A method and system for fatigue damage modeling of a cold in-place recycled asphalt mixture

Abstract

The application discloses a kind of fatigue damage modeling method and system of road cold recycling asphalt mixture, belong to highway asphalt mixture test technical field, construct the discrete element numerical simulation model of cold recycling asphalt mixture;Obtain the loading force-loading displacement curve of asphalt mortar test piece in different stress directions under different loading rates through test, determine the contact stiffness value of asphalt mortar test piece in different stress directions;Through fatigue loading test of different strain levels, the contact stiffness attenuation rate corresponding to different strain levels of asphalt mortar test piece is fitted, and the fatigue damage rate of asphalt mortar test piece is obtained;According to fatigue damage rate and contact stiffness value, combined with discrete element numerical simulation model, construct the fatigue damage model that particle contact stiffness attenuates with fatigue loading number.The fatigue damage model constructed by the method can finely simulate and characterize the fatigue damage process of asphalt mortar test piece.

Description

Technical Field

[0001] This invention relates to the field of highway asphalt mixture testing technology, and more specifically to a fatigue damage modeling method and system for cold recycled asphalt mixtures for roads. Background Technology

[0002] Cold recycling is a technology that involves milling and grading reclaimed asphalt pavement (RAP) from old road surfaces, then mixing it at room temperature with emulsified asphalt, new aggregates, cement, mineral powder, water, recycling agents (if necessary), and some additives to obtain cold recycled asphalt mixtures that meet specifications. Cold recycled asphalt mixtures are typically used in the base or lower layers of roads, and their fatigue resistance is one of the important design indicators. From the material composition of cold recycled asphalt mixtures, cement, emulsified asphalt, and fine aggregates form new asphalt mortar, which acts as a binder within the mixture, providing bond strength. The RAP material consists of old aggregates and a surface layer of aged asphalt mortar. The old and new aggregates form the skeleton structure of the mixture. Therefore, cold recycled asphalt mixtures mainly consist of three parts: new asphalt mortar, aged asphalt mortar, and aggregates.

[0003] In the field of road engineering, discrete element numerical simulation technology has become an important tool for studying the mechanical properties and fatigue damage behavior of cold recycled asphalt mixtures. By establishing discrete element numerical simulation models, the interaction mechanisms between particles within the material can be analyzed in depth, thereby more accurately predicting the material's performance under actual service conditions and providing a scientific basis for road design, construction, and maintenance.

[0004] Currently, existing technologies for constructing fatigue damage models and determining relevant parameters in discrete element numerical simulations mainly employ fatigue damage models based on the decay of the bond radius. In the discrete element model, the bond between particles is usually assumed to be simulated by bond elements with a certain radius. As fatigue loading continues, the radius of the bond elements gradually decreases according to a preset rule. When the bond radius decreases to a certain extent, it is considered that the bond between particles has been broken, which leads to damage inside the material. As the damage accumulates, it eventually causes fatigue failure of the material.

[0005] Due to the uncertainty of the bond radius decay law, the fatigue damage models in the existing technology lack sufficient theoretical basis and experimental verification for the assumptions on the bond radius decay law. In actual cold recycled asphalt mixtures, the development of fatigue damage is an extremely complex process, which is affected by a variety of factors. Simply assuming that the bond radius decays according to a certain fixed law is difficult to accurately reflect the real evolution process of fatigue damage under different working conditions, thus leading to a large deviation between the model prediction results and the actual situation. Summary of the Invention

[0006] To address the problems existing in the above-mentioned fields, this invention proposes a fatigue damage modeling method and system for road cold recycled asphalt mixtures. The constructed fatigue damage model can more accurately reflect the fatigue damage evolution process and significantly improve the prediction accuracy of fatigue damage behavior of cold recycled asphalt mixtures.

[0007] To address the aforementioned technical problems, this invention discloses a fatigue damage modeling method for cold recycled asphalt mixtures for roads, comprising the following steps: Construct a discrete element numerical simulation model for cold recycled asphalt mixtures; Obtain asphalt mortar specimens; obtain the loading force-displacement curves of asphalt mortar specimens under different loading rates and in different stress directions through experiments, and determine the contact stiffness values ​​of asphalt mortar specimens in different stress directions based on the ratio of peak loading force to displacement in different stress directions. The relationship between the cumulative deformation of asphalt mortar specimens and the number of loading times was obtained through fatigue loading tests at different strain levels. The contact stiffness attenuation rate corresponding to different strain levels of asphalt mortar specimens was fitted to determine the fatigue damage rate of asphalt mortar specimens. Based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimen, the model parameters of the discrete element numerical simulation model are updated, and a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles is constructed. Based on the fatigue damage model and the contact stiffness value of the asphalt mortar specimen at the current time step, the contact stiffness value of the asphalt mortar specimen at the future step is predicted.

[0008] Preferably, the relationship between the cumulative deformation and the number of loading cycles of the asphalt mortar specimen is obtained through fatigue loading tests at different strain levels. The contact stiffness attenuation rate corresponding to different strain levels of the asphalt mortar specimen is fitted to determine the fatigue damage rate of the asphalt mortar specimen. Specifically, this includes: Fatigue loading tests at different strain levels were conducted on asphalt mortar specimens to determine the cumulative deformation of the mortar specimens. d With load count N Exponential relationships; The exponential relationship is: ; In the formula: d 0 represents the coefficient of the functional relationship; Using cumulative deformation d With load count N By fitting the exponential relationship, the contact stiffness attenuation rate corresponding to different strain levels of asphalt mortar specimens was obtained. γ .

[0009] Preferably, based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimen, the model parameters of the discrete element numerical simulation model are updated to construct a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles, specifically including: The cold recycled asphalt mixture is discretized into individual particle units using a discrete element numerical simulation model. The obtained contact stiffness value is assigned to the particle contact of the corresponding type or position in the discrete element numerical simulation model as the initial contact stiffness. At the same time, the fatigue damage rate is used as the rate index of contact stiffness decay in the discrete element numerical simulation model. The fatigue damage model constructed in which the particle contact stiffness decreases with the number of fatigue loading cycles is as follows: ; In the formula: k t+Δt for t +Δ t Contact stiffness of asphalt mortar specimens corresponding to the time step; k t To step at the current time t The contact stiffness of the corresponding asphalt mortar specimen.

[0010] Preferably, the discrete element numerical simulation model for constructing cold recycled asphalt mixtures further includes using a linear parallel bonding model for the asphalt mortar specimens, assigning model micro-parameters to adjacent particles, wherein the model micro-parameters include contact parameters and strength parameters, wherein: Contact parameters include normal stiffness, tangential stiffness, and stiffness ratio, which characterize the interparticle stiffness of asphalt mortar specimens and characterize the interparticle stress by the change in the deformation of asphalt mortar specimens. Strength parameters include normal tensile strength and shear strength, which characterize the interparticle strength that asphalt mortar specimens can withstand; When the real-time normal tensile strength or real-time tangential strength between particles exceeds the corresponding ultimate strength, the contact between adjacent particles in the asphalt mortar specimen fails, and the asphalt mortar specimen fractures and breaks.

[0011] Preferably, the contact parameters include normal stiffness, tangential stiffness, and stiffness ratio, which characterize the interparticle stiffness of the asphalt mortar specimen. The change in deformation of the asphalt mortar specimen characterizes the interparticle stress, specifically including: Multiple sets of compression, tensile and shear loading tests with different loading rates were carried out on asphalt mortar specimens to obtain the loading force-displacement curves of the asphalt mortar specimens; The ratio of the peak loading force to the displacement in different stress directions is used as the contact stiffness value of the asphalt mortar specimen in the corresponding direction. Among them, the ratio of the peak ultimate force to the displacement in the shear direction is used as the particle contact shear stiffness of the asphalt mortar specimen. The stiffness value in the compression direction of the particles is determined by the compression stiffness, that is, the stiffness value of the asphalt mortar specimen in the compressed state, and the stiffness value in the tensile direction is determined by the tensile stiffness, that is, the stiffness value of the asphalt mortar specimen in the tensile state. The ratio of tensile stiffness to shear stiffness is used as the stiffness ratio. The compressive and tensile states of asphalt mortar specimens are determined by the reference spacing Δδ between adjacent particles. When Δδ>0, the adjacent particles are in a tensile state; when Δδ<0, the adjacent particles are in a compressive state. By fitting the compressive stiffness, tensile stiffness, and shear stiffness of asphalt mortar specimens as functions of loading rate, the loading rates corresponding to the compressive stiffness, tensile stiffness, and shear stiffness under different working conditions are obtained, and then the forces corresponding to the compressive stiffness, tensile stiffness, and shear stiffness under different working conditions are determined.

[0012] Preferably, when the real-time normal tensile strength or real-time tangential strength between particles exceeds the corresponding ultimate strength, the contact between adjacent particles in the asphalt mortar specimen fails, and the asphalt mortar specimen fractures and fails. Specifically, this includes: Based on the loading force-displacement curves of the asphalt mortar specimens in the tensile, compressive and shear directions under different loading rates, the tensile ultimate strength and shear ultimate strength of the asphalt mortar specimens corresponding to different loading rates are determined by the peak loading force. Based on the discrete element numerical simulation model, the real-time normal tensile strength between particles of asphalt mortar specimens is determined by simulation; when the obtained real-time normal tensile strength between particles exceeds the tensile limit strength, the particle contact of the new asphalt mortar specimen fails. Alternatively, based on multiple shear tests with different loading rates, the Mohr-Coulomb criterion is used for fitting to determine the interparticle cohesion and internal friction angle parameters of the asphalt mortar specimen; the cohesion and internal friction angle parameters of the asphalt mortar specimen are input into the discrete element numerical simulation model, and the real-time interparticle tangential strength of the asphalt mortar specimen is determined through simulation; when the real-time interparticle tangential strength exceeds the shear limit strength, the particle contact of the asphalt mortar specimen fails.

[0013] Preferably, the determination of the interparticle cohesion and internal friction angle parameters of the asphalt mortar specimen based on multiple shear tests with different loading rates using the Mohr-Coulomb criterion includes: The Mohr-Coulomb criterion includes two parameters: cohesion and internal friction angle. Based on direct shear tests of asphalt mortar specimens with multiple loading rates, the interparticle cohesion and internal friction angle of the asphalt mortar specimens corresponding to each direct shear test are obtained. Based on the interparticle cohesion and internal friction angle of the asphalt mortar specimens corresponding to each group of direct shear tests, the interparticle cohesion of the asphalt mortar specimens was obtained using a parameter fitting method. c and internal friction angle φ ; Fitting interparticle cohesion c The relationship between the loading rate and the particle cohesion is shown to change significantly as the loading rate increases. The particle cohesion can be equated to the cohesion of the asphalt mortar specimen.

[0014] Preferably, obtaining the asphalt mortar specimen specifically includes: Asphalt mortar specimens include new asphalt mortar specimens and aged asphalt mortar specimens; Colored emulsions are used as binders to give different colors to new asphalt mortar, aged asphalt mortar and aggregates in cold recycled asphalt mixtures. By determining the mix proportion of cold recycled asphalt mixtures, the amount of added water and asphalt emulsion is obtained. The amount of new asphalt mortar is equivalent to the amount of asphalt emulsion; the new asphalt mortar adopts a linear parallel bond model and is formed into new asphalt mortar specimens by Marshall or rotary compaction methods. For aged asphalt mortar, firstly, RAP material with a particle size of less than 1.18mm is placed in a high-temperature oven for heating and heat preservation. After being taken out, the RAP material is weighed and subjected to Marshall compaction molding. Then, it is demolded and cut into block, cylindrical or other shaped specimens to form aged asphalt mortar specimens. The same method was used to obtain the corresponding contact stiffness values ​​and fatigue damage rates for both new and aged asphalt mortar specimens.

[0015] Preferably, it also includes a fatigue damage modeling system for cold recycled asphalt mixtures for roads, comprising: The simulation model building module is used to build discrete element numerical simulation models of cold recycled asphalt mixtures. The contact stiffness value determination module is used to obtain asphalt mortar specimens; through experiments, the loading force-displacement curves of asphalt mortar specimens under different loading rates and in different stress directions are obtained, and the contact stiffness value of asphalt mortar specimens in different stress directions is determined based on the ratio of peak loading force to displacement in different stress directions. The fatigue damage rate determination module is used to obtain the relationship between the cumulative deformation of asphalt mortar specimens and the number of loading times through fatigue loading tests at different strain levels, fit the contact stiffness decay rate of asphalt mortar specimens at different strain levels, and determine the fatigue damage rate of asphalt mortar specimens. The fatigue damage model construction module is used to update the model parameters of the discrete element numerical simulation model based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimen, and to construct a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention proposes a fatigue damage modeling method for cold recycled asphalt mixtures used in roads. Since cold recycled asphalt mixtures are subjected to forces from different directions in actual roads, the fatigue damage mechanism and evolution rate of cold recycled asphalt mixtures vary under different strain levels. Furthermore, the contact state and interaction between particles play a crucial role in the mechanical properties of the material. With increasing fatigue loading cycles, the bond between particles gradually breaks down, and the contact stiffness gradually decreases. The fatigue damage model constructed in this invention comprehensively considers the force direction, loading rate, and strain level. By acquiring relevant data from asphalt mortar specimens under different loading rates, force directions, and strain levels, the contact stiffness value and attenuation rate are determined, and then the parameters of the discrete element numerical simulation model are updated to construct the fatigue damage model. This invention incorporates the contact stiffness attenuation rate obtained through experimental fitting into the discrete element numerical simulation model, enabling the constructed fatigue damage model to simulate the dynamic changes in interparticle contact stiffness during fatigue loading. This allows for a more realistic simulation of the changes in interparticle interaction within the material under fatigue loading, accurately reflecting the actual evolution of fatigue damage under different working conditions. Consequently, it significantly improves the prediction accuracy of fatigue damage behavior in cold recycled asphalt mixtures, making the prediction results closer to reality and providing more reliable technical support for the design, construction, and maintenance of cold recycled asphalt mixtures in road engineering. Attached Figure Description

[0017] Figure 1 This is a flowchart of the fatigue damage modeling method for cold recycled asphalt mixtures for roads proposed in this invention; Figure 2 A graph showing the relationship between deformation and loading cycles of a new asphalt mortar specimen provided in an embodiment of the present invention; Figure 3 The graph shows the relationship between the contact stiffness attenuation rate and strain level of the new asphalt mortar specimen provided in the embodiments of the present invention. Detailed Implementation

[0018] The following will refer to the appendices in the embodiments of the present invention. Figure 1-Figure 3 The technical solutions in the embodiments of the present invention will be clearly and completely described. It should be understood that the terminology used in the present invention is only for describing particular implementation methods and is not intended to limit the present invention.

[0019] Example like Figure 1 The diagram illustrates a fatigue damage modeling method for cold recycled asphalt mixtures for roads, proposed in this invention, comprising the following steps: S1: Construct a discrete element numerical simulation model for cold recycled asphalt mixtures; S2: Obtain asphalt mortar specimens; obtain the loading force-displacement curves of asphalt mortar specimens under different loading rates and in different stress directions through experiments, and determine the contact stiffness values ​​of asphalt mortar specimens in different stress directions based on the ratio of peak loading force to displacement in different stress directions. S3: Obtain the relationship between the cumulative deformation of asphalt mortar specimens and the number of loading times through fatigue loading tests at different strain levels, fit the contact stiffness attenuation rate of asphalt mortar specimens at different strain levels, and determine the fatigue damage rate of asphalt mortar specimens. S4: Based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimen, update the model parameters of the discrete element numerical simulation model and construct a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles.

[0020] Specifically, in step S1, a colored emulsion is used as the binder, and cold recycled asphalt mixture specimens are formed using Marshall or rotary compaction methods. New asphalt mortar, aged asphalt mortar, and aggregates are assigned different colors, with the aggregates being gray, the aged asphalt mortar being black, and the new asphalt mortar being the selected color. Digital image processing was used to scan the cross-section of the asphalt mixture specimen. Based on the color differences of each phase, MATLAB was used to generate code that could be recognized by discrete element software. Different colors were defined as the distribution of each phase and imported into the discrete element software to construct a discrete element numerical simulation model of cold recycled asphalt mixture.

[0021] Based on the discrete element method (DEM) numerical simulation model, contact model parameters are assigned to adjacent particles. In cold recycled asphalt mixtures, the aggregate contact model is set as a linear contact model, and the new asphalt mortar and aged asphalt mortar adopt a parallel bond model, specifically including: First, the mix design of cold recycled asphalt mixture is carried out, and the amount of added water and asphalt emulsion is calculated. Secondly, the portion of cold recycled asphalt mixture with a particle size less than 1.18 mm or 2.36 mm is considered as the aggregate portion of the mortar. The equivalent asphalt emulsion content of the new asphalt mortar is calculated using the specific surface area method. Based on the gradation curve of the cold recycled asphalt mixture, the percentage of each aggregate grade is statistically analyzed. According to the specific surface area values ​​of aggregates with different sieve sizes specified in the "Technical Specification for Construction of Highway Asphalt Pavement" (JTG F40-2004), the total surface area of ​​the new aggregates below 1.18 mm is calculated as shown in formula (1): (1) In the formula: RA This represents the total surface area of ​​the aggregates in the cold recycled mixture. R i The percentage of aggregate residue at different screen openings. A iThis represents the aggregate specific surface area coefficient corresponding to different sieve apertures.

[0022] according to RA The value is calculated according to formula (2) to obtain the equivalent asphalt emulsion content required for the new asphalt mortar, thereby obtaining the amount of new asphalt mortar.

[0023] The formula for calculating the equivalent asphalt emulsion content required for new asphalt mortar is: (2) In the formula: R m For new asphalt mortar, an intermediate-efficiency asphalt emulsion. RA <1.18mm The total surface area of ​​aggregates smaller than 1.18mm. R a The optimal asphalt emulsion content for cold recycled asphalt mixtures, R 1.18 The sieve aperture is 1.18.

[0024] For aged asphalt mortar, a certain mass of RAP material with a particle size less than 1.18mm is first placed in a high-temperature oven for heating and heat preservation. After removal, a certain mass of RAP material is weighed and subjected to Marshall compaction molding, followed by demolding and cutting. The material is then cut into block, cylindrical, or other shaped specimens to form aged asphalt mortar specimens.

[0025] New and aged asphalt mortar are cut into blocks, cylinders, or other shapes to form aged asphalt mortar specimens. This invention uses block specimens as an example, cutting both new and aged asphalt mortar into pieces with dimensions of 15mm × 10mm × 10mm, where the length is 15mm, the width and height of the cross-section are 10mm, and the cross-sectional area is 1e. - 4 m 2 The mechanical properties in the compression, tension and shear directions were tested using a high-precision mortar mechanical loading tester.

[0026] In step S2, a linear parallel bond model is used on both new and aged asphalt mortar specimens, assigning microscopic parameters to adjacent particles, including contact and strength parameters, wherein: Contact parameters include normal stiffness, tangential stiffness, and stiffness ratio, which characterize the stiffness values ​​between particles and the force between particles is characterized by the change in deformation. Strength parameters include normal tensile strength and shear strength, which characterize the strength force that particles can withstand. When the contact force between particles exceeds the maximum strength force, the contact between adjacent particles fails and fracture occurs.

[0027] Because cold recycled asphalt mixtures possess certain viscoelastic mechanical properties, meaning their stiffness and strength are functions of loading time, these parameters were calibrated using mortar loading tests at different loading rates for compression, tension, and shear.

[0028] This invention employs loading rates of 0.1 mm / s, 0.2 mm / s, and 0.4 mm / s to conduct three sets of compression, tensile, and shear loading tests on new and aged asphalt mortar specimens at different loading rates. Loading force-displacement curves are obtained. The peak loading force and the corresponding displacement value are selected, and the ratio of the peak loading force to the displacement is taken as the contact stiffness value in each direction. The ratio of the ultimate force peak value in the shear direction to the displacement is taken as the particle contact shear stiffness, and the ratio of tensile stiffness to shear stiffness is taken as the stiffness ratio.

[0029] Tables 1 and 2 show the peak loading force and displacement values ​​of new and aged asphalt mortar specimens under different loading rates.

[0030] Table 1 Contact parameters of new asphalt mortar specimens

[0031] Note: Table 1 y This indicates the contact stiffness corresponding to different directions. x This refers to the loading rate.

[0032] Table 2 Contact parameters of aged asphalt mortar specimens

[0033] For normal stiffness, since the stiffness values ​​in the compression and tension directions are different, the stiffness value in the compression direction of the particle is defined as the compression stiffness, that is, the stiffness value in the compression state, and the stiffness value in the tension direction is defined as the tension stiffness, that is, the stiffness value in the tension state, so as to achieve separation of tension and compression states in particle contact properties.

[0034] Among them, the reference spacing Δ between adjacent particles δ Determine the compression and tension states when Δ δ When Δ > 0, adjacent particles are in a stretched state; when Δ δ When <0, adjacent particles are in a compressed state.

[0035] This invention is based on the difference in stiffness values ​​under different loading rates. It defines compressive stiffness, tensile stiffness, and shear stiffness as functions of the loading rate through fitting. These functions can be linear or other types of nonlinear forms. As shown in Tables 1 and 2, this invention uses an exponential function form to regress the relationship between different loading rates and contact stiffness in each direction. The ratio of different tensile stiffnesses to shear stiffnesses is the stiffness ratio.

[0036] For the strength parameters used to determine interparticle contact failure, the tensile and shear strengths corresponding to the peak loading forces of the three sets of tensile and shear test curves with different loading rates were calculated, as shown in Table 3, which represents the contact strength parameters of new asphalt mortar specimens, and Table 4, which represents the contact strength parameters of aged asphalt mortar specimens.

[0037] Table 3. Contact strength parameters of new asphalt mortar joint specimens

[0038] Table 4 Contact strength parameters of aged asphalt mortar specimens

[0039] Note: In Tables 3 and 4 z For the contact strength corresponding to different directions, x This refers to the loading rate.

[0040] Based on the discrete element numerical simulation model of cold recycled asphalt mixture, the real-time normal tensile strength between particles is determined by simulation; when the real-time normal tensile strength between particles is higher than or exceeds the tensile limit strength, particle contact failure occurs. Alternatively, based on multiple shear tests with different loading rates, the Mohr-Coulomb criterion is used for fitting to determine the interparticle cohesion and internal friction angle parameters of new asphalt mortar specimens / aged asphalt mortar specimens; the cohesion and internal friction angle parameters of the mortar specimens are input into the discrete element numerical simulation model, and the real-time tangential strength between particles is determined through simulation; when the real-time tangential strength between particles exceeds the shear limit strength, particle contact failure occurs.

[0041] Based on the differences in strength values ​​under different loading rates, the tensile ultimate strength and shear ultimate strength are defined as functions of loading rate by fitting a function. The function can be linear or other types of nonlinearity. As shown in the last column of Tables 3 and 4, the functional relationships between different loading rates and strength are summarized. This invention adopts an exponential function form.

[0042] The real-time shear strength between particles is calculated by the Mohr-Coulomb criterion as shown in equation (3). The Mohr-Coulomb criterion includes two parameters: cohesion and internal friction angle. Based on three sets of direct shear tests of mortar specimens with different loading rates, the cohesion and internal friction angle between mortar particles corresponding to each set of direct shear tests are obtained.

[0043] Based on the cohesive force and internal friction angle between particles obtained for each group of direct shear tests, the interparticle cohesion was obtained using a parameter fitting method. c and internal friction angle φ .

[0044] By fitting the interparticle cohesion cThe relationship between the loading rate and the particle cohesion is as follows: as the loading rate increases, the particle cohesion increases. c Significant changes will occur in the internal friction angle. φ The variation amplitude is small; therefore, this invention uses interparticle cohesion to characterize the material's internal shear resistance.

[0045] This invention employs a direct shear testing device to apply shear stresses at different loading rates, without applying axial normal stress. σ c =0, then the shear strength in each test group τ Equal to cohesion c : (3) In the formula: τ Shear strength, c It is the cohesive force between particles. σc It is normal stress. φ It is the internal friction angle.

[0046] This invention applies shear loads at loading rates of 0.1 mm / s, 0.2 mm / s, and 0.4 mm / s respectively to obtain the interparticle cohesion strength of new and aged asphalt mortar specimens. c As shown in Table 5.

[0047] An exponential function was used as the interparticle cohesion of new asphalt mortar specimens / aged asphalt mortar specimens. c and loading rate x The fitting equation for the relationship is shown in Table 6. Furthermore, considering that the loading rate has a relatively small impact on the internal friction angle, this invention uses empirical methods to determine the internal friction angle. φ The value is 45º.

[0048] Table 5. Cohesion of two mortar specimens under different loading rates

[0049] Table 6. Functional relationship between mortar cohesion and loading rate

[0050] In step S3, fatigue loading tests with different strain levels are conducted on the two types of mortar specimens to obtain the relationship between the cumulative deformation of each mortar specimen and the number of loading cycles. The contact stiffness attenuation rate corresponding to different stress levels is obtained by fitting, and the fatigue damage rate is determined. Specifically, this includes: In this embodiment of the invention, a new asphalt mortar specimen is used as an example. Fatigue loading tests with different strain levels are conducted on the new asphalt mortar specimen, and the cumulative deformation of the mortar specimen is recorded. d With load count NTo determine the relationship between deformation and loading cycles, a graph showing the relationship between deformation and loading cycles is drawn, such as... Figure 2 As shown.

[0051] Similarly, a graph showing the relationship between the deformation and the number of loading cycles of aged asphalt mortar specimens can be drawn.

[0052] Using cumulative deformation d With load count N By fitting the exponential relationship, the contact stiffness attenuation rate corresponding to different strain levels was obtained. γ ; The exponential relationship is: (4) In the formula: d 0 represents the coefficient of the functional relationship; Using cumulative deformation d With load count N By fitting the exponential relationship, the contact stiffness attenuation rate corresponding to different strain levels of asphalt mortar specimens was obtained. γ .

[0053] It also includes the contact stiffness attenuation rate corresponding to different strain levels. γ The contact stiffness decay rate was obtained by fitting. γ and strain level ε The exponential function is: (5) In the formula: γ This is the contact stiffness decay rate, i.e., the fatigue damage rate. and These represent the coefficients of the exponential function; Among them, strain level ε The larger, γ The larger the value, the higher the strain level. ε The smaller, γ The smaller the value.

[0054] like Figure 3 The figure shows the relationship between the contact stiffness attenuation rate and strain level of a new asphalt mortar specimen, obtained through fitting. =2.367386, =0.005588.

[0055] Similarly, the relationship between the contact stiffness decay rate and strain level of aged asphalt mortar specimens can be fitted.

[0056] In step S4, based on the fatigue damage rate and the current contact stiffness value, a fatigue damage model is constructed to show the decay of particle contact stiffness with the number of fatigue loading cycles. Specifically, this includes: The fatigue damage model that constructs the particle contact stiffness decays with the number of fatigue loading cycles is as follows: (6) In the formula: k t+Δt for t +Δ t Contact stiffness of asphalt mortar specimens corresponding to the time step; k t This represents the contact stiffness at the current time step.

[0057] The fatigue damage model constructed in this invention can comprehensively consider multiple factors, specifically including: First, in actual road use, vehicle speeds vary, resulting in different loading rates for cold recycled asphalt mixtures. This method obtains the loading force-displacement curves of asphalt mortar specimens at different loading rates through experiments, thereby determining the contact stiffness values ​​in different force directions. This allows the constructed fatigue damage model to capture the influence of loading rate on the material's mechanical properties. For example, high-speed loading may result in a higher stiffness response, while low-speed loading may lead to more plastic deformation accumulation. This invention incorporates this contact stiffness information at different loading rates when constructing the fatigue damage model, enabling the model to simulate the evolution characteristics of fatigue damage in cold recycled asphalt mixtures under different vehicle speeds (i.e., different loading rates), including the initiation, development speed, and extent of the damage.

[0058] Secondly, cold recycled asphalt mixtures are subjected to forces from different directions in actual roads, such as vertical vehicle load pressure and horizontal shear force. This method determines the contact stiffness values ​​of asphalt mortar specimens in different stress directions, comprehensively considering the differences in the mechanical properties of the material in different stress directions. During fatigue loading, the damage development in different stress directions may differ. For example, fatigue damage in the vertical direction may lead to material compaction and cracking, while fatigue damage in the horizontal direction may trigger shear failure. The fatigue damage model constructed in this invention can more realistically simulate the distribution and evolution of fatigue damage in different directions under complex stress conditions based on the changes in contact stiffness in different stress directions.

[0059] Furthermore, the fatigue damage mechanism and evolution rate of cold recycled asphalt mixtures differ under different strain levels. This method obtains the relationship between the cumulative deformation and the number of loading cycles of asphalt mortar specimens through fatigue loading tests at different strain levels, and fits the contact stiffness decay rate corresponding to different strain levels. This indicates that the model can take into account the influence of strain level on fatigue damage. For example, at high strain levels, the material may reach the fatigue limit more quickly, and the contact stiffness decays faster; while at low strain levels, fatigue damage development is relatively slow. In this way, the fatigue damage model constructed in this invention can accurately simulate the complete evolution process of fatigue damage in cold recycled asphalt mixtures from initial minor damage to final failure under different strain levels.

[0060] The method provided by this invention uses asphalt mortar specimens as the research object and obtains key data through a series of actual tests. Asphalt mortar is an important component of cold recycled asphalt mixtures, and its performance directly affects the overall performance of the mixture. The data obtained through experiments, such as the loading force-displacement curve, contact stiffness value, relationship between cumulative deformation and loading cycles, and contact stiffness decay rate, are all based on the mechanical response of actual materials and can realistically reflect the mechanical properties and fatigue characteristics of asphalt mortar under different working conditions. Using these actual experimental data to construct a fatigue damage model makes the model's foundation more solid and can more accurately simulate the fatigue damage behavior of cold recycled asphalt mixtures in actual road environments.

[0061] The model parameters of the discrete element numerical simulation model are updated based on the fatigue damage rate and contact stiffness values ​​of the obtained asphalt mortar specimens. This data-driven parameter update method allows the model parameters to closely reflect the actual material performance changes. Compared with traditional methods that determine model parameters based on assumptions and experience, this method can dynamically adjust the model parameters according to actual experimental data, enabling the model to more accurately reflect the mechanical state and damage degree of the material at different fatigue loading stages. For example, as the number of fatigue loading cycles increases, the contact stiffness gradually decreases, and the model updates the parameters in a timely manner according to the actual measured decay rate, thereby more realistically simulating the continuous evolution process of fatigue damage.

[0062] The core of fatigue damage model construction is the decay of particle contact stiffness with the number of fatigue loading cycles. In cold recycled asphalt mixtures, the contact state and interaction between particles play a crucial role in the mechanical properties of the material. As the number of fatigue loading cycles increases, the bond between particles gradually breaks down, and the contact stiffness gradually decreases.

[0063] The method proposed in this invention experimentally fits the contact stiffness decay rate and incorporates it into a discrete element numerical simulation model, enabling the model to simulate the dynamic changes in interparticle contact stiffness during fatigue loading. This dynamic simulation can more realistically reflect the damage evolution process of the material's internal structure. For example, when the contact stiffness decreases to a certain level, the relative displacement between particles increases, and microcracks may propagate and connect within the material, ultimately leading to macroscopic fatigue failure.

[0064] The fatigue damage model constructed using this method, by considering the decay of contact stiffness, can more accurately simulate the mechanical behavior between particles at different fatigue stages. For example, in the early stages of fatigue, the contact stiffness between particles is high, the interaction is strong, and the material exhibits high strength and stiffness. As fatigue loading progresses, the contact stiffness gradually decreases, the interaction between particles weakens, and the material begins to undergo plastic deformation and damage accumulation. In the later stages of fatigue, the contact stiffness decays to a low level, the bond between particles is almost completely destroyed, and the material undergoes macroscopic failure. Through this accurate simulation of the mechanical behavior between particles, the constructed fatigue damage model can more realistically reflect the entire evolution process of cold recycled asphalt mixtures from the initial state to fatigue failure.

[0065] This invention also proposes a fatigue damage modeling system for cold recycled asphalt mixtures for roads, comprising: The simulation model building module is used to build discrete element numerical simulation models of cold recycled asphalt mixtures. The contact stiffness value determination module is used to obtain asphalt mortar specimens; through experiments, the loading force-displacement curves of asphalt mortar specimens under different loading rates and in different stress directions are obtained, and the contact stiffness value of asphalt mortar specimens in different stress directions is determined based on the ratio of peak loading force to displacement in different stress directions. The fatigue damage rate determination module is used to obtain the relationship between the cumulative deformation of asphalt mortar specimens and the number of loading times through fatigue loading tests at different strain levels, fit the contact stiffness decay rate of asphalt mortar specimens at different strain levels, and determine the fatigue damage rate of asphalt mortar specimens. The fatigue damage model construction module is used to update the model parameters of the discrete element numerical simulation model based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimen, and to construct a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles.

[0066] The fatigue damage modeling method for cold recycled asphalt mixtures proposed in this invention fully considers the viscoelastic mechanical properties of both new and aged asphalt mortar, enabling accurate calculation of fatigue damage characteristics and fatigue life of cold recycled asphalt mixtures under complex material compositions and different stress conditions. Through mortar tests under tensile, compression, and shear modes at different loading rates, the method achieves accurate characterization of contact and strength parameters of different material components in the discrete element numerical simulation model, solving the problem of non-uniqueness of microscopic contact and strength parameters caused by current trial-and-error methods.

[0067] This invention solves the problem of tension-compression anisotropy in particle contact stiffness by using a refined characterization method for separating the tension and compression states of contact parameters in discrete element numerical simulation models, and achieves the unification of tension and compression directions.

[0068] The discrete element numerical simulation fatigue damage model constructed in this invention can realistically consider the influence of different strain levels on the fatigue damage rate by establishing a fatigue damage rate function with strain level as an internal variable, thus filling the gaps in the physical meaning and calibration methods of current fatigue damage models.

[0069] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0070] Furthermore, unless otherwise stated, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. All references to this specification are incorporated by way of citation to disclose and describe methods relating to those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

Claims

1. A method of fatigue damage modeling of a cold-in-place recycled asphalt mixture, characterized in that, Includes the following steps: Construct a discrete element numerical simulation model for cold recycled asphalt mixtures; Obtain asphalt mortar specimens; obtain the loading force-displacement curves of asphalt mortar specimens under different loading rates and in different stress directions through experiments, and determine the contact stiffness values ​​of asphalt mortar specimens in different stress directions based on the ratio of peak loading force to displacement in different stress directions. The relationship between the cumulative deformation of asphalt mortar specimens and the number of loading times was obtained through fatigue loading tests at different strain levels. The contact stiffness attenuation rate corresponding to different strain levels of asphalt mortar specimens was fitted to determine the fatigue damage rate of asphalt mortar specimens. Based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimens, the model parameters of the discrete element numerical simulation model are updated, and a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles is constructed.

2. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 1, characterized in that, The relationship between the cumulative deformation and the number of loading cycles of asphalt mortar specimens obtained through fatigue loading tests at different strain levels is used to fit the contact stiffness attenuation rate of the asphalt mortar specimens at different strain levels, thereby determining the fatigue damage rate of the asphalt mortar specimens. Specifically, this includes: Fatigue loading tests at different strain levels were conducted on asphalt mortar specimens to determine the cumulative deformation of the mortar specimens. d With load count N Exponential relationships; The exponential relationship is: ； In the formula: d 0 represents the coefficient of the functional relationship; Using cumulative deformation d With load count N By fitting the exponential relationship, the contact stiffness attenuation rate corresponding to different strain levels of asphalt mortar specimens was obtained. γ .

3. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 2, characterized in that, The process involves updating the model parameters of the discrete element numerical simulation model based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimens, and constructing a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles. Specifically, this includes: The cold recycled asphalt mixture is discretized into individual particle units using a discrete element numerical simulation model. The obtained contact stiffness value is assigned to the particle contact of the corresponding type or position in the discrete element numerical simulation model as the initial contact stiffness. At the same time, the fatigue damage rate is used as the rate index of contact stiffness decay in the discrete element numerical simulation model. The fatigue damage model constructed in which the particle contact stiffness decreases with the number of fatigue loading cycles is as follows: ； In the formula: k t+Δt for t +Δ t Contact stiffness of asphalt mortar specimens corresponding to the time step; k t To step at the current time t The contact stiffness of the corresponding asphalt mortar specimen.

4. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 1, characterized in that, The discrete element numerical simulation model for constructing cold recycled asphalt mixtures also includes using a linear parallel bonding model for asphalt mortar specimens, assigning model micro-parameters to adjacent particles. These model micro-parameters include contact parameters and strength parameters, wherein: Contact parameters include normal stiffness, tangential stiffness, and stiffness ratio, which characterize the interparticle stiffness of asphalt mortar specimens and characterize the interparticle stress by the change in the deformation of asphalt mortar specimens. Strength parameters include normal tensile strength and shear strength, which characterize the interparticle strength that asphalt mortar specimens can withstand; When the real-time normal tensile strength or real-time tangential strength between particles exceeds the corresponding ultimate strength, the contact between adjacent particles in the asphalt mortar specimen fails, and the asphalt mortar specimen fractures and breaks.

5. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 4, characterized in that, The contact parameters include normal stiffness, tangential stiffness, and stiffness ratio, which characterize the interparticle stiffness of the asphalt mortar specimen. The changes in deformation of the asphalt mortar specimen characterize the interparticle stress, specifically including: Multiple sets of compression, tensile and shear loading tests with different loading rates were carried out on asphalt mortar specimens to obtain the loading force-displacement curves of the asphalt mortar specimens; The ratio of the peak loading force to the displacement in different stress directions is used as the contact stiffness value of the asphalt mortar specimen in the corresponding direction. Among them, the ratio of the peak ultimate force to the displacement in the shear direction is used as the particle contact shear stiffness of the asphalt mortar specimen. The stiffness value in the compression direction of the particles is determined by the compression stiffness, that is, the stiffness value of the asphalt mortar specimen in the compressed state, and the stiffness value in the tensile direction is determined by the tensile stiffness, that is, the stiffness value of the asphalt mortar specimen in the tensile state. The ratio of tensile stiffness to shear stiffness is used as the stiffness ratio. The compressive and tensile states of asphalt mortar specimens are determined by the reference spacing Δδ between adjacent particles. When Δδ>0, the adjacent particles are in a tensile state; when Δδ<0, the adjacent particles are in a compressive state. By fitting the compressive stiffness, tensile stiffness, and shear stiffness of asphalt mortar specimens as functions of loading rate, the loading rates corresponding to the compressive stiffness, tensile stiffness, and shear stiffness under different working conditions are obtained, and then the forces corresponding to the compressive stiffness, tensile stiffness, and shear stiffness under different working conditions are determined.

6. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 4, characterized in that, When the real-time normal tensile strength or real-time tangential strength between particles exceeds the corresponding ultimate strength, the contact between adjacent particles in the asphalt mortar specimen fails, and the asphalt mortar specimen fractures and fails. Specifically, this includes: Based on the loading force-displacement curves of the asphalt mortar specimens in the tensile, compressive and shear directions under different loading rates, the tensile ultimate strength and shear ultimate strength of the asphalt mortar specimens corresponding to different loading rates are determined by the peak loading force. Based on the discrete element numerical simulation model, the real-time normal tensile strength between particles of asphalt mortar specimens is determined by simulation; when the obtained real-time normal tensile strength between particles exceeds the tensile limit strength, the particle contact of the new asphalt mortar specimen fails. Alternatively, based on multiple shear tests with different loading rates, the Mohr-Coulomb criterion is used for fitting to determine the interparticle cohesion and internal friction angle parameters of the asphalt mortar specimen; the cohesion and internal friction angle parameters of the asphalt mortar specimen are input into the discrete element numerical simulation model, and the real-time interparticle tangential strength of the asphalt mortar specimen is determined through simulation; when the real-time interparticle tangential strength exceeds the shear limit strength, the particle contact of the asphalt mortar specimen fails.

7. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 6, characterized in that, The shear tests based on multiple sets of different loading rates were fitted using the Mohr-Coulomb criterion to determine the interparticle cohesion and internal friction angle parameters of the asphalt mortar specimens, specifically including: The Mohr-Coulomb criterion includes two parameters: cohesion and internal friction angle. Based on direct shear tests of asphalt mortar specimens with multiple loading rates, the interparticle cohesion and internal friction angle of the asphalt mortar specimens corresponding to each direct shear test are obtained. Based on the interparticle cohesion and internal friction angle of the asphalt mortar specimens corresponding to each group of direct shear tests, the interparticle cohesion of the asphalt mortar specimens was obtained using a parameter fitting method. c and internal friction angle φ ; Fitting interparticle cohesion c The relationship between the loading rate and the particle cohesion is shown to change significantly as the loading rate increases. The particle cohesion can be equated to the cohesion of the asphalt mortar specimen.

8. The fatigue damage modeling method for cold recycled asphalt mixtures for roads according to claim 1, characterized in that, The process of obtaining asphalt mortar specimens specifically includes: Asphalt mortar specimens include new asphalt mortar specimens and aged asphalt mortar specimens; Colored emulsions are used as binders to give different colors to new asphalt mortar, aged asphalt mortar and aggregates in cold recycled asphalt mixtures. By determining the mix proportion of cold recycled asphalt mixtures, the amount of added water and asphalt emulsion is obtained. The amount of new asphalt mortar is equivalent to the amount of asphalt emulsion; the new asphalt mortar adopts a linear parallel bond model and is formed into new asphalt mortar specimens by Marshall or rotary compaction methods. For aged asphalt mortar, firstly, RAP material with a particle size of less than 1.18mm is placed in a high-temperature oven for heating and heat preservation. After being taken out, the RAP material is weighed and subjected to Marshall compaction molding. Then, it is demolded and cut into block, cylindrical or other shaped specimens to form aged asphalt mortar specimens. The same method was used to obtain the corresponding contact stiffness values ​​and fatigue damage rates for both new and aged asphalt mortar specimens.

9. A fatigue damage modeling system for cold recycled asphalt mixtures for roads, characterized in that, include: The simulation model building module is used to build discrete element numerical simulation models of cold recycled asphalt mixtures. The contact stiffness value determination module is used to obtain asphalt mortar specimens; through experiments, the loading force-displacement curves of asphalt mortar specimens under different loading rates and in different stress directions are obtained, and the contact stiffness value of asphalt mortar specimens in different stress directions is determined based on the ratio of peak loading force to displacement in different stress directions. The fatigue damage rate determination module is used to obtain the relationship between the cumulative deformation of asphalt mortar specimens and the number of loading times through fatigue loading tests at different strain levels, fit the contact stiffness decay rate of asphalt mortar specimens at different strain levels, and determine the fatigue damage rate of asphalt mortar specimens. The fatigue damage model construction module is used to update the model parameters of the discrete element numerical simulation model based on the fatigue damage rate and contact stiffness value of the asphalt mortar specimen, and to construct a fatigue damage model in which the particle contact stiffness decays with the number of fatigue loading cycles.

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