Design method of asphalt pavement based on humidity environment

Abstract

The invention discloses a design method of an asphalt pavement based on a humidity environment. The design method comprises the following steps: (1) determining an asphalt pavement structure model; (2) determining material parameters of the asphalt pavement structure model; (3) establishing an asphalt pavement structure temperature field finite element model; (4) establishing a steam concentration field finite element model of the asphalt pavement structure; and (5) establishing a fatigue life model of the asphalt pavement structure in consideration of the humidity environment. Based on a finite element analysis method, a pavement structure temperature field model and a non-uniform water vapor concentration field model are established, the influence of water vapor factors is introduced into the pavement structure, the pavement structure without considering the water vapor state and the pavement structure without considering the non-uniform water vapor concentration state are compared, a pavement structure fatigue cracking model is further established, and the pavement structure fatigue cracking performance is improved. And the change of the fatigue life of the asphalt pavement under the two water vapor concentration states is analyzed, so that a scientific basis is provided for reducing the generation of crack diseases and constructing the long-life durable asphalt pavement.

Description

Technical Field

[0001] The invention belongs to the technical field of asphalt pavement design, and in particular relates to a design method for an asphalt pavement based on a humidity environment. Background Art

[0002] As the main structural form of the surface layer of my country's highways, asphalt pavement not only withstands the effects of vehicle loads and temperature for a long time, but also experiences the effects of humidity caused by the humidity gradient of the soil roadbed and the air. In the real environment, due to the existence of relative humidity, asphalt pavements are inevitably in an environment with different water vapor content. However, the current research on the humidity sensitivity of asphalt mixture performance is still scarce, and the water damage caused by water infiltration of asphalt pavement is mainly analyzed from the perspective of liquid water. However, in fact, gaseous water is more likely to invade the adhesion interface between asphalt and aggregate or the interior of asphalt mortar at the beginning of pavement service, reducing the adhesion binding energy and cohesive binding energy, causing asphalt to peel off from the aggregate surface and cause initial water damage. On the other hand, temperature and humidity are interrelated environmental factors. Therefore, it is necessary to study the performance of asphalt mixtures under temperature and humidity coupling conditions, in order to more accurately predict the development of fatigue cracking performance of asphalt pavement in the real service environment and more accurately predict the life of asphalt pavement.

[0003] Asphalt pavement is a pavement structure with asphalt mixture as the surface layer. Its base layer can be divided into flexible base layer, rigid base layer or semi-rigid base layer. In the actual service process, asphalt pavement often shows fatigue damage under repeated vehicle loads. At present, the international asphalt pavement design methods can be roughly divided into three types: empirical method, mechanical-empirical method and performance-based design method. Although the existing asphalt pavement design method takes into account the water stability of asphalt mixture, it does not consider the effect of environmental humidity on fatigue cracking of asphalt mixture, which causes the process of accelerated peeling of asphalt film from aggregate to be considered during the use of asphalt mixture in high humidity environment, resulting in accelerated damage of asphalt pavement in high humidity environment.

[0004] Some scholars have obtained the mechanical response of pavement structures under different water vapor conditions through numerical simulation methods, and have estimated the fatigue life of asphalt pavements by referring to the calculation method of pavement structure fatigue life in the current Chinese standards. However, the facts show that the prediction results obtained based on the mechanical response of the pavement structure have a poor correlation with the actual fatigue life of the asphalt pavement. This is mainly because fatigue damage of asphalt pavements is often accompanied by the generation of cracks, and the expansion of cracks will cause the simulation results to be inconsistent with the actual stress conditions of the pavement. Therefore, simple simulations are difficult to characterize the asphalt pavement structure under the multi-field coupling of humidity and humidity environments.

[0005] In summary, in the design process of asphalt pavement, the design parameters of asphalt pavement need to take into account the effect of humidity, so as to accurately reflect the structural performance of asphalt pavement during all-weather service. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a design method for asphalt pavement under a humid environment. Combined with the actual climatic conditions of the project, a pavement structure temperature field model is established based on the finite element software analysis method; on this basis, according to the accumulation type water vapor diffusion theory, a non-uniform water vapor concentration field model is established to obtain the water vapor concentration distribution of the pavement structure. The results of the temperature field and water vapor concentration field are used as predefined fields, and the influence of water vapor factors is introduced into the pavement structure. The pavement structure without considering the water vapor state (the water vapor concentration inside the pavement structure is zero) and the pavement structure under the non-uniform water vapor concentration state are compared. With the help of the improved Paris formula, a pavement structure fatigue cracking model is further established, and the changes in the fatigue life of the asphalt pavement under the two water vapor concentration states are analyzed, providing a scientific basis for reducing the occurrence of crack diseases and building long-life durable asphalt pavements.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A design method for an asphalt pavement under a humidity environment is disclosed. The design method simulates fatigue cracking changes of an asphalt pavement in finite element software under multi-field coupling in a humidity environment.

[0009] The design method of asphalt pavement specifically includes the following steps:

[0010] (1) Determine the asphalt pavement structure model: Use the typical asphalt pavement structure form in Jiangxi Province to establish a pavement structure model in the finite element software, determine the model size, and use the finite element software (partitioning tool) to complete the division of the pavement structure layer;

[0011] (2) Determine the material parameters of the asphalt pavement structure model: the material parameters include thermal parameters, mechanical parameters, fracture parameters and fatigue cracking parameters, the thermal parameters include thermal conductivity, heat capacity and material density, the mechanical parameters include viscoelastic parameters, elastic modulus and Poisson's ratio, the fracture parameters include fracture energy and tensile strength, and the fatigue cracking parameters include improved Paris formula parameters and limit fracture energy release rate;

[0012] (3) Establish a finite element model of the temperature field of the asphalt pavement structure: simulate the solar radiation, effective pavement radiation, air temperature and convective heat exchange of the pavement structure respectively, complete the definition of the thermal boundary conditions of the pavement structure in the finite element software, mesh the asphalt pavement structure model determined in step (1), input the thermal parameters of each structural layer in the finite element software, complete the finite element definition of air temperature, convective heat exchange and effective pavement radiation in the interaction module combined with the user subroutine FILM, and complete the finite element definition of solar radiation in the load module combined with the user subroutine DFLUX. Combined with the atmospheric temperature in low temperature weather in Jiangxi Province (refer to the data of the National Meteorological Science Data Center, the average atmospheric temperature of the lowest 10 days in January 2020 in Yichun City, Jiangxi Province) as the temperature data of the pavement structure, simulate the pavement structure temperature in the finite element to obtain the pavement structure temperature field;

[0013] (4) Establishing a finite element model of the water vapor concentration field of the asphalt pavement structure: First, determine and test the water vapor diffusion coefficient of the asphalt pavement structure and perform fitting to obtain water vapor diffusion coefficient data at different temperatures. The pavement structure temperature field established in step (3) is used as the predefined field, and the grid division is the same as the temperature field. In the material parameter module, Fick's law is selected as the mass diffusion law. The temperature and the corresponding water vapor diffusion coefficient data are input into the finite element software diffuseness module. Then, the upper and lower boundary conditions of the water vapor concentration of the pavement structure are defined and input. The water vapor diffusion of the pavement structure is simulated in the finite element to obtain the water vapor concentration field of the pavement structure under the influence of the temperature field.

[0014] (5) Establishment of fatigue life model of asphalt pavement structure under humidity environment: Based on the asphalt pavement structure model determined in step (1), the wheel load is set as a semi-sinusoidal dynamic load, and the wheel load function is defined. The material parameters determined in step (2) are set in the finite element software, and the temperature distribution corresponding to the lowest temperature (5h) in the temperature field finite element model established in step (3) is used as the temperature predefined field, and the water vapor concentration when reaching the equilibrium state (2400d) in the water vapor concentration field finite element model established in step (4) is used as the water vapor concentration predefined field of the pavement structure. The temperature and water vapor concentration are introduced into the pavement structure model, and finally a task is created and submitted in the operation module to obtain the fatigue life model of the asphalt pavement structure under humidity environment. At the same time, the case where the water vapor concentration of the entire pavement structure is zero is set as the control group, which is compared with the pavement fatigue cracking situation under humidity environment, and the fatigue cracking simulation of the asphalt pavement structure is performed.

[0015] The specific method for simulating fatigue cracking of asphalt pavement structure is as follows: based on the extended finite element XFEM method and the improved Paris formula, the fatigue cracking of asphalt pavement is simulated, and the expansion of cracks can be directly observed. When the cracks enter the unstable failure stage under the action of load, the cracks continue to expand and eventually penetrate the entire surface layer. The number of load cycles at this time is the fatigue life of the pavement structure. The fatigue life of the pavement under the humidity environment is compared with the control group, so as to adjust the design standards in the actual asphalt pavement design process to improve the pavement's ability to resist cracking.

[0016] Preferably, in step (1), the pavement structure model has a size of 3m in height and 3.75m in width. The pavement structure includes, from top to bottom, a surface layer, a base layer (cement-stabilized crushed stone), a subbase layer (graded crushed stone) and a soil base, the surface layer includes an upper layer (AC-13C asphalt mixture), a middle layer (AC-20C asphalt mixture) and a lower layer (AC-25C asphalt mixture), and the base layer includes a base layer 1 (5% cement-stabilized crushed stone) and a base layer 2 (3% cement-stabilized crushed stone).

[0017] Preferably, in step (2), the viscoelastic parameter is the relaxation modulus Prony series of the asphalt pavement surface layer. The dynamic modulus and phase angle master curve can be established by conducting an indoor uniaxial tensile dynamic modulus test of the asphalt mixture at different temperatures, different water vapor concentrations and different loading frequencies. At the same time, the relaxation modulus Prony series expression is determined based on the generalized Maxwell model. Then, the dynamic modulus is converted into the relaxation modulus, and the relaxation modulus Prony series of the asphalt mixture can be determined.

[0018] In the indoor uniaxial tensile dynamic modulus test, the different temperatures include 5°C, 20°C, 35°C, and 50°C, and the different water vapor concentrations include 0.35 g / m 3 , 8.64g / m 3 , 13.82g / m 3 , 17.27g / m 3 (corresponding to the four relative humidity levels of 2%, 50%, 80% and 100% at 20°C respectively), the different loading frequencies include 25Hz, 10Hz, 5Hz, 1Hz, 0.5Hz, 0.1Hz, and the final reference temperature is 20°C and the reference humidity is 0.35g / m 3 Master curve of lossless viscoelastic property parameters under conditions.

[0019] Preferably, the relaxation modulus Prony series expression is:

[0020]

[0021] Where E(t) is the relaxation modulus function, in MPa; t is time; Eg is the instantaneous relaxation modulus, in MPa; i = 1, 2, ..., m, where m is the total number of model elements in parallel; E i is the relaxation modulus of the i-th spring; τ i is the relaxation time of the i-th spring in the generalized Maxwell model, in seconds; the relaxation modulus Prony series is the parameter E g and E i .

[0022] Preferably, the method of converting the dynamic modulus into the relaxation modulus comprises:

[0023] S1. Determine the Prony series expressions of the complex modulus, storage modulus, and loss modulus respectively. First, perform Fourier transform on the Prony series expression of the relaxation modulus of the generalized Maxwell model. Let t = iw, w is the angular frequency, rad / s, and the complex modulus expression can be obtained as follows:

[0024] Then, according to the viscoelastic relationship, the expression of the storage modulus can be obtained as:

[0025] The expression for loss modulus is:

[0026] S2. Based on the dynamic modulus and phase angle data of asphalt mixture, solve the storage modulus and instantaneous modulus E g ;

[0027] S3. Use the configuration method to perform E on the parameters in the storage modulus expression. i Solve, select the preset relaxation time point to set the configuration point, and solve the parameter E corresponding to each relaxation time i ;

[0028] S4. The parameters E obtained are i Substitute it into the relaxation modulus expression and solve to obtain the relaxation modulus.

[0029] Preferably, in step (2), the fracture parameters are obtained by an indoor semicircular bending test of asphalt mixture at different temperatures and different water vapor concentrations, wherein the different temperatures include 5°C, 15°C, and 25°C, and the different water vapor concentrations include 0.35 g / m 3 , 8.64g / m 3 , 13.82g / m 3 , 17.27g / m 3 The test loading rate is 2mm / min, the prefabricated crack lengths are 0mm and 10mm respectively, the specimens with 0mm prefabricated crack length are used to test the tensile strength of the mixture, and the specimens with 10mm prefabricated crack length are used to test the fracture energy of the mixture.

[0030] The fracture parameters were fitted according to the GEP model to obtain the following fitting relationship:

[0031] Fracture energy of AC-13C asphalt mixture: y = 0.88x1 2 -67.19x1+4.65x2 2 -15.81x2+3204.96, R 2 =0.9821;

[0032] Tensile strength of AC-13C asphalt mixture: y = 0.005x1 2 -0.371x1-0.058x2+8.799, R 2 =0.9431;

[0033] Fracture energy of AC-20C asphalt mixture: y = -0.54x1 2 -16.85x1+3.93x2 2 -2.53x2+2592.53, R 2 =0.9842;

[0034] Tensile strength of AC-20C asphalt mixture: y = -0.008x1 2 -0.032x1 -0.006x2 2 +0.015x2+8.672, R 2 =0.9560;

[0035] Fracture energy of AC-25C asphalt mixture: y = -1.64x1 2 +17.13x1+2.44x2 2 +7.57x2+2117.36, R 2 =0.9798;

[0036] Tensile strength of AC-25C asphalt mixture: y = 0.007x1 2 -0.404x1 -0.003x2 2 +0.001x2+7.545, R 2 =0.9491.

[0037] Among them, y is the model dependent variable, corresponding to tensile strength and fracture energy respectively; x1 and x2 are the model independent variables, x1 is temperature (℃), x2 is water vapor concentration (g / m 3 ).

[0038] This fitting relationship can be used to obtain fracture parameters in more temperature and water vapor concentration ranges. The fracture parameters of the control group with zero water vapor concentration in step (5) can be calculated using the above fitting relationship.

[0039] Preferably, in step (2), the improved Paris formula is expressed by the fracture release rate G, specifically:

[0040]

[0041] is the crack growth rate, a is the crack growth length, N is the variable of the number of load cycles, ΔG is the change of the fracture release rate, and C′ and m′ are material-related parameters.

[0042] The expansion of fatigue cracks can be divided into three main stages: slow expansion stage, stable expansion stage and unstable failure stage. Under the action of load, the fracture energy release rate G generated at the crack tip continues to increase and reaches the fracture energy release rate threshold G t At this time, the crack extension conditions are met and the crack begins to expand due to fatigue. As the load continues to act, the fracture energy release rate at the crack tip continues to increase until it reaches the limit fracture energy release rate G. pl , the structure becomes unstable and damaged. Studies have shown that the remaining fatigue life of the third stage is much smaller than that of the first two stages and can be ignored.

[0043] Based on the improved Paris formula, the slow and stable expansion stage of crack propagation is described to judge the crack initiation. The judgment criteria are: Where: f is the fatigue crack initiation factor. When f≥1.0, the crack starts to develop and enters the stable expansion stage. N0 is the number of load cycles. c1 and c2 are both material-related parameters.

[0044] When the crack expansion enters the stable expansion stage, the crack expansion rate is mainly controlled by the improved Paris formula, as shown in the following formula: Where: c3 and c4 are parameters related to materials.

[0045] c1, c2, c3 and c4 are the parameters of the improved Paris formula, which can be obtained by fitting the standard test.

[0046] Preferably, in step (3),

[0047] The solar radiation is calculated according to the Fourier series principle, and the calculation formula is:

[0048] Where Q(t) is the daily variation function of solar radiation; t is the time variable; Q0 is the maximum daily solar radiation, usually Q0 = 0.131nQ t , n = 12 / c, Q t is the total solar radiation per day, J / m 2 ; c is the actual effective sunshine duration, h.

[0049] Fourier coefficient In this formula, t=0 is assumed at 6 a.m.;

[0050] The calculation formula for the effective radiation of the road surface is: e =εσ[(T r -T Z ) 4 -(T a -T Z ) 4 ]; where: q e is the effective radiation of the road surface, J / m 2 ; ε is the asphalt pavement emissivity, which is taken as 0.81; σ is the Boltzmann constant, which is taken as 5.670×10 -8 W / (m 2 ·K 4 );T r is the road surface temperature, °C; T a is the daily average temperature, ℃; T z The absolute zero value is -273℃.

[0051] The simulated relationship of daily temperature variation is:

[0052] T=T a +T m {0.96sin[ω(t-t0)]+0.14sin[2ω(t-t0)]}; where: T a is the daily average temperature, ℃; T m is the daily temperature amplitude, ℃; t0 is the initial phase, usually set to 9h; ω is the angular frequency, ω=π / 12; T max is the highest temperature of a single day, ℃; T min The lowest temperature of the day, ℃.

[0053] The heat exchange coefficient is calculated as: k c =3.7v+9.4; where: k c is the heat exchange coefficient, W / (m2·℃); ν is the daily average wind speed, m / s.

[0054] Preferably, in step (4), the water vapor diffusion coefficient is specifically an accumulation type water vapor diffusion coefficient. The accumulation type water vapor diffusion coefficient data at different temperatures can be obtained through indoor water vapor tests, and then the relationship between the accumulation type water vapor diffusion coefficient and temperature can be fitted through the Arrhenius equation to expand the water vapor diffusion coefficient of the asphalt surface layer.

[0055] The fitted model is:

[0056] D=A1e -E / RT; In the formula: D is the accumulation type water vapor diffusion coefficient, A1 is the pre-exponential factor, dimensionless; E is the reaction activation energy, J / mol, R is the universal gas constant, J / (K·mol); T is the temperature in Kelvin, K.

[0057] The fitting results are: The fitting results are: A1=3.753, E=39.197 J / mol, R=8.314 J / (K·mol).

[0058] In step (4), the upper boundary condition of the water vapor concentration in the pavement structure is the water vapor concentration in the layer below the asphalt pavement, which is calculated based on the temperature obtained from the results of the temperature field finite element model; the lower boundary condition of the water vapor concentration in the pavement structure is the water vapor concentration in the atmosphere, which is the atmospheric water vapor concentration under low temperature weather in Jiangxi Province (referring to the data from the National Meteorological Science Data Center, the water vapor concentration in the 10 days with the lowest temperature in January 2020 in Yichun City, Jiangxi Province is selected).

[0059] Preferably, in step (5), the wheel load function expression is:

[0060]

[0061] Where p(t) is the wheel load function; t is the time variable; p max is the value of the average step load (usually 700 kPa); T is the load action time (s), which is related to the driving speed v and the radius r of the tire contact area. According to the characteristics of the driving load, a reasonable assumption is made. When the load action point is more than 6r away from a certain point, it can be considered that the point is no longer affected by the load. Therefore, the load action time T = 12r / v, v is the vehicle speed (m / s), and r is the wheel load action radius (m). In this model, r = (0.213 / 2)m.

[0062] Technical effects of the present invention:

[0063] 1. The present invention combines the actual climatic conditions of the project and establishes a pavement structure temperature field model based on the finite element software analysis method; on this basis, according to the accumulation type water vapor diffusion theory, a non-uniform water vapor concentration field model is established to obtain the water vapor concentration distribution of the pavement structure. The results of the temperature field and water vapor concentration field are used as predefined fields, and the influence of water vapor factors is introduced into the pavement structure. The pavement structure without considering the water vapor state (the water vapor concentration inside the pavement structure is zero) and the pavement structure under the non-uniform water vapor concentration state are compared. With the help of the improved Paris formula, a pavement structure fatigue cracking model is further established, and the changes in the fatigue life of the asphalt pavement under the two water vapor concentration states are analyzed, providing a scientific basis for reducing the occurrence of crack diseases and building long-life durable asphalt pavements.

[0064] 2. Compared with the prior art, the mechanical response of the pavement structure under different water vapor conditions is obtained by numerical simulation method, and the fatigue life of the asphalt pavement is estimated by referring to the calculation method of the fatigue life of the pavement structure in the current specifications of my country. The present invention is based on the study of asphalt pavement material and structural performance under the multi-field coupling of humidity environment by finite element method, which can more accurately reflect the development law of fatigue cracking of asphalt pavement under high humidity environment, improve the use quality of asphalt pavement under high humidity environment, and effectively extend the service life of asphalt pavement. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 It is a finite element component diagram of a typical asphalt pavement structure of the present invention;

[0066] Figure 2 is the dynamic modulus master curve of the AC-13C asphalt mixture of the present invention;

[0067] Figure 3 is the phase angle master curve of the AC-13C asphalt mixture of the present invention;

[0068] Figure 4 is the dynamic modulus master curve of the AC-20C asphalt mixture of the present invention;

[0069] Figure 5 is the phase angle master curve of the AC-20C asphalt mixture of the present invention;

[0070] Figure 6 is the dynamic modulus master curve of the AC-25C asphalt mixture of the present invention;

[0071] Figure 7 is the phase angle master curve of the AC-25C asphalt mixture of the present invention;

[0072] Figure 8 A generalized Maxwell model diagram for describing the stress relaxation behavior of asphalt mixture in the present invention;

[0073] Fig. 9 Fracture energy of AC-13C asphalt mixture of the present invention under different test conditions;

[0074] Fig.10 is the fracture energy of the AC-20C asphalt mixture of the present invention under different test conditions;

[0075] Fig.11 is the fracture energy of the AC-25C asphalt mixture of the present invention under different test conditions;

[0076] Fig.12 is the tensile strength of the AC-13C asphalt mixture of the present invention under different test conditions;

[0077] Fig.13 is the tensile strength of the AC-20C asphalt mixture of the present invention under different test conditions;

[0078] Fig.14 is the tensile strength of the AC-25C asphalt mixture of the present invention under different test conditions;

[0079] Fig.15 It is a schematic diagram of mesh division of the temperature field finite element model of the present invention;

[0080] Fig.16 The 24-hour temperature field simulation results of the road surface and the bottom of each asphalt surface layer of the present invention;

[0081] Fig.17 The moisture concentration change at the bottom of each asphalt pavement surface layer during the humidity field simulation of the present invention;

[0082] Fig.18 A name recognition statement used to define different asphalt mixtures when establishing an asphalt pavement cracking model in the finite element software of the present invention;

[0083] Fig.19 The pavement structure temperature predefined field is set when the asphalt pavement cracking model is established in the finite element software of the present invention;

[0084] Fig. 20 It is a predefined field of water vapor concentration of the pavement structure set when establishing the asphalt pavement cracking model in the finite element software of the present invention;

[0085] Fig.21 The ENRRTXFEM field output setting is used for establishing the asphalt pavement cracking model in the finite element software of the present invention.

[0086] Fig. 22 The crack expansion conditions directly observed when simulating the cracking of asphalt pavement in the finite element software of the present invention [(a) is the simulation condition with N = 0 times and a = 2 cm, (b) is the simulation condition with N = 3.079 × 10 7 For the simulation case of a=8 cm, (c) is N=5.584×10 7 For the simulation case of a=14 cm, (d) is N=5.728×10 7 times, simulation case of a = 18 cm];

[0087] Fig.23 It is a schematic diagram of the relationship between the crack extension length and the number of load cycles when simulating the cracking of asphalt pavement in finite element software in the present invention. DETAILED DESCRIPTION

[0088] The above scheme is further described below in conjunction with specific implementations; it should be understood that these embodiments are used to illustrate the basic principles, main features and advantages of the present invention, and the present invention is not limited to the scope of the following embodiments; the implementation conditions adopted in the embodiments can be further adjusted according to specific requirements, and the implementation conditions not specified are usually the conditions in conventional experiments.

[0089] Unless otherwise specified in the following embodiments, the specific details of asphalt pavement design and construction refer to existing highway specifications, such as "Highway Asphalt Pavement Design Specifications" JTG D50-2017, "Highway Asphalt Pavement Construction Technical Specifications" (JTGF40), "Highway Engineering Asphalt and Asphalt Mixture Test Procedures" (JTG E20-2011), etc.

[0090] Example 1

[0091] This embodiment provides a design method for an asphalt pavement under a humidity environment, which is performed in a finite element software under the multi-field coupling effect of the humidity environment. The design method is specifically as follows.

[0092] 1. Determine the asphalt pavement structure model

[0093] The typical asphalt pavement structure in Jiangxi Province was adopted, and the pavement structure model was established in ABAQUS finite element software. The model size was 3m high and 3.75m wide. According to the project construction drawing design, the pavement structure was determined to be "4cm AC-13C asphalt mixture upper layer + 6cm AC-20C asphalt mixture middle layer + 8cm AC-25C asphalt mixture lower layer + 20cm5% cement stabilized gravel base + 18cm 3% cement stabilized gravel base + 20cm graded gravel subbase + soil base" (see Figure 1 ).

[0094] The material information of the three asphalt mixtures of the surface layer is shown in Table 1.

[0095] Table 1

[0096]

[0097] 2. Determine the material parameters of the asphalt pavement structure model

[0098] The material parameters of the asphalt pavement structure model include thermal parameters, mechanical parameters, fracture parameters and fatigue cracking parameters. The thermal parameters include thermal conductivity, heat capacity and material density. The mechanical parameters include viscoelastic parameters, elastic modulus and Poisson's ratio. The viscoelastic parameters are the Prony series of the relaxation modulus of the asphalt pavement surface layer. The fracture parameters include fracture energy and tensile strength. The fatigue cracking parameters include the improved Paris formula parameters of the asphalt pavement surface layer and the ultimate fracture energy release rate.

[0099] 1. Thermal parameters

[0100] Thermal parameters include thermal conductivity, heat capacity and material density. The thermal parameters of each layer of pavement material are shown in Table 2.

[0101] Table 2

[0102]

[0103] 2. Viscoelastic parameters of asphalt pavement surface layer

[0104] The Prony series of the relaxation modulus of asphalt mixture is a commonly used viscoelastic parameter in finite element analysis. The present invention establishes the dynamic modulus|E through an indoor uniaxial tensile dynamic modulus test. * | and phase angle The master curve is then converted into the relaxation modulus to determine the Prony series of the relaxation modulus of the asphalt mixture. Among them, the method for establishing the master curve of the dynamic modulus and phase angle under humidity environment can refer to the document 1 of the inventor's research group ("Study on the influence of humidity on the viscoelastic properties and fatigue cracking performance of asphalt mixture, Hou Qiang, master's degree thesis, Wuhan University of Technology, 2020).

[0105] (1) Indoor uniaxial tensile dynamic modulus test

[0106] According to the "Testing Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG E20-2011), uniaxial tensile dynamic modulus tests were carried out indoors on asphalt mixtures (AC-13, AC-20 and AC-25) specimens. The dynamic modulus tests were carried out at four temperatures of 5°C, 20°C, 35°C and 50°C, six loading frequencies of 25Hz, 10Hz, 5Hz, 1Hz, 0.5Hz and 0.1Hz, and 0.35g / m 3 , 8.64g / m 3 , 13.82g / m 3 , 17.27g / m 3 Four water vapor concentration levels (which are absolute humidity, corresponding to the four relative humidity levels of 2%, 50%, 80% and 100% at 20°C). This test uses a standard cylindrical asphalt mixture specimen with a diameter of 100mm and a height of 150mm. The properties of the asphalt mixture selected for the test have been verified by the Marshall test to meet the requirements. After the asphalt mixture specimens have been cured at four relative humidity levels (2%, 50%, 80% and 100%) for more than 6 months, indoor tests are carried out to measure the dynamic modulus of the asphalt mixture at different temperatures, different humidity and different loading frequencies. * | and phase angle φ data, the results are shown in Table 3-5.

[0107] Table 3 Dynamic test results of AC-13C asphalt mixture (dynamic modulus unit: MPa, phase angle unit: °)

[0108]

[0109]

[0110] Table 4 Dynamic test results of AC-20C asphalt mixture (dynamic modulus unit: MPa, phase angle unit: °)

[0111]

[0112] Table 5 Dynamic test results of AC-25C asphalt mixture (dynamic modulus unit: MPa, phase angle unit: °)

[0113]

[0114] As can be seen from Table 3-5, after humidity curing under different water vapor concentrations, the viscoelastic behavior of asphalt mixture under the same temperature and frequency conditions has changed. With the increase of water vapor concentration, the dynamic modulus of the mixture decreases and the phase angle increases. This phenomenon is particularly obvious under high temperature and low frequency test conditions. This is because the aggregate has a strong hydrophilicity. Under the continuous action of water vapor, water vapor invades the interface between asphalt and aggregate, resulting in a decrease in adhesion, thus showing a change in the entire viscoelastic behavior. In order to obtain the viscoelastic parameters of asphalt mixture outside the test set temperature, frequency and humidity, the measured dynamic modulus and phase angle will be processed based on the master curve model and the temperature-frequency-humidity joint shift factor to provide a reference for the determination of subsequent numerical simulation parameters.

[0115] (2) Drawing of dynamic modulus and phase angle master curve of asphalt mixture

[0116] a. Based on the time-temperature-humidity equivalence principle, the joint displacement factor equation of temperature-humidity coupling displacement is established based on the free volume theory and the Doolittle equation. Specifically:

[0117]

[0118] Among them, TH is the joint shift factor, C1, C2, C3 are shift parameters, C1 = B / 2.303f0, C2 = f0 / α T , C3=f0 / α H , T is the actual temperature, H is the actual humidity, T0 is the reference temperature, H0 is the reference humidity, B is the material parameter, f0 is the free volume fraction of the material at temperature T0 and humidity H0, α T , α Hare the coefficient of thermal expansion and the coefficient of moisture expansion respectively.

[0119] b. The generalized Sigma model is used as the dynamic modulus master curve model, and based on this, the phase angle master curve model is derived according to the approximate Kramers-Kronig relationship. Combined with the joint shift factor equation, the dynamic modulus and phase angle master curve models including temperature, humidity, and loading frequency are constructed.

[0120] The generalized sigmoid model is:

[0121] The phase angle master curve model is:

[0122] Among them, |E * (f)| is the dynamic modulus function, is the phase angle function, f is the test loading frequency, ζ TH is the joint shift factor, δ, α, γ, β, and λ are the model parameters of the master curve, δ and δ+α are the extreme values ​​of the dynamic modulus master curve, and γ, β, and λ are the shape parameters of the dynamic modulus master curve.

[0123] c. According to the least squares fitting method, the reference temperature is 20°C and the reference humidity is 0.35g / m 3 Master curves of dynamic modulus and phase angle under different conditions.

[0124] The reference temperature is selected as 20°C and the reference humidity is selected as 0.35g / m 3 The dynamic modulus and phase angle master curves, as well as the joint shift factor equation of temperature-humidity coupling shift are fitted to minimize the objective function value. The model parameters and shift parameters are obtained with the help of the planning and solving function of Excel software, as shown in Tables 6 and 7.

[0125] Table 6 Model parameters for dynamic modulus and phase angle master curve fitting

[0126]

[0127]

[0128] Table 7 Shift parameters and fitting determination coefficients of the joint shift factor equation for temperature and humidity coupled shift

[0129]

[0130] According to the model parameters and shift parameters in Tables 6 and 7, the reference temperature is 20 °C and the reference humidity is 0.35 g / m 3 The main curve of dynamic modulus and phase angle is shown in Figure 2-7, which are the dynamic modulus master curves and phase angle master curves of the three asphalt mixtures. The fitting determination coefficients of the dynamic modulus master curves and phase angle master curves of the three asphalt mixtures are all above 0.9, indicating that the master curves can better characterize the viscoelastic properties of asphalt mixtures.

[0131] (3) Determine the Prony series of relaxation modulus of asphalt mixture

[0132] a. First, determine the Prony series expression of the relaxation modulus of the generalized Maxwell model

[0133] The generalized Maxwell model is used to describe the stress relaxation behavior of asphalt mixture. The constitutive relationship of the model is as follows: Figure 8 (In the figure, E i is the relaxation strength of the i-th spring in the model, MPa; η i is the viscosity of the i-th viscosity pot, i = 1, 2, ..., n, n is the total number of model elements in parallel; Ee is the equilibrium modulus, MPa) as shown in Figure 2. The stress-strain relationship of asphalt mixture under load can be expressed by the Boltzmann superposition principle. Therefore, the relaxation modulus is expressed using the generalized Maxwell model, and the relaxation modulus Prony series expression is determined as follows:

[0134]

[0135] Where E(t) is the relaxation modulus function, in MPa; t is time; E g is the instantaneous relaxation modulus, in MPa; i = 1, 2, ..., m, where m is the total number of model elements in parallel; E i is the relaxation modulus of the i-th spring; τ i is the relaxation time of the i-th spring in the generalized Maxwell model, in seconds; the relaxation modulus Prony series is the parameter E g and E i .

[0136] b. Based on the dynamic modulus and phase angle master curves of the three asphalt mixtures obtained above, the dynamic modulus is converted into relaxation modulus in combination with the viscoelastic parameter conversion relationship and configuration method, thereby determining the relaxation modulus parameter of the Prony series expression of the relaxation modulus of the generalized Maxwell model, which is used as the viscoelastic parameter of the asphalt mixture in the finite element software.

[0137] The conversion relationship of viscoelastic parameters into dynamic modulus |E * |, complex modulus E * , storage modulus E', loss modulus E", phase angle The relationship between: E * =E'+iE",

[0138] The relaxation modulus of asphalt mixture can be converted from the expressions of complex modulus, storage modulus and loss modulus. The specific methods for converting dynamic modulus to relaxation modulus include:

[0139] S1. Determine the Prony series expressions of the complex modulus, storage modulus, and loss modulus respectively. First, perform Fourier transform on the Prony series expression of the relaxation modulus of the generalized Maxwell model. Let t = iw, w is the angular frequency, rad / s, and the complex modulus expression can be obtained as follows:

[0140] Then, according to the viscoelastic relationship, the expression of the storage modulus can be obtained as:

[0141] The expression for loss modulus is:

[0142] S2. Based on the dynamic modulus and phase angle data of asphalt mixture, solve the storage modulus and instantaneous modulus E g ;

[0143] S3. Use the configuration method to perform E on the parameters in the storage modulus expression. i Solve, select the preset relaxation time point to set the configuration point, and set the configuration point wi and the relaxation time τ i The relationship between them is wi×τ i =2; Use the planning and solving function in EXCEL to solve the parameter E corresponding to each relaxation time i ;

[0144] S4. The parameters E obtained are i Substitute it into the relaxation modulus expression and solve to obtain the relaxation modulus.

[0145] The Prony series of relaxation modulus of the asphalt pavement surface layer (asphalt mixture) obtained by the above method is shown in Table 8.

[0146] Table 8

[0147]

[0148] 3. Elastic modulus and Poisson's ratio

[0149] The linear elastic parameters are the elastic modulus and Poisson’s ratio of the asphalt pavement base, subbase and soil subgrade, which are fixed parameters, see Table 9 below.

[0150] Table 9

[0151]

[0152] 4. Fracture parameters

[0153] Fracture parameters include fracture energy and tensile strength. The cracking behavior of asphalt mixtures can be mainly divided into linear viscoelastic fracture mechanics and viscoelastic-plastic fracture mechanics. When the temperature is 10°C lower than the low temperature lower limit of the asphalt PG classification used, it is considered that the cracking behavior of the mixture conforms to the linear viscoelastic fracture mechanics theory. At this time, the fracture energy G is often used. f To characterize the crack resistance of the mixture; when the temperature is in the medium temperature range, the cracking behavior of the mixture is viscoelastic-plastic fracture behavior, and the fracture energy G f It is not possible to fully characterize the crack resistance of the mixture, and the tensile strength is also needed for comprehensive evaluation. Fracture energy and tensile strength are not only important indicators for evaluating the failure of asphalt mixtures, but also important parameters for mixture cracking simulation. Among them, fracture energy is used to define the damage evolution criterion of the model, and tensile strength is used to define the damage initiation criterion of the model. Both fracture energy and tensile strength need to be obtained through experimental testing and calculation.

[0154] (1) Semicircular bending test to determine the fracture energy and tensile strength

[0155] Referring to the semicircular bending test procedure of the American Association of State Highway and Transportation Officials (AASHTO TP 105-2013), semicircular bending tests were carried out indoors on asphalt mixture (AC-13, AC-20 and AC-25) specimens. The test temperature conditions were medium temperature (5°C, 15°C and 25°C), the relative humidity conditions were 2%, 50%, 80% and 100%, the loading rate was 2mm / min, and the prefabricated crack lengths were 0mm and 10mm. Among them, the specimens without prefabricated slits were used to test the tensile strength of the mixture, and the specimens with 10mm prefabricated slits were used to test the fracture energy of the mixture. For the case of a slit depth of 10mm, a hand-held cutting saw was used to cut the slit at the bottom centerline of the semicircular bending specimen, and the slit width was 1mm.

[0156] This test uses a semicircular bending specimen of asphalt mixture with a diameter of 150mm and a thickness of 50mm. The properties of the asphalt mixture selected for the test have been verified by the Marshall test to meet the requirements. The semicircular bending test was carried out after the asphalt mixture samples were cured at four relative humidity levels (2%, 50%, 80% and 100%) for more than 4 months. The fracture energy and peak load size data of the three asphalt mixtures under different conditions were measured, and the fracture energy and tensile strength were calculated according to the following calculation formula. The results are shown in Figure 9-14 .

[0157] The fracture energy calculation formula is as follows:

[0158] Where: G f is the fracture energy, mJ / mm 2 ; W fis the work of fracture, J; A act =(rc)×b,A act is the effective fracture area, mm 2 ; r is the radius of the semicircular bending specimen, mm; c is the length of the prefabricated crack, mm; b is the thickness of the semicircular bending specimen, mm.

[0159] The tensile strength calculation formula is as follows:

[0160] Where: R is the tensile strength, MPa; P is the peak load size, N; L is the support distance, mm, which is 0.8D in this test; t is the specimen thickness, mm; h is the specimen height (D / 2), mm; D is the diameter of the semicircular bending specimen, mm.

[0161] (2) Fracture parameter fitting

[0162] Based on the fracture parameter data obtained from the above tests, the GEP model was used to fit the fracture parameters of different types of asphalt mixtures, and the relative square root error (RRSE) and the determination coefficient (R) were used to calculate the fracture parameters of different types of asphalt mixtures. 2 The model was evaluated and the model fitting results are shown in Table 10. In the table, y is the model dependent variable, corresponding to tensile strength and fracture energy respectively; x1 and x2 are the model independent variables, where x1 is temperature (℃), x2 is water vapor concentration (g / m 3 ).

[0163] Among them, the fitness function based on the relative square root error is as follows:

[0164] Where: RRSE is the relative square root error; The data predicted by the model; y i The data obtained from the experiment are is the average value of the data obtained from the experiment.

[0165] The coefficient of determination is also a commonly used indicator for evaluating model fitting and prediction effects. The calculation method is shown in the following formula:

[0166] The meaning of the parameters is the same as the above RRSE calculation formula.

[0167] Table 10 Fracture parameter GEP model fitting results

[0168]

[0169] 5. Fatigue cracking parameters of asphalt pavement surface

[0170] The fatigue cracking parameters of asphalt pavement surface layer include the improved Paris formula parameters and the ultimate fracture energy release rate.

[0171] (1) Improved Paris formula

[0172] The classic Paris formula is mainly applicable to linear elastic materials and cannot accurately evaluate viscoelastic materials such as asphalt mixtures. On this basis, Reference 2 (Shen Qing. Research on fatigue expansion of reflective cracks in semi-rigid base asphalt pavement based on XFEM [D]. Changsha: Hunan University, 2018.) improved the Paris formula and used the fracture release rate G instead of the stress intensity factor, as shown in the following formula:

[0173]

[0174] is the crack growth rate, a is the crack growth length, N is the number of load cycles, ΔG is the difference between the maximum fracture release rate and the minimum fracture release rate, C′ and m′ are material-related parameters, which can be obtained by fitting the standard semicircular bending fatigue test.

[0175] (2) Limit fracture energy release rate of asphalt mixture

[0176] The expansion of fatigue cracks can be divided into three main stages: slow expansion stage, stable expansion stage and unstable failure stage. Under the action of load, the fracture energy release rate G generated at the crack tip continues to increase and reaches the fracture energy release rate threshold G t At this time, the crack extension conditions are met and the crack begins to expand due to fatigue. As the load continues to act, the fracture energy release rate at the crack tip continues to increase until it reaches the limit fracture energy release rate G. pl , the structure becomes unstable and damaged. Studies have shown that the remaining fatigue life of the third stage is much smaller than that of the first two stages and can be ignored.

[0177] (3) Improve the parameters of the Paris formula

[0178] The present invention describes the slow and stable expansion stage of fatigue crack expansion based on the phenomenological model, which is used to judge the crack initiation situation. The judgment criterion is shown in the following formula:

[0179]

[0180] Where: f is the fatigue crack initiation factor. When f≥1.0, the crack starts to develop and enters the stable expansion stage. N0 is the number of load cycles at the beginning of fatigue crack expansion. c1 and c2 are both material-related fatigue crack expansion initiation parameters.

[0181] When the crack expansion enters the stable expansion stage, the crack expansion rate is mainly controlled by the improved Paris formula, as shown in the following formula:

[0182]

[0183] Where: c3 and c4 are material-related parameters and can be obtained by fitting the standard semicircular bending fatigue test.

[0184] From reference 3 (Liu Yu. Research on dynamic response and fracture performance of asphalt mixture based on semicircular bending test [D]. Harbin: Harbin Institute of Technology, 2009.) and reference 4 (Liu Yu, Zhang Xiaoning, Zou Guilian. Analysis of fatigue crack propagation of asphalt mixture based on semicircular specimens [J]. Science Technology and Engineering, 2009, 19(9): 5712-5716.), it can be seen that there are certain differences in the fatigue cracking parameters of asphalt mixtures, but they are very close overall. In order to simplify the simulation process, the present invention assumes that the parameters of the Paris formula of the asphalt surface material remain unchanged under different environmental conditions. The experimental method and initial values ​​of the parameters are determined by selecting reference 2 (Shen Qing. Research on fatigue expansion of reflective cracks in semi-rigid base asphalt pavement based on XFEM [D]. Changsha: Hunan University, 2018.). The initial values ​​in the reference are fitted, and the planning solution in Excel obtains that the limit fracture energy release rate is 1800N / m, and the parameters of the improved Paris formula are: c1 is 0.5, c2 is -1.7, and c3 is 2.64×10 -8 , c4 is 1.16.

[0185] 3. Establishing the Finite Element Model of Temperature Field of Asphalt Pavement Structure

[0186] The following assumptions are made when simulating the road structure temperature field model:

[0187] (1) Except for the surface asphalt concrete, which is a viscoelastic material and is defined by viscoelastic parameters, the materials of the other layers are considered to be linear elastic materials and are defined by linear elastic parameters;

[0188] (2) The road structure layers are in complete contact with each other, and the temperature and heat flow between the layers change continuously, as well as the displacement change continuously.

[0189] 1. Heat transfer

[0190] The pavement structure is in the external environment, and with the change of seasons and day and night, it continuously transfers heat with the atmosphere, causing the temperature of the pavement structure to change. The heat transfer methods mainly include solar radiation, heat conduction and heat convection, which can be achieved by setting thermal boundary conditions in finite element software. The solar radiation, effective radiation of the pavement structure, air temperature and convection heat exchange are simulated respectively to complete the definition of the thermal boundary conditions of the pavement structure in the finite element software.

[0191] (1) Solar radiation

[0192] In finite element simulation, solar radiation refers to the heat radiation from the sun directly reaching the surface of the earth. The solar radiation is calculated according to the principle of Fourier series, and the calculation formula is:

[0193]

[0194] Among them, Q(t) is the daily variation function of solar radiation; t is the time variable; Q0 is the maximum daily solar radiation, usually Q0 = 0.131nQ t , n = 12 / c, Q t is the total solar radiation per day, J / m 2 ; c is the actual effective sunshine duration, h.

[0195] Fourier coefficient In this formula, t=0 is assumed at 6:00 a.m.

[0196] In actual situations, solar radiation is not completely absorbed by asphalt pavement, and some of it is reflected by the asphalt pavement. The solar radiation absorption rate of asphalt pavement is generally 0.9, so the solar radiation absorbed by the pavement is Qs = 0.9Q.

[0197] (2) Effective radiation of road surface

[0198] The effective radiation of the road surface is the difference between the long-wave radiation emitted by the road surface and the atmospheric back radiation absorbed by the road surface. It represents the net loss of long-wave radiation of the road surface and is mainly related to the road surface temperature, air temperature and atmospheric humidity. The boundary condition definition of the effective radiation of the road surface is realized by the following formula:

[0199] q e =εσ[(T r -T Z ) 4 -(T a -T Z ) 4 ];

[0200] Where: q e is the effective radiation of the road surface, J / m 2 ; ε is the asphalt pavement emissivity, which is taken as 0.81; σ is the Boltzmann constant, which is taken as 5.670×10 -8 W / (m 2 ·K 4 );T r is the road surface temperature, °C; T a is the daily average temperature, ℃; T z The absolute zero value is -273℃.

[0201] (3) Air temperature and convective heat exchange

[0202] In addition to the effect of solar radiation, the atmospheric temperature and heat exchange will also affect the temperature of the pavement structure, which can be simulated by the following formula:

[0203] T=T a +T m {0.96sin[ω(t-t0)]+0.14sin[2ω(t-t0)]}

[0204] T a =(T max +T min ) / 2

[0205] T m =(T max -T min ) / 2;

[0206] Where: T a is the daily average temperature, ℃; T m is the daily temperature amplitude, ℃; t0 is the initial phase, usually set to 9h; ω is the angular frequency, ω=π / 12; T max is the highest temperature of a single day, ℃; T min The lowest temperature of the day, ℃.

[0207] The heat exchange coefficient between the pavement structure and the atmosphere is affected by wind speed, and the expression is as follows:

[0208] k c =3.7v+9.4;

[0209] Where: k c is the heat exchange coefficient, W / (m 2 ·℃); ν is the daily average wind speed, m / s. According to the statistics of the National Meteorological Science Data Center, the annual average wind speed in Jiangxi Province is about 2.7 m / s.

[0210] 2. Temperature data selection of pavement structure

[0211] Studies have shown that in the medium and low temperature range, the main disease of asphalt pavement is cracking. In order to study the influence of water vapor factors on the crack resistance of asphalt pavement, the present invention selects the actual temperature, air humidity, wind speed and other climate data of a section of Yisui Expressway in Yichun City, Jiangxi Province, and conducts research on related contents. Referring to the real meteorological condition data, and in order to avoid the influence of a few bad weather conditions such as rainy weather, the present invention selects the average atmospheric temperature of the lowest 10 days in January 2020 in Yichun City, Jiangxi Province as the temperature data of the pavement structure, and lists them in Table 11.

[0212] Table 11 Average temperature at each hour on the coldest day in January in Yichun City, Jiangxi Province

[0213]

[0214] 3. Meshing of temperature field finite element model

[0215] When the finite element software is setting the pavement temperature field and water vapor concentration field, it is necessary to ensure that the meshes between the connected models are exactly the same in order to realize the calling of unit material parameters. In order to import the parameters of temperature and water vapor concentration into the pavement cracking model, the mesh division of the pavement temperature field and water vapor concentration field models must be consistent with the pavement cracking model. The finite element software limits the size of the model mesh by seeding. The present invention adopts a combination of global seeding and edge seeding to divide the model mesh, and sets the meshes into different densities. The surface layer has more meshes, and the subsoil has fewer meshes. In the crack extension area and the load action area, a denser mesh is set, and in other areas, a sparser mesh is set, so as to balance the calculation efficiency and calculation accuracy of the simulation. The model after meshing is as follows Fig.15 shown.

[0216] 4. Temperature field simulation

[0217] Enter the thermal parameters of each structural layer in the finite element software, complete the finite element definition of air temperature, convective heat exchange, and effective radiation of the pavement in the interaction module combined with the user subroutine FILM, and complete the finite element definition of solar radiation in the load module combined with the user subroutine DFLUX. According to the data of the National Meteorological Science Center, the total solar daily radiation of the pavement structure is determined to be 10.61MJ / m 2 , the average wind speed is 2.7m / s, and the temperature boundary, unit type, boundary constraint conditions, etc. of the model are set. Combining the atmospheric temperature in low temperature weather in Jiangxi Province as the temperature data of the pavement structure (Table 11), the pavement structure temperature is simulated in the finite element, and the simulation results of the road surface and the bottom of each asphalt surface layer are plotted as shown in the figure. Fig.16 As shown, the pavement structure temperature field model is obtained.

[0218] Depend on Fig.16 It can be seen that the degree to which the pavement structure temperature is affected by the external environment decreases with the increase of the pavement structure layer depth. The result file of the temperature field simulation is exported and used as a predefined field for the pavement structure water vapor diffusion simulation.

[0219] 4. Establishing the Finite Element Model of Water and Gas Concentration Field of Asphalt Pavement Structure

[0220] 1. Accumulation type water vapor diffusion coefficient

[0221] During the entire process of pavement construction, the asphalt mixture is always in a high temperature state above 100°C, which results in the mixture containing almost no moisture. At this time, the water vapor concentration inside the newly paved asphalt surface layer can be considered to be zero. The surface of the newly paved asphalt pavement is completely exposed to the external environment, and the internal water vapor concentration will change dynamically under the influence of the environment. The pavement structure is built on the roadbed as a whole. Studies have shown that the humidity of the roadbed soil is close to saturation humidity, always maintaining a relative humidity range of 98% to 100%; at the same time, the external environment also maintains a certain humidity, which causes the roadbed and the external environment above the asphalt pavement to form a water vapor concentration difference, driving the water vapor to continuously diffuse from the high concentration to the low concentration, and finally reaching a dynamic equilibrium state of water vapor diffusion between the roadbed soil, the pavement structure and the external environment. According to the law of water vapor concentration changes inside the newly paved asphalt pavement, the diffusion of water vapor in the pavement is divided into two types: accumulation type water vapor diffusion and penetration type water vapor diffusion. Existing studies have shown that when water vapor reaches accumulation equilibrium in the asphalt pavement surface layer, the water vapor distribution inside the pavement structure no longer changes significantly. It can be considered that the accumulation equilibrium state of water vapor is the real water vapor distribution of the pavement structure. Therefore, the accumulation type water vapor diffusion coefficient of the asphalt pavement surface layer is used in this invention to measure the ability of water vapor to reach accumulation type equilibrium inside the asphalt pavement, and the unit is m 2 / s or mm 2 / s.

[0222] Referring to document 5 (Xi Lei. Research on the influence of relative humidity on the pressure properties of asphalt mixture [D]. Wuhan: Wuhan University of Technology, 2021), document 6 (Tu Chongzhi. The influence law and simulation method of water vapor movement on asphalt-aggregate adhesion [D]. Wuhan: Wuhan University of Technology, 2021.), document 7 (Huang Tingting. Research on the mechanism of accumulation-type water vapor movement in asphalt mixture [D]. Wuhan: Wuhan University of Technology, 2018.) and document 8 (Liu Ziyao. Research on the influencing factors of penetrating water vapor diffusion in asphalt mixture [D]. Wuhan: Wuhan University of Technology, 2018.), the research team of the inventor found that the time required for the humidity curing of asphalt mixtures of three gradations of AC-13C, AC-20C and AC-25C to reach equilibrium state is roughly the same, and the accumulation-type water vapor diffusion coefficients of the three asphalt mixtures are in the same order of magnitude. It can be assumed that the accumulation-type water vapor diffusion coefficients of the three gradations of asphalt mixtures are the same. Based on the fact that the inventor's research group (reference 7) has measured the accumulation type water vapor diffusion coefficient of AC-20C limestone asphalt mixture, the accumulation type water vapor diffusion coefficients of three asphalt mixtures at different temperatures are obtained as shown in Table 12.

[0223] Table 12

[0224]

[0225] The research group of the present inventors only conducted tests on the accumulation type water vapor diffusion coefficient within a certain temperature range. In actual pavement structures, the temperature range inside the pavement structure will be larger. In order to obtain the accumulation type water vapor diffusion coefficient under more temperature conditions, the present invention fits the relationship between the accumulation type water vapor diffusion coefficient and temperature through the Arrhenius equation. The fitting model is as follows:

[0226] D=A1e -E / RT , where: D is the accumulation type water vapor diffusion coefficient, A1 is the pre-exponential factor, dimensionless; E is the reaction activation energy, J / mol, R is the universal gas constant, J / (K·mol); T is the temperature in Kelvin, K.

[0227] The water vapor fitting results are shown in Table 13, and the model fitting coefficients are: A1 = 3.753, E = 39.197 J / mol, R = 8.314 J / (K·mol). From the fitting results, the goodness of fit R of the accumulation type water vapor diffusion coefficient prediction model is 2 It can reach above 0.997, which proves that the relationship between water vapor diffusion coefficient and temperature obtained by fitting the Arrhenius equation has high accuracy.

[0228] Table 13

[0229]

[0230] The present invention can expand the water vapor diffusion coefficient of the asphalt surface layer through the above fitting model to obtain the accumulation type water vapor diffusion coefficient in a larger temperature range, as shown in Table 14 below, to provide material parameters for the water vapor concentration field model of the pavement structure.

[0231] Table 14

[0232]

[0233] 2. Parameter settings

[0234] The pavement structure temperature field simulation results established above are used as the predefined field. In the material parameter module, Fick's law is selected as the mass diffusion law, and the temperature and the corresponding water vapor diffusion coefficient are input in the finite element software diffusivity module. Solubility refers to the solubility of the diffused gas in the diffusion medium. In order to reduce the influence of gas dissolution on water vapor diffusion, the solubility is uniformly set to 10 in the model. -7 .

[0235] 3. Determine the upper and lower boundary conditions of water vapor concentration of pavement structure

[0236] Relative humidity and water vapor concentration can be converted from the ideal gas state equation. The conversion relationship is shown in the following formula:

[0237]

[0238] Where: R is the proportional constant, J / (K·mol); T is the Kelvin temperature, K; P T is the saturated vapor pressure of water at the corresponding temperature, Pa; m H2O is the relative molecular mass of water, 18.015 g / mol; C is the water vapor concentration, g / m 3 .

[0239] The entire pavement structure is built on the soil base. Some scholars have calculated that the relative humidity of the soil base is always maintained at about 100% based on soil suction. In the pavement structure selected by the present invention, the base layer is in direct contact with the soil base, the porosity of the base layer is relatively large, and its diffusion coefficient is much larger than that of the asphalt surface layer. In order to simplify the simulation calculation, it is assumed that the relative humidity of the base structure is maintained at 100%, and the lower boundary of the water vapor concentration of the pavement structure is the bottom surface of the layer below the asphalt. According to the temperature field calculation results, the temperature of the structural layer is stable at 7.34°C, and the water vapor concentration is calculated to be 7.93g / m 3 The upper surface of the asphalt surface layer is in direct contact with the air, and the water vapor concentration in the air will affect the water vapor distribution inside the pavement structure. This paper refers to the data from the National Meteorological Science Data Center to obtain the average relative humidity values ​​of the 10 days with the lowest temperature in Yichun City in January 2020 (see Table 15 below), and converts it into water vapor concentration values ​​through the conversion relationship between relative humidity and water vapor concentration, which is used as the upper boundary condition of water vapor concentration in the pavement structure. By calculation, the water vapor concentration in the air is 6.13g / m 3 .

[0240] Table 15 Average relative humidity values ​​for the coldest 10 days in January in Yichun City, Jiangxi Province

[0241]

[0242]

[0243] 4. Establish the water vapor concentration field of the pavement structure

[0244] The upper and lower boundary conditions of water vapor concentration of the pavement structure are input into the finite element method to simulate the non-uniform water vapor diffusion of the pavement structure and obtain the water vapor concentration field of the pavement structure under the influence of the temperature field.

[0245] The model uses mass diffusion analysis steps for analysis. Since the temperature conditions corresponding to the model are different every hour, multiple analysis steps need to be set according to the model running time. The water vapor diffusion of the pavement structure takes a long time to reach a relatively stable state. The present invention simulates the water vapor diffusion phenomenon of the pavement structure (upper layer and lower layer) under this environment. Fig.17The simulation results show that when the water vapor diffuses for about 2400 days, the water vapor concentration inside the pavement structure reaches a relatively stable state. Affected by the upper and lower water vapor concentration boundary conditions, the water vapor concentration of the asphalt surface layer is maintained at 6-7g / m 3 This shows that the pavement structure is in a relatively stable water vapor concentration environment. It is of great significance to study the influence of water vapor on the anti-cracking performance of asphalt pavement. In the future, the simulation results of temperature field and water vapor concentration field will be used as predefined fields to study the influence of water vapor on the anti-cracking performance of asphalt pavement.

[0246] 5. Establishment of fatigue life model of asphalt pavement structure under humidity environment

[0247] 1. Pavement structure model

[0248] The size of the pavement structure model refers to the first part above, which is consistent with the temperature field and water vapor concentration field models. The model is 3m high and 3.75m wide. Figure 1 The road structure dimensions specified in the specification are divided into various structural layers using a partitioning tool. The structural layers are asphalt surface layer, cement stabilized gravel base layer, graded gravel subbase layer and soil base from top to bottom. Under the action of vehicle load, reflective cracks will appear in the base layer, which will continue to expand along the asphalt surface layer until the cracks penetrate the entire surface layer structure. The present invention assumes that reflective cracks have already occurred in the base layer and reflected to the asphalt surface layer. The initial crack is set at the bottom of the base layer, and the crack is allowed to penetrate the entire base layer. After the load is applied, the crack begins to expand upward along the surface layer.

[0249] 2. Wheel load definition

[0250] The current asphalt pavement design specification in my country stipulates that the standard design axle load is a single-axle double-wheel load BZZ-100, with an axle load of 100kN and a tire ground pressure of 0.7MPa. In this model study, a double-wheel axle is used, and the wheel load on each side is represented by two equivalent circles with a diameter of 0.213m. In the ABAQUS finite element software, the definition of different vehicle loads can be achieved by setting the load action area, action time, and load size.

[0251] The wheel load is set as a semi-sinusoidal dynamic load to simulate the pavement structure. The wheel load function expression is: Among them, p(t) is the wheel load function; t is the time variable; p maxis the value of the average step load (usually 700 kPa); T is the load action time (s), which is related to the driving speed v and the radius r of the tire contact area. According to the characteristics of the driving load, a reasonable assumption is made. When the load action point is more than 6r away from a certain point, it can be considered that the point is no longer affected by the load. Therefore, the load action time T = 12r / v, v is the vehicle speed (m / s), and r is the wheel load action radius (m). In this model, r = (0.213 / 2)m.

[0252] 3. Setting of material parameters of asphalt pavement structure model

[0253] According to the material parameters of the asphalt pavement structure model determined in the first part above (thermal parameters, viscoelastic parameters, elastic modulus, Poisson's ratio, fracture parameters and fatigue cracking parameters), corresponding material parameters are set.

[0254] Among them, the finite element method realizes the definition of viscoelastic parameters and fracture parameters through UTRS and USDFLD subroutines. In order to be closer to the actual situation, the asphalt pavement surface layer uses viscoelastic parameters (relaxation modulus Prony series, see Mark 8) to define material parameters, and other structural layers such as base, subbase and soil subgrade are defined by linear elastic materials, and elastic modulus and Poisson's ratio parameters are selected (see Table 9). In addition, fracture energy and tensile strength (see Figure 9-14 ) to set the damage evolution criterion and damage initiation criterion of the model. The load is set to the standard vehicle load.

[0255] In the pavement structure, it is necessary to define three types of asphalt mixtures at the same time. At this time, it is necessary to use the name recognition statement to identify the mixture type, and then change the material parameters. The name recognition statement of the USDFLD subroutine is as follows Fig.18 shown.

[0256] Finite element software can define the fatigue cracking criterion of the model by editing keywords in the contact property module, thereby realizing fatigue analysis of asphalt pavement. To realize cyclic loading of loads, the model uses a cyclic analysis step based on the Fourier series method for analysis. To ensure that the number of cycles is sufficient to achieve fatigue failure of the asphalt pavement, the maximum number of cycles of the analysis step is set to 10. 7 The fatigue cracking criterion of the model is controlled by the parameters of the improved Paris formula and the ultimate fracture energy release rate of the asphalt mixture.

[0257] 4. Set up predefined fields

[0258] Before simulating the crack expansion of the pavement structure, it is also necessary to set the temperature field and water vapor concentration field of the pavement structure to realize the calling of the asphalt surface material parameters.

[0259] (1) Temperature predefined field

[0260] Research shows that at low temperatures, asphalt pavement is mainly damaged by cracking. Fig.16 From the simulation results of the temperature field, it can be seen that the temperature of the entire pavement structure is kept below 19°C. When the temperature field is an incremental step corresponding to 5h, the overall temperature of the pavement structure is the lowest. Therefore, this paper intends to use the temperature distribution corresponding to the temperature model of 5h as the temperature predefined field.

[0261] (2) Predefined field of water vapor concentration

[0262] from Fig.17 The simulation results of the water vapor concentration field show that the pavement structure reaches a water vapor accumulation equilibrium state after 2400 days of water vapor diffusion. This invention mainly studies the influence of water vapor concentration, so the predefined temperature field under different working conditions remains consistent; the water vapor concentration field needs to characterize the long-term water vapor distribution state of the pavement structure, so the water vapor concentration reaching the equilibrium state (2400 days) is selected as the predefined field of water vapor concentration of the pavement structure.

[0263] By setting the above predefined fields, the temperature and water vapor concentration factors can be introduced into the pavement structure model, such as Fig.19 , 20 As shown, these are the predefined fields of pavement structure temperature and moisture concentration.

[0264] 5. Set up a control group

[0265] In order to quantify the effect of water vapor on the crack resistance of asphalt pavement, the present invention sets a control group to analyze the cracking of pavement structure under different water vapor concentrations. The control group assumes that the water vapor concentration of the entire pavement structure is zero. The fracture parameters (fracture energy, tensile strength) of the model can be calculated by the GEP model fitting results of the fracture parameters in Table 10 above, and directly defined in the material parameter module; the dynamic modulus data tested under this concentration is used to define the viscoelastic parameters of the model.

[0266] 6. Job submission and post-processing

[0267] After completing the model settings, you need to import the user subroutine file in the manager to call the material parameters. After the task is completed, create a task in the job module and submit it.

[0268] In order to read the information of crack extension in post-processing, it is necessary to set the field output. However, the output of energy release rate cannot be defined in the CAE interface. The output of ENRRTXFEM can be realized by editing keywords, such as Fig.21 shown.

[0269] 7. Simulation of fatigue life of asphalt pavement under humidity environment

[0270] The present invention simulates the fatigue cracking of asphalt pavement based on the XFEM method and the improved Paris formula. The pavement structure model adopts a two-dimensional model, which can directly observe the expansion of cracks. During the simulation process, the length a of the crack expansion and the corresponding number of load cycles N change as follows: Fig. 22 (where (a) is the simulation case where N = 0 times and a = 2 cm, and (b) is the simulation case where N = 3.079 × 10 7 For the simulation case of a=8 cm, (c) is N=5.584×10 7 For the simulation case of a=14 cm, (d) is N=5.728×10 7 times, simulation case of a=18 cm).

[0271] Depend on Fig. 22 It can be seen that under the cyclic action of load, the cracks expand freely inside the unit and eventually penetrate the entire asphalt surface layer, which proves that the above method can realize the calculation of the fatigue life of asphalt pavement.

[0272] Assuming that the initial crack has extended to 2 cm below the layer. The pavement structure under two environmental conditions, considering non-uniform water vapor concentration (humidity) and the control group, is simulated. The results of the change in crack extension length and load cycle number are shown in Tables 16 and Fig.23 shown.

[0273] Table 16

[0274]

[0275]

[0276] From Table 16 and Fig.23 It can be seen that with the increase of the number of load cycles, when the fracture energy release rate at the crack tip reaches the threshold of fatigue cracking, the crack enters the stable expansion stage and the crack length increases steadily; with the further increase of the number of load cycles, the fracture energy release rate at the crack tip reaches the upper limit, and the crack enters the unstable failure stage. At this time, the crack grows rapidly and penetrates the entire asphalt surface layer. At this time, the number of load cycles is the fatigue life of the pavement structure. Among them, the lower layer and the middle layer area account for more than 98% of the fatigue life of the entire surface layer, and the upper layer area accounts for less than 2%. This shows that the fatigue life of the asphalt surface layer is greatly affected by the middle layer and the lower layer. When the crack expands to the upper layer, the crack expansion rate is the fastest. In the control case (water vapor concentration is zero), the fatigue life of the pavement structure is 6.544×10 7 times, while the fatigue life of the pavement structure considering the non-uniform water vapor concentration field is 5.728×10 7The fatigue life was reduced by 14.246%. This shows that as the water vapor concentration increases, the fatigue cracking resistance of the asphalt pavement decreases, and fatigue cracks are more likely to occur. In areas with high water vapor concentration, the design standard of the pavement fatigue life should be improved to improve the overall cracking resistance of the pavement.

Claims

1. A design method for asphalt pavement under humidity environment, characterized by: The design method is to simulate the fatigue cracking changes of asphalt pavement in finite element software under the multi-field coupling effect of humidity environment; The specific steps include: (1) Determine the asphalt pavement structure model: Use a typical asphalt pavement structure to establish a pavement structure model in finite element software, determine the model size, and use finite element software to complete the division of the pavement structure layer; (2) Determine the material parameters of the asphalt pavement structure model: the material parameters include thermal parameters, mechanical parameters, fracture parameters and fatigue cracking parameters. The thermal parameters include thermal conductivity, heat capacity and material density. The mechanical parameters include viscoelastic parameters, elastic modulus and Poisson's ratio. The fracture parameters include fracture energy and tensile strength. The fatigue cracking parameters include improved Paris formula parameters and limit fracture energy release rate. (3) Establish a finite element model of the temperature field of the asphalt pavement structure: simulate the solar radiation, effective pavement radiation, air temperature and convective heat exchange of the pavement structure respectively, complete the definition of the thermal boundary conditions of the pavement structure in the finite element software, mesh the asphalt pavement structure model determined in step (1), input the thermal parameters of each structural layer in the finite element software, and complete the finite element definition of solar radiation, effective pavement radiation, air temperature and convective heat exchange respectively. Combine the atmospheric temperature in low temperature weather in Jiangxi Province as the temperature data of the pavement structure, simulate the pavement structure temperature in the finite element, and obtain the pavement structure temperature field; (4) Establishing a finite element model of the water vapor concentration field of the asphalt pavement structure: First, determine and test the water vapor diffusion coefficient of the asphalt pavement structure and perform fitting to obtain water vapor diffusion coefficient data at different temperatures. The pavement structure temperature field established in step (3) is used as the predefined field. Fick's law is selected as the mass diffusion law. The temperature and corresponding water vapor diffusion coefficient data are input into the finite element software diffuseness module. Then, the upper and lower boundary conditions of the water vapor concentration of the pavement structure are defined and input. The water vapor diffusion of the pavement structure is simulated in the finite element to obtain the water vapor concentration field of the pavement structure under the influence of the temperature field. (5) Establishment of fatigue life model of asphalt pavement structure under humid environment: Based on the asphalt pavement structure model determined in step (1), the wheel load is set as a semi-sinusoidal dynamic load, and the wheel load function is defined. The material parameters determined in step (2) are set in the finite element software, and the temperature distribution corresponding to the lowest temperature in the temperature field finite element model established in step (3) is used as the temperature predefined field. The water vapor concentration when the equilibrium state is reached in the water vapor concentration field finite element model established in step (4) is used as the water vapor concentration predefined field of the pavement structure. The temperature and water vapor concentration are introduced into the pavement structure model. Finally, a task is created and submitted in the job module to obtain the fatigue life model of the asphalt pavement structure under humid environment.

2. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (1), the model size is: 3m high and 3.75m wide. The pavement structure includes a surface layer, a base layer, a subbase layer and a soil base from top to bottom. The surface layer adopts asphalt mixture, which includes an upper layer, a middle layer and a lower layer in sequence, and the asphalt mixtures AC-13C, AC-20C and AC-25C are adopted in sequence.

3. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (2), the viscoelastic parameter is the relaxation modulus Prony series of the asphalt pavement surface layer. The dynamic modulus and phase angle master curve can be established by conducting indoor uniaxial tensile dynamic modulus tests on asphalt mixtures at different temperatures, different water vapor concentrations, and different loading frequencies. At the same time, the relaxation modulus Prony series expression is determined based on the generalized Maxwell model. Then, the dynamic modulus is converted into the relaxation modulus, and the relaxation modulus Prony series of the asphalt mixture can be determined.

4. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (2), the fracture parameters are obtained through indoor semicircular bending tests of asphalt mixtures at different temperatures and different water vapor concentrations. The fracture parameters are fitted according to the GEP model to obtain the following fitting relationship: Fracture energy of AC-13C asphalt mixture: y=0.88x1 2 -67.19x1+4.65x2 2 -15.81x2+3204.96, R 2 =0.9821; Tensile strength of AC-13C asphalt mixture: y=0.005x1 2 -0.371x1-0.058x2+8.799, R 2 =0.9431; Fracture energy of AC-20C asphalt mixture: y=-0.54x1 2 -16.85x1+3.93x2 2 -2.53x2+2592.53, R 2 =0.9842; Tensile strength of AC-20C asphalt mixture: y=-0.008x1 2 -0.032x1 -0.006x2 2 +0.015x2+8.672, R 2 =0.9560; Fracture energy of AC-25C asphalt mixture: y=-1.64x1 2 +17.13x1+2.44x2 2 +7.57x2+2117.36, R 2 =0.9798; Tensile strength of AC-25C asphalt mixture: y=0.007x1 2 -0.404x1 -0.003x2 2 +0.001x2+7.545, R 2 =0.9491; Among them, y is the model dependent variable, corresponding to tensile strength and fracture energy respectively; x1 and x2 are the model independent variables; x1 is temperature, ℃; x2 is water vapor concentration; g / m 3 .

5. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (2), the improved Paris formula is expressed by the fracture release rate G, specifically: ； is the crack growth rate, a is the crack growth length, N is the variable of the number of load cycles, ΔG is the change value of the fracture release rate, C´ and m´ are material-related parameters; The improved Paris formula parameters include parameters of the slow and stable expansion stage of crack expansion and parameters of the stable expansion stage of crack expansion; Based on the improved Paris formula, the slow and stable expansion stage of crack propagation is described to judge the crack initiation. The judgment criteria are: ; Where: ƒ is the fatigue crack initiation factor. When ƒ≥1.0, the crack starts to crack and enters the stable expansion stage. N0 is the number of load cycles. c1 and c2 are both material-related parameters. When the crack expansion enters the stable expansion stage, the crack expansion rate is mainly controlled by the improved Paris formula, as shown in the following formula: ; Wherein: c3 and c4 are parameters related to materials; c1, c2, c3 and c4 are the parameters of the improved Paris formula, which can be obtained by fitting the standard test.

6. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (3), The solar radiation is calculated according to the Fourier series principle, and the calculation formula is: ; In the formula, Q(t) is the daily variation function of solar radiation; t is the time variable; Q0 is the maximum daily solar radiation, usually Q0=0.131nQ t , n=12 / c, Q t is the total solar radiation per day, J / m 2 ; c is the actual effective sunshine duration, h; Fourier coefficient , in which t=0 is set at 6 a.m.; The calculation formula of the effective radiation of the road surface is: ; Where: q e is the effective radiation of the road surface, J / m 2 ; ε is the asphalt pavement emissivity, which is taken as 0.81; σ is the Boltzmann constant, which is taken as 5.670×10 -8 W / (m 2 ·K 4 ) ; T r is the road surface temperature, °C; T a is the daily average temperature, ℃; T z For absolute zero, take -273 ℃; The simulated relationship of daily temperature variation is: ; Where: T a is the daily average temperature, ℃; T m is the daily temperature amplitude, ℃; t0 is the initial phase, usually set to 9h; ω is the angular frequency, ω=π / 12; T max is the highest temperature of a single day, ℃; T min is the lowest temperature of a single day, ℃; The heat exchange coefficient is calculated as: ; Where: k c is the heat exchange coefficient, W / (m2·℃); ν is the daily average wind speed, m / s.

7. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (4), the water vapor diffusion coefficient is specifically an accumulation type water vapor diffusion coefficient, and the accumulation type water vapor diffusion coefficient data at different temperatures can be obtained through indoor water vapor tests, and then the relationship between the accumulation type water vapor diffusion coefficient and temperature is fitted through the Arrhenius equation, so that the water vapor diffusion coefficient of the asphalt surface layer can be expanded; The fitted model is: ; Where: D is the accumulation type water vapor diffusion coefficient, A1 is the pre-exponential factor, dimensionless; E is the reaction activation energy, J / mol, R is the universal gas constant, J / (K·mol); T is the temperature in Kelvin, K; The fitting results are: A1=3.753, E=39.197 J / mol, R=8.314 J / (K·mol); The upper boundary condition of the water vapor concentration in the pavement structure is the water vapor concentration in the layer below the asphalt pavement, which is calculated based on the temperature obtained from the results of the temperature field finite element model; the lower boundary condition of the water vapor concentration in the pavement structure is the water vapor concentration in the atmosphere, which is the atmospheric water vapor concentration under low temperature weather in the area.

8. The method for designing asphalt pavement under humidity environment according to claim 1 is characterized in that: In step (5), the wheel load function expression is: ； Where p(t) is the wheel load function; t is the time variable; p max is the value of the uniform step load; T is the load action time, s; T=12r / v; v is the vehicle speed, m / s; r is the wheel load action radius, m.

9. The method for designing asphalt pavement under humidity environment according to claim 1, characterized in that: Step (5) also includes: setting the condition where the water vapor concentration of the entire pavement structure is zero as a control group, comparing it with the pavement fatigue cracking condition under the humidity environment, and simulating the fatigue cracking of the asphalt pavement structure; The specific method for simulating fatigue cracking of asphalt pavement structure is as follows: based on the extended finite element XFEM method and the improved Paris formula, the fatigue cracking of asphalt pavement is simulated, and the expansion of cracks can be directly observed. When the cracks enter the unstable failure stage under the action of load, the cracks continue to expand and eventually penetrate the entire surface layer. The number of load cycles at this time is the fatigue life of the pavement structure. The fatigue life of the pavement under the humidity environment is compared with the control group, so as to adjust the design standards in the actual asphalt pavement design process to improve the pavement's ability to resist cracking.

10. The method for designing asphalt pavement under humidity environment according to claim 3, characterized in that: The relaxation modulus Prony series expression is: ； Where E(t) is the relaxation modulus function, in MPa; t is time; E g is the instantaneous relaxation modulus, in MPa; i=1, 2, …, m, where m is the total number of model elements in parallel; E i is the relaxation modulus of the i-th spring; τ i is the relaxation time of the i-th spring in the generalized Maxwell model, in seconds; the relaxation modulus Prony series is the parameter E g and E i .

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