral powders, and flame retardants are all dried at 105℃±5℃ for 1 to 2 hours to ensure that the moisture content is ≤0.5%.

[0013] Furthermore, the preparation method of the flame retardant is as follows: Add 80-90 parts by weight of the basic flame retardant additive aluminum hydroxide; 150-200 parts of xylene; 0.08-0.15 parts of ferrocene methyl isobutylene ester (CAS: 31566-61-7); and 0.3-0.8 parts of vinyl-1,1-diphosphate tetraethyl ester to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. Stir at 300-400 rpm for 30-100 minutes to form a stable suspension system. Heat to 70-80℃, slowly add 0.5-1.2 parts of azobisisobutyronitrile, adjust the rotation speed to 500-600 r / min, and maintain this temperature for 3.5-5 hours. Separate the solid and liquid by vacuum filtration and collect the filter cake with a vacuum degree of -0.08 to -0.09 MPa. Finally, place the filter cake in a vacuum drying oven at 80-90℃ and dry for 4-6 hours with a vacuum degree of -0.07 to -0.08 MPa to obtain the modified flame retardant additive.

[0014] Furthermore, in step (2), a high-shear mixer is used during the mixing process. Initially, the mixer is premixed at a speed of 450-500 r / min for 1-2 minutes, then sheared at 2000-3000 r / min for 1-1.5 hours, and then mixed at 400-500 r / min for 4-6 minutes.

[0015] Furthermore, the frequency of the vibrating plate during the compaction process is 50Hz to 70Hz.

[0016] Furthermore, the compaction degree is 96-99%.

[0017] Flame retardant reaction mechanism: In the preparation of the flame retardant, aluminum hydroxide, a basic flame retardant additive, is first selected and mixed with xylene, ferrocene methanol isobutylene ester, and vinyl-1,1-diphosphate tetraethyl ester in a specific device according to a certain ratio. This ensures that the components are in full contact and form a stable suspension system. Subsequently, by raising the temperature and adding an initiator, and adjusting the stirring rate, the functional monomers in the system undergo a polymerization reaction around the basic flame retardant additive particles. The polymerization product uniformly coats the basic flame retardant additive particles. After the reaction is complete, solid-liquid separation is achieved by vacuum filtration, and the solid component is collected. Finally, the solid component is dried in a vacuum drying environment to obtain the modified flame retardant additive. Technical effects: 1. Balancing road performance and flame retardant properties By optimizing the flame retardant additives, controlling the proportions of various raw materials, and adjusting the mineral powder substitution ratio, combined with precise control of construction techniques, the modified flame retardant additives achieve good dispersion and synergistic effects within the asphalt mixture system. While significantly improving the combustion safety of the mixture, it effectively maintains its key engineering properties in areas such as high-temperature rutting resistance, low-temperature crack resistance, and water stability, successfully balancing the road surface's functional requirements with the needs for flame retardancy and smoke reduction. 2. Improve safety and reliability in fire environments The flame-retardant asphalt mixture prepared by this method exhibits excellent performance in fire scenarios involving heat radiation and combustion spread. The base flame-retardant additives in the modified flame-retardant additive work synergistically with the coating layer under high-temperature conditions. This is achieved by interrupting heat transfer paths, releasing inert gases to inhibit combustion reactions, and forming a carbonized barrier layer to block flame spread. These methods delay the ignition time of the mixture, inhibit the expansion of the combustion range, and reduce the generation of smoke and toxic gases, creating safer environmental conditions for ensuring personnel evacuation routes and fire rescue operations in tunnel fires. 3. When introducing modified flame-retardant additives into the traditional asphalt mixture construction process framework, no significant changes were made to the original core construction logic. By clarifying the operational specifications for each stage, such as optimizing the phased control of the mixing process based on raw material characteristics and determining the key operational requirements of the compaction process in conjunction with the dispersion requirements of the modified flame-retardant additives, the preparation process becomes standardized and controllable. This eliminates the need for large-scale modifications to existing road or tunnel construction equipment, facilitating rapid promotion and application within existing engineering systems and improving the feasibility and economy of technology implementation. Detailed Implementation

[0018] The features of the present invention are further illustrated below through embodiments, but the scope of protection of this patent is not limited to the embodiments.

[0019] Example 1: Preparation of flame-retardant asphalt mixture for tunnel pavement 1. Experimental Materials Coarse aggregate (10~20mm): 45kg Medium aggregate (5-10mm): 25kg Fine aggregate (0~5mm): 20kg Mineral powder (limestone powder, particle size <75μm): 2.0kg Flame retardant raw materials: 80 kg aluminum hydroxide, 150 kg xylene, 0.08 kg ferrocene methyl methacrylate (CAS: 31566-61-7), 0.3 kg tetraethyl vinyl-1,1-diphosphate, and 0.5 kg azobisisobutyronitrile. Road asphalt (70# natural petroleum asphalt): 5.0 kg 2. Experimental Procedure (1) Preparation of flame retardant 80 kg of aluminum hydroxide, 150 kg of xylene, 0.08 kg of ferrocene methyl isobutylene ester, and 0.3 kg of vinyl-1,1-diphosphate tetraethyl ester were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 300 rpm for 30 minutes to form a stable suspension. The temperature was raised to 70 °C, and 0.5 kg of azobisisobutyronitrile was slowly added. The stirring speed was adjusted to 500 rpm, and the reaction was maintained at this temperature for 3.5 hours. The solid and liquid were separated by vacuum filtration, and the filter cake was collected (vacuum degree -0.08 MPa). Finally, the filter cake was placed in an 80 °C vacuum drying oven and dried for 4 hours (vacuum degree -0.07 MPa) to obtain 0.1 kg of modified flame retardant.

[0020] (2) Raw material pretreatment 45 kg of coarse aggregate, 25 kg of medium aggregate, 20 kg of fine aggregate, 2.0 kg of mineral powder and 0.1 kg of the prepared flame retardant were placed in an oven at 105℃±5℃ and dried for 1 hour to ensure that the moisture content was ≤0.5%.

[0021] (3) Dry mixing (step 1) The dried coarse aggregate, medium aggregate, fine aggregate, mineral powder, and flame retardant are put into the mixing equipment and dry-mixed evenly to obtain dry-mixed material.

[0022] (4) Asphalt heating and mixing (step 2) Heat 5.0 kg of 70# road asphalt to 150°C to make it fluid; add the dry mix to the heated asphalt and control the mixing temperature at 145°C; use a high-shear mixer, initially premix at 450 r / min for 1 minute, then continue shearing at 2000 r / min for 1 hour, and then mix at 400 r / min for 4 minutes, for a total mixing time of 2 minutes.

[0023] (5) Spreading and compaction (step 3) The mixed material is quickly spread out and vibrated to compact it, with the frequency of the vibrating plate (50Hz) controlled to a compaction degree of 96%. The compaction process is completed before the mixture cools down to below 70°C.

[0024] (6) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0025] Example 2: Preparation of flame-retardant asphalt mixture for tunnel pavement 1. Experimental Materials Coarse aggregate (10~20mm): 47kg Medium aggregate (5-10mm): 30kg Fine aggregate (0~5mm): 24kg Mineral powder (granite powder, particle size <75μm): 2.8kg Flame retardant raw materials: 83 kg aluminum hydroxide, 165 kg xylene, 0.10 kg ferrocene methyl methacrylate (CAS: 31566-61-7), 0.5 kg tetraethyl vinyl-1,1-diphosphate, and 0.7 kg azobisisobutyronitrile. Road asphalt (70# natural petroleum asphalt): 5.3kg 2. Experimental Procedure (1) Preparation of flame retardant 83 kg of aluminum hydroxide, 165 kg of xylene, 0.10 kg of ferrocene methyl isobutylene ester, and 0.5 kg of vinyl-1,1-diphosphate tetraethyl ester were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 330 rpm for 50 minutes to form a stable suspension. The temperature was raised to 73°C, and 0.7 kg of azobisisobutyronitrile was slowly added. The stirring speed was adjusted to 530 rpm, and the reaction was maintained at this temperature for 4.0 hours. The solid and liquid were separated by vacuum filtration, and the filter cake was collected (vacuum degree -0.085 MPa). Finally, the filter cake was placed in a vacuum drying oven at 83°C and dried for 4.5 hours (vacuum degree -0.075 MPa) to obtain 0.2 kg of modified flame retardant.

[0026] (2) Raw material pretreatment 47 kg of coarse aggregate, 30 kg of medium aggregate, 24 kg of fine aggregate, 2.8 kg of mineral powder and 0.2 kg of the prepared flame retardant were placed in an oven at 105℃±5℃ and dried for 1.3 hours to ensure that the moisture content was ≤0.5%.

[0027] (3) Dry mixing (step 1) All the dried solid raw materials are put into the mixing equipment and mixed evenly to obtain the dry mixture.

[0028] (4) Asphalt heating and mixing (step 2) Heat 5.3 kg of 70# road asphalt to 155°C to make it fluid; add the dry mix to the asphalt and control the mixing temperature at 150°C; use a high-shear mixer, initially premix at 470 r / min for 1.2 minutes, then continue shearing at 2400 r / min for 1.2 hours, and then mix at 450 r / min for 5 minutes, for a total mixing time of 3 minutes.

[0029] (5) Spreading and compaction (step 3) Spread the mixed material and turn on the vibrating plate (frequency 55Hz) to vibrate and compact it, controlling the compaction degree to 97%; the compaction process is completed before the mixture cools down to below 70℃.

[0030] (6) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0031] Example 3: Preparation of Flame-Retardant Asphalt Mixture for Tunnel Pavement 1. Experimental Materials Coarse aggregate (10~20mm): 49kg Medium aggregate (5~10mm): 33kg Fine aggregate (0~5mm): 27kg Mineral powder (limestone powder, particle size <75μm): 3.5kg Flame retardant raw materials: 87 kg aluminum hydroxide, 185 kg xylene, 0.13 kg ferrocene methyl methacrylate (CAS: 31566-61-7), 0.7 kg tetraethyl vinyl-1,1-diphosphate, and 1.0 kg azobisisobutyronitrile. Road asphalt (90# natural petroleum asphalt): 5.7kg 2. Experimental Procedure (1) Preparation of flame retardant 87 kg of aluminum hydroxide, 185 kg of xylene, 0.13 kg of ferrocene methyl isobutylene ester, and 0.7 kg of vinyl-1,1-diphosphate tetraethyl ester were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 370 rpm for 80 minutes to form a stable suspension. The temperature was raised to 77°C, and 1.0 kg of azobisisobutyronitrile was slowly added. The stirring speed was adjusted to 570 rpm, and the reaction was maintained at this temperature for 4.5 hours. The solid and liquid were separated by vacuum filtration, and the filter cake was collected (vacuum degree -0.088 MPa). Finally, the filter cake was placed in a vacuum drying oven at 87°C and dried for 5.5 hours (vacuum degree -0.078 MPa) to obtain 0.4 kg of modified flame retardant.

[0032] (2) Raw material pretreatment 49 kg of coarse aggregate, 33 kg of medium aggregate, 27 kg of fine aggregate, 3.5 kg of mineral powder and 0.4 kg of the prepared flame retardant were placed in an oven at 105℃±5℃ and dried for 1.7 hours to ensure that the moisture content was ≤0.5%.

[0033] (3) Dry mixing (step 1) The dried solid raw materials are put into the mixing equipment and mixed evenly to obtain the dry mixture.

[0034] (4) Asphalt heating and mixing (step 2) Heat 5.7 kg of 90# road asphalt to 165°C to make it fluid; add dry mix to the asphalt and control the mixing temperature at 160°C; use a high-shear mixer, initially premix at 490 r / min for 1.5 minutes, then continue shearing at 2700 r / min for 1.4 hours, and then mix at 480 r / min for 5.5 minutes, for a total mixing time of 3.5 minutes.

[0035] (5) Spreading and compaction (step 3) Spread the mixed material and turn on the vibrating plate (frequency 65Hz) to vibrate and compact it, controlling the compaction degree to 98%; the compaction process is completed before the mixture cools down to below 70℃.

[0036] (6) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0037] Example 4: Preparation of Flame-Retardant Asphalt Mixture for Tunnel Pavement 1. Experimental Materials Coarse aggregate (10~20mm): 50kg Medium aggregate (5-10mm): 35kg Fine aggregate (0~5mm): 30kg Mineral powder (granite powder, particle size <75μm): 4.0kg Flame retardant raw materials: 90 kg aluminum hydroxide, 200 kg xylene, 0.15 kg ferrocene methyl methacrylate (CAS: 31566-61-7), 0.8 kg tetraethyl vinyl-1,1-diphosphate, and 1.2 kg azobisisobutyronitrile. Road asphalt (90# natural petroleum asphalt): 6.0 kg 2. Experimental Procedure (1) Preparation of flame retardant 90 kg of aluminum hydroxide, 200 kg of xylene, 0.15 kg of ferrocene methyl isobutylene ester, and 0.8 kg of vinyl-1,1-diphosphate tetraethyl ester were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 400 rpm for 100 minutes to form a stable suspension. The temperature was raised to 80 °C, and 1.2 kg of azobisisobutyronitrile was slowly added. The stirring speed was adjusted to 600 rpm, and the reaction was maintained at this temperature for 5 hours. The solid and liquid were separated by vacuum filtration, and the filter cake was collected (vacuum degree -0.09 MPa). Finally, the filter cake was placed in a 90 °C vacuum drying oven and dried for 6 hours (vacuum degree -0.08 MPa) to obtain 0.5 kg of modified flame retardant.

[0038] (2) Raw material pretreatment 50 kg of coarse aggregate, 35 kg of medium aggregate, 30 kg of fine aggregate, 4.0 kg of mineral powder and 0.5 kg of the prepared flame retardant were placed in an oven at 105℃±5℃ and dried for 2 hours to ensure that the moisture content was ≤0.5%.

[0039] (3) Dry mixing (step 1) All the dried solid raw materials are put into the mixing equipment and mixed evenly to obtain the dry mixture.

[0040] (4) Asphalt heating and mixing (step 2) Heat 6.0 kg of 90# road asphalt to 170℃ to make it fluid; add dry mix to the asphalt and control the mixing temperature at 165℃; use a high shear mixer, initially premix at 500 r / min for 2 minutes, then continue shearing at 3000 r / min for 1.5 hours, and then mix at 500 r / min for 6 minutes, for a total mixing time of 4 minutes.

[0041] (5) Spreading and compaction (step 3) Spread the mixed material and turn on the vibrating plate (frequency 70Hz) to vibrate and compact it, controlling the compaction degree to 99%; the compaction process is completed before the mixture cools down to below 70℃.

[0042] (6) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0043] Comparative Example 1: Preparation of Flame-Retardant Asphalt Mixture for Tunnel Pavement 1. Experimental Materials Coarse aggregate (10~20mm): 45kg Medium aggregate (5-10mm): 25kg Fine aggregate (0~5mm): 20kg Mineral powder (limestone powder, particle size <75μm): 2.0kg Flame retardant raw material: 0.1 kg aluminum hydroxide Road asphalt (70# natural petroleum asphalt): 5.0 kg 2. Experimental Procedure (1) Raw material pretreatment Place 45 kg of coarse aggregate, 25 kg of medium aggregate, 20 kg of fine aggregate, 2.0 kg of mineral powder and 0.1 kg of flame retardant aluminum hydroxide in an oven at 105℃±5℃ and dry for 1 hour to ensure that the moisture content is ≤0.5%.

[0044] (2) Dry mixing (step 1) The dried coarse aggregate, medium aggregate, fine aggregate, mineral powder, and flame retardant are put into the mixing equipment and dry-mixed evenly to obtain dry-mixed material.

[0045] (3) Asphalt heating and mixing (step 2) Heat 5.0 kg of 70# road asphalt to 150°C to make it fluid; add the dry mix to the heated asphalt and control the mixing temperature at 145°C; use a high-shear mixer, initially premix at 450 r / min for 1 minute, then continue shearing at 2000 r / min for 1 hour, and then mix at 400 r / min for 4 minutes, for a total mixing time of 2 minutes.

[0046] (4) Spreading and compaction (step 3) The mixed material is quickly spread out and vibrated to compact it, with the frequency of the vibrating plate (50Hz) controlled to a compaction degree of 96%. The compaction process is completed before the mixture cools down to below 70°C.

[0047] (5) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0048] Comparative Example 2: Preparation of Flame-Retardant Asphalt Mixture for Tunnel Pavement 1. Experimental Materials Coarse aggregate (10~20mm): 45kg Medium aggregate (5-10mm): 25kg Fine aggregate (0~5mm): 20kg Mineral powder (limestone powder, particle size <75μm): 2.0kg Flame retardant raw materials: 80 kg aluminum hydroxide, 150 kg xylene, 0.3 kg tetraethyl vinyl-1,1-diphosphate, 0.5 kg azobisisobutyronitrile. Road asphalt (70# natural petroleum asphalt): 5.0 kg 2. Experimental Procedure (1) Preparation of flame retardant 80 kg of aluminum hydroxide, 150 kg of xylene, and 0.3 kg of vinyl-1,1-diphosphate tetraethyl ester were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 300 rpm for 30 minutes to form a stable suspension. The temperature was raised to 70 °C, and 0.5 kg of azobisisobutyronitrile was slowly added. The stirring speed was adjusted to 500 rpm, and the reaction was maintained at this temperature for 3.5 hours. The solid and liquid were separated by vacuum filtration, and the filter cake was collected (vacuum degree -0.08 MPa). Finally, the filter cake was placed in an 80 °C vacuum drying oven and dried for 4 hours (vacuum degree -0.07 MPa) to obtain 0.1 kg of modified flame retardant.

[0049] (2) Raw material pretreatment 45 kg of coarse aggregate, 25 kg of medium aggregate, 20 kg of fine aggregate, 2.0 kg of mineral powder and 0.1 kg of the prepared flame retardant were placed in an oven at 105℃±5℃ and dried for 1 hour to ensure that the moisture content was ≤0.5%.

[0050] (3) Dry mixing (step 1) The dried coarse aggregate, medium aggregate, fine aggregate, mineral powder, and flame retardant are put into the mixing equipment and dry-mixed evenly to obtain dry-mixed material.

[0051] (4) Asphalt heating and mixing (step 2) Heat 5.0 kg of 70# road asphalt to 150°C to make it fluid; add the dry mix to the heated asphalt and control the mixing temperature at 145°C; use a high-shear mixer, initially premix at 450 r / min for 1 minute, then continue shearing at 2000 r / min for 1 hour, and then mix at 400 r / min for 4 minutes, for a total mixing time of 2 minutes.

[0052] (5) Spreading and compaction (step 3) The mixed material is quickly spread out and vibrated to compact it, with the frequency of the vibrating plate (50Hz) controlled to a compaction degree of 96%. The compaction process is completed before the mixture cools down to below 70°C.

[0053] (6) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0054] Comparative Example 3: Preparation of Flame-Retardant Asphalt Mixture for Tunnel Pavement 1. Experimental Materials Coarse aggregate (10~20mm): 45kg Medium aggregate (5-10mm): 25kg Fine aggregate (0~5mm): 20kg Mineral powder (limestone powder, particle size <75μm): 2.0kg Flame retardant raw materials: 80 kg aluminum hydroxide, 150 kg xylene, 0.08 kg ferrocene methyl methacrylate (CAS: 31566-61-7), 0.5 kg azobisisobutyronitrile (AIOBR). Road asphalt (70# natural petroleum asphalt): 5.0 kg 2. Experimental Procedure (1) Preparation of flame retardant 80 kg of aluminum hydroxide, 150 kg of xylene, and 0.08 kg of ferrocene methyl isobutylene ester were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 300 rpm for 30 minutes to form a stable suspension. The temperature was raised to 70 °C, and 0.5 kg of azobisisobutyronitrile was slowly added. The stirring speed was adjusted to 500 rpm, and the reaction was maintained at this temperature for 3.5 hours. The solid and liquid were separated by vacuum filtration, and the filter cake was collected (vacuum degree -0.08 MPa). Finally, the filter cake was placed in an 80 °C vacuum drying oven and dried for 4 hours (vacuum degree -0.07 MPa) to obtain 0.1 kg of modified flame retardant.

[0055] (2) Raw material pretreatment 45 kg of coarse aggregate, 25 kg of medium aggregate, 20 kg of fine aggregate, 2.0 kg of mineral powder and 0.1 kg of the prepared flame retardant were placed in an oven at 105℃±5℃ and dried for 1 hour to ensure that the moisture content was ≤0.5%.

[0056] (3) Dry mixing (step 1) The dried coarse aggregate, medium aggregate, fine aggregate, mineral powder, and flame retardant are put into the mixing equipment and dry-mixed evenly to obtain dry-mixed material.

[0057] (4) Asphalt heating and mixing (step 2) Heat 5.0 kg of 70# road asphalt to 150°C to make it fluid; add the dry mix to the heated asphalt and control the mixing temperature at 145°C; use a high-shear mixer, initially premix at 450 r / min for 1 minute, then continue shearing at 2000 r / min for 1 hour, and then mix at 400 r / min for 4 minutes, for a total mixing time of 2 minutes.

[0058] (5) Spreading and compaction (step 3) The mixed material is quickly spread out and vibrated to compact it, with the frequency of the vibrating plate (50Hz) controlled to a compaction degree of 96%. The compaction process is completed before the mixture cools down to below 70°C.

[0059] (6) Cooling and curing (step 4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for 24 hours to obtain flame-retardant asphalt mixture for tunnel pavement.

[0060] Test method: 1. High-temperature rutting test The ideal rutting test according to ASTM D8360-22 was conducted on a compacted cylindrical specimen prepared in the laboratory.

[0061] The specimen diameter is 150mm, the height is adjusted according to the specifications, and the test temperature is generally set to 60℃.

[0062] Record the depth of the specimen after a predetermined rutting load / cycle number to evaluate its resistance to rutting at high temperatures.

[0063] Table 1 Results of High Temperature Rutting Test Depth of ruts caused by high temperatures (mm) Example 1 4.2 Example 2 4.2 Example 3 4.1 Example 4 4.1 Comparative Example 1 4.3 Comparative Example 2 4.2 Comparative Example 3 4.2 2. Low-temperature crack resistance test The crack resistance was determined using the indirect tensile crack test of ASTM D8225-19 under medium and low temperature conditions.

[0064] The test temperature was set to -10℃, and a specimen with a diameter of 150mm and a thickness of 62mm was used.

[0065] The maximum crack strain or the strain / displacement at the crack initiation point is measured to evaluate the low-temperature crack resistance.

[0066] Table 2 Results of Low-Temperature Crack Resistance Test Maximum strain (με) in low-temperature cracks Example 1 2210 Example 2 2212 Example 3 2213 Example 4 2213 Comparative Example 1 2208 Comparative Example 2 2210 Comparative Example 3 2209 3. Water stability test Marshall stability or separation rate was determined by immersion-freeze-thaw cycles or immersion saturation.

[0067] After the specimen is saturated, it is cyclically subjected to specified conditions (24-hour immersion in water + several cycles of freeze-thaw), and then the stability retention rate or strength loss is measured.

[0068] Table 3 Results of water stability test Water stability retention rate (%) Example 1 82.2 Example 2 82.3 Example 3 82.3 Example 4 82.4 Comparative Example 1 82.0 Comparative Example 2 82.1 Comparative Example 3 82.1 4. Combustion / Smoke Release and Heat Release Test Combustion specimens were tested in the laboratory using a standard cone calorimeter. Test parameters included combustion initiation time, smoke generation time, peak heat release rate (PHRR), and total smoke release (TSR).

[0069] The specimens were taken from compacted blocks of the same formulation, and after curing, they were cut into suitable combustion test sizes.

[0070] Table 4 Results of Combustion / Smoke Emission and Heat Emission Tests Combustion initiation time (s) Smoke generation time (s) PHRR (kW / m²) TSR(m² / m²) Example 1 5.1 6.4 522 127 Example 2 5.4 6.1 518 124 Example 3 5.6 5.9 515 122 Example 4 5.9 5.6 511 119 Comparative Example 1 4.2 7.2 533 135 Comparative Example 2 4.6 6.9 529 130 Comparative Example 3 4.8 6.8 527 130 The test results from the above specific embodiments show that the road performance (rutting depth, crack strain, and water stability) of the flame-retardant asphalt mixture prepared by this method is almost unaffected, and even slightly better than the comparative example. Furthermore, with the increase of flame retardant dosage, the combustion initiation time is prolonged, the smoke generation time is shortened, and the PHRR and TSR values ​​decrease, indicating that the addition of flame retardant effectively improves combustion safety and smoke reduction performance.

[0071] The core of this flame-retardant asphalt mixture preparation technology for tunnel pavement revolves around "performance balance and engineering adaptability," and its operability is further enhanced by clarifying the raw material combination and polymerization coating process. At the material level, modified flame-retardant additives are prepared by polymerization coating with aluminum hydroxide as a base, combined with specific functional monomers. This solves the problems of poor compatibility and uneven dispersion between traditional flame retardants and asphalt mixtures, and also enhances the flame-retardant effect through the synergistic effect of each component, achieving a synergistic improvement in both flame-retardant function and road performance. At the process level, the traditional asphalt construction process is optimized based on the characteristics of the raw materials, clarifying the key points of operation at each stage, avoiding the impact of new technologies on the existing construction system, and lowering the threshold for project promotion. From an application perspective, this technology specifically addresses the fire safety hazards of pavement materials in enclosed spaces such as tunnels, while ensuring the long-term stability of the pavement. It provides a feasible solution with clear composition and controllable process for the safe upgrading of pavement materials for underground transportation infrastructure, combining technological innovation with engineering practicality.

[0072] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. Other modifications can be easily made by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and the illustrations shown and described herein.

Claims

1. A method for preparing flame-retardant asphalt mixture for tunnel pavement, characterized in that, Includes the following steps: (1) Mix the aggregate, mineral powder, and modified flame retardant additives by mass parts according to the following proportions: 45-50 parts coarse aggregate; 25-35 parts medium aggregate; 20-30 parts fine aggregate; 2.0-4.0 parts mineral powder; and 0.1-0.5 parts flame retardant. (2) Heat 5.0 to 6.0 parts of road asphalt to 150°C to 170°C, add dry mix and mix at 145°C to 165°C for 2 to 4 minutes; (3) Spread and compact the mixed material, turn on the vibrating plate to vibrate, and complete the compaction before the temperature cools down to below 70℃. (4) After the mixture is compacted, it is allowed to cool naturally to room temperature and cured for at least 24 hours to become flame-retardant asphalt mixture for tunnel pavement. The modified flame retardant additive is prepared by reacting aluminum hydroxide, ferrocene methanol isobutylene ester, vinyl-1,1-diphosphate tetraethyl ester, and azobisisobutyronitrile.

2. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The coarse aggregate particles are 10-20 mm; the medium aggregate particles are 5-10 mm; and the fine aggregate particles are 0-5 mm.

3. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The mineral powder is limestone powder or granite powder with a particle size of less than 75 μm.

4. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The asphalt used for the road is conventional natural petroleum asphalt, grade 70 or 90.

5. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The aggregates, mineral powders, and flame retardants were all dried at 105℃±5℃ for 1 to 2 hours, with a moisture content ≤0.5%.

6. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The preparation method of the modified flame retardant is as follows: According to the mass percentages, add 80-90 parts of the basic flame retardant additive aluminum hydroxide; 150-200 parts of xylene; 0.08-0.15 parts of ferrocene methanol isobutylene ester; and 0.3-0.8 parts of vinyl-1,1-diphosphate tetraethyl ester to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. Stir at 300-400 r / min for 30-100 minutes to form a stable suspension system. Heat to 70-80℃, slowly add 0.5-1.2 parts of azobisisobutyronitrile, adjust the rotation speed to 500-600 r / min, and maintain this temperature for 3.5-5 hours. Separate the solid and liquid by vacuum filtration and collect the filter cake with a vacuum degree of -0.08 to -0.09 MPa. Finally, place the filter cake in a vacuum drying oven at 80-90℃ and dry for 4-6 hours with a vacuum degree of -0.07 to -0.08 MPa to obtain the modified flame retardant additive.

7. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: In step (2), a high-shear mixer is used during the mixing process. Initially, the mixer is premixed at a speed of 450-500 r / min for 1-2 minutes, then sheared at 2000-3000 r / min for 1-1.5 hours, and then mixed at 400-500 r / min for 4-6 minutes.

8. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The frequency of the vibrating plate during the compaction process is 50Hz to 70Hz.

9. The method for preparing a flame-retardant asphalt mixture for tunnel pavement according to claim 1, characterized in that: The compaction degree is 96-99%.