Influence of temperature and humidity coupling on rutting deformation of asphalt pavement

3.2.1 Test materials

The AC-13C and AC-20C asphalt mixes selected SBS-modified asphalt and diabase aggregate, and AC-25C selected 70# matrix asphalt and limestone aggregate. The asphalt was tested according to the relevant test specifications [14], and the test results were all in line with the specifications. The results are listed in Table 2. The test results of the asphalt mixture ratio are shown in Figures 6–8.

Figure 6

Grading curve of the AC-13C asphalt mixture.

Figure 7

Grading curve of the AC-20C asphalt mixture.

Figure 8

Grading curve of the AC-25C asphalt mixture.

According to the above gradation, the Marshall test was conducted according to the test procedure [14], and the optimal oil–stone ratio of the three asphalt mixtures was determined to be 5.0, 4.5, and 3.9%. Marshall specimens were prepared according to the optimum oil–stone ratio and design grade, and the test results are shown in Table 3.

3.2.2 Moisture diffusion test

Referring to the relevant literature research methods [15], the moisture diffusion test device of the asphalt mixture penetration type is designed according to the gravimetric method, as shown in Figure 9. The main part of the device includes a glass container (diameter 70 mm, height 100 mm), specimen, and distilled water. Silicone gaskets are added between the specimen and glass container and sealed with caulking wax. The device is placed in an environmental chamber so that the moisture concentration difference is formed on both sides of the specimen (relative humidity RH2 is controlled by the environmental chamber).

According to the design and grading of the asphalt mixture, the test piece is prepared by the rotating compaction method. The specific method is as follows: a cylindrical specimen with a diameter of 150 mm and a height of 170 mm is formed by a rotary compacting instrument. Then, a standard specimen with a diameter of 100 mm and a height of 150 mm is cut by a core drilling machine and cutting machine. Finally, a thin slice with a thickness of 15 mm ± 1 mm is cut from the middle of the standard specimen as the test specimen. Assemble the test device according to Figure 9. The specific assembly steps are as follows:

1. Mark the glass every 1 cm, and inject distilled water to a 3 cm mark;

2. Apply vacuum silicone grease on the upper edge of the glass container and stick the silicone gasket on the upper edge of the glass container;

3. Apply the melted paraffin wax evenly on the silicone gasket, and quickly cover the specimen sheet with the silicone gasket to seal the opening of the glass container. In this process, the specimen should avoid sliding;

4. Molten paraffin wax is used to fill the side of the specimen, the joint between the specimen and the silicone gasket, and the joint between the silicone gasket and the edge of the glass container so that only the upper surface of the specimen is in contact with the outside world;

5. Smooth the whole device with knives to prevent paraffin chips from falling off and affecting the test results during the later weighing process (Figure 10).

Figure 9

Schematic diagram of the penetrating moisture diffusion coefficient measuring device.

Figure 10

Assembly process of moisture diffusion device. (a) Inject distilled water into the container, (b) paste the circular silicone gasket, (c) paste the test piece, and (d) test device finished product.

In this article, five groups of specimens are set up at intervals of 5°C within 10–30°C. According to the literature, after injecting distilled water into the device, the relative humidity in the device can reach balance after 24 h, at which time the relative humidity in the device is about 100% [15]. The whole device is placed in an environment chamber with 50% relative humidity. At this time, due to the existence of the moisture concentration difference on both sides of the specimen, moisture diffuses from the side with high concentration to the side with low concentration, thus reducing the weight of the whole device. The mass reduced by the device is the moisture mass reduced by moisture diffusion.

3.2.3 Analysis of test results

Fick’s first law is often used to describe penetrating moisture diffusion, which can be expressed in the following form:

where J is the diffusion flux, g/m² s. D is the effective diffusion coefficient, m²/s. d C d x is the internal concentration gradient of the mixture, which is approximately the concentration difference divided by the thickness of the specimen.

The relationship between moisture concentration and relative humidity is shown as follows:

where C is the moisture concentration (g/m³). P _T is the saturated vapour pressure of water at the corresponding temperature (Pa). R: 8.314 J/(K mol). RH is the relative humidity (%) m H 2 O is the relative molecular weight of water (18.015 g/mol). T is the Kelvin temperature, and its value is Celsius +273.15 (K).

The moisture concentration at different temperatures and relative humidities can be calculated from equation (4), and the results are shown in Table 4.

The diffusion flux J is the variation in water and air per unit area in unit time, and its expression is

where A is the specimen diffusion area, and “−” indicates that the diffusion direction is the opposite direction of the concentration gradient. d W H 2 O d t is the moisture mass loss rate.

The expression of the diffusion coefficient can be obtained from equations (3) and (5):

where L is the specimen thickness, mm. C _i and C _j are the moisture concentrations on both sides of the specimen, g/mm³.

By regularly measuring the mass loss in the device, the penetration moisture diffusion coefficient can be obtained by calculating from equations (6). The diffusion coefficients of the three asphalt mixtures at different temperatures are shown in Table 5.

By comparing the diffusion coefficients of the three asphalt mixtures at different temperatures, it can be found that the diffusion coefficient of the AC-25C asphalt mixture is the largest, followed by the AC-20C asphalt mixture, and the AC-13C asphalt mixture is the smallest. This may be because there are more pores in the mixed material with a larger particle size, which easily forms moisture channels, which is manifested as a larger moisture diffusion coefficient. When the temperature increases from 10 to 30°C, the moisture diffusion coefficients of the AC-13C, AC-20C, and AC-25C asphalt mixtures increase by 1.676, 1.465, and 1.364, respectively. Thus, the diffusion coefficient of the AC-13C asphalt mixture is more sensitive to temperature change.

3.3.1 Diffusion coefficient of each structure layer of asphalt pavement

According to Section 2.3, the maximum temperature of the asphalt pavement surface is close to 60°C. According to the test results in Section 3.2.3, the moisture diffusion coefficient of the surface structure at 10–60°C is obtained, as shown in Table 6. The diffusion coefficient of the base material is referred to in the data in the literature [12].

where A ₁ is the preexponential factor. R = 8.314 J/(K mol). T is the Kelvin temperature.

3.3.2 Model boundary condition setting

This article considers the variation of non-uniform moisture concentration field on asphalt pavement under the influence of all-weather temperature and humidity and therefore introduces ambient humidity as a boundary condition into the calculation model. It has been confirmed that the relative humidity of soil foundation is above 97% [16]. In this article, the pavement structure base is graded gravel with a large porosity, which is considered to be the same as the ambient humidity of the soil foundation. Therefore, it is assumed that the relative humidity at the boundary under the model is 100%, and the moisture concentration can be calculated as 28.33 g/m³ by using equation (4). The boundary conditions of the model are set equal to the ambient humidity. The calculated results are shown in Table 7 (the saturated vapour pressure of water at different temperatures is calculated by the linear interpolation method).

3.3.3 Temperature humidity coupling

The upper and lower moisture concentration boundary conditions of pavement structure are set into 24 analysis steps, and 24 predefined fields are created to introduce the 24 h daily temperature field simulated in Section 2.3. This part imports the all-weather temperature and humidity environment. By submitting the analysis, the non-uniform moisture concentration field of the road structure under all-weather temperature and humidity can be obtained.

3.3.4 Analysis of moisture concentration field results

Cyclic loading for 7 days under the same environmental conditions is carried out. The calculation results of the bottom of the upper layer, the bottom of the middle layer, and the bottom of the lower layer are taken to draw the curve diagram of the asphalt pavement moisture concentration field, as shown in Figure 11.

As shown in Figure 11, after the moisture concentration field inside the asphalt pavement reaches equilibrium, the moisture concentration at the bottom of the upper layer changes periodically, while the moisture concentration at the bottom of the middle and bottom layers basically remains stable. The variation range of moisture concentration inside the asphalt pavement is consistent with the ambient humidity range.

Figure 11

Moisture concentration curve of asphalt pavement at different times.

Seven different time points within 24 h are selected to analyse the variation rule of moisture concentration along the depth direction of the pavement structure. The variation curve is shown in Figure 12. As shown in Figure 12, after 7 days of moisture accumulation, the influence of environmental humidity on the moisture concentration of asphalt pavement is mainly concentrated in the upper layer and the middle layer. The moisture concentration of the upper layer varies greatly, while that of the lower layer is less affected.

Figure 12

Curve chart of moisture concentration changing with pavement structure depth at different times.

4.1.1 Creep model

This article selects the creep model for simulating asphalt pavement rutting. There are two common ways to describe the creep characteristics of materials: the time-hardening creep model and strain-hardening creep model. The time-hardening creep model is applicable to situations where the stress state remains constant during the analysis process, while the strain-hardening creep model is applicable to situations where the stress state changes during the analysis process. Due to the fact that the occurrence of asphalt pavement rutting is always accompanied by changes in stress state, a strain hardening creep model is used for rutting analysis. The model equation can be expressed as follows:

where ε ̇ _cr is the uniaxial equivalent creep strain rate, q is the stress, t is the time. A, n, m is the model parameter, determined through indoor creep tests. A, n ＞ 0, −1＜ m ≤ 0.

4.1.2 Boundary

Academic x-direction displacement on both sides of the model. Fixed at the bottom of the model.

4.1.3 Coupled analysis

This article considers the impact of ambient temperature and humidity on asphalt pavement rutting deformation. The temperature field and moisture concentration field affect rutting by influencing pavement material parameters. Set predefined temperature and moisture concentration fields in the “load” module. First, import the temperature field calculation results at different times (select 5 h, 9 h, 11 h, and 14 h) and calculate the asphalt pavement rut depth at four different temperatures. For ambient humidity, import the calculation results of the asphalt pavement moisture concentration field at four different times and submit the analysis to obtain the asphalt pavement rut depth under different temperatures and moisture concentrations.

4.4.1 Temperature

Considering the influence of ambient temperature, temperature fields at different times are introduced. The rutting deformation of asphalt pavement under the action of different temperature fields is shown in Figure 13 (unit: m).

Figure 13 shows the vertical deformation diagram of asphalt pavement under the action of the temperature field at different moments. Introducing 5 and 11 h temperature fields, under 500,000 axial actions, the vertical deformation of asphalt pavement is 1.082 and 1.751 mm, respectively. When 11 and 14 h temperature fields are introduced, the vertical deformation of asphalt pavement is 4.732 and 8.274 mm, respectively. The ambient temperature has a great influence on asphalt pavement rutting deformation. When the ambient temperature is low, the vertical deformation of asphalt pavement increases slightly. When the ambient temperature exceeds 30°C, the rutting deformation of asphalt pavement increases obviously. This is mainly due to the fact that the asphalt mixture is a temperature-sensitive material.

Figure 13

Vertical deformation diagram of asphalt pavement under different temperature fields: (a) 5 h, (b) 9 h, (c) 11 h, and (d) 14 h.

4.4.2 Ambient humidity

On the basis of Section 4.4.1, the calculation results of 5, 9, 11, and 14 h moisture concentration fields are imported. The rutting deformation of asphalt pavement under the action of different temperature and moisture concentration fields is shown in Table 10.

According to the simulation results, it can be seen that in the low-temperature environment, the ambient humidity has less influence on the rutting deformation of asphalt pavement. When the road surface temperature reaches the maximum at 14 h, considering the influence of environmental humidity, the rutting deformation of asphalt pavement increases by 67%.

Figure 14 shows the vertical displacement curve of each structural layer of asphalt pavement under the action of a 14 h temperature field and moisture concentration field. As can be seen Figure 14, the rutting deformation of asphalt pavement mainly occurs in the upper and middle surfaces. Under the action of load, the asphalt pavement under the tire has settlement deformation. Additionally, due to the extrusion action on both sides of the tire, the pavement structure on both sides of the tire is uplifted. The rutting deformation of the upper and middle layers is a “W” shape and changes to a “V” shape with increasing structure layer depth. The ambient humidity has a great influence on the rutting deformation of the upper and middle layers of asphalt pavement.

Figure 14

Vertical displacement curve of each structural layer of asphalt pavement under different factors: (a) surface, (b) upper floor, (c) middle bottom, and (d) bottom layer.