A method for dividing the RAP aggregate interface transition zone in recycled asphalt mixtures

Abstract

This invention belongs to the field of recycled asphalt mixture technology, specifically disclosing a method for delineating the RAP aggregate interface transition zone in recycled asphalt mixtures. The method includes the following steps: constructing a dual-condition constraint RAP aggregate identification method; forming the final tracer asphalt; preparing Marshall specimens of recycled asphalt mixtures with different RAP contents and obtaining SEM-EDS observation results; subdividing the boundary region of the RAP aggregate into the RAP aggregate zone, aged asphalt zone, asphalt fusion zone, and asphalt mortar zone; and realizing the delineation of the RAP aggregate interface transition zone. This invention employs the aforementioned method for delineating the RAP aggregate interface transition zone in recycled asphalt mixtures, based on marker element tracing and electron microscopy for microscopic observation, to determine the delineation method of the interface transition zone near the RAP aggregate in recycled asphalt mixtures, and to determine the lengths of different regions near the RAP aggregate boundary, thereby achieving the subdivision of the interface micro-regions in recycled asphalt mixtures.

Description

Technical Field

[0001] This invention belongs to the field of recycled asphalt mixture technology, specifically relating to a method for dividing the transition zone of RAP aggregate interface in recycled asphalt mixture. Background Technology

[0002] In recent years, recycled pavement has been widely used in road construction and maintenance due to its significant economic and environmental benefits. By incorporating recycled asphalt pavement (RAP), the demand for new aggregates and asphalt can be effectively reduced, and the environmental impact of waste materials can also be minimized. However, while high-volume RAP applications enhance economic and environmental benefits, they also pose challenges to the durability and crack resistance of recycled asphalt mixtures. The microstructure of asphalt mixtures has a significant impact on their macroscopic properties, and the heterogeneity of RAP makes the microstructure of recycled mixtures more complex.

[0003] However, current research largely focuses on the macroscopic properties of recycled asphalt mixtures, with limited attention paid to the interfacial characteristics of RAP aggregates. Therefore, determining a method for defining the interfacial transition zone in recycled asphalt mixtures is crucial for improving the performance of high-RAP-content recycled mixtures. While current research includes methods for defining the interfacial transition zone (ITZ) in ordinary asphalt mixtures, the microscopic interfacial structure of RAP aggregates in recycled asphalt mixtures remains unexplored.

[0004] Therefore, there is a need in this field to develop a method for dividing the RAP aggregate interface transition zone in recycled asphalt mixtures, which can effectively solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide a method for dividing the transition zone of RAP aggregate interface in recycled asphalt mixtures. Based on the development of marker element tracing and electron microscopy, the method for dividing the interface transition zone near RAP aggregate in recycled asphalt mixtures is determined, and the length of different regions near the boundary of RAP aggregate is determined, thereby realizing the subdivision of interface micro-regions in recycled asphalt mixtures.

[0006] To achieve the above objectives, the present invention provides a method for dividing the transition zone of RAP aggregate interface in recycled asphalt mixtures, comprising the following steps:

[0007] Step S1: Select new limestone aggregate and diabase RAP aggregate, and construct a dual-condition constraint RAP aggregate identification method based on the differences in the color and characteristic elements contained in the aggregates.

[0008] Step S2: Select nano-sized titanium dioxide with a particle size of less than 30 nm as a tracer, and form the final tracer pitch through a three-stage preparation process;

[0009] Step S3: Prepare Marshall specimens of recycled asphalt mixtures with different RAP content, and prepare scanning electron microscope (SEM) section specimens after freeze cutting to obtain SEM-EDS observation results;

[0010] Step S4: Based on Step S1, construct a dual-condition constraint RAP aggregate identification method to locate the boundary region of RAP aggregate; based on backscattering and EDS element surface / line scanning mode, subdivide the boundary region of RAP aggregate into RAP aggregate area, aged asphalt area, asphalt fusion area and asphalt mortar area; among them, the aged asphalt area and asphalt fusion area constitute the RAP aggregate interface transition zone, and the boundary of the area is divided by the change of characteristic element concentration to realize the division of the RAP aggregate interface transition zone.

[0011] Preferably, step S1 specifically includes:

[0012] Step S11: Select new and old aggregates of different lithological types based on differences in color and aggregate element composition, that is, divide them into new aggregates and RAP aggregates;

[0013] Among them, the new aggregate is limestone, the cross-section of which is grayish-white, and its characteristic element is Ca; the RAP aggregate is diabase aggregate, the cross-section of which is grayish-green, and its characteristic element is Si.

[0014] Step S12: Based on the differences in color and aggregate elements, construct a dual-condition constraint RAP aggregate identification method for aggregate type identification and positioning;

[0015] a. By using the color difference between limestone and diabase, the vicinity of the RAP aggregate to be scanned can be initially located;

[0016] b. Switch to EDS elemental surface scanning mode to perform surface scanning at a scale of 1000x and 50μm, identify characteristic elements Si and Ca, distinguish RAP aggregates from new aggregates, and locate the boundary of RAP aggregates.

[0017] Preferably, in step S2, the three-stage preparation process specifically includes:

[0018] Step S21: Heat the SBS asphalt to a fluid state at 165°C and maintain a constant temperature.

[0019] Nano-sized titanium dioxide was selected as the tracer, wherein the particle size of titanium dioxide is less than 30nm; the tracer was added to SBS asphalt that was kept at a constant temperature; the mixture was manually stirred with a glass rod for 2-3 minutes until the nano-sized titanium dioxide was initially dispersed and submerged below the liquid surface, thus obtaining the initially mixed tracer asphalt;

[0020] Step S22: The pre-mixed tracer bitumen is sheared using a shearing machine to disperse the titanium dioxide evenly, thus obtaining a uniformly dispersed tracer bitumen.

[0021] Step S23: Put the evenly dispersed tracer bitumen into a mixer and stir it to eliminate air bubbles, thus obtaining the final tracer bitumen; set the final tracer bitumen aside for later use.

[0022] Preferably, in step S21, the mass ratio of the tracer to the SBS asphalt kept at a constant temperature is 1:4 to 1:6.

[0023] Preferably, in step S22, the rotation speed of the shearing machine is 4950-5050 rad / min, and the shearing time is 25-30 min;

[0024] In step S23, the speed of the mixer is 1490 rad / min-1510 rad / min, and the mixing time is 10 min-15 min.

[0025] Preferably, step S3 specifically includes:

[0026] Step S31: Determine the tracer bitumen content for each RAP dosage;

[0027] a. Prepare standard Marshall specimens according to the Marshall design method;

[0028] b. The mixture is a dense-graded asphalt mixture, with RAP content as the control. The mixing temperature and compaction temperature of the mixture are 160℃-165℃ and 155℃-160℃, respectively.

[0029] c. Standard Marshall specimens were prepared with different asphalt contents, ranging from 3.5% to 5.5% in the mixture, with intervals of 0.5%.

[0030] d. Finally, the tracer bitumen content at each RAP dosage is determined based on the volume index, Marshall stability, and flow value of the mixture.

[0031] Step S32: The tracer bitumen content for each RAP dosage is determined and formed into cylindrical samples at 155°C using a Superpave rotary compactor.

[0032] Step S33: After freezing the cylindrical sample in a refrigerator for 24 hours, cut the cylindrical sample into cuboid blocks using a cutting machine.

[0033] Step S34: Spray gold onto the surface of the cut cuboid sample block, and set it aside for later use to obtain the scanning electron microscope section sample.

[0034] Step S35: Perform SEM-EDS observation on the obtained scanning electron microscope section sample to obtain the SEM-EDS observation results.

[0035] Preferably, in step S32, the diameter of the cylindrical sample is The porosity of the cylindrical samples is 3%-5%; several cylindrical samples are formed for different RAP dosages.

[0036] In step S33, the length and width of the cuboid test block are both 15mm, and the thickness is 5mm.

[0037] Preferably, step S4 specifically includes:

[0038] Step S41: Initially locate RAP aggregates by color;

[0039] The cross-section of the scanning electron microscope sample was observed in backscatter mode. By the color difference between limestone and diabase, the vicinity of RAP aggregate was initially located. Further observation was conducted at the RAP-mortar interface between RAP aggregate and new aggregate.

[0040] Step S42: Switch to EDS elemental surface scanning mode to perform surface scanning at a scale of 1000x and 50μm to identify characteristic elements Si and Ca, thereby locating diabase RAP aggregate and limestone new aggregate, selecting the RAP-mortar interface between diabase RAP aggregate and limestone new aggregate, and locating the boundary of RAP aggregate.

[0041] Step S43: Switch to EDS line scan mode, select a test path with a length of 100μm along the vertical boundary direction from the RAP aggregate area to perform EDS line scan, and export the element count rate CPS curve data of all measuring points of this EDS line scan, where there are 300 measuring points; thereby obtaining the element change trend on the entire test path.

[0042] Step S44: Based on the element distribution characteristics between different phases of asphalt mixture, the RAP aggregate interface transition zone is divided.

[0043] Preferably, step S43 specifically involves: using equations (1) and (2) to statistically analyze the distribution of S and Ti elements in the RAP aggregate zone.

[0044] (1);

[0045] (2);

[0046] In equations (1) and (2), The normalized mean of a certain element; The normalized value of a sample of a certain element; The number of elements in the sample; is the normalized standard deviation of the element.

[0047] Preferably, step S44 specifically includes:

[0048] a. The RAP aggregate interface transition zone in recycled asphalt mixture is defined as the small area at the RAP-mortar interface between RAP aggregate and new aggregate, that is, the transition zone from the aged asphalt area at the edge of the RAP aggregate area to the mixed asphalt in the asphalt mortar area.

[0049] In traditional asphalt mixtures, the aggregate interface transition zone (ITZ) refers to the micro-region between the aggregate interface and the mortar interface. In recycled asphalt mixtures, the region between RAP aggregates and new aggregate mortar can be further subdivided into the RAP aggregate zone, aged asphalt zone, asphalt fusion zone, and asphalt mortar zone. The aged asphalt zone and the asphalt fusion zone together constitute the RAP aggregate interface transition zone. This division can be used to better describe the gradual transition process from the aged asphalt at the edge of the RAP aggregates to the fused asphalt in the mortar, which helps to understand the microstructure and elemental characteristics of recycled asphalt mixtures.

[0050] b. C, O, and Si are used to characterize the RAP aggregate zone, S and Ti are used to characterize the aged asphalt zone and the asphalt fusion zone, and C, O, Si, Ca, S, and Ti are used to characterize the asphalt mortar zone.

[0051] c. Delineation of boundaries for each region:

[0052] The boundary between the RAP aggregate zone and the aged asphalt zone is defined as follows: the S element content is equal to the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, and the Ti element concentration is lower than the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, i.e., there is S element but no Ti element.

[0053] The boundary between the aged asphalt zone and the asphalt fusion zone is the position where the Ti element content is equal to the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, i.e., there is both S and Ti elements present.

[0054] The boundary between the asphalt fusion zone and the asphalt mortar zone is the position where the Si concentration in the RAP aggregate decreases from the average value in the RAP aggregate zone to the average value in the asphalt mortar zone, i.e., it leaves the boundary controlled by the RAP aggregate.

[0055] The width of the interface transition zone is defined as the range between the boundaries of each region; specifically, the width of the interface transition zone is the distance between the boundary between the RAP aggregate zone and the aged asphalt zone and the boundary between the asphalt fusion zone and the asphalt mortar zone.

[0056] Therefore, the present invention adopts the above-mentioned method for dividing the RAP aggregate interface transition zone in recycled asphalt mixture. Based on the tracer of marker elements and the microscopic observation of electron microscopy, the method for dividing the interface transition zone near RAP-aggregate in recycled asphalt mixture is determined, and the length of different regions near the RAP aggregate boundary is determined, thereby realizing the subdivision of interface micro-regions in recycled asphalt mixture.

[0057] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0058] Figure 1 This is a diagram showing the result of normalizing the count rate values ​​of each element in an embodiment of the method for dividing the RAP aggregate interface transition zone in a recycled asphalt mixture according to the present invention.

[0059] Figure 2 This is a schematic diagram of the interface transition zone division of ordinary asphalt mixture in a comparative example of the RAP aggregate interface transition zone division method in the recycled asphalt mixture of the present invention. Detailed Implementation

[0060] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0061] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0062] Example

[0063] Step S1: Select new limestone aggregate and diabase RAP aggregate. Based on the differences in the color and characteristic elements contained in the aggregate, construct a dual-condition constraint RAP aggregate identification method.

[0064] Step S11: Select new and old aggregates of different lithological types based on the differences in color and aggregate element composition, that is, divide them into new aggregates and RAP aggregates.

[0065] The new aggregate is limestone, with a grayish-white cross-section and its characteristic element being Ca. The RAP aggregate is diabase aggregate, with a grayish-green cross-section and its characteristic element being Si.

[0066] Step S12: Based on the differences in color and aggregate elements, construct a dual-condition constraint RAP aggregate identification method for aggregate type identification and positioning.

[0067] a. By using the color difference between limestone and diabase, the vicinity of the RAP aggregate to be scanned can be initially located.

[0068] b. Switch to EDS elemental surface scanning mode to perform surface scanning at a scale of 1000x and 50μm, identify characteristic elements Si and Ca, distinguish RAP aggregates from new aggregates, and locate the boundary of RAP aggregates.

[0069] Step S2: Select nano-sized titanium dioxide with a particle size of less than 30nm as a tracer, and form the final tracer pitch through a three-stage preparation process.

[0070] Step S21: Heat the SBS asphalt to a fluid state at 165°C and maintain a constant temperature.

[0071] Nano-sized titanium dioxide was selected as the tracer, and the mass ratio of the tracer to SBS asphalt kept at a constant temperature was 1:5. The particle size of the titanium dioxide was less than 30 nm. The tracer was added to the SBS asphalt kept at a constant temperature and stirred manually with a glass rod for 2-3 minutes until the nano-sized titanium dioxide was initially dispersed and submerged below the liquid surface, thus obtaining the initially mixed tracer asphalt.

[0072] Step S22: The pre-mixed tracer bitumen is sheared using a shearing machine to disperse titanium dioxide evenly. The rotation speed of the shearing machine is 5000 rad / min, and the shearing time is 25-30 min, thus obtaining uniformly dispersed tracer bitumen.

[0073] Step S23: Place the evenly dispersed tracer bitumen into a mixer and mix it at a speed of 1500 rad / min for 10-15 minutes. The mixing process eliminates air bubbles, yielding the final tracer bitumen. Set the final tracer bitumen aside for later use.

[0074] Step S3: Prepare Marshall specimens of recycled asphalt mixtures with different RAP content, and prepare scanning electron microscope (SEM) section specimens after cryogenic cutting to obtain SEM-EDS observation results.

[0075] Step S31: Determine the tracer bitumen content for each RAP dosage.

[0076] a. Prepare standard Marshall specimens according to the Marshall design method.

[0077] b. The mixture is a dense-graded asphalt mixture, with RAP content as the control factor. The mixing temperature and compaction temperature of the mixture are 160℃-165℃ and 155℃-160℃, respectively.

[0078] c. Standard Marshall specimens were prepared with different asphalt contents, ranging from 3.5% to 5.5% in the mixture, with intervals of 0.5%.

[0079] d. Finally, the tracer bitumen content at each RAP dosage is determined based on the volume index, Marshall stability, and flow value of the mixture.

[0080] Step S32: The tracer bitumen content at each RAP dosage is determined and molded into cylindrical specimens using a Superpave rotary compactor at 155°C. The diameter of the cylindrical specimen is... The porosity of the cylindrical samples was 3%-5%. Several cylindrical samples were formed for different RAP dosages.

[0081] Step S33: After freezing the cylindrical sample at 10°C for 24 hours, cut the cylindrical sample into pieces using a cutting machine. Rectangular test block.

[0082] Step S34: Spray gold onto the surface of the cut cuboid sample under a high vacuum environment. After the gold spraying, set it aside for later use to obtain the scanning electron microscope section sample.

[0083] Step S35: Perform SEM-EDS observation on the obtained scanning electron microscope section sample to obtain the SEM-EDS observation results.

[0084] Step S4: Based on Step S1, a dual-condition constraint RAP aggregate identification method is constructed to locate the boundary region of the RAP aggregate. Based on backscattering and EDS elemental surface / line scanning modes, the boundary region of the RAP aggregate is subdivided into the RAP aggregate zone, the aged asphalt zone, the asphalt fusion zone, and the asphalt mortar zone. The aged asphalt zone and the asphalt fusion zone constitute the RAP aggregate interface transition zone, and the boundary of the zone is defined by the change in characteristic element concentration, thus realizing the division of the RAP aggregate interface transition zone.

[0085] Step S41: Initially locate RAP aggregates by color.

[0086] The cross-section of the sample was observed in backscatter mode using a scanning electron microscope. The color difference between limestone and diabase was used to initially locate the area near the RAP aggregate. Further observation was conducted at the RAP-mortar interface between the RAP aggregate and the new aggregate.

[0087] Step S42: Switch to EDS elemental surface scanning mode to perform surface scanning at a scale of 1000x and 50μm to identify characteristic elements Si and Ca, thereby locating the diabase RAP aggregate and the new limestone aggregate. Select the RAP-mortar interface between the diabase RAP aggregate and the new limestone aggregate to locate the boundary of the RAP aggregate.

[0088] Step S43, as Figure 1As shown, switching to EDS line scan mode, a 100μm long test path is selected along the vertical boundary direction from the RAP aggregate area for EDS line scan. The element count rate CPS curve data of all measuring points in this EDS line scan section are exported, with 300 measuring points. This allows the element variation trend along the entire test path to be obtained.

[0089] The distribution of S and Ti elements in the RAP aggregate zone was statistically analyzed using equations (1) and (2).

[0090] (1);

[0091] (2);

[0092] In equations (1) and (2), The normalized mean of a certain element; The normalized value of a sample of a certain element; The number of elements in the sample; is the normalized standard deviation of the element.

[0093] Step S44: Based on the element distribution characteristics between different phases of asphalt mixture, the RAP aggregate interface transition zone is divided.

[0094] a. The RAP aggregate interface transition zone in recycled asphalt mixture is defined as the area between RAP aggregate and new aggregate at the RAP-mortar interface, that is, the transition zone from the aged asphalt zone at the edge of the RAP aggregate zone to the mixed asphalt in the asphalt mortar zone.

[0095] b. C, O, and Si are used to characterize the RAP aggregate zone, S and Ti are used to characterize the aged asphalt zone and the asphalt fusion zone, and C, O, Si, Ca, S, and Ti are used to characterize the asphalt mortar zone.

[0096] c. Delineation of boundaries for each region:

[0097] The boundary between the RAP aggregate zone and the aged asphalt zone is defined as follows: the S element content is equal to the average concentration of the element in the RAP aggregate zone plus twice the standard deviation (95% confidence interval), and the Ti element concentration is below the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, i.e., there is S element but no Ti element.

[0098] The boundary between the aged asphalt zone and the asphalt fusion zone is the position where the Ti element content is equal to the average concentration of this element in the RAP aggregate zone plus twice the standard deviation, i.e., there is both S and Ti elements.

[0099] The boundary between the asphalt fusion zone and the asphalt mortar zone is the position where the Si concentration in the RAP aggregate decreases from the average value in the RAP aggregate zone to the average value in the asphalt mortar zone, i.e., it leaves the boundary controlled by the RAP aggregate.

[0100] The width of the interface transition zone is defined as the range between the boundaries of each zone. Specifically, the width of the interface transition zone is the distance between the boundary between the RAP aggregate zone and the aged asphalt zone and the boundary between the asphalt fusion zone and the asphalt mortar zone.

[0101] Comparative Example

[0102] The method for dividing the transition zone at the aggregate interface of ordinary asphalt mixtures includes the following steps:

[0103] S1. Sample preparation: Cut a Marshall specimen to form a 15mm×15mm×15mm cube to obtain a scanning electron microscope section sample that meets the requirements of SEM-EDS observation.

[0104] S2. Morphological observation: The interface area between aggregate and mortar was scanned using the secondary electronic imaging mode (SE mode) of FESEM. Based on the difference in grayscale in the image, the area of ​​structural density transition was identified, and it was determined that there is a grayscale transition zone between aggregate and mortar, which is the ITZ. The width range of ITZ was measured to be 5μm-20μm using a scale tool.

[0105] S3, EDS Elemental Scan: Switch to EDS line scan mode, set the scan path to vertically traverse the aggregate to the mortar area, with a typical length of 100μm and no less than 100 scan points, and record the X-ray count rate (CPS) curves of elements such as C, O, Si, and Ca as a function of position.

[0106] S4. ITZ Confirmation and Definition: Combining the grayscale transition image and the characteristics of the EDS element distribution curve, the region where the element content gradually transitions from aggregate to mortar is identified, namely the interface transition zone (ITZ). Further, by combining the locations of abrupt changes in the concentrations of elements such as Ca and Si, the boundaries of the ITZ are determined, which corroborates its width and distribution pattern.

[0107] like Figure 2 As shown, compared with the comparative examples, the embodiments of this invention are the first to address the complex structure of the RAP-aggregate interface in recycled asphalt mixtures. They innovatively combine titanium dioxide tracer with aggregate tracer to construct a fusion zone identification system that combines S and Ti element dual identifiers. Furthermore, a quantitative boundary determination method based on the statistical law of element content is proposed, refining the interface structure from the traditional three-zone to four-zone (RAP aggregate zone, aged asphalt zone, fusion zone, and mortar zone). This significantly improves the identification accuracy and structural analysis capability, breaking through the technical bottlenecks of existing ITZ identification methods that cannot be applied to recycled multiphase interfaces and lack quantitative standards, and has good promotional value.

[0108] Therefore, the present invention adopts the above-mentioned method for dividing the RAP-aggregate interface transition zone in recycled asphalt mixtures. Based on the tracer of marker elements and the microscopic observation of electron microscopy, the method for dividing the interface transition zone near the RAP-aggregate in recycled asphalt mixtures is determined, and the length of different regions near the RAP-aggregate boundary is determined, thereby realizing the subdivision of interface micro-regions in recycled asphalt mixtures.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for dividing the transition zone at the RAP aggregate interface in recycled asphalt mixtures, characterized in that, Includes the following steps: Step S1: Select new limestone aggregate and diabase RAP aggregate, and construct a dual-condition constraint RAP aggregate identification method based on the differences in the color and characteristic elements contained in the aggregates. Step S2: Select nano-sized titanium dioxide with a particle size of less than 30 nm as a tracer, and form the final tracer pitch through a three-stage preparation process; The three-stage preparation process is as follows: Step S21: Heat the SBS asphalt to a fluid state at 165°C and maintain a constant temperature. Nano-sized titanium dioxide was selected as the tracer, wherein the particle size of titanium dioxide is less than 30nm; the tracer was added to SBS asphalt that was kept at a constant temperature; the mixture was manually stirred with a glass rod for 2-3 minutes until the nano-sized titanium dioxide was initially dispersed and submerged below the liquid surface, thus obtaining the initially mixed tracer asphalt; The mass ratio of tracer to SBS bitumen kept at a constant temperature is 1:4-1:6; Step S22: The pre-mixed tracer bitumen is sheared using a shearing machine to disperse the titanium dioxide evenly, thus obtaining a uniformly dispersed tracer bitumen. The rotation speed of the shearing machine is 4950 rad / min-5050 rad / min, and the shearing time is 25 min-30 min; Step S23: Put the evenly dispersed tracer bitumen into a mixer and stir it to eliminate air bubbles, thus obtaining the final tracer bitumen; set the final tracer bitumen aside for later use. The mixer speed is 1490 rad / min-1510 rad / min, and the mixing time is 10 min-15 min; Step S3: Prepare Marshall specimens of recycled asphalt mixtures with different RAP content, and prepare scanning electron microscope (SEM) section specimens after freeze cutting to obtain SEM-EDS observation results; Step S4: Based on Step S1, construct a dual-condition constraint RAP aggregate identification method to locate the boundary region of RAP aggregate; based on backscattering and EDS element surface / line scanning mode, subdivide the boundary region of RAP aggregate into RAP aggregate area, aged asphalt area, asphalt fusion area and asphalt mortar area; among them, the aged asphalt area and asphalt fusion area constitute the RAP aggregate interface transition zone, and the boundary of the area is divided by the change of characteristic element concentration to realize the division of the RAP aggregate interface transition zone. Step S41: Initially locate RAP aggregates by color; The cross-section of the scanning electron microscope sample was observed in backscatter mode. By the color difference between limestone and diabase, the vicinity of RAP aggregate was initially located. Further observation was conducted at the RAP-mortar interface between RAP aggregate and new aggregate. Step S42: Switch to EDS elemental surface scanning mode to perform surface scanning at a scale of 1000x and 50μm to identify characteristic elements Si and Ca, thereby locating diabase RAP aggregate and limestone new aggregate, selecting the RAP-mortar interface between diabase RAP aggregate and limestone new aggregate, and locating the boundary of RAP aggregate. Step S43: Switch to EDS line scan mode, select a test path with a length of 100μm along the vertical boundary direction from the RAP aggregate area to perform EDS line scan, and export the element count rate CPS curve data of all measuring points of this EDS line scan, where there are 300 measuring points; thereby obtaining the element change trend on the entire test path. The distribution of S and Ti elements in the RAP aggregate zone was statistically analyzed using equations (1) and (2). （1）； （2）； In equations (1) and (2), The normalized mean of a certain element; The normalized value of a sample of a certain element; The number of elements in the sample; The normalized standard deviation of the elements; Step S44: Based on the element distribution characteristics between different phases of asphalt mixture, the RAP aggregate interface transition zone is divided. a. The RAP aggregate interface transition zone in recycled asphalt mixture is defined as the area between RAP aggregate and new aggregate at the RAP-mortar interface, that is, the transition zone from the aged asphalt zone at the edge of the RAP aggregate zone to the mixed asphalt in the asphalt mortar zone. b. C, O, and Si are used to characterize the RAP aggregate zone, S and Ti are used to characterize the aged asphalt zone and the asphalt fusion zone, and C, O, Si, Ca, S, and Ti are used to characterize the asphalt mortar zone. c. Delineation of boundaries for each region: The boundary between the RAP aggregate zone and the aged asphalt zone is defined as follows: the S element content is equal to the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, and the Ti element concentration is lower than the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, i.e., there is S element but no Ti element. The boundary between the aged asphalt zone and the asphalt fusion zone is the position where the Ti element content is equal to the average concentration of the element in the RAP aggregate zone plus twice the standard deviation, i.e., there is both S and Ti elements present. The boundary between the asphalt fusion zone and the asphalt mortar zone is the position where the Si concentration in the RAP aggregate decreases from the average value in the RAP aggregate zone to the average value in the asphalt mortar zone, i.e., it leaves the boundary controlled by the RAP aggregate. The width of the interface transition zone is defined as the range between the boundaries of each region; specifically, the width of the interface transition zone is the distance between the boundary between the RAP aggregate zone and the aged asphalt zone and the boundary between the asphalt fusion zone and the asphalt mortar zone.

2. The method for dividing the RAP aggregate interface transition zone in recycled asphalt mixture according to claim 1, characterized in that, Step S1 is as follows: Step S11: Select new and old aggregates of different lithological types based on differences in color and aggregate element composition, that is, divide them into new aggregates and RAP aggregates; Among them, the new aggregate is limestone, the cross-section of which is grayish-white, and its characteristic element is Ca; the RAP aggregate is diabase aggregate, the cross-section of which is grayish-green, and its characteristic element is Si. Step S12: Based on the differences in color and aggregate elements, construct a dual-condition constraint RAP aggregate identification method for aggregate type identification and positioning; a. By using the color difference between limestone and diabase, the vicinity of the RAP aggregate to be scanned can be initially located; b. Switch to EDS elemental surface scanning mode to perform surface scanning at a scale of 1000x and 50μm, identify characteristic elements Si and Ca, distinguish RAP aggregates from new aggregates, and locate the boundary of RAP aggregates.

3. The method for dividing the RAP aggregate interface transition zone in recycled asphalt mixture according to claim 1, characterized in that, Step S3 is as follows: Step S31: Determine the tracer bitumen content for each RAP dosage; a. Prepare standard Marshall specimens according to the Marshall design method; b. The mixture is a dense-graded asphalt mixture, with RAP content as the control. The mixing temperature and compaction temperature of the mixture are 160℃-165℃ and 155℃-160℃, respectively. c. Standard Marshall specimens were prepared with different asphalt contents, ranging from 3.5% to 5.5% in the mixture, with intervals of 0.5%. d. Finally, the tracer bitumen content at each RAP dosage is determined based on the volume index, Marshall stability, and flow value of the mixture. Step S32: The tracer bitumen content for each RAP dosage is determined and formed into cylindrical samples at 155°C using a Superpave rotary compactor. Step S33: After freezing the cylindrical sample in a refrigerator for 24 hours, cut the cylindrical sample into cuboid blocks using a cutting machine. Step S34: Spray gold onto the surface of the cut cuboid sample block, and set it aside for later use to obtain the scanning electron microscope section sample. Step S35: Perform SEM-EDS observation on the obtained scanning electron microscope section sample to obtain the SEM-EDS observation results.

4. The method for dividing the RAP aggregate interface transition zone in recycled asphalt mixture according to claim 3, characterized in that: In step S32, the diameter of the cylindrical sample is The porosity of the cylindrical samples is 3%-5%; several cylindrical samples are formed for different RAP dosages. In step S33, the length and width of the cuboid test block are both 15mm, and the thickness is 5mm.

Images